Economic Geology News

Oxide Gold Hides Primary High-Grade Ore, and a New Book (November 30, 2022)

Lately, I wrote a book-review for Applied Earth Science, a joint journal of the IoM3 and the AusIMM. The book was 'Formation of Gold Deposits' by Neil Phillips, published by Springer earlier this year. Reading it, I was enthused. But I'll come to that further down.

In his book, Neil recounts briefly the history of Fosterville Gold Mine near Bendigo in the Victorian gold province (Australia) that used to be a small open pit oxide ore operation. A few years ago, and with new owners, sulfide-hosted and high-grade native gold was found down to about 500 m below surface and still open continuation. I am convinced that this success should be an important lesson for similar cases! Always be attentive for primary gold ore shoots underneath the supergene zone!

Neil informs that the site was one of the thousands of minor gold deposits discovered after the Victorian gold rushes in 1851. The shallow free-milling oxidized heap-leaching Au ore at Fosterville had earlier been wrongly interpreted as of hydrothermal-epizonal origin, alike to most Carlin (USA) ore. In the Carlin gold province, the presence of primary sulfide ore beneath a thick regolith of friable clays and iron oxides was first recognized in the Screamer zone beneath the Goldstrike deposit, marked by auriferous quartz veins, calcite, pyrite, and arsenopyrite. Similarly at Fosterville, deep gold ore was predicted, based on 30 years of published research, including Neil's. Deep drilling from 2015-2020 yielded multiple core intersections of primary pyrite-arsenopyrite and native gold-stibnite ore locally grading over 1000 g/t Au. Today, a low-cost high-grade underground mine is working. In 2020, the operation produced a record 640,467 ounces of gold at an average grade of 33.9 g/t Au and average recoveries of 98.9% (Fuller & Hann 2019).

Fuller & Hann (2019) stand here for a full NI 43-101 Technical Report on Fosterville Gold Mine, written for a Canadian Stockmarket in order to support a formal resources and reserves statement. Some of my readers may be surprised, how much information about the mine can be found in this kind of report, and it is OPEN ACCESS. Is this not amazing?

Allow me to cite a brief characterisation of the Fosterville Deep gold deposit (Fuller & Hann 2019):

The geodynamic setting of the gold province was a Paleozoic subduction-related active margin of Gondwana (Taylor et al 2017). Victorian deposits are of the 'gold-only' class (Phillips 2022). 'Orogenic type' would be the common term; I call this class 'metamorphogenic' (Pohl 2020, 2022). In my Economic Geology book (2020) on page 236, you may read a brief description of the Victorian gold province. Mineralization at Fosterville is controlled by post-folding brittle faulting. These faults are generally steeply west-dipping reverse faults with a series of moderately west-dipping reverse splay faults formed in the footwall of the main Fosterville fault. There are also moderately east-dipping faults. Primary gold mineralization occurs as disseminated arsenopyrite and pyrite forming a selvage to veins in a quartz-carbonate veinlet stockwork. The mineralization is structurally controlled with high-grade zones localized by a geometric relationship between the bedding of turbitites, folds and faults. Mineralized shoots are typically 4 to 15 metres thick, 50 to 150 metres up/down dip and 300 to 1,500 metres down plunge, and have average grades of 5 to 10 g/t gold, with individual assays up to 60 g/t gold.

Graphical
Figure 7-4 Fosterville fault zone schematic cross section looking North (Fuller & Hann 2019)

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Fuller & Hann (2019) provide a detailed history of earlier exploration at Fosterville, by half a dozen different companies. The last owners did use some of the common methods in their latest mode, but the real difference is the amount of drilling, which is known as the proverbal key to success.

Fuller & Hann (2019) do not mention magnetotelluric data (MT), which reveal that Victorian gold deposits are underlain by a source zone at a crustal depth of >20 km, and by pathway zones of low resistivity that lead up to gold deposits with >1 t total Au production (Heinson et al. 2021). At about 440 Ma, orogenic amphibolite metamorphism at depth mobilized fluids and HS- ligands for Au, and CO2. Organic carbon of the marine sediments was graphitized, explaining the low resistivity. The heat source (such as a slab window) that triggered the fluid release was probably in the mantle but this is not imaged by the authors.

Useful features of the 'Formation of Gold Deposits' by Neil Phillips include two pages of Terms and Abbreviations after the Preface, and examples of 3 maps showing A. gold-only and gold-plus provinces, B. gold-only and gold-plus goldfields and deposits, and C. map with figures. Page xiv provides a geological time scale of Earth history and the timing of 9 important gold provinces. Appendices explain A: Production, Endowment, Reserves and Resources; B: Regolith Science; C: Disagreements in Gold Geoscience; D: Approaches to Research of Gold Deposits; E: Some Useful Reading.

The 'Formation of Gold Deposits' by Neil Phillips is a great book. It is very original, e.g. in proposing the bimodal classification of gold-only and gold-plus deposits. It is strong in the explanation of processes, using clear language and often chemical arguments. Accesses to the Springer book website (URL below) until this day count nearly 6000. If you work in gold geoscience or elsewhere in the gold sector, I fear, you'll have to buy it.

References

Fuller T & Hann I (2019) Fosterville Gold Mine updated NI 43-101 Technical Report for Kirkland Lake Gold Ltd. 244 pp. Numerous figures and Tables: Download from https://www.agnicoeagle.com/English/operations/operations/Fosterville-Gold-Mine/default.aspx

Heinson G, Duan J, Kirkby A, et al (2021) Lower crustal resistivity signature of an orogenic gold system. Nature Sci Rep 11:15807. DOI https://doi.org/10.1038/s41598-021-94531-8 Open Access

Phillips, Neil (2022) Formation of Gold Deposits. 291 pp. 141 Figures, 21 Tables. Springer Singapore. DOI https://doi.org/10.1007/978-981-16-3081-1 eBook (PDF) ISBN978-981-16-3081-1 $ 85.59 VAT within EU Hardcover ISBN978-981-16-3080-4 $ 109.99

Pohl WL (2022a) Metallogenic models as the key to successful exploration -- a review and trends. Mineral Economics (2022) : 36 pp. Springer. https://doi.org/10.1007/s13563-022-00325-3 Open Access

Pohl WL (2022b) Supplementary Information: The online version contains supplementary material (Figures and Subtitles) available at https:// doi. org/ 10. 1007/ s13563- 022- 00325-3

Pohl WL (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons - an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. Print www.schweizerbart.de/9783510654352 (Soft Cover), www.schweizerbart.de/9783510654413 (Hard Cover). E-book (PDF) ISBN 9783510654369 sold by https://www.lehmanns.de/shop/naturwissenschaften/57770431-9783510654369-economic-geology

Taylor DH, Willman, CE, Hughes MJ & Boucher RK (2017) Recent gold mining and exploration in Victoria. Pp 807-810 in Australian Ore Deposits (ed GN Phillips), Australasian Institute of Mining and Metallurgy


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Formation of Superhydrous Melts at the MOHO: A NEW EXPLORATION TOOL (October 17, 2022)

ABSTRACT (Urann et al. 2022):

Magmatic volatiles (for example, water) are abundant in arc melts and exert fundamental controls on magma evolution, eruption dynamics and the formation of economic ore deposits. To constrain the H2O content of arc magmas, most studies have relied on measuring extrusive products and mineral-hosted melt inclusions. However, these methods have inherent limitations that obfuscate the full range of H2O in arc magmas. Here, we report secondary-ion mass spectrometry measurements of volatile (H2O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan palaeo-arc (northwestern Pakistan) and determine H2O abundances of melts from which the cumulates crystallized. Pyroxenes retained magmatic H2O abundances and record damp (less than 1 wt% H2O) to hydrous (up to 10 wt% H2O) primitive melts. Subsequent crystal fractionation led to formation of superhydrous melts with approximately 12-20 wt% H2O, predicted petrologically yet virtually absent from the melt-inclusion record. Porphyry copper deposits are probably a natural eventuality of fluid exsolution from superhydrous melts, corroborating a growing body of evidence.

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I suggest that the recognition and petrologic modelling of the formation of superhydrous melts by Urann et al. (2022 - see Abstract above) is a great step ahead for a full understanding of suprasubduction-magmatic-hydrothermal ore classes such as Cu, Cu-Au or gold only porphyry deposits. Together with the reform proposals on a modern understanding and use of the term 'Metallogeny' employing the case of GOLD for the study that I published recently (Pohl 2022a and b), metallogenic models for exploration will be much improved.

In order to illustrate the logic place where the new model of high water contents in residual melts fits, I briefly list below some important points, which I made in my 2022 review-paper, concerning the source-to-trap process chain that leads to porphyry ore formation:

a Metallogenic classes of ore and mineral deposits (not types) (page 5 of Pohl 2022)

b Subduction of lithospheric plates at convergent ('destructive') plate boundaries (page 10)

c Metallogenic models: SOURCE of METALS (page 13) Increasingly, the source of orogenic and of magmatic-hydrothermal porphyry gold, copper or Cu-Au deposits is suggested to have been metasomatized, hydrated and/or fertilized mantle. The fertilization may be caused by asthenospheric partial melts, by devolatilization, dehydration and partial melting of subducting oceanic crust and sediments, or by plume-derived melts (Pohl 2022b: ANNEX Figure 5).

Graphical
Figure 5 Schematic sketch of major plate tectonic settings of gold metallogeny: At the metallogenic subduction factory underneath an active continental margin, the slab is devolatized by low T/high P metamorphism. The mafic crust along the slab top surface is the main source of water released at subarc depths. Metalliferous supercritical fluids and melts may metasomatize ('fertilize') the mantle wedge or subcontinental lithospheric mantle (SCLM). Enriched, or 'metasomatized' mantle (mSCM) is the source of hydrous and metalliferous magmas and fluids that rise toward the MOHO where they intrude and cool down to ambient temperatures, forming pyroxene cumulates: The MOHO underplating system (red circles). Residual melts display a high concentration of water and other volatiles, of metals, and a supercritical state. They rise toward the surface where they form porphyry, epithermal and orogenic gold ore deposits. The hydrous nature of alteration zones and higher conductivity of mineralized bodies can be detected and imaged by modern geophysics such as magneto-tellurics.

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d Mobilized by heat pulses, hydrous melts and fluids intrude (underplate) below the MOHO (page 15)

e NEW HARD DATA by Urann et al. (2022): "Dry" pyroxene crystal fractionation leads to formation of cumulates and residual superhydrous melts with approximately 12-20 wt% H2O (see Abstract above)

f Metalliferous supercritical superhydrous melts rise through the continental crust, creating or following vertically extensive conduit systems (UPFLOW); exsolving fluids precipitate matter and form ore deposits (TRAP) (page 16)

g Metallogeny in gold exploration practice (page 17) This chapter of my paper briefly touches most methods used in modern exploration. Here, I'll only mention current work on holistic joint analysis of seismic, magnetotelluric (MT) and potential field methods for the next generation of exploration in Australia. For searching and mapping deep conductive zones, magnetotellurics is eminently useful. As throughout my review (Pohl 2022a), brief presentation of case locations facilitates further studies.

h The role of metallogeny in mineral systems analysis (MSA) (page 24): By integrating innovations in geosciences, metallogenic understanding at all scales grows ceaselessly; some of this growth may provide novel keys that can be used in exploration. MSA chooses from the metallogenic toolbox those features that may be effective new search criteria. The new understanding of the formation of superhydrous melts at the base of the continental crust by Urann et al. (2022) is, in my opinion, one of these useful innovations. It explains the concentration of water, of other volatiles, of metals, and the supercritical state of such melts. The hydrous nature of alteration zones and elevated conductivity of mineralized bodies can be detected and imaged by modern geophysics such as magnetotellurics (g).

