Variation in copper‐gold styles through time in the Proterozoic Cloncurry goldfield, Mt Isa Inlier: A reconnaissance view

Abstract

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 (Gilded Rose‐style) formed during regional D1 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 D2/D3‐shears within metasediments. Brittle‐ductile D3 to post‐D3/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 D1‐ and D2‐related deposits are uncertain, but deposits that formed during D3 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.