Olympic Dam ore genesis; a fluid-mixing model

Abstract

Numerical modeling shows that fluid mixing was probably the dominant ore-forming process in the Olympic Dam Cu-U-Au deposit. The deposit is hosted by the Olympic Dam Breccia Complex, which is within the Roxby Downs Granite. The granite and the breccia complex are coeval with the Gawler Range Volcanics-Hiltaba Suite volcano-plutonic association, and all are products of a major Middle Proterozoic thermal event on the Gawler craton, South Australia. In the Olympic Dam Breccia Complex, early magnetite (+ or - hematite), chlorite, sericite, siderite, and minor pyrite, chalcopyrite, and uraninite mineralization (association I) is extensively overprinted by hematite, sericite, chalcocite, bornite, pitchblende, barite, fluorite, and chlorite (association II). The paragenetically latest major mineral association consists of hematite, or hematite + granular quartz + or - barite (association III). All three mineral associations locally display complex overlapping and indistinct boundaries. Rock relations, breccia textures, ore mineral textures, and mineral parageneses all provide evidence of repetitive brecciation and mineralization events, indicating that ore genesis was complex and multistage. The mineral associations and their zonation, combined with fluid inclusion and isotopic data, indicate that mixing of a hotter magmatic or deeply circulated meteoric water and a cooler meteoric water was probably responsible for ore genesis. Ore mineral textures, the abundance of hematite, and the close association of sulfides and pitchblende with hematite, all suggest that ore precipitation was caused by reduction of sulfate coupled with oxidation of iron during mixing. Fluid inclusion salinities, and the absence of evidence for boiling during precipitation of associations II and III, are consistent with the cooler meteoric water having originated as saline ground water or playa lake water within the volcanic succession inferred to have been extensively developed above and in the vicinity of the Olympic Dam Breccia Complex. U-Pb geochronology and textural studies show that mineralization accompanied brecciation, dike intrusion, and regional mafic and felsic volcanism on the Gawler craton...