OLYMPIC DAM ORE GENESIS - A FLUID-MIXING MODEL

Numerical modeling shows that fluid mixing was probably the dominant o re-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 D owns 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 G awler craton, South Australia. In the Olympic Dam Breccia Complex, ear ly magnetite (+/- hematite), chlorite, sericite, siderite, and minor p yrite, chalcopyrite, and uraninite mineralization (association I) is e xtensively overprinted by hematite, sericite, chalcocite, bornite, pit chblende, barite, fluorite, and chlorite (association II). The paragen etically latest major mineral association consists of hematite, or hem atite + granular quartz +/- barite (association III). All three minera l associations locally display complex overlapping and indistinct boun daries. Rock relations, breccia textures, ore mineral textures, and mi neral parageneses all provide evidence of repetitive brecciation and m ineralization events, indicating that ore genesis was complex and mult istage. The mineral associations and their zonation, combined with flu id inclusion and isotopic data, indicate that mixing of a hotter magma tic or deeply circulated meteoric water and a cooler meteoric water wa s probably responsible for ore genesis. Ore mineral textures, the abun dance of hematite, and the close association of sulfides and pitchblen de with hematite, all suggest that ore precipitation was caused by red uction of sulfate coupled with oxidation of iron during mixing. Fluid inclusion salinities, and the absence of evidence for boiling during p recipitation of associations II and III, are consistent with the coole r meteoric water having originated as saline ground water or playa lak e water within the volcanic succession inferred to have been extensive ly developed above and in the vicinity of the Olympic Dam Breccia Comp lex. U-Pb geochronology and textural studies show that mineralization accompanied brecciation, dike intrusion, and regional mafic and felsic volcanism on the Gawler craton. Numerical chemical modeling was used to simulate mixing of a hotter saline water and a selection of cooler meteoric waters at 250 degrees and 150 degrees C, respectively. The co mposition of the hotter water was constrained by minerals observed in association I and in the Roxby Downs Granite. Compositions of the cool er meteoric waters were determined by simulated evaporation and heatin g of ground waters derived from continental basaltic and granitic prov enances, respectively. A simulated mixing of the cooler and hotter wat ers generated mineral assemblages and parageneses nearly identical to those in the Olympic Dam deposit. Titration of Roxby Downs Granite int o the mixed water also generated ore mineral associations comparable t o those observed in the deposit, with weight ratios of ore mineral com ponents differing by factors less than or equal to 10. The waning stag es of each mixing event were simulated by titrating additional cooler meteoric water into the mineral assemblages produced by the prior mixi ng and granite interaction. This generated a leached assemblage compos ed mainly of anhydrite, hematite, fluorite, muscovite (sericite); quar tz, and barite adjacent to zoned gold-, uraninite-, and chalcocite-bea ring assemblages. Simulated boiling of the hotter water generated an a ssemblage containing abundant fluorite, magnetite, hematite, sericite, and quartz and a trace of copper-iron sulfide. The modeling supports the hypothesis that precipitation of magnetite, hematite, sulfides, an d uraninite resulted from coupled sulfate reduction and ferrous iron o xidation. The pH decrease caused by hematite precipitation was suffici ent to generate the large sericite +/- chlorite halo observed in the R oxby Downs Granite. The modeling also supports the hypothesis that the cooler meteoric waters were oxidized and were derived from a provenan ce containing mafic volcanic rocks with or without a felsic volcanic c omponent. We conclude that association I precipitated in the early sta ges of each mixing event, followed by production of II as more cooler water mixed with the hotter water. Lateral and downward flow of additi onal cooler water in the waning stage of each mixing event resulted in partial or complete oxidation of associations I and II. This generate d the observed vertical and lateral zonation from the upper and centra l hematite + quartz + sericite +/- barite zones (III), through variabl y hematitic rocks locally enriched in gold and/or containing native co pper, to zones containing hematite, sericite, quartz, fluorite, chalco cite, and pitchblende (i.e., the more oxidized parts of II) to zones t ransitional between II and I. The vertical and lateral zonation patter ns were made more complex by brecciation and fluid mixing in subsequen t mineralizing events. The repetitive brecciation generated complex sp atial anti paragenetic relations between the associations, as illustra ted by the common occurrence of clasts of III within breccias containi ng I and II. We also conclude that the Olympic Dam Breccia Complex con tains a major Cu-U-Au orebody because it formed within a reservoir of saline ground water in contact with mafic and felsic volcanics and sub volcanic intrusions. The ground water was responsible for transport of Cu, U, Au, and most of the S into the breccia complex, where it inter acted with the hotter water which introduced most of the Fe, F, Ba, an d CO2 from below. The absence of economic Cu +/- U mineralization in m any magnetite +/- hematite deposits apparently analogous to Olympic Da m can be attributed to the absence of a contemporaneous near-surface r eservoir of appropriately oxidized saline ground water. Many of these deposits presumably resulted from boiling of a water analogous to the hotter water, or from mixing of this water with cooler, oxidized meteo ric waters that were either nonsaline or saline but comparatively redu ced.