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

Abstract

The temperature and compositional parameters of the parental magma of ore-bearing apophysis DV10 of the Yoko-Dovyren massif are estimated by the method of geochemical thermometry based on results of thermodynamic modeling of the equilibrium crystallization of the melts of 24 rocks. The thermometric calculations were carried out using the COMAGMAT-5.3 program with increments of 0.5 mol % to a maximum degree of crystallization 75–85%, under oxygen fugacity controlled by the QFM buffer. The model crystallization sequence of minerals was as follows: olivine (Ol) + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate−sulfide immiscibility was calculated to occur mostly before the onset of plagioclase crystallization, which is consistent with initial sulfide saturation of the parental magma. The calculation results demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185^oC. When applied to the average composition of apophysis DV10, this temperature indicates the existence of suspension of the original crystals, including 52.1 wt % cumulus olivine (Fo_83.6), 2.3 wt % plagioclase (An_79.7), 0.24 wt % 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). Therewith the sulfur concentration at sulfide saturation (SCSS) was estimated at 0.083 wt %. This heterogeneous system had a viscosity of 4.71 log units (Pa s) and 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 is the localized accumulation of olivine in the trough-shaped part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.