Base metal - platinum-group element sulfides from the Urals and the Eastern Alps: Characterization and significance for mineral systematics

A mineralogical classification of sulfides containing base metals (BM) and platinum group elements (PGE) is proposed based on BM-PGE ratios. Group A c omprises BM sulfides carrying PGE as trace or minor elements (e.g., pentlan dite). Group B is characterized by BM/PGE > 1 comprising kharaelakhite and some poorly defined minerals (thiospinels and monosulfides) which are descr ibed in detail. In group C, all sulfides with BM/PGE<1 are summarized, comp rising PGE-rich thiospinel, minerals related to the thiospinel group (e.g. xingzhongite, konderite, inaglyite), and the Pd-Pt+/-Ni sulfides. A number of BM-PGE sulfides are described from podiform chromite occurrence s in ultramafic portions of ophiolite complexes in the southern Urals (Kemp irsai, Kazakhstan) and the Eastern Alps (Kraubath, Austria). Copper- and (I r, Rh, Pt)-rich thiospinel (general formula AB(2)S(4), with A = Cu, Ni, Fe and B = Ir, Rh, Pt) is present in complex assemblages in Kraubath, usually intergrown with laurite, Pt-Fe alloy and Rh sulfide. These thiospinels are commonly associated with lamellae and inclusions of Ni-and/or Fe-rich (Ir, Rh) sulfide showing either monosulfide or BM-rich thiospinel stoichiometry. In massive chromitite from Kempirsai, (Ni,Cu,Fe,Ir,Rh,Os) sulfides are int ergrown with laurite-erlichmanite, Ir-Os alloy, and rarely, PGE sulfarsenid es (e.g. irarsite), and usually have monosulfide (BM,PGE)S compositions. A small number of grains have (BMI+/-PGE)/S matching PGE-rich thiospinel (cup roiridsite) and BM-rich thiospinel (Ni,Cu,Fe)(1.5)(IrRh)(1.5)S-4 In the occ urrences studied, monosulfides exhibit sulfur-deficient stoichiometries (e. g., (BM,PGE)(1-x)S) and are characterized by BM/PGE ranging from 0.8 to 2.2 . Although anisotropic in reflected light, their reflectance spectra (Y% = 33-38) differ only slightly from those of isotropic cuproiridsite and cupro rhodsite (Y% = 36-38). At least three groups of monosulfides can be disting uished on chemical grounds using literature data: monosulfides dominated by Ni and Ir ("iridian millerite") with BM/PGE ranging from 1.6 to 5.9, monos ulfides dominated by Fe and Rh ("rhodian pyrrhotite") with BM/PGE ranging F rom 1.6 to 7.1, and monosulfides dominated by Cu, Ir or Rh ( "xingzhongite" -type) with BM/PGE ranging from 0.6 to 1.1. While the first two types presu mably crystallize in a hexagonal NiAs structure and exhibit extensive solid solution between each other, xingzhongite is cubic (BM-rich thiospinel?) a nd usually poor in Ni and Fe. Monosulfides and thiospinel may form from PGE -rich base metal sulfide liquids after cooling and equilibration in chromit e-precipitating magmatic systems.