Applied Earth Science: Transactions of the Institute of Mining and Metallurgy

Goldfields extend for 300 km around the margin of the Archaean Witwatersrand Basin of South Africa associated with regional greenschist facies metamorphism and deformation. Metamorphic mineral assemblages involving pyrophyllite-chloritoid are associated with gold in all goldfields, reflecting low pressure and 300 to 400°C conditions, and indicating high geothermal gradients. The origin of Witwatersrand gold can be explained by metamorphic devolatilisation to generate auriferous fluids beneath and outside the Witwatersrand Basin, followed by passage of these fluids along large structures and into the Basin. In this model, generation of the fluid is a consequence of the transition to amphibolite facies of mafic rocks such as Archaean greenstone belts, as in . In a grain-by-grain scale devolatilisation process, the lower temperature minerals have released their H2O, CO2 and H2S to form a metamorphic fluid which at the time of its formation has already dissolved gold from these unenriched rocks (say at the ∼2 ppb Au level). A substantial volume of auriferous fluid is inferred given that regional metamorphism is on the scale of thousands of cubic kilometres, all evolving a few per cent of fluid. In the model, this auriferous fluid migrated from the immediate source region along grain boundaries, then in shear zones, and into the Witwatersrand Supergroup via the major thrust faults adjacent to all goldfields. Once in the Supergroup, fluid migration was via bedding planes and especially unconformity surfaces, along cross-cutting faults, shear zones and reef packages. The few thicker shale units and especially the overlying Archaean Ventersdorp basalt pile acted as barriers to fluid flow and facilitated fluid focussing. Deposition of gold from this auriferous fluid occurred where there was abundant carbon and/or iron. The distribution of these two elements was pre-determined by sedimentary distribution, and diagenetic migration. Many bedding features were preserved or mimicked during the hydrothermal replacement process because the volumetric strain in most of the sequence was low. Critical to the success of the hydrothermal process in producing gold deposits were the widespread local concentrations of C and Fe on unconformity surfaces, the reef packages, the major thrust faults adjacent to each goldfield, source rocks of greenstone-like character that were once at (sub)-greenschist facies grade with assemblages that included chlorite–calcite–pyrite, and a regional metamorphic event of high geothermal gradient. Moreover, critical to the exposure and preservation of the goldfields has been the limited erosion of the Basin since the hydrothermal deposition of the gold. The metamorphic devolatilisation–hydrothermal replacement model predicts many features of Witwatersrand ores, and can explain quantitatively the origin of its ∼100 000 t gold endowment. Calculations suggest that there is adequate gold at background (2 ppb) levels in inferred source rocks to form the necessary auriferous hydrothermal fluids; in contrast, there appears to have been insufficient particulate gold to form a significant placer accumulation. Without a viable source region for detrital gold, and indeed no plausible sorting mechanism to produce such deposits, it would be unwise to accept a placer model.

G Neil Phillips

R Powell

Applied Earth Science Transactions of the Institutions of Mining and Metallurgy Section B

Origin of Witwatersrand gold: a metamorphic devolatilisation–hydrothermal replacement model

威特沃特斯兰德金矿的成因：变质脱挥发分-热液置换模型

Shale-hosted Ni–Cu–platinum group element (PGE) sulphide occurrences represent significant resources of not only Ni, Cu and the PGEs but also, in some cases, other elements such as Mo, Zn, Mn, V and U. The largest known deposit of this type that is currently being mined is the Talvivaara Zn–Ni–Cu–Mn–U shale deposit in Finland. The total resource of this deposit is ∼1·550 Bt of ore, at 0·22%Ni, 0·13%Cu, 0·49%Zn, 200 ppm Co and 17 ppm U, all of which can be extracted using a bioheapleaching mineral processing approach. Other significant examples of this deposit type are the Mo–Ni–Zn–Au–PGE deposits of the Niutitang Formation in China and the Ni–PGE–Zn–Mo–Re–Ag mineralisation of the Nick deposit in the Selwyn Basin, Yukon, Canada. However, although they provide insights into the processes responsible for mineralisation in shale-hosted Ni deposits, the Chinese and Selwyn Basin deposits are of too low a tonnage to make them high priority exploration targets. Other potential resources discussed here include the Okcheon shales in Korea, the European Alum Shale and the Chattanooga Shale of the Appalachian Basin. Possible modern analogues for the shale-hosted Ni–Cu–PGE deposits at Talvivaara, Nick, and in China include the Rainbow and Logatchev vent fields of the Mid-Atlantic Ridge, where hydrothermal fluids interact with ultramafic rocks, stripping them of metals before venting them at the seawater–ocean floor boundary, and sapropel formation in the Mediterranean, where organic carbon-rich sediments are interlayered with organic carbon-poor sediments. These organic carbon-rich layers are associated with significantly elevated base metal and sulphur concentrations when compared to the carbon-poor layers, indicating preferential deposition of base metals during sapropel sedimentation. Two genetic models for shale-hosted Ni–Cu–PGE mineralisation are currently favoured: synsedimentary mineralisation, with metals being scavenged from the overlying water column, and exhalative hydrothermal mineralisation, with metals being deposited into sedimentary basins from hydrothermal vents. This paper focuses on shale-hosted Ni mineralisation and the potential for these shales as an economic source of unconventional Ni, Cu and the PGEs.

