macla nº 16. junio 12 242 revista de la sociedad española de mineralogía
Platinum Group-Minerals in the Gossan from the Key West Cu-Ni-Pt Sulphide Occurrence
(Nevada, USA) / SAIOA SUÁREZ (1, 2, 3*), HAZEL M. PRICHARD (1), PETER C. FISHER (1), IAIN McDONALD (1) (1) Dpto. de Mineralogía y Petrología. UPV/EHU, Leioa, Spain. (2) IKERBASQUE, Basque Foundation for Science, Bilbao, Spain. (3) School of Earth and Ocean Sciences, Cardiff University, Wales, U.K.
INTRODUCTION by overlying sediments composed of ppb Pt, 2612 ppb Pd and 4 6 ppb Au. typical basin debris. The Pt/Pd ratio varies bet een 0.1 and Eluvial deposits including gossans and 1.2 (average of 0.5), ith the lo est laterites have long attracted attention as The ore deposit is associated ith a values in the most oxidised samples. having the potential to be economically lenticular hornblendite body hich viable unconventional platinum group- follo s a steeply dipping fault zone that About 400 PGE-grains have been element (PGE)-deposits (Green Peck, strikes N65º to 75ºE, parallel to the located in the eathered samples. Most 2005). Ho ever, the mineralogical gneiss structure. Mineralisation occurs of them are tiny grains that occur in changes to primary platinum-group as disseminated small patches including groups close to larger PGM, and only 70 minerals (PGM), and the secondary chalcopyrite, pyrite, pyrrhotite, millerite, PGE-grains over 5 m in size have been processes that redistribute precious pentlandite and magnetite, ith observed. These occur disseminated in metals in the surface environment are associated Pt and Pd as sulpharsenides goethite box ork and at the edges of still relatively unexplored. and tellurides. Supergene oxidation of goethitic or quartz infillings, often the ore is almost complete to a depth of concentrated along thin veins filled by The eathering of PGE-bearing ores is about 12 m and forms a gossan zone late Fe-oxides/oxyhydroxides. More likely to vary bet een climatic regions grading 2.5 Cu, 1.3 Ni and 2. g/t Pt rarely, grains are associated ith relic orld ide, affecting the behavior of PGE (Carpenter, 1 54) composed of rusty silicates and relic oxides (i.e. ilmenite, in supergene conditions and thus, their rocks covered by a very thin soil magnetite). More than 0 Au-bearing potential accumulation in valuable (Prichard, 2006). grains have been found spatially related surface deposits. In this study e to PGM. present the preliminary results of the METHODS PGE-mineralogy in the gossan capping PGE-bearing phases recognised in the the ey West Cu-Ni-Pt occurrence, in the Sixteen samples ere collected in 2002 ey West gossan are diverse. Most are desert of southern Nevada (USA). The in the ey West pit and nearby trenches. secondary Pd-, Pt- and Fe-Pd/Pt-rich main objective is to assess ho PGM Eight samples belong to the oxidised oxides (62 ), and the rest are primary and PGE evolve in gossans developed in material from the surface gossan and PGM, mainly Ir-PGM, that remain these extremely arid regions. the rest are fresh samples of cumulate unaltered. rocks ith interstitial sulphides. THE KEY WEST DEPOSIT Samples ere analysed for Pt, Pd and Residual PGM Au using fire assay and ICP-MS (Ultra The ey West occurrence is part of the Trace Pty. Ltd. Laboratories, Australia). Irarsite (Ir,Pt,Rh,Ru)AsS is a idespread large Bunkerville Mining District in Clark PGM ere searched for and analysed residual PGM in the gossan (n 7 County, Nevada. Originally kno n as the qualitatively in 12 polished blocks using grains located). It mainly occurs as Copper ing district on the est side a Veeco FEI L30 field emission gun equant grains ithin goethite or quartz, of the Virgin Mountains, it hosts other environmental scanning electron although elongate irarsite in laths is also commodities including Ni, Pt, Pd, Au, Ag, microscope fitted ith an Oxford common. These irarsites are sometimes W, Co, U, Ti, gypsum, Be and mica. ey Instruments INCA energy-dispersive - broken and may appear as subrounded West is a small Cu-Ni-PGE sulphide ray spectrometer (Cardiff University, U ). small fragments ith corroded edges in deposit (31.000 t at 2.5 Cu, 1. Ni, 4 goethite. Sizes are variable from less g/t Pt) that as sporadically mined PRELIMINARY RESULTS than 1x1 to 15x7 m. bet een 1 0 and 1 53 for Cu and Ni (Tingley, 1 ). The ey West gossan is composed of Accessory inherited PGM are laurite, goethite, hematite, quartz, limonite, and osarsite and scarce holling orthite. Rocks exposed at the ey West mine Cu- and Ni-rich carbonates and silicates These phases occur as small grains in consist of a belt of Precambrian gneiss in shear zones. The main accessory goethite and together total 50 grains. Pt- and schist that trends northeast and is phases are PGM, electrum, native Ag, Pd-(Bi)-tellurides are rare and only one intruded by both hornblendite dikes and barite, zircon, ilmenite, and secondary relic grain of keithconnite ith altered plugs, and by aplite and pegmatite Ag-halide minerals (embolite series). edges as found in goethite. Tellurides dikes. This belt is bounded by tilted and Average precious metal contents occur at the contact bet een sulphides faulted Paleozoic sedimentary rocks and recorded in eathered samples are 71 or associated ith magnetite, ilmenite
palabras clave: ey West, Nevada, Sulfuros Cu-Ni-Pt, PGM, Gossan, key words: ey West occurrence, Nevada, Cu-Ni-Pt-sulphides, PGM, xidos de PGE Gossan, PGE-bearing oxides resumen SEM/SEA 2012 * corresponding author: [email protected]
macla nº 16. junio 12 revista de la sociedad española de mineralogía 243
or silicates in the primary sulphide zone, subhedral to subrounded ( x6 m) and gossan and are altered in situ to form b( ut are rare in the supergene zone. sho strongly oxidised edges (Fig. 1). Pd-rich oxides. The smaller ones are Grains ith more Pt are larger ( 1 x6.5 being partially replaced by larger Fe-Pd- Secondary PGE-Bearing Phases m) although very scarce (n 4), and oxides, hich form a thick rim around occur as anhedral grains in the Fe- grain cores that may act as a local Pd-rich oxides (n 10 ) are the most oxides. barrier to Pd dispersion (Fig. 1). The Pt- abundant PGE-bearing phases in the oxides preserved in the gossan have gossan and can be tentatively divided Commonly PGE- and Fe-oxides in ey also been mostly altered to Fe-Pt-rich into the follo ing subgroups according West contain variable Cl- and to a lesser oxides, but these are scarce and occur to their composition and textural extent Br- in their compositions. as deformed grains embedded in the features: goethite groundmass.