This is one example, how the science of ore formation (metallogeny) is a never-ceasing source of new discovery models and tools in MSA applied to exploration.

Alas, the paper by Urann et al. (2022) is not OPEN ACCESS. Free is, however, a very valuable list of References and the usual bibliographic data.

References

Pohl, W.L. (2022a) Metallogenic models as the key to successful exploration -- a review and trends. Mineral Economics (2022):36 pp. Springer. https://doi.org/10.1007/s13563-022-00325-3 Creative Commons Licence Open Access

Pohl, W.L. (2022b) Supplementary Information: The online version contains supplementary material (Figures and Subtitles) available at https:// doi. org/ 10. 1007/ s13563- 022- 00325-3

Urann B.M., Le Roux V., Jagoutz O., et al. (2022) High water content of arc magmas recorded in cumulates from subduction zone lower crust. Nat. Geosci. 15, 501-508 (2022). https://doi.org/10.1038/s41561-022-00947-w

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MINERAL ECONOMICS "What we have learned from the past and how we should look forward" . Wisdom from a professional leader (September 19, 2022)

The recent paper by Friedrich-Wilhelm Wellmer (2022) provides profound geological wisdom and surprising solutions concerning some of the fundamental discussions in mineral and metals economics. To this day, Springer Nature counts about 1000 accesses to this paper. It is marked by the editors as 'open access' and 'high impact'.

The author's Abstract, Keywords and Conclusions illustrate best the coverage and width of his treatment:

Abstract

From the vantage point of more than 50 years' work in the raw material field, as well as working in the private sector, in the German federal ministry of economics, at a geological survey, and engaged in teaching and supervising research at a university, I draw a number of conclusions about the following aspects of the fields: development of long-term prices, the long-term supply situation, especially the expectation of an imminent peaking of supply, the frequent and mistaken prediction of shortfalls in supply, our understanding of reserves and resources, and the cyclic nature of success in exploration. I am solely dealing with geological aspects, not taking into account political inferences and supply disruptions. This is followed by an attempt to look into the future of raw materials demand within the framework of the accelerating green energy transition. These conclusions are:

Keywords: Price developments · Technological breakthroughs · Growth rates · Reserve to production ratio · Exploration success · Raw materials for the green energy transition · Innovation · Substitution · Learning curves.

For an example of the depth and style of Wellmer's treatment, read the following paragraph on "How the balance between success in exploration and exploitation technology can keep up with rising production and result in a stable relationship between reserves and production (R/P-ratios), declining grades and constant prices in real terms (Conclusion 9)".

" Reserves (Figure 8) are one side of the coin of mineral supply. Nature offers us mineral enrichment in the Earth's crust to produce tools for needed functions. The other side of the coin is the mining companies which develop resources, bring them to the market and satisfy supply needs. Supply shortfalls had been forecast during the last 50 years again and again. Typically, graphs were produced, indicating not a shortage of reserves, but a shortage of projects needed to come on stream to cover the required supplies. During my time in the German Federal Ministry of Economics such graphs were regularly presented, for example by the uranium and electricity industry, which was afraid of not being able to supply the envisaged future reactors with nuclear fuel. Recent examples are the graphs by the International Energy Agency for copper, lithium and cobalt, important raw materials for the green energy transition. These predicted shortages rarely occurred. The free market mechanism functioned as expected: prices were a reliable signal to reflect demand and supply. Essential is the feedback control cycle of mineral supply: If a shortage occurs, prices increase which can be painful as long as they last. Or as the Economist wrote: " high prices are the best cure for high prices " (NN 2021). Rising prices cause reactions on the supply and the demand side. Shortages contain the seeds of their own destruction. On the supply side, the usual response to sharply higher prices is to increase both the primary and the secondary production. The first is achieved by bringing in new deposits, or lowering the cut-off grade, and mining lower grade parts of an orebody. The second, by reprocessing lower grade scrap. On the demand side, consumers develop and use alternative materials or substitute technologies that use entirely different materials, or simply find ways to get the same output with less material input. After these reactions on both the supply and demand side, a new price equilibrium is again established".

Resources
Figure 8 The Total Resource Box showing the dynamic system of resource categories (x-axis: general trend of increasing knowledge, going from left to right; y-axis: general trend of increasing economic viability going from bottom to top)

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For professionals working in the exploration, mining and trading sector this paper is highly recommended. The PDF can easily be searched using the Adobe Acrobat Reader DC. I am convinced that it will be a useful tool in your work.

References

NN (2021) Chipmaking. The chip shortage is a self-solving problem. The Economist 7.8.2021

Wellmer, FW (2022) What we have learned from the past and how we should look forward. Miner Econ (published March 10, 2022). 31 pp. 22 Figures. URL https://doi.org/10.1007/s13563-021-00296-x Open Access


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LITHIUM NOVEL EXPLORATION KEYS & REPORTING (17 August 2022)

In the new-age energy technology, Lithium (Li) is a key metal. World supply must grow rapidly. Prices are steeply rising. Consequently, new sources are needed and must be found and developed. Investors are keen to support promising projects. Current producers of economic significance are restricted to few genetic classes (see more detail in Section 2.5.10 pages 289-292 in Pohl 2020):

o Rare element (Li-Cs-Ta) pegmatites with lithium minerals and often, with exploitable tenors of Sn, Ta (>Nb) and Be (e.g. Greenbushes, W.A.); essentially, these mines sell concentrates of lithium minerals but Tanco (Canada), for example, is a unique spodumene-Cs-Rb-Ta producer;

o Evaporative lithium brines of playa lakes or from their subsurface aquifers (e.g. Salar de Atacama, Northern Chile; Salar de Uyuni, Bolivia); operations market lithium carbonate or chloride.

Read novel publications that may assist scientists and practicioners in lithium exploration:

Lithium brines can be exploited and processed at much lower cost compared with hard rock minerals. Brine deposits occur in the western USA, Argentina, China (Quaidam salt basin and Tibet) and Chile. Vast potential deposits are known in Afghanistan, Bolivia and Peru.

The Salar de Atacama in the Chilean Altiplano contains a significant part of the world's known lithium resources. The salar is a dry depression at 2300 m altitude with an area of 2900 km2. A massive halite facies occurs in the south-centre of the basin. Halitite in this area contains brines with 2.55 g/l Li, 27.4 g/l K (as well as traces of Rb and Cs) and 0.82 g/l B (Houston et al. 2011). The brines are pumped into shallow constructed ponds where solutes are concentrated by natural evaporation to a final tenor of ~4% Li. The enriched brine is chemically processed. Products are Li2CO3, KCl, K2SO4 and H3BO3. Recently, the primary source of most of the lithium in the Atacama brine district was determined in marginal ignimbrites using lithium isotopic geochemistry (Álvarez-Amado et al. 2022).

The ignimbrites are moderately enriched in Li (between 20 and 50 ppm, averaging 33 ppm) and display δ7Li values from -1.5 ‰ to +12.8 ‰. Different processes together with geological features and the hydrologic and climatic history of the region caused the formation of three subzones marked by distinct lithium isotope fractionation (see Graphical Abstract: Álvarez-Amado et al. 2022).

Crustal 7Li/6Li ratios are relatively uniform (δ7Li ~0 ‰). Li isotopes are not fractionated through redox reactions and biological processes. Yet, the large relative mass difference between the two stable isotopes causes considerable fractionation. In hydrothermal processes in the crust, fractionation of stable lithium isotopes 6Li (7.59%) and 7Li (92.41%) serves as an efficient tracer. Near-surface processes may cause distinct fractionation, as shown by lithium isotopes in the Atacama basin (Álvarez-Amado et al. 2022).

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Atacama
Figure Graphical Abstract: Geochemical and isotopic lithium domains in the Atacama basin, Chile (Álvarez-Amado et al. 2022)

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Spodumene-bearing pegmatites are the most important and easily exploitable Li deposits, typically containing 0.5 Mt Li (Bowell et al. 2020). Associated with granitic rocks ore comprises spodumene (LiAl(SiO3)2) and some other economic minerals. Prospecting for lithium-pegmatites was broadly and instructively described by Gao et al. (2020) and in-depth documented by Bradley et al. (2017).

The prospecting campaign described by Gao et al. (2020) in the Bayankala Fold Belt in NW-China is characterized by high mountains and rugged steep topography, which make it extremely difficult to explore. The authors chose a combination of geochemical methods, geological mapping, and remotely multispectral imagery in order to spot potential Li-mineralisation. The target was pegmatite clusters potentially containing the mineral assemblage of spodumene-muscovite-albite and quartz. This required processing high-resolution and multispectral remote sensing data from Worldview-2, Worldview-3 and ASTER. This strategy led to discovery of many large Li mineral occurrences. Steps of data processing are in-depth explained.

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Spodumene
FIGURE 2 | Hyperspectral curve for spodumene-bearing pegmatites at Greenbushes (Australia) and Dahongliutan, and tourmaline-bearing pegmatites at Dahongliutan. Samples were analyzed by an Analytical Spectral Device (ASD) FieldSpec spectrometer with a spectral range of 350-2,500 nm and a nominal resolution of 1 nm (Gao et al. 2020).

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FIGURE 9 | Model of pegmatite swarms in the Dahongliutan area (NW-China) (Gao et al. 2020), showing: (A) cross-section of the Triassic granite-pegmatite system; and (B) plan view of the pegmatite zonation. Host rocks are Late-Triassic deep-water turbiditic clastic sediments and minor carbonate rocks.

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Excluding U.S. production (withheld), worldwide lithium production contained in mineral concentrates and brines in 2021 increased by 21% to approximately 100,000 tonnes from 82,500 tonnes in 2020 in response to strong demand from the lithium-ion battery market and increased prices of lithium (USGS 2022). Global consumption of lithium in 2021 is estimated to be 93,000 tonnes, a 33% remarkable increase from 70,000 tonnes in 2020.

In 2021, Australia dominated world output with ~55% of the total contained in spodumene, followed by Chile (~26% of world production), China (14%) and Argentina (6%), producing from natural brines. Global identified resources (~90 Mt) and reserves (22 Mt) are large. USGS data are given in metric tonnes of contained lithium unless otherwise noted. The media provide ample information on the rapidly growing need for lithium, because it is one of the key metals for new age green energy technologies, especially for electricity storage. Currently, global end-use markets are estimated to consume about 74% of world Li for batteries (USGS 2022).

For practicioners, the open-access paper by Scogings et al. 2016 is most valuable. It "examines lithium production markets and prices, reporting of exploration results, special considerations that should be applied to the reporting of pegmatite Mineral Resources and issues around Competence for the public reporting of lithium exploration results and resources." For example, ore for manufacturing heat-resistant glass, pyroceramics and enamels must be very low in impurities of Fe, Mn, Ti and Cr.

References

Álvarez-Amado F, Rosales M, Godfrey L, et al. (2022) The role of ignimbrites and fine sediments in the lithium distribution and isotopic fractionation in hyperarid environments: Insights from Li-isotopes in the Atacama Desert. J Geochemical Exploration 241:107062, ISSN 0375-6742. Https://doi.org/10.1016/j.gexplo.2022.107062.

