Metasomatic and petrologic evidence of polymetallic sulfide melts and their effect on the ore package of the Broken Hill lead-zinc-silver deposit, New South Wales Australia

The Broken Hill lead-zinc-silver deposit is the largest such deposit in the world and the type locality for Broken Hill Type (BHT) deposits. Sedimentary-exhalative deposits were deformed and metamorphosed to granulite-grade conditions during meso-Proterozoic time. Deformation has obliterated most of the original rock textures and fabrics and thus their true genesis remains a topic of debate. The ore package includes the ore lenses and a suite of rocks surrounding the lead-rich orebody which exhibits a distinctive chemical and mineralogical zonation. Two models exist for the origin of this zonation. One is that they are metamorphic products of a zoned protolith and the compositional gradient is inherited from a primary gradient formed during formation of the orebody, leading to local metasomatism. The other possibility is that the zoning is the result of reactions between a melted sulfide orebody and the country rock, leading to metasomatism on the scale of tens of meters. Observed mineral assemblages and compositional zoning in individual garnet grains of the ore package rocks indicate a gradient in activity which decreases with distance from the orebody. Calcium and manganese enriched garnet overgrowths suggest that melt-derived fluids added these elements to the ore package system during and after peak metamorphism, resulting in reverse-zoning in garnet overgrowths, consistent with the latter of the two models.