S M Jowitt

R R Keays

Shale-hosted Ni–(Cu–PGE) mineralisation: a global overview

页岩型镍-(铜-铂族元素)矿化作用：全球概述

"A novel mechanism for the formation of tropically weathered REE ion adsorption deposits." Applied Earth Science, 126(2), p. 63

Liam Hardy

Billy Nason

Martin Smith

A novel mechanism for the formation of tropically weathered REE ion adsorption deposits

热带风化REE离子吸附矿床形成的新机制

"The petrology, mineralogy and geochemistry of the large Fen Carbonatite Complex (Norway) REE-Nb resource." Applied Earth Science, 126(2), p. 74

Christian Marien

Arjan H. Dijkstra

Colin Wilkins

The petrology, mineralogy and geochemistry of the large Fen Carbonatite Complex (Norway) REE-Nb resource

大型芬碳酸岩杂岩（挪威）稀土-铌资源的岩石学、矿物学和地球化学

"Crustal sequestration of magmatic sulphur dioxide as an ore-forming process." Applied Earth Science, 126(2), pp. 76–77

John Mavrogenes

Jon Blundy

Crustal sequestration of magmatic sulphur dioxide as an ore-forming process

地壳中岩浆二氧化硫的隔离作用作为一种成矿过程

ABSTRACT The discovery and development of world-class Lithium–Caesium–Tantalum (LCT) spodumene-bearing pegmatites in Western Australia underpins growth of a significant new sector of its mining industry. Recently, several new spodumene discoveries have been delineated in the Yilgarn and Pilbara Cratons. Contrary to exploration narratives that new economic mineral discoveries will generally be made at increasingly greater depths, beneath barren cover rocks, or in remote geological environments, all new lithium discoveries have clear surface expressions in relatively ‘mature’ greenstone belts. The exploration implication is that the search space for pegmatite-hosted spodumene deposits in Western Australia remains immature. These recently discovered LCT pegmatites have geological features relevant to exploration including their age, amphibolite-facies metamorphic setting and syn-metamorphic timing, and 3D geometry, particularly their typically gentle dips, that match other such world-class pegmatites globally. Further spodumene discoveries within pegmatites at or near surface are likely in the Archean terranes of Western Australia based on these consistent exploration criteria and supportive capital market conditions.

Zoe Phelps-Barber

Allan Trench

David I. Groves

Applied Earth Science

Recent pegmatite-hosted spodumene discoveries in Western Australia: insights for lithium exploration in Australia and globally

西澳大利亚近期发现的锂辉石伟晶岩：对澳大利亚及全球锂勘探的启示

Hallbauer DK, Joughin NC. 1973. The size distribution and morphology of gold particles in Witwatersrand reefs and their crushed products. Journal SAIMM. 73:395–405. Harmon RS, Russo RE, Hark RR. 2013. Applications of laserinduced breakdown spectroscopy for geochemical and environmental analysis: a comprehensive review. Spectrochimica Acta Part B. 87:11–26. Minter WEL, Goedhart M, Knight J, Frimmel HE. 1993. Morphology of Witwatersrand gold grains from the Basal reef; evidence for their detrital origin. Econ Geol. 88:237–248. Rifai K, Laflamme M, Constantin M, Vidal F, Sabsabi M, Blouin A, Bouchard P, Fytas K, Castello M, Kamwa BN. 2017. Analysis of gold in rock samples using laser-induced breakdown spectroscopy: matrix and heterogeneity effects. Spectrochimica Acta Part B. 134:33–41. Storrar CD. 1981. South African mine valuation. Johannesburg: Chamber of Mines of South Africa.