Bowell RJ, Lagos L, de los Hoyos CR & Declercq J (2020) Classification and characteristics of natural lithium resources. Elements 16:259-264. Htpps: //doi.org/10.2138/gselements.16.4.259

Bradley, D.C., McCauley, A.D. & Stillings, L.M. (2017) Mineral-deposit model for lithium-cesium-tantalum pegmatites. U.S. Geological Survey Scientific Investigations Report 2010-5070-O, 48 pp. OPEN ACCESS https://doi.org/10.3133/sir20105070O.

Gao Y, Bagas L, Li K, Jin M, Liu Y & Teng J (2020) Triassic lithium deposits in the Dahongliutan Area, North-West China: A case study for the detection of lithium-bearing pegmatite deposits in rugged terrains using remote-sensing data and images. Front. Earth Sci. 8:591966. doi: 10.3389/feart.2020.591966 OPEN ACCESS

Pohl, WL (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons - an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. Print www.schweizerbart.de/9783510654352 (Soft Cover), www.schweizerbart.de/9783510654413 (Hard Cover). E-book (PDF) ISBN 9783510654369 sold by https://www.lehmanns.de/shop/naturwissenschaften/57770431-9783510654369-economic-geology

Scogings, A., Porter, R. & Jeffress, G. (2016) Reporting exploration results and mineral resources for lithium mineralised pegmatites. AIG Journal Paper N2016-001, 9 pp. Australian Institute of Geoscientists. OPEN ACCESS www.aigjournal.aig.org.au

USGS, United States Geological Survey (2022) USGS minerals information webpages. Annual mineral commodities summaries. Lithium 2022. https://www.usgs.gov/centers/national-minerals-information-center/commodity-statistics-and-information Accessed in August 2022.


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'Metallogeny' - My Open Access Review employing the case of GOLD for the study (Published 14 June 2022; Blog Update 25 July 2022)

Pohl, W.L. (2022) Metallogenic models as the key to successful exploration -- a review and trends. Mineral Economics (2022):36 pp. Springer. https://doi.org/10.1007/s13563-022-00325-3 Creative Commons Licence Open Access

How welcome is it to learn from the publisher's production team: "Congratulations. We're pleased to inform you that we've published your article and it's available to view online." 'Viewing' includes that the PDF can be downloaded. Readers of the article will also be able to use enhanced PDF features such as search and annotation tools, one-click supplements, citation file exports and article metrics. To this day (September 19, 2022), Springer Nature counts about 1400 accesses to this paper.

I have spent most of the year 2021 collecting published, and my own thoughts about metallogeny, and writing this review paper that should provide access to the present scientific essence of metallogeny and its application in exploration. It is confined to gold metallogeny, because this sector of the industry is of foremost economic importance. The review is based on numerous publications, most of the recent past, which reflect current global understanding of the term metallogeny and how it is employed. Older literature is less often cited, in order to avoid ending up with a full-scale book.

To this day, Springer Nature counts about 1000 accesses to this paper.

A short definition of metallogeny goes like this, taken from the Abstract of my Review:

"Metallogeny is the science of ore and mineral deposit formation in geological space and time. Metallogeny is interdisciplinary by nature, comprising elements of natural science disciplines such as planetology to solid state physics and chemistry, and volcanology. It is the experimental forefront of research and bold thinking, based on an evergrowing foundation of solid knowledge. Therefore, metallogeny is not a closed system of knowledge but a fast-growing assemblage of structured and unstructured information in perpetual flux."

Metallogeny acompanied my professional life as an economic geologist from the beginning, initially nudged by my teacher and mentor Walter E. Petrascheck. I hope that my views (and the many examples referenced) might help to clarify the core concepts, systems, models, and metallogeny's place in current terminology and practice.

Here, I'll abstain from reporting details of the contents in the review, just read it. But allow me to highlight the 11 figures (with their subtitles) of this paper that have been put into a PDF file, which you will find under:

Supplementary Information (Annex) . The PDF and the online version contains a link that leads to the supplementary material (a PDF of 1913 KB) at

https:// doi. org/ 10. 1007/ s13563- 022- 00325-3

My favourite is Figure 5 (Mantle_Gold_Fertilization_Pohl.jpg) that shows novel concepts of process systems of gold metallogeny at active continental margins (Pohl 2022):

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Gold
Fig 5 Schematic sketch of major plate tectonic settings of gold metallogeny: Mid-oceanic Cu-Zn-Au-Ag mineralization (triangles) is being carried towards the metallogenic subduction factory underneath an active continental margin, where the slab is devolatized by low T/high P metamorphism. The mafic crust along the slab top surface is the main source of water released at subarc depths (Holt & Condit 2021). The eclogitized slab breaks and founders downwards. Metalliferous supercritical fluids and melts may metasomatize ('fertilize') the mantle wedge or subcontinental lithospheric mantle (SCLM). Enriched, or 'metasomatized' mantle (mSCM) is the source of hydrous and metalliferous magmas and fluids that rise toward the surface where they form porphyry, epithermal and orogenic gold ore deposits. The great mass of calc-alkaline magma builds the continental, or Cordilleran, volcanic arc. The back-arc basin may display bimodal alkaline igneous rocks, and IOCG (iron oxide copper gold) or gold-rich VMS (volcanic massive sulfide) deposits. An approaching plume can provide matter and heat, or even gold to this system, or may mobilize matter from older fertilized mantle.

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Extract from the Conclusions

Science and practice of metallogeny ultimately serve to supply mineral raw materials that are needed for a dignified life of humankind. Earth has a much larger endowment of mineral resources than is often claimed. Proponents of impending 'limits to growth' have been proved wrong by scientific arguments (Wellmer and Scholz 2018, 2017), and indeed, by the mining industry that reliably satisfies global requirements. There is no reason to assume a shortage of raw materials, if the mining sector is allowed to do its work. This paper is a call for the appreciation of metallogeny as the holistic science of ore formation, and in that role, as a fundamental toolbox for exploration. The body of metallogenic knowledge grows at a quick pace. Like human ingeniousness, this growth is unlimited.


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The Metallogenesis of Orogenic Gold Deposits: A remarkable contribution from China (May 18, 2022) (OPEN ACCESS)

Once more, we are informed of a new metallogenic enlightenment derived by investigations targeting a gold deposit in the North China Craton (NCC: see earlier News items): The giant high-grade Dongping gold deposit with reserves of 120 t Au (Fan et al 2022).

The authors provide the following Abstract of their paper:

Lode gold deposits, which are currently the world's major gold supply, have been shown to be generated mostly by phase separation of metamorphic fluids and/or interaction between these fluids and wall rocks. Here we use garnet oxygen isotopes by secondary ion mass spectrometry to document the crucial role of magmatic hydrothermal fluids and their mixing with meteoric water in the formation of the world-class Dongping gold deposit in the North China Craton. Garnet grains from quartz veins of various paragenetic stages and the mineralized alteration envelope at Dongping have dynamic δ18O variations of 3.8 to 211 ‰, with large intragrain fluctuations up to 5.3 ‰. These values correspond to calculated δ18O values of 6 to 29 ‰ for the hydrothermal fluids from which the garnet formed. The isotope data, notably the cyclic alternation in δ18O within individual garnet grains, are best interpreted to reflect multiple pulses of magmatically derived fluids and mixing of each pulse with variable amounts of meteoric water. The results presented here allow us to quantify the significant interplay between magmatic hydrothermal fluids and meteoric water that spanned the entire mineralization history and triggered gold deposition of a lode gold deposit. This study highlights the potential use of in situ oxygen isotope analysis of garnet in tracing the origin and evolution of hydrothermal fluids in the Earth's crust that may have formed giant ore deposits.

Study the authors' explanatory figures:

China
Fig 4. Schematic illustration showing the hydrothermal fluid system from which various minerals precipitated to form the Dongping gold deposit. (A) Magmatic fluid pulses were repeatedly exsolved from the underlying magma chamber. Faults and fractures facilitated ascension of magmatically derived fluids and higher up their mixing with meteoric water, causing gold deposition and associated veining. Disseminated mineralization in alteration envelopes occurred in less-fractured wall rocks where the infiltration of meteoric water was minor. Various mineral assemblages in pre- (S1), syn- (S2-a), and postore (S3) veins and the mineralized alteration envelopes (S2-b) formed in response to fluid mixing. (B) Evolution trend of δ18Owater values of garnet grains from each paragenetic stage indicating pulsed injection of magmatic fluids [red bar; δ18Owater = 5 to 10 ‰; (21, 22)] and incursions of meteoric water [blue bar; average δ18Owater = -12 ‰; (30, 31)]. The range and variations of oxygen isotopic ratios for each stage are defined using δ18Owater values shown in Fig. 3 and SI Appendix, Fig. S5. The brown and beige solid lines correspond to the brown and beige garnet grains, respectively (Fan et al 2022).

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In the main text, the authors mention that earlier stable isotope investigations of bulk fluid inclusions in quartz at Dongping had already indicated that the average δ18O might reflect a mixture of magmatic and meteoric water. Their high-precision data confirm this but illuminate the wide range of mixtures. For comparison, you might look up other published δ18O values. In principle we know that part of the orogenic gold is magmatic-hydrothermal in origin (Pohl 2020) but this paper is first in providing hard analytical data.

Interesting are the authors' speculations that mixing and cooling of hot upflow by meteoric water precipitated gold and the gangue. Do you think this is valid?

Very useful is the concise description of 'Methods' at the end of the paper, with the headings: Sample Preparation. - SEM and EPMA. - Oxygen Isotope Analysis by SIMS. - Trace Element Analysis by LA-ICP-MS. - Calculation of δ18O Values of Garnet-Forming Aqueous Fluid. - Estimation of Meteoric Water Contribution.

In addition, this article contains voluminous supporting information online at https://www.pnas.org/doi/full/10.1073/pnas.2116380119#supplementary-materials

You did notice that the paper is OPEN ACCESS. The read is highly recommended, even if your quest is not oxygen isotopes, and you do not have all the fancy equipment anyway!

References

Fan G-H, Li J-W, Valley J-W, et al (2022) Garnet secondary ion mass spectrometry oxygen isotopes reveal crucial roles of pulsed magmatic fluid and its mixing with meteoric water in lode gold genesis. PNAS 119 (19) e2116380119 | https://doi.org/10.1073/pnas.2116380119 OPEN ACCESS

Pohl WL (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons – an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. ISBN 978-3-510-65435-2. eBook ISBN 9783510654369. www.schweizerbart.de/9783510654352


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The Essential Metallogenic Role of the Sub-Continental Lithospheric Mantle (SCLM) in Eastern China (March 28, 2022) OPEN ACCESS

In a recent open access paper, Yang et al (2022) describe and reconfirm the outstanding metallogenic role of the sub-continental lithospheric mantle (SCLM) based on data gathered in Eastern China. The importance of the SCLM as a source of ore metals has earlier been recognized by Griffin et al (2013) who wrote that "models for ore genesis and exploration need to incorporate the entire lithosphere to be effective" . The paper by Yang et al (2022) applies the principles of enriched SCLM sourcing metallogenic systems to the Precambrian North China and Yangtze cratons. Their main theme, however, is the preferred location of great ore deposits within a 100 km wide zone along the present margins of these cratons.

