COMPARISON BETWEEN LOWER AMPHIBOLITE FACIES GARNET-GAHNITE-BEARING ROCKS IN THE MUNDI MUNDI PLAIN AND GRANULITE FACIES ROCKS AT THE BROKEN HILL PB-ZN-AG DEPOSIT, AUSTRALIA

In the Mundi Mundi Plain (MMP) of the Proterozoic Curnamona Province, laminated sulfides occur in the Bimba Formation (a transitional oxidized to reduced carbonate evaporitic unit) and in overlying garnet- and gahnite-rich layers, pelite, psammite, psammopelite, black shale and calc-silicate nodules (Broken Hill Group equivalent). The Bimba Formation (Olary Domain) is the stratigraphic equivalent of the Ettlewood Calc-silicate Member in the Broken Hill Domain (BHD). The garnet-gahnite-bearing layers in the MMP were metamorphosed to the lower amphibolite facies and show remarkable resemblance to so-called "lode rocks," metamorphosed to granulite facies, which are spatially associated with the giant Broken Hill Pb-Zn-Ag deposit (located 60 km SE of the MMP). The best mineralized intersection in the MMP contains 8.3% Zn, 1.0% Pb, and 17 g/t Ag over 4m but one 209 m interval contains 0.34% Pb+Zn. Sulfides identified to date are sphalerite, pyrrhotite, galena, pyrite, and chalcopyrite. Poikilitic gahnite (gahnite_57-65%hercynite_24-37%spinel_5-8%) and Mn-bearing ilmenite (up to 9 wt. % MnO) are spatially associated with sulfides in the metasedimentary rocks overlying the Bimba Formation. Biotite and retrograde chlorite contain up to 1.4 and 3 wt. % MnO, respectively. Garnets (alm_40-90% gross+andr_2-25%spess_10-60%) in garnet-gahnite-rich layers in the metasedimentary rocks are compositionally zoned and show a decrease in Mn (spess_40-24%) and Ca (gross+andr_16-9%) and an increase in Fe (alm_46-66%) towards the rim, which is consistent with growth during prograde metamorphism. In contrast to these garnets, those in calc-silicate rocks are more variable in composition and are enriched in Ca and Mn (alm_0-66%gross+andr_18-88%spess_12-70%). The variability in garnet and gahnite compositions in rocks in the MMP is similar to that for the same minerals in garnet- and gahnite-bearing rocks in the Broken Hill deposit. We consider that precursors to conformable spessartine garnet-gahnite layers and some associated carbonate and calc-mangan silicate rocks in the MMP formed as chemical precipitates on the ocean floor by hydrothermal exhalative or inhalative processes and were subsequently metamorphosed. The same processes are responsible for the formation of lode rocks in the Broken Hill deposit.