Prolonged existence of sulfide melt in the Broken Hill ore body, New South Wales, Australia

• Published in: Canadian mineralogist Vol. 43; pp. 479 - 493
• Main Authors: Frost, B. Ronald, Swapp, Susan M, Gregory, Robert W
• Format: Journal Article
• Language: English; French
• Published: Mineralogical Association of Canada 01-02-2005
• Subjects: absolute age; Australasia; Australia; Broken Hill Block; Broken Hill Group; Broken Hill Mine; Clevedale Migmatite; dates; deformation; dikes; Economic geology; facies; faults; fluid phase; folds; geochemistry; geologic barometry; geologic thermometry; gneisses; granulite facies; high-grade metamorphism; hydrothermal alteration; igneous and metamorphic rocks; intrusions; lithostratigraphy; major elements; massive deposits; massive sulfide deposits; melting; melts; Mesoproterozoic; metal ores; metallogeny; metamorphic rocks; metamorphism; metapelite; metasedimentary rocks; metasomatism; mineral assemblages; mineral composition; mineral deposits, genesis; mineral exploration; nesosilicates; New South Wales Australia; ore bodies; orthosilicates; P-T conditions; Paleoproterozoic; Paragon Group; petrography; Petrology; phase equilibria; Precambrian; Proterozoic; sedex-type deposits; silicates; structural analysis; sulfides; Sundown Group; tectonics; Thackaringa Group; Thorndale Gneiss; U/Pb; upper Precambrian; Willyama Supergroup; zinc ores; zircon; zircon group; zoning
• ISSN: 0008-4476
• DOI: 10.2113/gscanmin.43.1.479

The Broken Hill orebody, in Australia, is hosted in pelitic and psammitic gneisses that underwent metamorphism in the granulite facies. Peak metamorphism in the area occurred around 1600 Ma and exceeded temperatures of 850°C at pressures of 5-7 kilobars. The orebody is located in an area of intense deformation that occurred 5 to 15 million years after the peak metamorphism. Highly foliated pelitic schist directly adjacent to the Broken Hill orebody contains assemblages ranging from quartz-K-feldspar - biotite - garnet - sillimanite to quartz - muscovite - biotite - staurolite - garnet - chlorite. The latter assemblage equilibrated at temperatures as low as 600°C. Silicates in the low-temperature assemblages are intimately associated with galena that, in places contains clusters rich in Ag and Sb alloys, sulfides, and sulfosalts. One-bar experiments in the system Ag-Sb-S indicate that the assemblages seen in these rocks would have been molten at 500°C. These relations suggest that a polymetallic melt persisted at Broken Hill for up to twenty million years after peak metamorphism.