Read Yang et al's (2022) Abstract for the essential information

The current margins of the North China and Yangtze Cratons provide arguably the best examples globally of anomalously high mineral endowment within a 100 km buffer zone, hosting 66 diverse world-class to giant ore systems that help explain China's premier position as a producer of multiple metal and mineral commodities. After the cratonization of these crustal blocks during the Neoarchean-Paleoproterozoic, with incorporation of iron ores on assembled micro-block margins, the margins of the cratons experienced multiple convergence and rifting events leading to metasomatism and fertilization of their underlying sub-continental lithospheric mantle. The rifted margins with translithospheric faults provided pathways for Cu-Au (Mo-W-Sn)-bearing felsic to Ni-Cu-bearing ultrabasic intrusions and REE-rich carbonatite magmas, and for the development of marginal sedimentary basins with both Cu-Pb-Zn-rich source units and reactive carbonate or carbonaceous host rocks. There was diachronous formation of hydrothermal orogenic gold, antimony, and bismuth systems in the narrow orogenic belts between the cratons. Complexity in the Mesozoic Paleo-Pacific subduction systems resulted in asthenosphere upwelling and lithosphere extension and thinning in the North China Craton, leading to anomalous heat flow and formation of orogenic gold deposits, including those of the giant Jiaodong gold province on its north-eastern margin. These gold deposits, many of which formed from fluids liberated by devolatilization of previously metasomatized sub-continental lithospheric mantle, helped propel China to be the premier gold producer globally. The thick sub-continental lithospheric mantle of the cold buoyant cratons helped the preservation of some of the world's oldest porphyry-skarn and epithermal mineral systems. Although craton margins globally control the formation and preservation of a diverse range of mineral deposits, China represents the premier example in terms of metal endowment due to the anomalous length of its craton margins combined with their abnormally complex tectonic history.

End of Abstract

Yang et al (2022) mapped the distribution of 66 world-class to giant metal deposits (as defined by Singer 1995) located within ~100 km of these margins; the sites contain ore-grade concentrations of Ag, Au, Cu, Fe, Mo, Nb, Ni, Pb, REE, Sb, Sn, W, and/or Zn. Most are renowned mining districts such as Bayan Obo, Daqiao, Jiaodong, Jinding and Shizhuyuan (Pohl 2020). The Supplementary Data Table S1 provides summary information and key references for each of the 66 deposits. Salient features of the 17 largest and most diverse deposits are summarized in Table 1. Figure 1 provides an overview of the tectonic framework of Eastern China and the position of the 66 great metalliferous ore deposits mapped. Figure 2 is of outstanding interest, because it summarizes the tectonic, geodynamic and metallogenic history of the major components of the North China and Yangtze cratons, along a time axis from the formation of continental nuclei soon after 3.8 Ga to the present; this allows to check, which one of the giants were formed at times of supercontinent formation or breakup, of orogeny or rifting, and various phases of re-working. Episodic tectonic events from ~2.3 Ga to ~700 Ma produced translithospheric extensional and transpressional faults in marginal zones that increased local crustal permeability. The margins were further modified from 520 to 200 Ma by diachronous Paleozoic to Triassic orogenies related to subduction of the Proto-Tethys (Shangdan), paleo-Asian, and Paleo-Tethys (Mianlue) oceans. From ~240 to 130 Ma, the North China craton (NCC) margin experienced a unique period of 'decratonization' by the complex changes of the Jurassic to Cretaceous subduction geometry of the paleo-Pacific. Slab break-off and roll-back of the paleo-Pacific Plate induced delamination and thinning of the lithosphere in the NCC, inducing asthenospheris upwelling, increasing thermal gradients, widespread magmatism, and formation of the highly endowed Jiaodong Peninsula gold province at ~120 Ma (Figure 1; Yang et al 2022).

Strangely, the Neoarchean Wilson Cycle of the Trans-North orogen crossing the NCC described by Zhong Y et al (2021) (see my Blog below, dated December 15, 2021) is not referenced; apparently, some journals are quicker to publish than others ...

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Figure 1. Tectonic framework of Eastern China showing 66 world-class to giant mineral deposits within ~100 km of the margins of the North China and Yangtze cratons: compiled by Yang et al (2022). Abbreviations: OGD = orogenic gold deposits; OGD-JD = orogenic gold deposits-Jiaodong; ORG = orogenic Au-Sb deposits. Courtesy Yang et al (2022)

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Yang et al (2022) describe the Jiaodong gold event in their Chapter 7 (here shortened and modified): Arguably, the most important metallogenic event in the NCC, at least in terms of gold mineralization, was related to the Upper-Jurassic to Lower Cretaceous sub-continental lithospheric mantle (SCLM) thinning and delamination due to the complex history and geometry of subduction related to convergence of the Paleo-Pacific Plate (Yang and Santosh 2020; see their Fig 2 in my blog below "GOLD * GOLD * GOLD * Remarkable Publications! April 12, 2021"). This resulted in asthenospheric upwelling, granitic magmatism, and mesozonal to epizonal orogenic gold mineralization at 120 Ma. The Jiaodong Gold Province contains >35% (>5000 t gold) of China's gold resource, in 17 giant orogenic deposits sited within 100 km of the margins of the NCC and YC. The auriferous ore fluids most likely had a non-crustal source. In terms of sub-crustal fertility, the significant older (>2000 my) pre-mineralization crustal metamorphism, combined with sulfur, lead and strontium isotope ratios that are inconsistent with crustal sources of these elements, indicate a dominant subcrustal source for the ore components of the Jiaodong gold deposits, consistent with evidence for volatile- and probable gold-fertilized metasomatized mantle lithosphere beneath at least part of the NCC. Helium-Ar and C-O isotope ratios also implicate mantle sources of ore fluids. For the Jiaodong gold province, the ore fluids are interpreted to have been derived from metasomatized SCLM on the NCC margin that was fertilized by volatiles and gold during earlier Triassic subduction of gold-enriched pyritic sedimentary rocks from the northern margin of the YC. In the Cretaceous, asthenospheric upwelling related to complex subduction of the Paleo-Pacific plate caused devolatilization of the metasomatized and fertilized SCLM to release auriferous ore fluids. These fluids advected via lithosphere-scale faults on the NCC margin and were focussed into subsidiary faults and shear zones to form the Jiaodong gold deposits. The deposits were preserved due to relatively slow exhumation despite the previous lithosphere delamination.

If you should be interested in details of one of the Jiaodong gold deposits, you may consult the recently published paper by Lan et al (2022).

References

Condie KC (2022) Chapter 4 - The mantle. Pp 81-125 in Earth as an Evolving Planetary System (4th ed Kent C. Condie) Academic Press, ISBN 9780128199145. DOI https://doi.org/10.1016/B978-0-12-819914-5.00010-X

Griffin WC, Begg GC & O'Reilly SY (2013) Continental-root control on the genesis of magmatic ore deposits. Nature Geoscience 6:905-910. DOI https://doi.org/10.1038/ngeo1954

Lan T, Fan Y, Lu J, et al (2022) Origin of the Dayingezhuang gold deposit in the Jiaodong district, eastern China: Insights from trace element character of pyrite and C-O-S isotope compositions. J of Geochemical Exploration 236:106986. ISSN 0375-6742. DOI https://doi.org/10.1016/j.gexplo.2022.106986

Pohl WL (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons – an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. ISBN 978-3-510-65435-2. eBook ISBN 9783510654369. www.schweizerbart.de/9783510654352

Singer DA (1995) World class base and precious metal deposits -- A quantitative analysis. Economic Geology Bull 90:88-104. 10.2113/gsecongeo.90.1.88

Yang LQ, Deng J, Groves DI, et al (2022) Metallogenic 'factories' and resultant highly anomalous mineral endowment on the craton margins of China. Geoscience Frontiers 13/2:101339. ISSN 1674-9871. https://doi.org/10.1016/j.gsf.2021.101339 Creative commons license Open Access

Zhong Y, Kusky Th, Wang L, et al (2021) Alpine-style nappes thrust over ancient North China continental margin demonstrate large Archean horizontal plate motions. Nature Communications 12:6172. DOI https://doi.org/10.1038/s41467-021-26474-7 Creative Commons Attribution 4.0 International License Open Access


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Late Archean Alpine-Style Full Wilson Cycle Plate Tectonics in Northern China (Open Access) (March 1st, 2022)

The emergence of modern plate tectonics is a long-standing open issue in Earth Science. A recent paper by Zhong Yating et al. (2021) reports detailed observations and data that constitute a solid proof of modern-style plate tectonics in the North China Craton. There, the authors demonstrate that modern plate tectonics was operating since about 2.7 Ga very much alike, for example, to the Mesozoic orogeny in the Eastern Alps of Central Europe.

Read the Authors' Abstract for the essentials: "Whether modern-style plate tectonics operated on early Earth is debated due to a paucity of definitive records of large-scale plate convergence, subduction, and collision in the Archean geological record. Archean Alpine-style sub-horizontal fold/thrust nappes in the Precambrian basement of China contain a Mariana-type subduction-initiation sequence of mid-ocean ridge basalt blocks in a 1600-kilometer-long mélange belt, overthrusting picritic-boninitic and island-arc tholeiite bearing nappes, in turn emplaced over a passive margin capping an ancient Archean continental fragment. Picrite-boninite and tholeiite units are 2698 ± 30 million years old marking the age of subduction initiation, with nappes emplaced over the passive margin at 2520 million years ago. Here, we show the life cycle of the subduction zone and ocean spanned circa 178 million years; conservative plate velocities of 2 centimeters per year yield a lateral transport distance of subducted oceanic crust of 3560 kilometers, providing direct positive evidence for horizontal plate tectonics in the Archean".

Of course, it remains unknown if the Central Orogenic Belt in the Neoarchean North China Craton ( Figure ) dates the absolutely earliest instance of Plate Tectonics on Earth, or if that has happened ages before.

Schematic cross sections through the Central Orogenic Belt in the Neoarchean North China Craton (Credit: Zhong, YL. et al. Figs 10a and b)

Windley et al. (2021), for example, suggest that (1) 'Accretionary Cycle Plate Tectonics' of small protocontinents with short boundaries started from 4 Ga, followed by (2) 'Wilson Cycle Plate Tectonics' with its long plate boundaries beginning by about 2.7 Ga. Yet, accretion in front of drifting protocontinents with limited subduction is part of Bédard's (2018) lid tectonic model (find more in my News Archive January 05, 2018). By the way, Windley et al. (2021) dismiss lid tectonics and similar models of Archean Earth. The case remains undecided; is it a matter of terms only? My opinion gained from published literature is that lid tectonics did exist and gradually evolved into plate tectonics.

Metallogenic systems in terms of lid tectonics are sketched as follows (Bédard 2018): Within the basalt-komatiite units, thick BIF (Algoma type, or volcanogenic exhalative basalt-related banded iron formations) and chert sequences are intercalated. Komatiites (Arndt et al 2008) host orthomagmatic rivers of nickel sulfide in submarine lava channels or as disseminated bodies in subvolcanic intrusions (Barnes et al 2017). Archean volcanogenic polymetallic massive base metal-sulfide (VMS) deposits may have formed from fluids mobilized by reheating of the hydrated submarine volcanic pile, caused by new magma pulses. The deep sections of imbricated and accreted foreland terranes were the source of gold-carrying metamorphic fluids and magmas, resulting in orogenic Au deposits.

