Supergene gold and sulphide mineralisation in sediment hosted deposits.
- Prof. Dave Craw, Department of Geology, University of Otago
- Assoc. Prof. Louisa Lawrance (School of Earth and Geographical Sciences. The University of Western Australia.)
Otago University PhD Scholarship
Brenda Shore award, Otago Branch of NZ Federation of graduate women
Jack Bradshaw Scholarship
SEG (Newmont research award)
Belle-Brook Holdings Ltd
AngloGold Ashanti Australia Ltd
Institute of Geological and Nuclear Sciences (QMAP Fiordland)
Donna's email: firstname.lastname@example.org
Southern South Island, New Zealand
My PhD research focuses on supergene processes that occur in sediment-hosted gold deposits. Specifically I am concentrating on two gold placers that contain abundant secondary gold mineralisation. The sites investigated are broadly similar in that they are all from unmetamorphosed fluvial sediments. My main sites occur in the Waimumu district, eastern Southland, New Zealand, at Belle-Brook and Parker Road, in Miocene to recent quartz pebble conglomerates (QPC). Both sites are small-scale alluvial gold mining operations. Belle-Brook is characterised by diagenetic sulphides, particularly marcasite, whereas the Parker Road site contains only rare diagenetic sulphide. My second site is an anomalously rich occurrence of gold mineralisation hosted within an Eocene palaeochannel superimposed on the Sunrise Dam Deposit 55km south of Laverton in the Yilgarn Craton, Western Australia. Although the palaeochannel directly overlies the Sunrise Dam Gold Mine (owned by AngloGold Ashanti, Australia), the gold within the palaeochannel is not obviously detrital, and nor is it colluvial. In order to determine the genesis of this palaeochannel gold, detailed geochemistry will be carried out on primary gold from the mineralised shear zones of the Sunrise-Cleo deposit to ascertain what, if any, contribution the underlying mineralisation has had on the mineralisation that occurs in the palaeochannel.
The chief aim of studying the mineralisation at these sites is to delineate the supergene processes that occur at lower-temperatures in unmetamorphosed sediments. It is envisaged that textural and geochemical analysis will also go some way to distinguishing the physical/textural/geochemical expression of dissolution and precipitation processes that occur during gold remobilisation. There are significant similarities and differences between the sites chosen, hence, an integrated approach that investigates both ‘traditional’ placer and supergene mineralisation will lead to greater understanding and overall simplification of the processes that occur in sediment-hosted deposits, regardless of their wider geological framework.
The most significant implication of this research will be its application to understanding supergene gold mobility, especially the role of diagenetic sulphides, in sediment-hosted deposits and in particular, Witwatersrand-style mineralisation. The interpretation of Wits-style mineralisation in the Witwatersrand is complicated by hydrothermal overprinting associated with metamorphism. Only through studying unmetamorphosed sediments can truly syngenetic processes be established. The sites chosen for study both will contribute significant new data towards the understanding of ore mineral diagenesis in Wits-style QPC deposits.
In addition to my research into supergene mineralisation I am also investigating gold and associated sulphide mineralisation that occurs in the historical Preservation Inlet Goldfield located in southwest Fiordland, New Zealand. This goldfield is hosted in Paleozoic metasedimentary rocks that include carbonaceous argillites and sulphide-rich shales of the Preservation Formation (475-490 Ma), that have been metamorphosed to upper greenschist facies and intruded by the Revolver granite (130 Ma). The aim of this research is firstly to constrain the geochemistry of alluvial gold derived from Fiordland into Southland. Secondly, detailed morphological and geochemical analysis will be carried out on the both detrital (beach placer) and primary gold to establish what, if any, relationship between them. Thirdly, to derive a genetic model for Preservation Inlet style mineralisation.
MSc title: Sediment-hosted Gold and Sulphide Mineralisation, Belle-Brook, Southland, New Zealand.
MSc Abstract: (pdf download)
Nonmarine ‘lignite measures’ sediments in the Waimumu District, eastern Southland are preserved in a basinal setting which is dissected by a series of steeply dipping N-NE striking, reverse faults of the Dunsdale Fault System. Belle-Brook, the main site investigated, is situated adjacent the Hedgehope Fault of this system. Belle-Brook is characterised by anomalous coarse gold and diagenetic iron sulphide mineralisation hosted within Miocene-Pliocene quartzose gravels.
New palynological data indicates an early Pliocene age for carbonaceous material overlying quartz pebble conglomerate (QPC) that occurs at Parker Road. An unusual abundance ofMicrocachrys (Podosporites parvus) distinguishes this QPC from other regional quartzose gravels. There is an apparent association betweenMicrocachrys occurrence and the inferred fault controlled deposition of recycled upper Gore Lignite Measures (GLM) along components of the Dunsdale Fault System.
