Anorthosites of the low-sulfide platiniferous horizon (Reef I) in the upper riphean Yoko-Dovyren massif (Northern Cisbaikalia): new data on the composition, PGE-Cu-Ni mineralization, fluid regime and formation conditions

Abstract

The carried out studies based on new data allowed to give mineralogical, petro- and geochemical characteristics to anorthosites, which are the main link and the major concentrator of PGE and Au in the composition of low-sulfide platinum metal mineralization, localized in a specific taxitic horizon (Reef I) of the Yoko-Dovyren massif. The revealed features of the composition and structure of this horizon indicate that the formation of anorthosites is caused by both the actual magmatic and the late- and postmagmatic processes with a high activity of volatile components. The horizon occurrence can be explained in terms of the “compaction” hypothesis and thermal shrinkage phenomenon. At the boundary of the rocks contrasting in composition and characteristics, when they are cooled, weakened zones form up to cracks and cavities, into which the interstitial leucocratic melt and volatiles squeezed out of the underlying horizons of the massif sucked as a result of the decompression effect. The revealed patterns of changes in the compositions of Pl (82-88% An), Ol (78-81% Fo), Cpx (40-44% En, 9-18% Fs, 41-47% Wo) and Opx (74-78% En, 16-24% Fs, 2-5% Wo) indicate fractional crystallization of the detrital melt. The processes of fluid-magmatic interaction led to a considerable heterogeneity of anorthosites and other rocks, the formation of disequilibrium mineral associations and concentration of ore-generating components. Sulfide associations are considered as products of the subsolidus transformation of solid solutions (mss and iss + poss) formed during the crystallization of an immiscible sulfide liquid enriched in Cu. It is demonstrated that noble metals were associated not only with a limited amount of sulfide liquid. The major part of noble metals with “crust” components (Sn, Pb, Hg, Bi, As, Sb, Te, S, etc.) entered the anorthosite cavities along with volatile components and chlorine, thus causing an abundance of native minerals among platinoids. The decisive role of reduced gases (H2, CH4, CO), H2O and Cl in the genesis of precious metal minerals is estimated.