Metallogenic consequences include that (1) plate tectonic models may be fully trusted and employed in applications such as exploration from the Neoarchean (~2.8 Ga) until the present; (2) pushing back or abandoning lid tectonics totally relegates the potential of this hypothesis to lower ranks. Yet, I have the impression that Windley's et al. (2021) 'Accretionary Cycle Plate Tectonics' may not be very different from Bédard's (2018) large oceans of lid tectonics dotted by drifting protocontinents that accreted, eventually amalgamated and finally formed megacontinents. In the Late Archean, the stage was set for deep subduction of cool and dense oceanic crust, and for the plate tectonic metallogenic factories operating as we know them now.

References

Bédard, J.H. (2018) Stagnant lids and mantle overturns: implications for Archaean tectonics, magma genesis, crustal growth, mantle evolution, and the start of plate tectonics. Geoscience Frontiers 9, 19-49. Open Access https://doi.org/10.1016/j.gsf.2017.01.005

Windley, B.F., Kusky, T. & Polat, A. (2021) Onset of plate tectonics by the Eoarchean. Precambrian Res. 352, 105980 (2021). https://doi.org/10.1016/j.precamres.2020.105980

Zhong, Y., Kusky, Th., Wang, Lu, et al. (2021) Alpine-style nappes thrust over ancient North China continental margin demonstrate large Archean horizontal plate motions. Nature Communications 12:6172. Open Access Creative Commons Attribution 4.0 International License. https://doi.org/10.1038/s41467-021-26474-7


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Deep Crustal Magneto—Telluric Features of Giant Gold Fields in the Victorian Gold Province of SW Australia (Open Access) (October 30, 2021)

Metallogeny investigates the formation of ore deposits from source to trap. Much is known about the latter, but the source(s) of gold remain disputed and many different models are discussed; main opinion groups search the source (1) either in the crust or, (2) increasingly in current papers, in the mantle. For individual gold deposits, districts and provinces, the source of gold remains a recurring quest as metallogenic understanding is fast expanding.

A remarkable recent open access paper by Heinson et al. (2021) reports magneto-telluric data and interpretations from the Victorian gold province hosting the Bendigo and Ballarat fields, once fabulously rich gold producers in SW Australia. Historic gold production from all sources (eluvial, alluvial, paleoplacer and rock hosted lodes) is throught to have delivered ~2500 t of gold (Taylor et al. 2017b) and an equal mass may remain hidden beneath younger cover (Taylor et al. 2017a).

In non-genetic terms, Victorian gold is 'shale, or turbidite hosted'. In current common nomenclature it is of the'orogenic type'. I would prefer to call the deposits members of the 'metamorphogenic-hydrothermal clan' (Pohl 2020: p. 236).

Victorian gold deposits consist of quartz vein fields in anticlinoria, trapped along vertical fluid escape zones. Structural and lithological control of gold quartz veins are equally important. Early veins form an interconnected fracture mesh controlled by folds, bedding planes, cleavage and reverse faults and are partly deformed. Most gold mineralization was synchronous with peak compressional deformation and metamorphism but extended into late tensional strain. Muscovite 40Ar/39Ar geochronological data of auriferous veins at Ballarat reveal a duration of deformation and mineralization from 445 to 370 Ma. Fluids were reduced, CO2-rich, of low salinity and reached temperatures of 350 ± 25 degrees C. Ore paragenesis comprises free gold, pyrite, arsenopyrite and pyrrhotite in quartz with ankerite and albite. Host rock alteration is macroscopically visible because of proximal bleaching and a wide halo of siderite spots; microscopic features include the introduction of sericite, chlorite, carbonates and pyrite-arsenopyrite. High-grade ore shoots occur at the contact with pyritic and graphitic beds ("indicator beds") and lumps of native gold were found that reached 18.8 kg. Since 1851, Victoria State produced a total of 2500 t of gold, of which ~40% were derived from quartz veins. The larger part was extracted from placer deposits that included some famously rich bonanzas, especially in 'deep leads' (buried paleoriver courses). The heaviest nugget weighed 71 kg.

For an overview, read the Abstract of Heinson et al. (2021a)

"Orogenic gold deposits provide a significant source of the world's gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade facies, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world's foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with  over  one tonne of total gold production and a  less than 20 Ωm low-resistivity region at crustal depths of more than 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth releasing HS- ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world's richest orogenic gold provinces."

MT
Fig 2 (c) from Heinson et al. 2021. Resistivity depth slice near the lithosphere-asthenosphere boundary (~ 150 km). Black circles are long-period MT and GDS observation sites used in the three-dimensional inversion; blue circles are broadband MT transects that were not used in the inversion. Large yellow circles represent gold mines with total production of more than one tonne; smaller white circles show production less than one ton Au. Solid black lines represent the boundaries of major tectonic elements, and the white lines show coastlines and bathymetry contours at 1000 m depth intervals. The colour scale bar on the right side represents resistivity values within the range of 10 to 10,000 Ωm.

The tectonic setting of the Victorian Gold Province was a subduction related active margin of Gondwana. Heinson's et al. (2021a) results appear to demonstrate that the Bendigo-Ballarat low-resistivity anomaly extends down to the boundary between the sub-continental lithospheric mantle (SCLM) and the asthenosphere (their Figure 2C above), although at values about 100 Ωm. The authors attribute the very low values of less than 32 Ωm of crust below the Bendigo-Ballarat zone to flake graphite in the rocks, which would be absent in the lithosphere. They refrain from any interpretation, however, and solely report that a heat anomaly caused by slab break-off and intrusion of hot asthenosphere may have acted on the crust (Vos et al. 2007). In one of the slides, Heinson (2021b) showed an arrow from below marked 'plume' but did not comment on it. Of course, there are now a number of potential explanations how heat, or fluids or melts (with or without gold) may impact the SCLM from below and cause or contribute to ore formation.

Is it likely that these magneto-telluric (MT) data may assist exploration? My quick answer is 'not directly'. But in the case of the Victorian Gold Province, the results reported by Heinson definitely refine the metallogenic model and generally, better models lead to improved exploration. Would you agree or reject this conclusion?

Yet, generally, deep imaging of crust and mantle by MT is an extremely promising exploration tool, especially for cases of buried hydrous or otherwise conductive sources, such as metasomatized subcontinental lithospheric mantle (SCLM). If the Victorian gold would have been buried similar to giant Olympic Dam IOCG (below 400 m cover), Heinson et al. (2021) might have reported a giant propicious anomaly.

References

Heinson, G., Duan, J., Kirkby, A. et al. (2021a) Lower crustal resistivity signature of an orogenic gold system. Nature Sci Rep 11, 15807 (2021). https://doi.org/10.1038/s41598-021-94531-8 Open Access (license http://creativecommons.org/licenses/by/4.0/).

Heinson, G. (2021b) Lower crustal resistivity signature of an orogenic gold system. Ore Deposits Hub October 20, 2021. Public YouTube Recording link: https://youtu.be/u3GZfsyF9so

Pohl W.L. (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons – an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. 32 Colour Plates, 305 Figures, 32 Tables, 25 Boxes, 81 Equations. Schweizerbart Science Publishers, Stuttgart.(Soft Cover)

Taylor, D.H., Willman, C.E., Hughes, M.J. & Boucher, R.K. (2017a) Recent gold mining and exploration in Victoria. Pp 807-810 in Australian Ore Deposits (ed G.N. Phillips), Australasian Institute of Mining and Metallurgy.

Vos, I.M.A., Bierlein, F.P. & Heithersay, P.S. (2007) A crucial role for slab break-off in the generation of major mineral deposits: insights from central and eastern Australia. Miner. Deposita 42, 515-522. https://doi. org/ 10.1007/s00126- 007- 0137-3


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Delamination – Induced HT – Metamorphism and A–Type Magmatism in the Eastern Kunlun Belt, China (September 15, 2021)

Most economic geologists will be aware of the metallogenetic role of granites and granitoids, and that their source is a strong control on possibly related mineralization. Investigation by petrological and geochemical methods allows determination of their origin by melting of three major 'source' components, which include (1) Peridotites of the Earth's upper mantle (asthenosphere, metasomatized lithosphere); (2) Intermediate volcanic or intrusive magmatic and metamorphic rocks of the deep continental crust (infracrustal); and (3) Clastic sediments and metamorphic equivalents (supracrustal).

Mainly mantle-derived (1) are A-type, or ferroan granitoids. They are characterized by iron–rich mafic mineralogy, ferroan, alkali–calcic to alkaline affinities, high LILE + HFSE abundances, typically by exsolving OH–F bearing fluids and crystallizating under reduced or oxidized conditions (Bonin 2007). A-type granites are formed under alkali–rich, anhydrous, and anorogenic conditions by melting of source rocks under low–pressure and high–temperature conditions (Frost & Frost 2019). They occur within plates and at distensive plate boundaries. They are often exposed at a subvolcanic level in ring complexes together with mafic igneous rocks. The origin of A–type granites was originally sought in extreme differentiation of basaltic melt; currently, derivation from lower crustal or lithospheric mantle sources is considered more common (Bonin 2007). A–type granites occur mainly in the Early to Middle Mesoproterozoic, a period that is marked not only by A–type granites, but also by anorthosites and by the absence of arc magmatism, subduction indicators, passive margins and seafloor spreading that characterize modern plate tectonics.

Different ore associations occur with A–type granitoids (Pohl 2020): i) Sodium–rich granites, striking because of attached albitite bodies, are related to concentrations of niobium, uranium, thorium, rare earth elements and some tin; and ii) potassium–rich granites with profuse hydrothermal silicification, tourmalinization and acidity produce deposits of molybdenum, tin, tungsten, lead, zinc and fluorspar. The second association may occur within the granite body (endogranitic greisen, pegmatite, and porphyry stockwork ore) or as vein fields in country rocks (exogranitic). A–type granites genetically related to magmatic–hydrothermal iron oxide–copper–gold (IOCG) deposits constitute a third economically important variety (Mueller & Groves 2019, Skirrow et al. 2019).

A–type granites are the alkali granites of continental rifts, for example the Jurassic W–Sn granites of the Nanling metallogenetic province in China or the Mesoproterozoic rapakivi granites in Finland with their large red perthitic alkali feldspars mantled by green plagioclase in a fine-grained groundmass. A suite of distinct post–collisional hornblendite, gabbro, granite and mafic granulite (ferroan granitoids?) was identified in the eastern Kunlun Orogen in China (Wang et al. 2022). This 3000–km–long W–E striking mountain belt formed by closure of the Proto–Tethys ocean that dated from the breakup of the Rodinia Supercontinent and by Late Paleozoic collision of Gondwana and Siberia. The authors provide petrological data such as zircon U–Pb geochronology, zircon in situ Hf isotope, mineral and whole–rock geochemistry, and whole–rock Sr–Nd isotope compositions. Strangely, Wang et al. (2022) fail to identify A–type granites in the text (and omit any reference to related mineralization), but in one of the figures (see below) they provide a geodynamic model sketch of the post–collisional extensional phase in the Late Devonian, when lithospheric delamination and upwelling of asthenosphere caused heating, deep metamorphism, partial melting and intrusion of S–type granites and A–type granitoids. This model may be useful as input into future research on the origin of A–type granitoids and their metallogeny.

Kunlun
Sketch showing post–collisional hornblendite, gabbro, S–type granite and A–type granitoid intrusions identified in the eastern Kunlun Orogen in China, caused by delamination and upflow of asthenosphere (Wang et al. 2022)

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A quick search for an English language description of the Kunlun metallogeny was not successful. Maybe you have more luck?

