SYNTHESIS OF PLATINUM-GROUP MINERALS UNDER HYDROTHERMAL CONDITIONS

Abstract

A number of experiments have been carried out using Cl-, Na-, K-, As-, Sn-, Bi-, Pb-, Te-bearing solutions at temperatures between 240 and 500°C in order to synthesize platinum-group minerals (PGM) and to study their relationships under hydrothermal conditions. Metallic platinum, palladium, PtAs2, Pt(As,Sn)2, and PdPb ("Pb-polarite") were synthesized directly from hydrothermal solutions in the presence of graphite as reducing component. Phase transformations were observed under hydrothermal conditions for some pre-synthesized Pd compounds: Pd bismuthides replace PdS in Cl-Sb-Bi-bearing solutions. However, no Pt-Sb or Pt-Bi phases crystallized under these conditions. Recrystallization of Pt- and Pd-sulphides in slightly acidic solutions with Sb- or Bi-chlorides at 240°C yielded complex multiphase associations including Sb2S3, BiSb3 and Bi2S3. The reactions leading to their formation did not include any platinum. However, froodite (PdBi2) was obtained by recrystallization of PdS + Fe0.88S + FeS2 assemblages in BiCl3 solution at 240°C. Experiments to investigate if PGE-bearing sulphides can be transformed to sulpharsenides by recrystallization gave negative results. Platinum-group elements (PGE) redistribution under hydrothermal conditions at controlled sulphur fugacity shows that PtS is stable at log/S2 > -3.4 and (Pt,Fe) is very poor in Fe at log/S2 < -6.5 (at 562°C). Experimental data obtained for some PGE-phases (Pt/Pd- and Ir-bearing sulphides) are in good agreement with observations on natural deposits.