Sphalerite and hexagonal pyrrhotite geobarometer: experimental calibration and application to the metamorphosed sulfide ores of Broken Hill, Australia

Abstract

Sphalerite (sp) and hexagonal pyrrhotite (po) were recrystallized in a eutectic LiCl-KCl salt flux at 450°C to 750°C and 1 to 6 kbars. The activity of FeS (aFeS), controlled by the composition of excess pyrrhotite, varied from 0.5 to 1.0. Out data show that FeS activity-sphalerite composition relationships for any pressure are independent of temperature, ie the sphalerite composition (XspFeS) is a function of the FeS activity only, consistent with results of earlier investigations at low pressure. Sulfide ores of the North mine, Broken Hill, Australia, where NpoFeS = 0.966 and XspFeS = 0.021: at 650°C, P = 6.6 kbars; and at 800°C, P = 6.7 kbars. Pressures calculated from coexisting sphalerite and hexagonal pyrrhotite, at 650°C, form the New Broken Hill Consolidated mine, the C lode and the Zinc Corporation mine; and the Pinnacles mine (12 km southwest of the main Broken Hill lode), indicate pressures of 6.8, 5.5, 6.2, and 6.3 kbars, respectively. These pressures are in excellent agreement with estimates based on silicate assemblages in adjacent host rocks. -from Authors