Genetic interpretation of the distribution of PGE and chalcogens in sulfide-mineralized ultramafic rocks from the Yoko-Dovyren layered intrusion

The paper presents newly acquired data on concentrations of chalcophile elements and chalcogens (Se and Te) in sulfide-bearing rocks and Cu–Ni ores from the bottom portion of the Yoko-Dovyren Massif, northern Baikal area, Russia. Positive covariations between Pd, Pt, Au, S, and Te in the samples highlight sulfide control on the behavior of these elements, which was related to the redistribution of essentially Fe–Ni sulfide liquids at a magmatic stage. The character of relationships between Pd, Pt, Te, Cu, and S in the rocks led us to distinguish two groups of genetic trends: the first group combines samples from the chilled zone, plagioperidotites, and olivine gabbronorites that compose underlying sills in the central part of the intrusion, and the second one comprises poor and high-grade ores in the northeastern termination of the intrusion (Ozernyi Prospect). We put forward the hypothesis that the first-group trends reflect different degrees of accumulation of crystallization products of the most primitive sulfide liquids, whereas the trends of the second group pertain to sulfide matter significantly depleted in Cu, Te, and PGE. New data on Fe, Ni, Cu, Co, Se, Te, Zn, Mo, Ag, Cd, Sb, Pb, Rh, PGE, and Au concentrations in sulfides from the chilled gabbronorite and ores of the Baikalskoe deposit are presented. Results of thermodynamic modeling (with the COMAGMAT-5 program package) of sulfide saturation in the intercumulus of a primitive orthocumulate are used to reproduce the composition (Cu, Pd, Pt, Au, and Te) of the parental sulfide liquid. The model concentrations of noble metals in the sulfide are demonstrated to be one to two orders of magnitude higher than the concentrations in the “average sulfide” estimated by LA–ICP–MS. More realistic estimates for the composition of the parental sulfide liquids can be obtained by normalizing the bulk concentrations of these elements to 100% sulfide mass. These estimates are in good agreement with results from thermodynamic simulations.