References

Bonin, B. (2007) A–type granites and related rocks: Evolution of a concept, problems and prospects. Lithos 97, 1–29.

Frost, B.R. & Frost C.D. (2019) Essentials of Igneous and Metamorphic Petrology. 2nd ed. 362 pp. ISBN 978-1-108-71058-9. Cambridge University Press.

Mueller, D. & Groves, D.I. (2019) Potassic igneous rocks and associated copper–gold mineralization. 5th ed. 398 pp. Mineral Resource Reviews. Springer.

Pohl W.L. (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons – an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. ISBN 978-3-510-65435-2. www.schweizerbart.de/9783510654352

Skirrow, R.G., Murr, J., Schofield, A., et al. (2019) Mapping iron oxide Cu–Au (IOCG) mineral potential in Australia using a knowledge–driven mineral systems–based approach. Ore Geology Reviews 113, 103011. OPEN ACCESS https://doi.org/10.1016/j.oregeorev.2019.103011

Wang, Q., Zhao, J., Zhang, Ch., et al. (2022) Paleozoic post–collisional magmatism and high–temperature granulite–facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt, NW China. Geoscience Frontiers 13, 101271, ISSN 1674-9871. https://doi.org/10.1016/j.gsf.2021.101271 OPEN ACCESS


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Brian McNulty's Book Review of my Economic Geology 2nd ed (Pohl 2020) (August 06, 2021)

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Brian McNulty is Postdoctoral Researcher in Economic Geology at the University of Nevada, Las Vegas, with a Ph.D. from the University of Tasmania — CODES, Australia (2019). Find a link to his professional profile below, if you are interested; it is certainly remarkable how it took him across the globe.

Brian wrote this review for the Society of Economic Geology (SEG) Bulletin, where it appeared first online a few days ago (citation below). SEG has about 10,000 members across the world. I am a member of this Society for more than two decades; so I asked them to arrange a review. In quality, the SEG Bulletin is arguably the globally leading journal in Economic Geology.

Brian's review of my book is remarkable, one of the best I have ever seen. I don't mean that he lauds my Economic Geology, which he does, but his text of two pages is an excellent description that adresses all questions, which anyone might wish to have answered in a book review. I'll cite a couple of paragraphs:

"I thoroughly enjoyed the applied aspect of this textbook, specifically "Part III: The Practice of Economic Geology." We all have our own origin story and, for some of us, this geology adventure has led us to lifelong careers as explorers, resource modelers, engineers, or environmental scientists. The life cycle of natural resources from discovery, beneficiation, and through to remediation is wonderfully presented in this section and highlights the numerous areas of expertise that encompass the multidisciplinary field of economic geology. Pohl captures the essence of economic geology so eloquently with the following text":

"Never forget that exploration is not only a scientific and technical enterprise but foremost an investment" (p. 447)

Brian also writes: "The format of the book, which includes informative yet condensed chapters with ample suggested readings, makes for a great choice of textbook for undergraduate and postgraduate students, as well as an excellent addition to the libraries of professional geoscientists interested in understanding and applying aspects of economic geology to their field of interest."

"Overall, I appreciate the different ways this book could be utilized by students, various academic researchers, and industry professionals. The book is a valuable resource in introducing the geologic processes involved in metal, nonmetal, and fossil fuel formation, including explanatory petrogenetic and tectonic deposit models, as well as how to apply this knowledge to exploration. The book also summarizes the current understanding of metals, elements, and minerals on an individual basis and does not shy away from the fact that society, industry, and academia need to work together to provide solutions toward sustainable natural resource management."

Thank you for your efforts, Brian!

References

Brian McNulty Professional Profile https://geoscience.unlv.edu/post-doctoral-researchers/brian-a-mcnulty/

McNulty, B.A. (2021) BOOK REVIEW Walter L. Pohl: Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons — Introduction to Formation and Sustainable Exploitation of Mineral Deposits (2nd revised edition): Schweizerbart Science Publishers, Stuttgart, 2020, hard-cover (ISBN 978-3-510-65441-3; price Euro 94.00), soft cover (ISBN 978-3-510-65435-2; price Euro 79.00) and ebook (ISBN 978-3-510-65436-9). Economic Geology (2021) 116 (6): 1485-1486. ISSN 0361-0128; doi:10.5382/econgeo.116.6.br02


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Anthropocene Revisited — Biologists Foresee a Managable Future for the Entire Planetary Ecosystem. Open access paper (July 20, 2021)

Economic Geology strives to supply human societies with needed Earth materials. Geologists assist in discovery and exploitation of ores and minerals, and in controlling damage to stakeholders and the environment. If you are interested in the great frame of our work, the well-being of humanity and our planet, you may be interested to read this paper by Lehman et al. (2021).

The main content of the paper concerns population dynamics of hominins. Introductory remarks concerning the Anthropocene and geologists are somewhat perfunctory such as "...prompting geologists to begin applying the term Anthropocene to recognize the present moment" (Abstract see below). Are we concerned with moments? In fact, although accepted by the Working Group of the IUGS Stratigraphic Commission, the term has not yet been formally accepted (http://quaternary.stratigraphy.org/working-groups/anthropocene/).

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Planet Earth. Courtesy European Space Agency (ESA) https://www.esa.int/

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The paper by Lehman et al. 2021 being OPEN ACCESS, you may skip the rest of this blog and go online to read it fully. For the others, here are some excerpts:

Abstract: Human populations have grown to such an extent that our species has become a dominant force on the planet, prompting geologists to begin applying the term Anthropocene to recognize the present moment. Many approaches seek to explain the past and future of human population growth, in the form of narratives and models. Some of the most influential models have parameters that cannot be precisely known but are estimated by expert opinion. Here we apply a unified model of ecology to provide a macroscale summary of the net effects of many microscale processes, using a minimal set of parameters that can be known. Our models match estimates of historic and prehistoric global human population numbers and provide predictions that correspond to some of the more complicated current models. In addition to fitting the data well they reveal that, amidst enormous complexity in our human and prehuman past, three key ecological discontinuities have occurred in turn: 1) becoming dominant competitors of large predators rather than their prey, 2) becoming mutualists with food species rather than acting as predators upon them, and 3) changing from a regime of uncontrolled population growth to one of controlled fertility instead. All three processes have been interlinked with cultural evolution and all three ushered in developments of the Anthropocene. Understanding the trajectories that have delivered us to this stage can help guide prudent paths into the future.

The Possibilist Agenda

Considering what is possible, and cutting on a separate plane across the range of attitudes from cynicism to pessimism to optimism to Pollyannaism, is "possibilism". In approaching self-inflicted problems of global scale, and also much more local problems, possibilism recognizes that the course of human events largely is not the domain of probability. Probability derives from combinations of many connected steps beyond our influence or knowledge. In human events, outcomes often depend on only a few major steps, often not beyond our influence or knowledge.

Therefore, in following the possibilist agenda, one first evaluates and eliminates what is impossible—what cannot occur by the laws of the universe. What remains is the tentatively possible, including both the desirable and the undesirable. Following the possibilist agenda means working tirelessly to imagine both possibilities and impossibilities and then laying plans to arrange events so that the desirable can be realized and the undesirable avoided — working to avoid unintended consequences. In this way, by superposing such thoughts onto recognized physical–biological–social problems of the world, including those described above, seemingly intractable problems may have possibilist solutions.

Directions Forward

Accompanying our domination and disruption of the planet has begun a conscious awareness of the magnitude of our powers to help guide us to prudent paths into the future. Ours is the first species to become aware of our global scope, the first to organize global communication and satellite monitoring of the planet as a whole, and the first consciously to consider how to create a sustainable planet.

We now face our next daunting challenge — learning to manage our role on the planet for our continued existence and — we can hope and expect — for that of our fellow creatures. Will our modern existence be a minor blip on the geological timescale, as indicated by the suffix " -cene" in " Anthropocene" ? Or will we be able — as a geologist suggested a century ago — to elevate our existence to the dignity of an era and advance the Anthropocene into the Anthropozoic?

There is reason for hope. It may seem unimaginable that we can learn to manage consciously the entire planetary ecosystem. We should, however, remember that throughout our relatively short history, the unimaginable repeatedly has morphed into the commonplace.

Reference

Lehman, Cl., Loberg, Sh., Wilson, M., Gorham, E. (2021) Ecology of the Anthropocene signals hope for consciously managing the planetary ecosystem. Proceedings of the National Academy of Sciences 118 (28) e2024150118; DOI: 10.1073/pnas.2024150118 OPEN ACCESS


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Book Review 4 of my Economic Geology 2nd ed (June 15, 2021)

CITATION OF THE BOOK REVIEW: Frank Melcher (2021) Walter L. Pohl: Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons - Introduction to Formation and Sustainable Exploitation of Mineral Deposits (2nd revised edition): Schweizerbart Science Publishers, Stuttgart, 2020, hard-cover (ISBN 978-3-510-65441-3; price Euro 94.00), soft cover (ISBN 978-3-510-65435-2; price Euro 79.00) and ebook (ISBN 978-3-510-65436-9). Mineralogy and Petrology. 10.1007/s00710-021-00754-y. OPEN ACCESS

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Prof. Frank Melcher is Full Professor of Economic Geology at the Montanuniversität Leoben in Austria. He is an excellent teacher and scientist. I know him for remarkable papers on Columbite (Ta) deposits in Africa. The last publication in this series is:

Wouters, S., Hulsbosch, N., Kaskes, P., Claeys, P., Dewaele, S., Melcher, F., Onuk, P. & Muchez, P. (2020) Late orogenic gold mineralization in the western domain of the Karagwe-Ankole Belt (Central Africa): Auriferous quartz veins from the Byumba deposit (Rwanda). Ore Geology Reviews 125, 19 pp., 103666.

You may find more about Prof Melcher and his research at https://pure.unileoben.ac.at/portal/en/persons/frank-melcher

In his review of my EG2, Prof. Melcher characterizes my book by ... "Economic geology is the application of geoscience to the supply of metals, minerals, and energy to society. The subject plays a central role in the exploration, evaluation, and development of deposits as well as in the mining and processing of raw materials. Economic geology is an indispensable discipline that is needed to meet mankind's increasing demand for nonrenewable raw materials and to further develop our society. In this expanded edition, Pohl is able to include current topics and crosslink them with future problems in the raw material supply. Thus, aspects of sustainable "green" mining, conservation of nature and the environment, and consequences for climate change are addressed within an ethical framework ..."

Melcher goes on with ... "Economic Geology, Principles and Practice is divided into four comprehensive Parts. In the first, the deposits of the metals are dealt with over 300 pages. In keeping with previous editions, the processes leading to the accumulation of metals in the Earth's crust and mantle ("metallogenesis") are treated first, and are described in detail and with the most up-to-date sources. Important subchapters are dedicated to the investigation methods for hydrothermal deposits as well as an outline of metallogenesis in the course of the Earth's history. This is followed by a chapter that is subdivided by the individual metallic raw materials, which are organized under the subheadings: iron and steel metals, nonferrous metals, precious metals, light metals as well as minor metals and speciality metals (including uranium and thorium), including many of the so-called critical raw materials. These individual chapters contain a veritable flood of interdisciplinary information and are a treasure trove for students, professors, professional geologists, and mineral enthusiasts. All relevant areas are represented including: mineralogy and geochemistry, deposit types and classification (numerous current examples), economic aspects, and notes on mining and processing ..."

