Economic Geology News

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).

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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.

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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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