Geochemical Thermometry of Ore-Bearing Gabbronorites from the Apophisis of the Yoko-Dovyren Massif: Composition, Amount of Olivine, and Conditions of Sulphide Saturation in the Parental Magma

Abstract

The temperature and compositional parameters of the parental magma of the ore-bearing apophysis DV10 from the Yoko-Dovyren massif are estimated based on the results of thermodynamic modeling the equilibrium crystallization of melts of 24 samples, following the method of geochemical thermometry. Thermometric calculations were carried out using the COMAGMAT-5.3 program with a step of 0.5 mol. % to a maximum degree of crystallization 75–85% under oxygen fugacity controlled by the QFM buffer. The model order of mineral crystallization corresponds to the sequence: olivine (Ol) + + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate-sulfide immiscibility was modeled to occur mostlybefore the onset of plagioclase crystallization, being consistent with sulfide saturation of the parental magma. The results of calculations demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185°C. When applied to the average composition of the DV10 apophysis, this temperature indicates the existence of a suspension of the original crystals (52.1 wt. % cumulus olivine (Fo83.6), 2.3 wt. % plagioclase (An79.7), 0.24% clinopyroxene (Mg# 88.8), 1 wt. % aluminochromite (Cr# 0.62)) and about 0.2% sulfide liquid in a moderately magnesian melt (53.6 wt. % SiO2, 7.4 wt. % MgO). At that the solubility of sulfide sulfur (SCSS) was estimated to be 0.083 wt. %. This heterogeneous system had a viscosity of 4.71 log. units (Pa · s) and an integral density of 2929 kg/m3. Such rheological properties do not contradict the possibility of the migration and emplacement of the protocumulus mush from the main Dovyren chamber. However, a more probable scenario includes a localized accumulation of olivine in the trough-like part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.