GAHNITE-GARNET-STAUROLITE RELATIONS TO PROTEROZOIC BROKEN HILL-TYPE PB-ZN-AG MINERALIZATION IN THE MUTOOROO AREA, SOUTH AUSTRALIA

The Broken Hill Domain of the Curnamona Province, New South Wales (Australia) hosts the Proterozoic Broken Hill deposit, the largest Pb-Zn-Ag deposit in the world. It is the type example of a class of ores known as Broken Hill-type (BHT) deposits, which are characterized by stratiform/stratabound Zn-Pb-Ag mineralization hosted in amphibolite/granulite terrains that contain elevated Mn-Ca-Fe and depleted S contents. Quartz-garnetite, garnetite, blue quartz-gahnite (ZnAl₂O₄) rocks, and "lode pegmatite" constitute the so-called "lode horizon" and they, along with Potosi gneiss, are intimately associated with the Broken Hill deposit and are a primary empirical exploration tool for BHT deposits. Exploration in the Mutooroo area (South Australia) during the 1990s identified BHT mineralization in the Broken Hill Group, which also hosts the Broken Hill deposit (60 km to the NE). Dismembered garnet- and gahnite-rich rocks (<2 m wide) and associated Pb-Zn-Ag mineralization are best developed at the Dingo Ridge, Two Mile Ridge, Horseshoe Ridge, Emu Ridge, and Berta Tank prospects in psammites, psammopelites, and pelites metamorphosed to the granulite facies. However, folded zones of lode horizon are up to 40 m wide. In places, this mineralization grades into Potosi gneiss. Surface samples of sulfide mineralization contain up to 1 wt. % Zn, 6.3 wt. % Pb, and 31 g/t Ag. The composition of gahnite (67 to 76 mole % ZnAl₂O₄, 18-28 mole % FeAl₂O₄, 4 to 7 mole % MgAl₂O₄) and garnet (enriched in spessartine and almandine) in lode rocks is almost identical to that in lode rocks associated with the Broken Hill deposit. The precursors to gahnite and garnet in lode horizon rocks at Mutooroo are considered to be products of exhalation or inhalation at or near the surface of a rifted Proterozoic sea floor. Zn-rich staurolite (up to 5.6 wt. % ZnO) at Berta Tank occurs as a breakdown product of gahnite whereas abundant coarse Zn-poor staurolite adjacent to the Dingo Ridge prospect is present in a shear zone that formed during an amphibolite facies, retrograde metamorphic event (Delamerian?). Lead-zinc-silver mineralization in the Mutooroo area constitutes an excellent example of minor BHT mineralization at the south-western extremity of the Broken Hill Domain.