Textural evidence for extensive melting of the broken hill orebody

Galena, pyrrhotite, and chalcopyrite in samples of Zn-rich ore from the Broken Hill orebody show textures similar to those in partially melted metamorphic rocks, including strings of low melting phases separating lithons of minerals likely to be restite, low dihedral angles between low melting-temperature minerals and restitic minerals, and enlargement of these dihedral angles due to partial annealing. Analysis of existing experimental work from the system Fe-Pb-Zn-S indicates that at 1 bar, galena and sphalerite coexisting with pyrrhotite of the compositions found at Broken Hill would have been partially meltedat temperatures of about 750°C; at 5 kbars, this melting would occur at 780°C. This temperature is well within the range of metamorphic temperatures reported from the area. From these data we conclude that the Broken Hill orebody was partially molten at peak metamorphism, with much of the Pb, Cu, Ag, Sb, and As in the melt and Fe and Zn residing mostly in the restite. Differentiation during cooling enriched the residual melt in Sb, As, Ag, and other low melting-temperature chalcophile elements. The main sulfide melt froze after the last penetrative deformation to affect the orebody, at temperatures below 720°C and possibly as low as 650°C. Sulfosalts crystallized out of the remaining melt at temperatures of 600° to 450°C, and the final melt crystallized native bismuth at ca. 250°C.