Cobaltite-gersdorffite solid solution as a collector of platinum-group elements in the Ban Phuc Ni-Cu-(PGE) sulfide deposit, northern Vietnam

The Ban Phuc Ni-Cu-(PGE) sulfide deposit in northern Vietnam contains no or few platinum-group minerals (PGM) in massive sulfide ores, and most PGM grains occur in altered disseminated sulfide ores, including sperrylite (PtAs2), irarsite-orarsite-hollingworthite [IrAsS, OsAsS, (Rh,Ir)AsS], and froodite-sobolevskitesudburyite [PdBi2, PdBi, Pd(SbBi) and PdSb]. Discrete PGM grains are enclosed within serpentines and base metal sulfides (BMS) of the disseminated ores. In contrast, small amounts (0.004 vol%) of cobaltite-gersdorffite solid solution (CGss) in massive sulfide ores commonly contain high PGE concentrations relative to the BMS. Textural relationships reveal that the CGss grains crystallize earlier than the host BMS in massive sulfide ores. The CGss grains are found to contain all six PGE and act as the major hosts for the PGE in the massive sulfide ores. The fast Fourier transform (FFT) patterns of high resolution TEM images show that the irarsite (IrAsS) enclosed within the CGss shares the same zone axis of (2 1 2), which results from the exsolution of irarsite from the CGss. On the other hand, the CGss grains without visible PGM show an even distribution of PGE in the crystal and respective FFT patterns of CGss crystals. This is an example of a growing number of cases that massive sulfide ores have been found to contain PGE in As-bearing minerals, either PGM including sperrylite and members of the irarsite-orarsite-hollingworthite solid solution series or CGss. These results argue that PGE in the sulfide melt may enter the CGss at high temperature before the crystallization of monosulfide solution, rather than soluble in BMS, even if the sulfide melt was not As-saturated.