Abstract

Skarn lenses at the Ban Ban zinc deposit, Queensland, are conformably enclosed in steeply dipping siliceous metasedimentary rocks, with adjacent metabasaltic rocks and minor limestone. Adamellite intrusives occur nearby. Prograde skarn development is represented by a garnet stage, dominated by medium- to coarse-grained grandite garnet (Ad (sub 40-100) ) with minor quartz and calcite. Local assemblages rich in manganoan hedenbergite occur near skarn peripheries. The garnet stage is crosscut and replaced by sulfide stage assemblages which, although dominated by sphalerite, pyrite, and, to a lesser extent, magnetite, contain a variety of other sulfides, hematite, Ca(-Fe-Mn-Al-Mg) silicates, quartz, and calcite. Mineralogical and metal zonation is demonstrated in sulfide stage assemblages by a pyrite + or - magnetite assemblage containing low Cu to the north and at depth, passing sharply along strike into a sphalerite + Fe sulfide + or - magnetite assemblage containing high Zn and low Cu, Pb, and Cd in central and near-surface areas, which in turn grades into a low sulfide, low metal assemblage to the south. A late retrograde chlorite stage is represented by the replacement of garnet and sulfide stage assemblages by Fe-rich chlorite and calcite. Initial prograde skarn formation may have resulted from reaction between carbonate beds and magmatically derived hydrothermal fluids, under generally rather oxidizing conditions. The sulfide stage may have formed after the metamorphic peak, reflecting decreasing temperature of the fluids and possible ingress of meteoric water. The chlorite stage is probably indicative of continuing retrogression at low temperature.

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