Lithogeochemistry of Pegmatites at Broken Hill: An Exploration Vector to Mineralisation

Abstract

A sampling program was undertaken to assess pegmatite geochemistry with the view to using the data to vector towards economic mineralisation. The work has shown that pegmatites hosted in high grade metamorphic rocks (granulite and upper amphibolite facies) are either leucogranites, quartz-feldspar-mica pegmatites or feldspar-rich pegmatites. In contrast those that occur within lower grade metamorphic rocks (lower amphibolite to greenschist facies) are feldspar poor and quartz-muscovite rich, locally highly sheared, with tourmaline on their margins and as replacements of surrounding host sediments. Element classification diagrams suggest a geochemical continuum from poorly evolved pegmatites hosted in the highest grade metamorphic rocks to highly evolved pegmatites in the lowest grade metamorphic rocks. Spatially, this trend is broadly from the south and southwest toward the north and northeast and from west to east. Economic element content is similarly divided into two broad groups; 1. Pegmatites within higher grade metamorphic rocks host elevated lead-zinc-silver-manganese. 2. Pegmatites within lower grade metamorphic rocks contain elevated tin, tungsten, lithium, niobium and tantalum. The presence of abundant tourmaline in association with these indicates highly elevated boron. Elevated base metals and silver in the granulite and amphibolite facies rock is attributed to the elevated nature of these elements in the metasedimentary protolith being largely the Broken Hill Group. Our interpretation suggests these are probably not allochthonous but are nearly in-situ bodies resulting from partial melting of the Broken Hill Group. Their chemistry reflects that of the surrounding rock. Analysis of the spatial distribution of pegmatites with elevated lead-zinc-silver-manganese indicates many of these are located close to known base metal and/or silver occurrences of the Broken Hill types (BHTs). In contrast the concentration of tin, tungsten, lithium and other incompatible elements in pegmatites which are allochthonous and intrude rocks of lower metamorphic grade, suggests these elements have accumulated as fractionation has taken place. This study has highlighted three areas of elevated base metal-silver within pegmatites where no known mineralisation is documented either as an occurrence or in historic exploration data. Systematic pegmatite sampling at Broken Hill has the potential to enable vectoring toward buried silver-lead-zinc ores of the Broken Hill type.