Zoning and substitutions in Co-Ni-(Fe)-PGE sulfarsenides from the Mount General'skaya layered intrusion, Arctic Russia

Compositions, zoning patterns and element correlations in platinum-group el ement (PGE)-Co-Ni-Fe sulfarsenides from the Mount General' skaya complex, i n Arctic Russia, reveal the following. (1) Members of the cobaltite-gersdor ffite series (MCGS) form an extensive solid-solution series between relativ ely S-rich CoAsS and (Ni0.667Fe0.333)AsS, and Ni-Fe order probably exists i n their crystal structure. Co increases, whereas Ni-(Fe), PGE, X-Rh [100Rh/ (Rh + Pd)] and the ratio As/S (atom.%) decrease toward the edge in the cryp tically zoned MCGS. (2) The PGE are relatively enriched in the earlier sulf arsenide phases (typically in the core). (3) Ir-(Pt) is more effectively pa rtitioned into the earliest phase than Rh; Rh is more strongly partitioned into the core than Pd, and transition metals are concentrated in the latest (marginal) phases. (4) The PGE substitute for Co, not for Ni and Fe in the MCGS. (5) Similar to Ni and Fe, the PGE correlate positively with As (and the As/S) and negatively with S, and they are selectively incorporated in r elatively As-rich (S-poor) MCGS. In contrast, Co displays a strong negative correlation with As, and a positive one with S. A link seems to exist betw een cation and (di)-anion substitutions in the MCGS, and the incorporation of the PGE and other metals probably is controlled by charge balance. The h igher temperature of crystallization of Ir-(Pt)AsS versus RhAsS, of RhAsS v ersus (Rh,Pd)AsS, and of (PGE)AsS versus MCGS may be crucial to form the zo ning patterns and compositional trends of these sulfarsenides.