The last chapter summarizes the two pages by ..."localities index, which helps in the targeted search for the about 1000 deposits and locations referenced in the EG2. The work is of an astonishing breadth and comprehensiveness. Basic knowledge of geology and mineralogy as well as some technical knowledge will make it easier for beginners to get started with this book. But with prior knowledge, it will captivate anyone interested in raw materials. This is a wonderful reference textbook that you will find yourself rereading again and again, whether to study a chapter in more detail, to look something up, or simply to learn something new. I highly recommend it to all mining geologists, reservoir geologists, and applied geoscientists ...."

Thank you, Frank, for your time in writing the review. Dear Reader, note that the book review is OPEN ACCESS (see above). One additional comment — the e-book version of my Economic Geology should soon be available. It will display all figures in colour.


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A Heap of Gold — the Tiny Volcanic Island and Giant Lihir (Ladolam) Deposit of Papua New Guinea (May 18, 2021)

If you are interested in Plate Tectonics, Volcanic Island Arcs, Mantle hydration and fertilization, and extraordinary gold deposits, this is for you. If this is not your thing, forgive me. Nowadays, I am writing a paper on gold metallogeny and cannot help being fascinated and sharing this with you!

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Figure 1 -- Newcrest's Lihir gold mine in PNG, sited in the caldera of Luise volcano (Courtesy itNews 2017) URL https://www.newcrest.com/our-assets/lihir

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The supergiant LIHIR (OR LADOLAM) alkalic epithermal, magmatic-hydrothermal gold deposit was discovered by rock chip exploration in 1982, locating auriferous alunite (Cooke et al. 2020). In 2019, total resources were estimated to 690 Mt at 2.3 g/t Au, equivalent to 50 Moz in situ gold. Ore bodies occur on the floor of a large volcanic amphitheater in Luise volcano, a Pleistocene stratovolcano built of shoshonitic volatile-rich, silica-undersaturated, and highly oxidized igneous rocks (Cooke et al. 2020) comprising alkaline lavas and tuffs of trachybasalt, basaltic trachyandesite and latite. The deposit is unique for the overprinting of epithermal gold mineralization over earlier porphyry-style veins and altered rocks with abundant anhydrite and carbonate but low-grade Au. The transition was probably caused by the collapse of a sector of Luise volcano into the sea (Sillitoe 1994). In an extensional setting from 0.6 Ma onwards, the mineralization changed to epithermal-style with sulfide and adularia alteration, during which the main resource was emplaced. High-T geothermal activity is ongoing and the recovered energy is used in the mine.

Epithermal activity produced at least six discrete mineralized zones, and each of these is dominated by refractory gold in arsenian pyrite. Gold is associated with adularia-pyrite-carbonate-anhydrite ± illite alteration assemblages, cemented breccias, and veins that overprinted the early porphyry-style features. Bonanza gold grades are associated with late-stage quartz and/or anhydrite veins.

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Figure 2 -- Schematic cross-section through the present-day New Ireland Basin (from SW to NE) in the Lihir area (upper plate only). This block model is to illustrate the links between collision, microplate interaction, tectonic structure, regional magmatism and the focusing of melts and fluids. Tectonics control the regional metal endowment (melting of a metasomatized lithospheric mantle source described by McInnes et al. 2001) but local structure is responsible for the focusing of metalliferous liquids and fluids into an ore-forming system. Note Solvara-1 in the Southwest, a high-grade massive sulfide (SMS) seafloor mining prospect of copper, gold, zinc and silver that was abandoned in 2019. Courtesy Brandl et al. (2020).

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Lihir lies in a broad, complex deformation zone caused by convergence of the Pacific and the Australian plate. The geodynamic setting is described as both postcollisional and back arc by Cooke et al. (2020). Brandl et al. (2020) place it into a former forearc followed by displacement to its current location in a rear- or backarc setting relative to active subduction along the New Britain Trench. The zone broke into microcontinents when colliding with the Ontong Java Plateau (OJP). Protracted, transtensional motion between distinct crustal blocks controls the location and timing of magmatism and mineralization (Figure 2).

Allow me to comment: One crucial point is missing in all this — the energy source. Energy is needed to initiate melting of the fertile mantle. Without energy, 2 billion years may pass by without ore formation. Look at the Jiaodong story in the blog before this one.

References

Brandl, Ph.A., Hannington, M.D., Geersen, J., et al. (2020) The submarine tectono-magmatic framework of Cu-Au endowment in the Tabar-to-Feni island chain, PNG. Ore Geology Reviews 121, 103491. ISSN 0169-1368. OPEN ACCESS. https://doi.org/10.1016/j.oregeorev.2020.103491

Cooke, D.R., Sykora, St., Lawlis, E., et al. (2020) Lihir alkalic epithermal gold deposit, Papua New Guinea. Pp. 579–597 in Geology of the World's Major Gold Deposits and Provinces (eds Sillitoe, R.H., Goldfarb, R.J., Robert, F. & Simmons, S.F.), SEG Special Publications 23, Society of Economic Geologists. Doi: 10.5382/SP.23.28.

McInnes, B.I.A., Gregoire, M., Binns, R.A., et al. (2001) Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths. Earth Planet. Sci. Lett. 188, 169–183.

Sillitoe, R.H. (1994) Erosion and collapse of volcanoes: causes of telescoping in intrusion-centered ore deposits. Geology 22, 945-948.


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GOLD * GOLD * GOLD * Remarkable Publications! (April 12, 2021)

If you work in the gold sector, you should acquire the following recent publications (references below). You need them in order to be up to date and for future reference at your work place. They are singular!

The first is the multi-authored SEG volume 'Geology of the World's Major Gold Deposits and Provinces', edited by Richard H. Sillitoe, Richard J. Goldfarb, F. Robert, and Stuart F. Simmons (2020a). The book packs 895 pages, numerous tables, coloured maps and sections, and provides descriptions of 29 of the world's largest gold deposits or districts, and of 7 overviews of great gold provinces including terrane-scale geologic parameters and their controls on the localization, styles, and timing of gold mineralization. Each description summarizes exploration history and regional and local geologic settings preparatory to synthesizing the salient lithologic, structural, alteration, and mineralization features of the deposit itself.

An Epilogue contains a paper on epithermal, Carlin and orogenic gold deposition — a must for mine geologists.

In the Foreword to the book, Mark Bristow Ph.D., the President and Chief Executive of Barrick Gold Corporation, who sponsored the publication, writes:

This special volume gives geology its deserved due and provides a timely insight into the world's major gold deposits and provinces. It will be a highly valuable, long-lasting reference for all geoscience practitioners of this and future generations. I (WLP) would add that SEG Special Publication 23 will also serve as a useful source for academic teaching and research libraries.

My second recommendation concerns the Introduction to the SEG-volume by Sillitoe (2020b) that provides a thumbnail sketch of each important gold deposit type, including geologic and economic characteristics and widely accepted genetic models, as well as briefly discussing aspects of their spatial and temporal associations and distributions. Currently, ore deposits are often differentiated as types that are haphazardly named, not following a logical system. Deposit types of gold, for example, in order of decreasing endowment and overall economic importance, comprise:

Paleoplacer, orogenic, porphyry, epithermal, Carlin, geologically young placer, reduced intrusion related, volcanogenic massive sulfide (VMS), skarn, carbonate replacement, and iron oxide-copper-gold (IOCG) (Sillitoe 2020b). Although in many respects alike to typical orogenic gold, the giant Jiaodong province deviates in some features and is by some considered as a new 'Jiaodong gold depost type' (Qiu et al. 2020). It has a premining gold resource exceeding 4,500 metric tonnes (t). Jiaodong is a rare case where relatively young gold ores (ca. 120 Ma) were formed in terranes that are billions of years older (ca. 2.9— 1.9 Ga). Host rocks are Mesozoic granitoids. Orebodies are mainly quartz-pyrite veins, veinlets and disseminated mineralization, controlled by complex faults. Pinkish K-feldspar alteration forms an outer halo to the quartz-pyrite-sericite inner alteration halo that is intimately associated with gold ore. Jiaodong gold metallogeny occurred within a broad period of transition from a circum-Pacific oblique compressional to an extensional tectonic regime between ca. 160 and 90 Ma (Qiu et al. 2020).

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One possible solution for the plate tectonic setting of major Mesozoic gold deposit formation in the North China Craton; the Jiaodong province shown on right hand side. Note the gold source in metasomatized 'churning' mantle below (red dots). Courtesy Yang & Santosh 2020.

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The SEG volume is not cheap but worth the expense. For those of you who find the cost too high I add below an open access source offering a different view about the possible new Jiaodong gold deposit type (Yang & Santosh 2020).

References

Qiu, K.-F., Goldfarb, R.J., Deng, J., et al. (2020) Gold Deposits of the Jiaodong Peninsula, Eastern China. Pp. 753— 773 in Geology of the World's Major Gold Deposits and Provinces (eds Sillitoe, R.H., Goldfarb, R.J., Robert, F., Simmons, S.F.), Spec. Publication 23, Society of Economic Geologists (SEG). Doi: 10.5382/SP.23.35.

Sillitoe, R.H., Goldfarb, R.J., Robert, F., Simmons, S.F. (eds) (2020a) Geology of the World's Major Gold Deposits and Provinces. Spec. Publication 23, 1-859 pp. Society of Economic Geologists (SEG).

Sillitoe, R.H. (2020b) Gold Deposit Types: An Overview. Pp. 1— 28 in Geology of the World's Major Gold Deposits and Provinces (eds Sillitoe, R.H., Goldfarb, R.J., Robert, F., Simmons, S.F.), Spec. Publication 23, Society of Economic Geologists (SEG). doi: 10.5382/SP.23.01.

Yang, Ch.-X. & Santosh, M. (2020) Ancient deep roots for Mesozoic world-class gold deposits in the north China craton: An integrated genetic perspective. Geoscience Frontiers 11, 203— 214. doi: 10.1016/j.gsf.2019.03.002. OPEN ACCESS under CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).


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Safe Tailings Facilities? What are the Challenges? How to Approach Zero Risk? (March 20, 2021)

Tailings facility failures may endanger human lives, property, water resources, and the environment. They make great media news that shame the whole mining industry. This must end!

If you agree and if you are one of the many professionals who work in mining or in a mining-related job, in practice or in research, a suite of 25 papers that recently appeared in Mine Water and the Environment, the Journal of the International Mine Water Association (IMWA), is a great source of valuable technical information. The whole issue seems to be OPEN ACCESS. I was able to download some papers but I could not find an affirmative information. So just try. If this should not work, join the Association. The annual membership fees are very moderate.

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Mine Water and the Environment, the Journal of the International Mine Water Association (IMWA), Volume 40, issue 1. Tailings Storage: Challenges & Technologies, 25 articles in this issue. ?OPEN ACCESS??

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In the Introduction, Fernández & Kleinmann (eds) (2021) conclude their short presentation of each of the 25 high class papers in the issue by warning words:

However, many challenges remain, including

Rubio mentions the term 'upstream dam construction'. This is explained by the figure below, which is taken from a hydrogeological article in this issue (Morton 2021).

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Three construction variants of sequentially raised tailings dams or Tailings Storage Facilities (Morton 2021); note that the upstream variant has morphed toward continuous (not stepped) raising operation. In my Economic Geology you may find a photograph and explanation of this method (Pohl 2020 page 485 Fig. 5.19).

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So how to approach zero risk? Let's act!