(i) Pd-oxides Te-Bi. These have the This lack of Pt-PGM in the gossan highest Pd and Te contents and a compared to Pd-PGM is unusual as Pd is common composition similar to 2O. typically more remobilised and These occur as relic grains in goethite dispersed than Pt in the surface ( x4 m) and may sho a skeleton environment. Ho ever, the lo Pt/Pd texture here they have been ratios of the eathered rocks as ell as replaced by quartz. These grains the textural and mineralogical features appear to be altered Pd-Bi-tellurides of the PGM in the ey West gossan associated ith magnetite and suggest a different behavior of the main ilmenite relicts in the gossan. PGE, as has been pointed out already by (ii) Pd-Cu-rich oxides. These form a Prichard (2006). It is possible that a scarce group of grains ith a notable greater mobility than usual of the Pt Cu content in a composition close to characterises the PGE exogenic cycle in O2. These occur as evenly eathered these very dry regions. In any case, equant and subhedral grains ( x5.5 fig 1. Pd-Fe-oxide altered to Fe-Pd-rich oxide. Gt these preliminary observations still need m) ithin goethite or along late veins goethite, ox. oxide. to be checked by laser ablation analysis that cross-cut the rocks. to compare the dispersion of each PGE (iii) Pd-Fe-rich oxides. These are the DISCUSSION more idely in the gossan. most abundant Pd-rich oxides ith a variable Fe content and a consistant The PGM in the ey West gossan sho a ACKNOWLEDGEMENTS O2 composition. The grains ith less variable evolution during eathering Fe are smaller ( .3x7 m) and occur ith Ir-, Os-, Ru- and Rh- minerals much We thank the Department of Education, ithin quartz or at the contact better resisting oxidation than the Pd- Universities and Research of the Basque bet een quartz and goethite. Those and Pt-rich ones. Only one relic primary Government (Refs. IT340-10, BFI-2011- ith more Fe are larger ( 22.5x14 Pd-PGM has been located hereas Pd- 254). m) and appear more eathered oxides are the chief PGE-bearing sho ing mottled surfaces or highly minerals in the gossan. In contrast, Pt- REFERENCES eroded edges, sometimes outlining C- rich minerals other than Pt-bearing shapes. These occur ithin goethite, irarsite are rare, ith only a fe small Carpenter, A. F. (1954): Report on the Key sometimes along box ork structures, Pt-Fe-oxides having been located. West Mine. Bunkerville Mining District, Clark County, Nevada, 19 p (Unpublished). and randomly distributed ithin late Green, T., Peck, D. (2005): PGE Exploration: Fe-oxide-filled veins. There are a fe It is kno n that relatively acidic fluids in Economic Considerations and Geological less dense Pd-Fe-oxides (n ) ith a supergene conditions, such as the ones Criteria. In Exploration for PGE Deposits . significant Pt content, all distributed originating during oxidation of Ni-Cu- Mungall JE (ed). Mineral Assoc Canada. ithin the same thin vein together sulphide orebodies, may dissolve Pt and Short Course 35, 247-274; ith the Pd-Cu-oxides. Pd (as ell as Au) and transport them Prichard, H. M. (2006): Natural and Artificial most likely in hydroxide- or chloride- Platinum and Palladium Occurrences Pt-rich oxides are sparse (n 24) and complexes (i.e. Wood et al., 1 2). Worldwide. In Advances in earth science: Supergene solutions in ey West ere from earthquakes to global warming . they sho a composition similar to O2 Royal Society Series on Advances in ith significant Fe and traces of Pd and probably enriched in chlorine, as Science. Imperial College Press, London, Te. These are small subhedral to revealed both by the presence of halide 273-291. anhedral grains ( 6.5x4 m), often minerals, and the Cl ( Br) content Tingley, J.V. (1989): Mineral resources of the mottled or ith serrated edges in recorded in the main Fe-oxides and in Overton 30' by 60' quadrangle. Nevada goethite. Only one euhedral, mottled Pt- the secondary PGM in the gossan. Bureau of Mines and Geology. Report 45, oxide has been located and this occurs Ho ever, this transport mechanism is 20 p. at the contact ith ilmenite. The not supported by the observation that no Wood SA, Mountain BW, Pan P (1992): The PGE have been recorded in halide aqueous geochemistry of Pt, Pd and Au; majority of the Pt-oxides found occur all recent experimental constraints and are- together in the same sample, ithin minerals themselves. Also, there is no evaluation of theoretical predictions. Can. goethite surrounding relic silicates. clear evidence for recrystallization of Mineral., 30, 955–982. ne secondary PGM a ay from the Fe-Pd/Pt-(Cu-Si-Te)-oxides (n 40) sho primary PGM. variable Pd and a lesser Pt content. Those grains ith more Pd are It seems that Pd-PGM persist in the