THE ORIGIN OF GARNET-RICH ROCKS AT RAVEN HILL (CATHEDRAL ROCK), OLARY DOMAIN, CURNAMONA PROVINCE, AUSTRALIA, AND THEIR GENETIC RELATIONSHIP TO BROKEN HILL-TYPE MINERALIZATION

The Proterozoic Broken Hill deposit occurs in the Broken Hill Domain of the Curnamona Province and is the largest Pb-Zn-Ag deposit in the world. It occurs in the Broken Hill Group and was metamorphosed to the granulite facies. Sulfides are most closely associated with quartz-garnetite, garnetite, blue quartz-gahnite rocks, and "lode pegmatite." In the adjacent Olary Domain, garnet-rich rocks are stratigraphically restricted, but are relatively common in the Walparuta and Outalpa Inliers, at Hunters Dam, and below the Mundi Mundi Plain. They are particularly common in a discontinuous horizon that extends for >10km from Raven Hill to Meningie Well. This horizon is in the Saltbush Subgroup of the Strathearn Group, which is the stratigraphic equivalent of the Broken Hill Group or the basal Sundown Group in the Broken Hill Domain. Quartz garnetite and garnetite at Raven Hill were metamorphosed to the amphibolite facies and contain garnet, quartz, biotite, ilmenite, pyrophanite, apatite, and chlorite. Whole rock analyses show that these rocks contain between 0.2 and 18.1 wt %. MnO, but mostly between 2 and 7 wt. % MnO. Garnet is enriched in the spessartine (up to 50 mole %) and almandine (up to 80 mole %) components but it also contains up to 23 mole % of combined andradite + grossular. Whole rock rare earth element (REE) plots of these rocks are light REE enriched and heavy REE depleted with negative Eu anomalies. By contrast normalized REE patterns, using LA-ICP-MS techniques, of garnet in garnetite and quartz-garnetite show very low LREE/HREE ratios, generally flat to slightly increasing and decreasing HREE signatures, and low overall REE contents (<100 - 200 ppm). They also exhibit both positive and negative Eu anomalies. Apatite is LREE enriched and HREE depleted and shows a weakly negative Eu anomaly. The REE signature of garnet in these rocks is characteristic of metamorphic garnet with Eu anomalies reflecting relative inputs of detrital to hydrothermal components in a premetamorphic protolith, fO₂ conditions and T. The REE chemistry of garnet and apatite is consistent with mineralogical, petrological and structural studies that demonstrate garnet-rich rocks are metamorphosed exhalites/inhalites, similar to those at Broken Hill. These rocks are not products of partial melting as has been proposed recently for manganiferous garnet-rich rocks at Broken Hill.