Mineralogical occurrence and texture of gold, platinum group minerals and other phases in heavy liquid fractions and diamond drill core samples, from Coronation Hill, Northern Territory

Abstract

This is an expansion on the mineragraphic report provided in CR1987-0341. Mineragraphic studies of drill core from holes CHDDHs 5,6,7,8,10,13,14,17,18,27 and 29 are also recorded. Gold/platinum group/ rare earth and uranium mineralisation occurs in altered host rocks including chloritic carbonaceous tuffaceous siltstone/ argillite, sericitised quartz feldspar porphyry (extrusive and intrusive) and sericitised/ chloritised quartz diorite/ micrographic granitoid. Sericitic alteration has destroyed the primary feldspars so exact classification of intrusives is difficult. Mineralisation occurs within microfractures, microveinlets and veinlets within altered sometimes hematised host rock, where the veinlet paragenesis is complex. Some mineralisation is also disseminated throughout the alteration matrix. Precious metal mineralisation is present in approximately the following order of decreasing abundance: gold(silver bearing variety> pure variety), stibiopalladinite (Pd5Sb2), sudburyite (PdSb), native palladium, palladium antimony selenide phase (PdSbSe; possibly an unrecorded mineral), platinum palladium selenide [(Pt,Pd)Se2, possibly a new mineral phase], rare palladseite (Pd17Se15), rare porpezite (Au Pd phase), rare Bi Pd Sb phase, rare Bi Pd Sb selenide, rare Pt Pd (Fe) alloy phase, rare Bi Ag selenide, rare Pd Hg selenide, and very rare Pd As selenide. Other selenide minerals recorded include: clausthalite (PbSe), Ni Co (Fe) selenide, timeannite (HgSe), and very rare Hg Sb Ni selenide. Iron and basemetal sulphides/ arsenides include pyrite/ nickeliferous pyrite, marcasite, galena, chalcopyrite, sphalerite, rare Ni Co Fe As sulphide (gersdorffite), rare Ni Cu sulphide, rare bornite, rare pyrrhotite, and rare chalcocite. Uranium and rare earth minerals present include uraninite, coffinite, rare U Th Al phosphate phase, rare U Th phosphate silicate phase and rare U Th Ra(??) oxide, florencite [(Ce,La,Ca)Al3 (PO4)2 (OH)6], xenotime (YPO4), and rare monazite [(Ce,La)PO4]. Intergrowths of gold/ clausthalite/ stibiopalladinite are common. Pt mineralisation comprises microveinlets and porous patches of PtPd selenide, generally closely associated with florencite crystals and clausthalite. Native Pt was only recorded once. The Au and Pt group mineral phases show a distinct selenide association. Another association is also present where gold/ clausthalite/ stibiopalladinite are associated with with replacive pyrite in some of the altered igneous rocks. These are later than pyrite in the paragenesis. There is an inference that precious metal mineralisation post dates pyrite and uraninite phase of mineralisation. Gold is observed in one case in association with coffinite, and coffinite conceivably post- dates uraninite due to later alteration effects. Some recommendations for mineral dressing processes are made based on observed minerals and textures. Of primary consideration is the insolubility of selenide phases in cyanide and the effect this has on platinoid recoveries.