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Hughes M.J.,University of Vic | Phillips G.N.,Phillipsgold Pty Ltd | Phillips G.N.,University of Melbourne | Phillips G.N.,Stellenbosch University
Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science | Year: 2015

Mineralogical domains use hypogene minerals (i.e. minerals not modified by weathering) and related geochemical characteristics of mineral occurrences, not only ore deposits, to subdivide large mineralised regions. Their use in the Victorian gold province is described using readily available historical data and field checking, and this is a scheme that has not required modification since 1997.The Victorian province is typical of sediment-hosted hydrothermal ores in metamorphic terrains (often termed orogenic gold deposits). Five distinctly different mineral assemblages are used to subdivide all Victorian gold occurrences into eight domains up to hundreds of kilometres in length and tens of kilometres in width. These parallel the regional structural trend and most are closely associated with, or sharply bounded by, major regional-scale faults. Seismic work has shown these faults to be listric thrusts, which flatten into a zone of duplexed greenstones overlying older basement rocks in the deeper crust. Although not defined genetically or temporally, mineralogical domains provide an additional variable related to fluid flow to assist genetic interpretation such as the scale at which a combination of processes operates, permitting predictions as to the origin of the fluids and their pathways. The variations in mineralogy in Victorian gold occurrences indicate that ore fluid compositions differed significantly between adjacent domains, and between areas overlying different regions of deeper crust. The pattern of domains gives clues to the existence of multiple mineralising events and to the degree of overprinting of these events. Domains also assist genetic comparisons by projection into similar adjoining regions to create new domains, for example Tasmania (Mathinna domain and Lefroy sub-domain), NSW (Cobar domain) and New Zealand (Reefton domain). The domainal pattern has application to mineral exploration, metallurgy and environmental issues. Mineralogical domains could be applied elsewhere, particularly in the study of difficult-to-subdivide sedimenthosted gold ores and Archaean greenstone-hosted gold, and possibly for other commodities, especially those that occur as hydrothermal ores. © 2015 Institute of Materials, Minerals and Mining and The AusIMM.

Phillips G.N.,University of Melbourne | Phillips G.N.,Phillipsgold Pty Ltd | Powell R.,University of Melbourne
Journal of Metamorphic Geology | Year: 2010

A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate-belts and other gold-only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist-amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre-scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold-thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon-bearing host rocks and/or by sulphidation of wallrocks to generate iron-bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite-bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock-dominated depositional systems, not fluid-dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold-only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher-grade metamorphism and deformation, and/or (palaeo)-weathering may provide many of the most-obvious features of goldfields including their mineralogy, geochemistry, geometry, small-scale timing features, geophysical response and even mesoscopic gold distribution. © 2010 Blackwell Publishing Ltd.

Neil Phillips G.,University of Melbourne | Neil Phillips G.,Phillipsgold Pty Ltd | Neil Phillips G.,Stellenbosch University | Powell R.,University of Melbourne
Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science | Year: 2012

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 Phillips and Powell (2010). In a grain-by-grain scale devolatilisation process, the lower temperature minerals have released their H 2O, CO 2 and H 2S 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. © 2012 Institute of Materials, Minerals and Mining and The AusIMM.

Phillips G.N.,University of Melbourne | Phillips G.N.,Phillipsgold Pty Ltd | Phillips G.N.,Stellenbosch University | Powell R.,University of Melbourne
Ore Geology Reviews | Year: 2015

Well-considered schemes that classify ore deposits can be effective contributors to exploration success, understanding of deposit genesis, mining geology, engineering, metallurgy, property valuations and stock market analysis. An effective classification scheme needs to be easy to apply without ambiguity and is much more powerful if based on sound scientific principles. For deposits with economic gold, a useful classification recognises gold-only and gold-plus deposits based upon whether they also have economic base metals. The scheme is easy to apply and in most cases unambiguous for specific deposits and whole provinces. The subdivision has a sound scientific basis that involves the chemistry of gold in relation to metal complexing, salinity of ore-forming fluids and redox state. Forward modelling from chemical principles can be integrated with inverse modelling from observations to add confidence in this classification and thus be used to create exploration opportunities. The Jiaodong gold province of eastern China comprises deposits mostly in granitic rocks with economic gold but low levels of base metals; the inferred ore fluids are of low to moderate salinity. From their gold-only character, several other aspects of their development can be inferred, though the tectonic and metamorphic history remains unresolved. The granitic host rocks appear to reflect a chemically and mechanically favourable site for deposition, not a source for the gold or the ore fluid neither of which have been established yet. © 2014 Elsevier B.V.

Phillips G.N.,University of Melbourne | Phillips G.N.,Phillipsgold Pty Ltd | Phillips G.N.,Stellenbosch University | Powell R.,University of Melbourne
Ore Geology Reviews | Year: 2015

Alteration is recorded throughout much of the 3. km thick Upper Witwatersrand succession in South Africa, and extends for 300. km around the basin and 50. km into the basin. The evidence for this alteration comes from petrography, mineralogy, geochemistry, rock fabrics and stratigraphy. The alteration, conceived as a giant halo enveloping each major goldfield, requires a major hydrothermal event focused on the auriferous Upper Wits (Central Rand Group). The scale and nature of the alteration, and its relationship to gold distribution on various scales, strongly support a hydrothermal model for gold deposition. Such a model can account for the 50,000. t of gold mined from the Witwatersrand goldfields. The model works within the context of the major advances in understanding of the geological framework of the Witwatersrand Basin including the recognition of the importance of post-burial processes. It is highly significant that the age of Witwatersrand sedimentation is much older than 2700. My, and thus the host rocks for gold were deposited at least 100. My before the major global Archean greenstone gold event.The shale units from 40 major mines covering the seven goldfields are examined to establish the nature and distribution of the alteration, and to make the connection between the fluid infiltration that caused the alteration, and the introduction of gold into the Witwatersrand Basin. In the hydrothermal model described here, an auriferous low salinity fluid is produced at about 500. °C during the metamorphism of mafic material that was below and adjacent to the Witwatersrand Basin. Large shear zones in the footwall of goldfields are the inferred fluid pathways into the Upper Wits succession, and then unconformities, smaller shear zones, veins and the reef packages guide local fluid flow. In this model, gold precipitation occurs in response to the reaction of the fluid with carbon- and iron-bearing rocks that are abundant in the reef packages immediately overlying unconformity surfaces. Gold precipitation occurred at about 350. °C and 10. km depth with the implication of a high geothermal gradient approaching 35. °C per km. The alteration of the shales, but lack of abundant gold in them, is predicted from the paucity of appropriate rocks or minerals in the shale units to precipitate gold, but fluid infiltration still changed their composition fundamentally. The source of the Witwatersrand gold, its present distribution and the alteration are explained within this hydrothermal model, with similarities to the models invoked for other major gold provinces.It is worth noting that quartz-pebble conglomerate is a poor term for this gold type and has almost certainly limited exploration opportunities globally. © 2014 Elsevier B.V.

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