Geochemistry and Petrology of Protosulfide Melts in an Ore-Bearing Apophysis of the Yoko-Dovyren Intrusion

— The paper presents results of petrological and geochemical studies of sulfide-bearing rocks from a bottom apophysis in the central part of the Yoko-Dovyren massif, northern Transbaikalia, Russia. It is composed of plagioperidotite and was sampled in a bulge up to 300 m thick throughout the whole vertical section, from its upper to lower contact. The observed diversity of sulfide-bearing rocks is subdivided into four types: (1) picrodolerite with droplet-shaped sulfides, (2) olivine gabbronorite with sulfides varying from droplet-shaped to irregular globules, (3) olivine gabbronorite with larger globules and patchy net-textured domains, and (4) net-textured ores. These varieties occur in the basal zone of the apophysis, in the interval up to 30 m inward from the lower contact. Petrographic features of these rocks correspond to olivine ortho- and mesocumulates, with the only exception of the quenched picrodolerite with an ophitic groundmass. The association of sulfides (pyrrhotite, chalcopyrite, pentlandite, and cubanite) has a pretty little varying composition, except only a relative decrease in the proportion of chalcopyrite in samples with more than 5% sulfides. The results of X-ray computed tomography show an increase in the sulfide connectivity in the rocks (from 25 to 95%) with increasing sulfide abundance. These characteristics, supported by visualized 3D models, allowed us to identify a morpho-structural trend tended from the rocks with sulfide droplets to the net-textured ores. The first ever data were obtained on the chemostratigraphy (PGE and tellurium) of rocks composing this apophysis. Covariations of Au, Pt and Pd with S and Te indicate that the precious metals have a single sulfide precursor. The 100%-sulfide compositions calculated for these rocks indicate that the most primitive and PGE rich sulfides correspond to droplets it the lowerest part of the apophysis, whereas the net-textured ores are depleted in these elements. The geochemical estimates are supported with COMAGMAT-5 modeling which made it possible to calculate the average Au, PGE, and Te contents in the protosulfide melt (in ppm): 2.9 Au, 14.4 Pt, 25.1 Pd, and 44.3 Te. Results of the geochemical studies of different fragments of the net-textured ores reveal a heterogeneity in the form of domains relatively enriched in copper, PGE, and tellurium, whose characteristic size is a few dozen centimeters. This can be considered as a signature of the porous migration and accumulation of the crystallization products of the original sulfide liquid at the subsolidus stage of cooling down the ore-bearing cumulates.