The primary magmatic concentration and secondary remobilisation of platinum-group elements in Ni-Cu sulphide ores

Knight, Robert (2014) The primary magmatic concentration and secondary remobilisation of platinum-group elements in Ni-Cu sulphide ores. Cardiff University, School of Earth & Ocean Sciences, Doctoral Thesis, 1141pp.

Abstract

The PGE mineralisation in the Fazenda Mirabela ultramafic-mafic and Jinchuan ultramafic intrusions has been characterised by determining the platinum-group mineralogy of each complex and the concentration of PGE in solid solution in the base metal sulphides (BMS). The Mirabela intrusion is largely unaltered and hosts two PGE occurrences from which 128 Au- and Ag-bearing minerals and 716 PGM have been identified; predominantly Pt-Pd-Ni tellurides in the Santa Rita sulphide ore deposit and to a lesser extent in the underlying sulphur-poor dunite. A localised Pd-Cu±Pb alloy assemblage is identified in the dunite in the central zone of the intrusion. The PGM assemblages at the edges of the intrusion are relatively As-rich containing sperrylite (PtAs2); arsenic may have been introduced through crustal assimilation. Two orebodies (#1 and #24) have been studied from the Jinchuan intrusion which has undergone extensive greenschist facies alteration. A total of 64 Au- and Ag-bearing minerals and semimetal alloys, and 93 PGM have been identified including michenerite (PdTeBi), froodite (PdBi2), members of the hollingworthite-irarsite-platarsite solid solution series ([RhIrPt]AsS), sperrylite and maslovite (Pt[BiTe]2) in decreasing order of abundance. The PGM vary across these two orebodies as a result of sulphide fractionation, with the Ni/MSSrich orebody #1 hosting early crystallising sulpharsenides and As-bearing PGM whereas the relatively Cu/ISS-rich orebody #24 hosts more Pd-bearing PGM with Pd partitioning into the Cu-rich sulphide liquid during MSS crystallisation. These studies show that Bi- and Te-bearing PGM ([PtPdNi]Te2, PdBi2, PdBiTe) may exsolve from the BMS during sub-solidus cooling after these elements have partitioned into the BMS at high temperatures whereas As-bearing PGM (PtAs2, [RhIrPt]AsS) and sulpharsenides (gersdorffite-cobaltite [NiAsS-CoAsS]) may crystallise early directly from an As-bearing immiscible sulphide melt. In the Mirabela intrusion, the IPGE and Rh have partitioned into MSS, from which pentlandite, pyrrhotite and pyrite have exsolved. Osmium and Ir preferentially partitioned into pyrite (with Co) whereas Ru and Rh partitioned equally between pentlandite and pyrite. In the Jinchuan intrusion, Ir, Rh and Pt have preferentially partitioned into early crystallising sulpharsenides (from which [RhIrPt]AsS PGM exsolve) depleting MSS in these elements. Palladium is identified in pentlandite in both complexes studied and may have diffused from ISS/chalcopyrite or partitioned into MSS at an earlier magmatic stage. Platinum does not usually partition into BMS (excluding sulpharsenides) and instead forms PGM. However, Pt does partition into pentlandite in the Mirabela sulphur-poor dunite where semimetal concentrations are very low and Pt-bearing PGM form only in low quantities, if at all. Minor localised serpentinisation of the Mirabela intrusion does not remobilise the PGE with the primary magmatic distribution of PGE and PGM preserved; the latter are predominantly associated with interstitial BMS and occur in sulphide stringers shown to be of magmatic origin. Pervasive greenschist facies hydrothermal alteration at Jinchuan altered and oxidised the BMS during a process of sulphur loss, resulting in the formation of secondary magnetite and the liberation of Pd, Bi and Te which coalesce to form secondary froodite and michenerite in situ at the edges of these replacement oxides; however, the PGE are not extensively remobilised. Both complexes show that the semimetal content of the ore-forming magma is critical in controlling the distribution of PGE into BMS and/or PGM.

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