J. J. Wilkinson

A. Pacey

L. A. Hart-Madigan

J. Longridge

D. R. Cooke

M. J. Baker

A. J. Boyce

C. C. Wilkinson

A new paradigm for the origin of propylitic alteration in porphyry ore systems

斑岩矿床中绿泥石化蚀变起源的新范式

The Dalradian Supergroup of Britain and Ireland is mineralised by gold-tellurium vein deposits. The host succession includes carbonaceous, pyritic shales (pelites) which were a source of trace elements, including gold and tellurium. LA-ICP-MS mapping of pyrite crystals shows that late stages are enriched in gold, tellurium and lead, representing concentration of these elements during metamorphism and related hydrothermal activity. The sulphur isotope composition of the pyrite varies with stratigraphic position, reflecting an origin for the pyrite in the depositional environment through microbial sulphate reduction. Where pyrite was converted to pyrrhotite, trace element contents are much lower, indicating element liberation during metamorphism. These observations are consistent with a model of black shale protoliths for orogenic gold deposits.

J. Parnell

M. Perez

J. Armstrong

L. Bullock

J. Feldmann

A black shale protolith for gold-tellurium mineralisation in the Dalradian Supergroup (Neoproterozoic) of Britain and Ireland

英国和爱尔兰新元古代达拉德超群中金碲矿化的黑色页岩原岩

Validating and updating geological models is important in mining to reduce the errors between the predicted and mined values. In the Banded Iron Formation-hosted iron ore deposits in the Hamersley Ranges Western Australia, geological models are based on widely spaced exploration data, resulting in errors on smaller scales. Closely spaced chemical assays from production blast hole drilling become available for updating models, but do not routinely provide mineralogy. This study identifies mineralogy by applying linear unmixing methods to chemical assays. Linear spectral mixture analysis (LSMA), extended linear mixing model (ELMM) and spectral unmixing within a multi-task GP framework (SUGP) successfully identified the mineralogy in synthetic mixtures, with RMSEs of 13.75%, 11.78% and 11.97%. LSMA had consistently poorer results. SUGP was more accurate when using fewer endmember examples. ELMM was easier to train and required less processing power. Therefore either SUGP or ELMM should be chosen to identify the mineralogy depending on the application.

Katherine L. Silversides

Arman Melkumyan

Mineralogy identification through linear unmixing of blast hole geochemistry

通过爆破孔地球化学的线性解混进行矿物学识别

Misclassification of ore grades results in lost revenues and the need for representative sampling procedures in open-pit mining is increasingly important in all mining industries. This study evaluated possible improvements in sampling representativity with the use of Reverse Circulation (RC) drill sampling compared to manual blast hole (BH) sampling in the Leveäniemi open-pit mine, northern Sweden. The results of the variographic experiment showed that sampling variability was lower for RC than for BH sampling. However, the total costs for RC drill sampling are significantly more than for manual BH sampling; other benefits may motivate the introduction of RC drilling. The main conclusion is that manual BH sampling can be fit-for-purpose in the studied open-pit mine. However, with so many mineral commodities and mining methods in use globally, there is no universal best practice for open-pit drill sampling and each case must be evaluated individually.

Karin Engström

Kim H. Esbensen

Optimal grade control sampling practice in open-pit mining – a full-scale blast hole versus reverse circulation variographic experiment

露天矿山最佳品位控制采样实践——全尺度爆破孔与反循环变异图实验对比

短名	Appl. Earth Sci.
Journal Impact	0.87
国际分区	GEOSCIENCES, MULTIDISCIPLINARY(Q4)

出版信息	出版商: SAGE Publications Inc.，出版周期: ，期刊类型: journal，开源期刊: 否
基本数据	创刊年份: 2018，原创研究文献占比: ，自引率:， Gold OA占比:

Applied Earth Science: Transactions of the Institute of Mining and Metallurgy

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