Diagenetic lump and spheroidal marcasite, and to a lesser extent, framboidal pyrite dominate the sulphide suite at Belle-Brook. Where observed together, paragenetic relations indicate that pyrite (composed of unordered octahedral microcrysts) is a precursor to marcasite formation. SEM textures suggest a possible transformation between framboidal pyrite and recrystallised spheroidal marcasite. Spheroidal marcasite is of a fundamentally similar habit to framboidal pyrite. Lump marcasite characteristically contains anomalously high concentrations of As (1,600 ppm); Ni (5,700 ppm); Cr (400 ppm); Co (3,200 ppm) and to a lesser extent, Zn, Pb, Cu and W. With the possible exception of rare discrete Ni-Co oxide euhedral crystals, these trace metals are assumed to occur in solid solution. Nickel and cobalt typically occur in various colloform-style bands of nickeliferous marcasite, as well as altered, variously oxidised and hydrated, equivalents of the marcasite – nickeliferous marcasite (bravoite) solid solution series. The style and distribution of nickel-rich bands as shown by element mapping may imply a secondary remobilisation of nickel that is potentially associated with marcasite recrystallisation. It is speculated that a desorptive expulsion of solid solution metals (Ni, Co, As?) occurs. It is further speculated that this expulsion may proceed via the reaction transport mechanism of geochemical self-organisation.
Spheroidal marcasite - Colloform banded marcasite (truncated bands are adjacent quartz grains)
Suspected authigenic arsenopyrite microcrystals
Preliminary findings, based of field occurrences and SEM textural analysis suggest that at least some of the diagenetic marcasite and pyrite is of recent authigenic origin. Sulphide dissolution and inferred precipitation textures are documented. The occurrence of pristine arsenopyrite without alteration to scorodite, as well as potentially authigenic microcrystalline arsenopyrite, implies previously undocumented arsenopyrite stability. The occurrence of detectable nickel in pristine microcrystalline arsenopyrite occurring along the cleavage of host arsenopyrite grains (without nickel), may lend weight to an authigenic origin for these micro-arsenopyrite crystals.
Sulphur isotope data for marcasite is within the light isotope range which characterises bacterial sulphate reduction in a nonmarine environment (∂34S -12.2 to -28.8 ‰). However, diagenetic recrystallised pyrite is atypically enriched (∂34S -2.0 ‰), as is pyrite associated with lignite (∂34S -6 to -11.0 ‰). No attempt was made to distinguish between fine-grained framboidal pyrite and spheroidal marcasite. Detrital arsenopyrite (∂34S -9.9 ‰) is not entirely consistent with an Otago Schist derivation. Detrital cubic pyrite data are variable, with an unusual enrichment (∂34S +13.3 ‰) precluding a derivation from the Otago Schist, whereas a second sample is consistent with an Otago Schist provenance (∂34S -0.9 ‰).
Decomposition of marcasite makes a significant contribution to the development of acidification at Belle-Brook (down to pH of 2.4). Elevated nickel (1.08 -17.9 ppm), zinc (1.63 - 27 ppm), cobalt (3.56 ppm) and manganese (0.64 -6.65 ppm) occur in the resultant acid water. There is a rapid attenuation of this metal load downstream with increasing pH. Jarosite is the predominant secondary mineral precipitate that occurs on outcrop surfaces. Aged jarosite contains significant nickel (6,064 ppm) and zinc (2,607 ppm), whereas concentration of metals to the same extent has not developed in fresh jarosite samples (i.e. Ni av. 85 ppm; Zn av. 82 ppm). In jarosite a consistent and often two-fold increase in concentration is shown by rare earth elements; Sr, La, Ce, Nd, and Pb, relative to the underlying substrate material.
Spheroidal marcasite (Ni element map)
Lump marcasite (Ni element map)
Electrum grain with Ag enriched rim (Ag element map)
Morphologically two distinct gold types exist. Gold is predominantly fine-grained (<1 mm) and consistent with an Otago Schist source. A second gold type occurs only at Belle-Brook that is coarse grained and commonly intergrown with quartz. The source of Belle-Brook ‘nuggety’ gold is unknown but its overall morphology implies that it is potentially local.
Compositionally, all gold analysed occurs as one of three optically indistinguishable gold types. Approximately 75 % of fine-grained gold occurs as Au-Ag alloy, 25 % alpha-phase Au-Ag-Hg alloy, 3 % pure gold. Silver depleted rims are a feature of both alloy types. Less commonly, silver enriched rims of up to 5 wt % Ag (relative to core) occur in 28 % of Au-Ag alloy and 6 % of Au-Ag-Hg alloy particles. Element mapping indicates both silver dissolution as well as gold precipitation has taken place to create silver depleted rims. Regular and irregular silver and mercury dissolution signatures reflect different geochemical, and or, transport histories of gold particles. Geochemical signatures of the fine-grained gold are generally consistent with gold derived from the Otago Schist (Caples Terrane). A single grain of electrum (32 wt % Ag) with well developed silver enriched rims is more enigmatic, with no known source in Otago or Southland.
