Abstract

Garnet-rich rocks are locally associated with the giant Paleoproterozoic Broken Hill Pb-Zn-Ag deposit and hundreds of smaller Broken Hill-type occurrences in the southern Curnamona Province, Australia; however, others are present unrelated to sulfides. Mn-bearing garnet-rich rocks are conformable to bedding in metasedimentary rocks of the Paleoproterozoic Willyama Supergroup and, together with sulfide-bearing zones, are spatially related to amphibolite and/or felsic volcanic rocks or shallow granite intrusions. The stratiform garnet-rich rocks can be subdivided into two groups: Mn-rich varieties typical of the stratigraphic hanging wall of the Broken Hill deposit and Mn-poor and Fe-rich varieties that are typical of the footwall and are regionally widespread. These garnet-rich rocks formed on the floor of a rifted basin as a mixture of chemical precipitates and detrital sediments and were subsequently metamorphosed to the amphibolite or granulite facies during the Olarian orogeny. A plot of Fe/Ti versus Al/(Al + Fe + Mn) ratios of these rocks suggests equal proportions of hydrothermal and detrital components in the precursor phases. Low Co + Cu + Ni contents are consistent with the input of a small hydrogenous component. Quartz garnetite and garnet-gahnite rocks in the stratigraphic footwall of the Broken Hill deposit (Zn-rich, Cu-poor B and C lodes), and garnet-rich rocks elsewhere in the southern Curnamona Province that are Mn poor and Fe rich, exhibit chondrite-normalized rare earth element (REE) patterns with negative Eu anomalies, which indicate that the precursor minerals formed from hydrothermal fluids at <300°C and under relatively low fO2 conditions. Garnetite and hedenbergite-, bustamite-, and rhodonite-bearing garnet-rich rocks from the stratigraphic hanging wall of the Broken Hill deposit (Pb-rich 2 and 3 lenses and the A lode) are Mn and Ca rich, Fe poor, and have positive Eu anomalies. Precursors to these Mn-rich rocks formed from relatively cool hydrothermal fluids (~250°C) at fO2 conditions higher than those that formed Mn-poor, Fe-rich garnet- and gahnite-rich rocks at Broken Hill. Correlations among various elements show that the presence of positive or negative Eu anomalies in garnet-rich rocks depends on the composition of the precipitate (Fe, Mn, and trace metal content) as well as the chemical, crystallographic, or sorption control that the precursor Fe-Mn oxyhydroxides/oxides/carbonates had upon the incorporation of Eu. Garnet-rich rocks from the Broken Hill deposit have higher average Mn, K, Rb, Eu, Ga, Cs, Cu, Pb, Zn, As, Cd, Sb, Ag, W, and Au contents than those from other locations in the southern Curnamona Province, and these are more enriched in Fe, Mg, Ni, P, V, Co, Sc, Sr, Y, and U, which indicates a larger hydrogenetic component to their precursor minerals. Enrichments in Mn, S, Ga, Eu, Cu, Pb, Zn, As, Cd, Sb, Ag, and Au, and a positive Eu anomaly in garnetite can be used as exploration guides in the search for Broken Hill-type deposits by indicating close proximity to mineralization.

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