References

Fernández Rubio & Kleinmann, B. (eds) (2021a) Mine Water and the Environment Journal of the International Mine Water Association (IMWA), Volume 40, issue 1. Tailings Storage: Challenges & Technologies, 25 articles in this issue. ?OPEN ACCESS??

Fernández Rubio & Kleinmann, B. (2021b) Introduction to Special Issue on Tailings Storage: Challenges and Technologies. Mine Water Environ 40, 1–5. https://doi.org/10.1007/s10230-021-00757-3

Morton, K.L. (2021) The Use of Accurate Pore Pressure Monitoring for Risk Reduction in Tailings Dams. Mine Water Environ 40, 42-49 (2021). https://doi.org/10.1007/s10230-020-00736-0

Pohl W.L. (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons — an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart.


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Newly Ordered ARCHIVED NEWS on my Website for Ease of Access and for Finding Links to Topics. Go to https://www.walter-pohl.com/news_archive.html (March 15 2021)

NEXT Book Review of Economic Geology 2nd ed. by Walter L. Pohl: Written by Dr. Richard H. Sillitoe (February 15, 2021)

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Pohl, W.L. (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons – an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart.

Dearly I would like to acquaint you with the whole of Richard's review, which is a paradigm of an informed, informative and useful critique, but copyright rules do not permit this. Accordingly, I cite here the last paragraph:

Economic Geology, Principles and Practice has something to offer to diverse audiences. It provides a useful introduction to the subject for senior undergraduates not only in economic geology but also in mining engineering, metallurgy, environmental studies and related fields. Graduate students and earlycareer professionals could dip into the volume for up-to-date introductions to topics beyond their specialist fields, and —if this reviewer is representative —even seasoned practitioners will surely find something new and of interest. The book is also strongly recommended to policy-makers, government officials, NGOs and the investment community needing a balanced overview of economic geology and the pivotal role it will play in the 'greening' of the world economy.

You do know that Dr. Sillitoe is one of the truly great economic geologists of our time?

Sillitoe, R.H. (2021) Walter L. Pohl: Economic geology, principles and practice: metals, minerals, coal and hydrocarbons — an introduction to formation and sustainable exploitation of mineral deposits (2nd ed.). Mineralium Deposita 56:619–620. https://doi.org/10.1007/s00126-021-01043-6


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Gold Assay Methods — Precise and Accurate for Reporting to Stock Exchanges? (February 10, 2021)

Public companies, the shares of which are traded on stockmarkets are obligated to report regularly on progress in their operations. Any figures stated (grade, core intersections, resources) must be of highest quality. This is central to issues that are regulated by codes such J0RC (2012) in Australia.

Analytical data in exploration geochemistry need not in all cases be equal to the absolute element content in a sample, or in other words, accuracy may not be essential. Deviations from the absolute content (e.g. an international laboratory standard) may be tolerated, if the relative error remains within narrow limits. Accuracy is assessed by employing certified reference materials (standards). In contrast, excellent reproducibility of results from duplicates, that is high precision, is absolutely required. Precision is the base for any data evaluation, especially if the contrast between background and anomalies is small. In all analytical programmes error control (in practice called QAQC — quality assurance and quality control) is a fundamental aspect. Errors may be introduced during sampling, sample processing and transport, and in the laboratory. Always, samples should be randomized before submission to the laboratory in order to avoid analysing them in the same sequence as collected. Also, it is good practice to repeat at least 10% of sampling. Analytical errors are revealed by repeatedly inserting duplicates, blanks or a standard of known composition such as international reference materials into the series. Control by another laboratory is advisable. Based on this kind of data, it is possible to calculate total error margins and the confidence interval.

If you are working for a gold miner, you know that getting precise and accurate data for gold tenors in ore (Figure 1) or in prospecting samples is not easy. Ore grades are low (ppm) and ppb may be required in exploration. Gold-specific problems often arise during sampling and analysis because of three points (Pohl 2020):


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Figure 1 — Gold-Quartz-Sulfide Ore, Fairbanks, Alaska

In every individual project it is extremely important to experiment and work out a procedure that minimizes resulting errors. For gold, time-tested fire assays and INAA (Instrumental Neutron Activation Analysis) are analytical standard methods. A newly adapted gamma-activation analysis (developed by CSIRO, Australia, and marketed by Chrysos since 2016) provides equal precision and accuracy, and accepts samples weighing several hundred grams, which avoids errors induced by subsampling. Meanwhile, Chrysos TM Photon Assay has been chosen by many mines across gold producers globally. Durance et al. (2014) wrote that

the majority of pXRF studies are conducted by private industry and are regarded as proprietary; it appears that the same applies today to Chrysos Photon Assay.


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Figure 2 — ChrysosTM Photon Assay is applicable for commercial analytical laboratories and for mine site use; this image shows sample containers on the conveyor belt feeding the analyzer. Courtesy © Chrysos https://www.chrysos.com.au/

The paper by Durance et al. (2014) provides an exemplary investigation of pitfalls in using a novel analytical tool, in their case of pXRF (portable X-ray fluorescence spectroscopy). They stress the importance to regard the rules of choosing a matched matrix, and of not analysing samples and standards through paper packets, as is often done instead of XRF-suitable film, impairing the precision and accuracy of pXRF data obtained; generally, the pXRF signal is attenuated by the paper. Also, the authors propose a best practice approach that corrects pXRF data using factors obtained from laboratory-based analysis (such as XRF or ICP-MS) of representative samples derived from the same project area so that they are matrix- and concentration-matched.


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Figure 3 — Olympus VantaTM portable X-Ray Fluorescence analyser https://www.olympus-global.com/

The project area was a former gold mine East of Kalgoorlie in Western Australia. About 900 samples were pulverised (to 90% passing 75 mm). Moisture content of samples for pXRF analysis should not exceed 20%. Standards employed were fresh, oxidised, mafic and felsic lithologies that were originally designated for JORC-compliant QAQC reporting for high-and low-grade gold detection by bulk fire assay and ICP-MS analytical methods. Two National Institute of Standards & Technology (NIST) soil standards and a silica blank were supplied with the pXRF unit. Note that many standards are not certified for all elements, which are produced by pXRF; these " results " may be useless and not acceptable for JORC-compliant QAQC reporting.

The authors demonstrate such pitfalls and conclude: Portable XRF undoubtedly provides a rapid and cost-effective means of assessing geochemical changes down-hole and in the field, but the technique generates data containing uncertainties that need to be identified and removed prior to any detailed field or analytical campaign.

References

Chrysos (2021) Chrysos Photon Assay. URL https://www.chrysos.com.au/ Accessed February 2021.

CSIRO (2021) Commonwealth Scientific and Industrial Research Organisation. URL https://www.csiro.au/ Accessed February 2021.

Durance, P., Jowitt, S.M. & Bush, K. (2014) An assessment of portable X-ray fluorescence spectroscopy in mineral exploration, Kurnalpi Terrane, Eastern Goldfields Superterrane, Western Australia. Applied Earth Science 123:3, 150-163. doi.org/10.1179/1743275814Y.0000000052

JORC (2012) Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves (The JORC Code). Available from (The Joint Ore Reserves Committee of The Australasian Institute of Mining and Metallurgy, Australian Institute of Geoscientists and Minerals Council of Australia). Accessed February 2021.

Pohl, W.L. (2020) Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons — an Introduction to Formation and Sustainable Exploitation of Mineral Deposits. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart. Citing from section 2.3.1 Gold (pages 224 — 240); 5.2.4 Geochemical Exploration (454 — 460). https://www.schweizerbart.de/9783510654352 (Soft Cover)


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A Diamond for your Loved One? As Proof for Deep Down-Cycling of Carbon into Earth? (January 6th, 2021)

How about an extremely rare diamond as a small gift? Although this is virtual, it stands for one of the numerous geoscientific discoveries of recent years. The majority of common diamonds (type I) formed at the boundary of lithosphere and astenosphere of the Earth (at 150-240 km depth), but this one shown in Figure 1, belonging to rare blue boron-bearing (type IIb) diamonds, came from the lower mantle (more than 660 km deep).

blue
Figure 1 — A high quality 25.5 carat blue diamond recovered at Cullinan S.A. in April 2013. Credit: Petra Diamonds Limited

Smith et al. 2018 analysed trace minerals trapped in Clippir and in 46 samples of blue diamonds across the globe, and from this worked out that the gemstones must have formed in the lower mantle. This is confirmed by inclusions of bridgmanite (MgSiO3), the high-pressure form of olivine (called bridgmanite) in the deep mantle. Clippir crystallised from a melt made of Fe-Ni-C-S, the blue diamonds from boron-bearing fluids. Seawater-serpentinized oceanic crust and lithosphere are suggested to be the source of boron.

Both diamond types are some of the deepest ever found. Moreover, they reveal a pathway that extends from the oceanic crust at Earth's surface to the lower mantle, and a potential route for the ultra-deep cycling of carbon and water in our planet.

In an OreDepositHub talk in July 2020, Evan Smith showed a sketch of oceanic slab subduction and sinking, which provides this pathway (Figure 2). This is one branch of Carbon downcycling into the Earth, whereas volcanism is the largest source of Carbon upcycling from inner Earth into the atmosphere. Clippir, and the blue diamonds can be lifted upwards by mantle plumes that impinge on the subcontinental lithospheric mantle (SCLM) where they may be haphazardly mixed into kimberlite melts that erupt through mantle and crust, possibly forming diamondiferous pipes.

blue
Figure 2 — Subduction of oceanic slab sinking into the lower mantle provides volatiles and matter that form type IIa Clippir and IIb boron diamonds. Upwelling can lift the deep diamonds to the base of continental lithosphere where they may mix into nascent diamondiferous kimberlite melt (Evan Smith ODH029 2020).

Ore Deposits Hub at oredepositshub.com was founded because of Covid-restrictions to scientific exchange and is sponsored by the professional economic geology societies SGA, SEG, & IAGOD in order to provide Open Science Talks. If you register you may profit from the services they offer. Take the chance to meet fellow scientists!

And how can we fit this into the Petrogenetic-Tectonic Classification of ore deposit formation, which I am promoting? — Well, the petrogenesis of these diamonds is metamorphism passing into anatexis. Considering the high pressures at these depths, I suggest that they crystallised from a supercritical hydrous melt (page 131) also called a supercritical fluid/melt phase (as described by Thomas & Davidson 2016) on pages 35-36 in my Economic Geology 2nd ed. (Pohl 2020). Note that generally in present-day geoscience, the existence of this kind of phase is sadly disregarded. The tectonic setting may be called subduction and down-sinking of oceanic slabs.

References

Pohl, W.L. (2020) Economic Geology, Principles and Practice. 2nd ed. 755 pp. Schweizerbart Science Publishers, Stuttgart.

Smith, Evan (2020) How the biggest and best diamonds defy exploration. Gemmological Institute of America. Ore Deposits Hub (Youtube video ODH029) July 15 2020.

Smith, E.M., Shirey, St.B., Richardson, St.H., et al. (2018) Blue boron-bearing diamonds from the Earth's lower mantle. Nature 560, 84—87. doi: 10.1038/d41586-018-05830-6

Thomas, R. & Davidson, P. (2016) Revisiting complete miscibility between silicate melts and hydrous fluids, and the extreme enrichment of some elements in the supercritical state — Consequences for the formation of pegmatites and ore deposits. Ore Geol. Rev. 72, 1088-1101.


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