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

Recent investigations have made it possible to update the mineralogical, petrochemical, and geochemical characteristics of anorthosites, which are considered the main link and main PGE and Au concentrator in low-sulfide PGM mineralization localized in a specific taxitic horizon (Reef I) of the Yoko–Dovyren massif. The recognized compositional and structural features of this horizon suggest that the anorthosites formed as a result of not only magmatic processes proper, but also late magmatic and postmagmatic processes with extremely high volatile-component activity. The origin of this horizon can be explained by the “compaction” hypothesis and thermal shrinking phenomenon. Zones of weakness up to fractures and cavities formed at the interface of rocks with contrasting compositions and properties while they cooled. Due to the decompression effect, these zones “sucked in” the interstitial leucocratic melt and the volatiles squeezed out of the massif’s deeper horizons. The established trends of variation in mineral composition—Pl (82–88% An); Ol (78–81% Fo); Cpx (40–44% En, 9–18% Fs, and 41–47% Wo); and Opx (74–78% En, 16–24% Fs, and 2–5% Wo)—suggest fractional crystallization of the residual liquor. Fluid–magma interaction processes led to the significant heterogeneity of anorthosites and other rocks, the formation of nonequilibrium mineral assemblages, and the concentration of ore-bearing components. The sulfide assemblages were viewed as products of the subsolidus transformation of solid solutions (mss and iss + poss) that formed during the crystallization of a Cu-rich immiscible sulfide fluid. It was demonstrated that noble metals were associated not only with the limited volume of the sulfide fluid. The bulk of the noble metals with “crustal” components (Sn, Pb, Hg, Bi, As, Sb, Te, S, etc.) were supplied to anorthosite cavities together with volatile components and chlorine, and this accounts for the abundance of platinoid minerals among the other forms of platinoid occurrence. The leading role of reduced gases (H₂, CH₄, and CO), H₂O, and Cl has been established in the genesis of noble metal minerals.