Variation in Copper-gold styles through time in the Proterozoic Cloncurry goldfield, Mt Isa Inlier: a reconnaissance view

The Cloncurry copper-gold field, Mt Isa Inlier, contains a variety of copper-gold mineralisation styles, in which Cu/Au ratios vary radically from Cu- to Au-only. Several of these mineralisation styles are not recognised elsewhere in the Mt Isa Inlier and are rare in Proterozoic metallogeny. They developed concurrently with intense, high-temperature, saline metasomatism exposed in the Cloncurry metasomatic zone 13 km to the southwest and may represent mineralisation styles that occur lateral to and above such albite-actinolite alteration. Mineralisation varied through time in response to changing structural style. Initially small quartz vein-hosted Au-only deposits (Glided Rose-style) formed during regional D 1 thrusting. These are known only from one east-west oriented antiform in the Soldiers Cap Group, and do not display the high-temperature saline fluid-related alteration of subsequent systems. Quartz vein/replacement-hosted Cu-Au ± Zn-As-Co-Bi-Mn deposits (Eloise-style) were formed in ductile D 2/D 3-shears within metasediments. Brittle-ductile D 3 to post-D 3/syn-granite deformation produced the most common deposit styles, including stratabound iron-formation-hosted Cu-Au-U-Co-Ba-Mn-P-Zn-rich deposits (e.g. Monakoff, Hot Rocks), albitite-associated Cu-Au-Co vein networks (e.g. Evening Star), and, most abundantly, Cu-Au-Co vein/replacement deposits in amphibolite (Mt Freda-style). The sources of ore components for D 1 and D 2-related deposits are uncertain, but deposits that formed during D 3 have a close spatial relationship to inferred isolated Williams Batholith intrusions, which supports some magmatic contribution for these. The albitite-related deposits have some features in common with porphyry copper systems with histories of external saline fluid interaction. For Mt Freda-style deposits, which mainly contain reduced pyrrhotite-pyrite assemblages, pH change that was induced by wall-rock reaction, with concurrent fluid unmixing in dilatant zones, is favoured to account for ore deposition. Oxidation by magnetite-bearing wall rocks, occurring concurrently with pH change, can account for unusual magnetite-bearing Mt Freda-style examples such as Great Australia, but this difference produces a fluid path which greatly favours Cu deposition over Au, producing Cu-only orebodies.