It is inferred that gold mobilisation surface textures indicating precipitation and dissolution can be differentiated. Inferred precipitation textures include: budded chains and masses, spheroidal forms, spheroids with five-fold symmetry; rare euhedral crystals (triangular plates), ultra-fine wispy veils and gold threads and possible bacterioform gold. Ultra-fine gold and two-phase colloidal gold is documented for the first time from a placer environment. Inferred dissolution textures reflect preferential dissolution that occurs along grain boundaries. Spatially associated gold mobilisation textures imply an almost synchronous dissolution and reprecipitation of gold within a distance of two microns. Complexes responsible for gold remobilisation remain unconstrained. Silver depleted rims may indicate gold remobilisation as a chloride complex, however, grains without rim development may signify gold mobility as a thiosulphate complex. The involvement of organic complexes is suggested by the occurrence of ultra-fine, and or, colloidal gold forms.
Based on the occurrence of broadly similar diagenetic iron sulphides both texturally and compositionally, as well as similar detrital and inferred authigenic gold, it is speculated that Belle-Brook may be analogous to an embryonic Witwatersrand-style deposit.
Selected images inferred to show textures associated with authigenic gold mobility from a placer environment
Triangular gold plate with spatially associated dissolution and precipitation features
Interpenetrating gold - void budded gold mass
Colloidal gold composed of microcrysts and a dispersed background phase (semi electron transparent)
Ultra-fine gold bridging gap between budded gold grains.
- Falconer, D.M. (1987) "Detrital and authigenic gold from Belle-Brook, Southland" Unpublished PGDip. Sci. thesis, University of Otago, Dunedin, New Zealand.
- Falconer, D.M. (2003) "Sediment-hosted gold and sulphide mineralisation, Belle-Brook, Southland, New Zealand" MSc thesis, University of Otago, Dunedin, New Zealand.
Conference Oral Presentations
- Falconer, D.M., Youngson, J., Craw, D., Faure, K., (2007) Gold Placers in southern New Zealand: a modern analogue for "Witwatersrand-style" gold mineralization. Advances in the understanding of supergene processes, held in conjunction with Ores and Orogenesis, A symposium honoring the career of William R Dickinson. 23-30 Sept, 2007, Tucson, Arizona.
- Clough, D.M., and Craw, D. (1989) "Authigenic gold-marcasite association – evidence for nugget growth by chemical accretion in fluvial gravels, Southland, New Zealand." Economic Geology (84):953-958p
- Craw, D., Falconer, D.M., and Youngson, J.H. (2003) "Environmental arsenopyrite stability and dissolution: theory, experiment and field observations. Chemical Geology (199) p71-82 (download pdf file)
- D.M. Falconer and D. Craw, (2005), Fluvial quartz pebble conglomerates as a source of acid rock drainage and trace elements: a case study from Belle-Brook, Southland. In Moore et al., (Eds) Metal contaminants in New Zealand: From sources and effects on human health and ecology to rehabilitation and restoration. Caxton Press, Christchurch. 139-152p
- Falconer, D.M., Craw, D., Youngson, J.H., and Faure, K., (2006), Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand. Ore Geology Reviews, Special publication on placer deposits. (28) 525-545p
- Donna M. Falconer and Dave Craw, (2006), Authigenic gold and surface textures indicative of authigenic gold remobilization on placer gold, Waimumu, New Zealand.
- JH Youngson, D Craw & DM Falconer, (2006) Evolution of Cretaceous-Cenozoic quartz-pebble conglomerate gold placers during basin formation and inversion, southern New Zealand. Ore Geology Reviews, Special publication on placer deposits (28) 451-474p.
- Frank Reith, Maggy F Lengke, Donna Falconer, David Craw and Gordon Southam. (2007) The geomicrobiology of gold. The ISME Journal 1, 567-584p.
- Donna M. Falconer and Dave Craw, in review, Supergene gold mobility: a textural and geochemical study from gold placers in southern New Zealand. Economic Geology
- Donna M. Falconer and Dave Craw, in prep, Morphology and composition of diagenetic and detrital sulphides in a quartz pebble conglomerate gold placer, Belle-Brook, New Zealand.
- 4th Asia Pacific Symposium on Environmental Geochemistry. Perth, Western Australia Jan 18-20 2005.
- Geological Society of Nevada "Symposium 2005 - Window to the World". Reno, Nevada May 11-21 2005.
- SEG 2004 Perth.
- AUSIMM, Greymouth, 2003 and NZ Geochemical Group, Kaikoura, 2003
- AUSIMM Auckland, 2002