COLLECTORS OF PT, PD AND RH IN A S-POOR FE-NI-CU SULFIDE SYSTEM AT 760°C: EXPERIMENTAL DATA AND APPLICATION TO ORE DEPOSITS

Abstract

The phase relations within the Fe9S8-Ni9S8-Cu9S8 section of the system Fe-Ni-Cu-S at 760similar toC were investigated in silica glass tubes. A complete "quaternary" solid-solution (Hz-Iss) between heazlewoodite solid-solution (Ni,Fe)(3+/-x)S-2 and intermediate Cu1+/-xFe1+/-yS2 solid-solution was established. The possibility of direct crystallization of pentlandite (Pn) from Cu-containing sulfide melt is uniikely, as no primary Pn was found to be stable in the high-temperature associations. Experiments performed at the same conditions with Pt, Pd or Rh added in small quantities reveal the contrasting behavior of platinum-group elements (PGE). Pt preferentially forms its own minerals, either Pt-Fe alloys in association with Fe-rich base-metal sulfides (BMS) or platinum sulfides in Ni- and Cu-rich assemblages. At 760degreesC, sulfur fugacity [f(S-2)] varies from -10.5 [log f(S-2)] where gamma(Fe,Ni,Pt) alloy is stable to log f(S-2) greater than or equal to -1.1 where Cu(Pt,Ni)(2)S-4 is present. Palladium enters sulfide solid-solutions, with up to 1.5 at.% in Hz-Iss and 0.9-1.1 at.% in Ni-rich monosulfide solid-solution (Mss), The behavior of Rh is remarkable; it may concentrate in BMS, especially Mss, accounting for up to 2.6 at.% Rh, or form Rh alloys or sulfides, under very low or very high f(S-2), respectively. The application of the present experiments to PGE-bearing sulfide deposits indicates that sulfide solid-solutions were in most cases the temporary collectors of the light PGE before the appearance of Pn. During subsolidus recrystallization of the high-temperature BMS, Pd and Rh partition into Pn or form their own secondary minerals. PGE deposits with a predominance of Pt-Fe alloys, such as those occurring in mantle rocks exposed in ophiolites (Corsica. New Caledonia) or kimberlites, and in layered complexes (e.g., the Merensky Reef, Bushveld Complex) are consistent with derivation from a S-poor sulfide melt, yielding early-formed Pt-Fe alloys. A similar origin may be inferred for the alloy type of Pt mineralization, which occurs in some crustal sequences of ophiolites and in Alaskan-type complexes, despite the very low amount of BMS present. Even in these low-S deposits, high f(S-2) may be reached locally as a result of the appearance of a Cu-rich sulfide liquid, derived by fractionation or by immiscibility from an original Fe-rich sulfide melt.