Abstract

Metabasalts surrounding the metasediment-hosted base metal deposit at Mount Isa have been modified by metamorphism, polyphase deformation, and several stages of hydrothermal alteration. This study aims to understand the large-scale hydrothermal processes that determined the chemical composition of syn to postmetamorphic ore brines, prior to being focused into chemicaly anomalous metasediments that acted as a chemical trap for localized copper precipitation. Field mapping of the metabasalts provided the framework to determine the relative timing of district-scale alteration events, which were then studied by mineralogic observations and bulk mass balance measurements. Microthermometry, Raman microspeetrometry and Br/Cl analyses of fluid inclusions, and H, O, C, and S stable isotope measurements on vein minerals were used to characterize the hydrothermal fluids associated with the alteration events. All fluids are moderately to highly saline, variably Ca enriched with high Br/Cl compared to modern seawater, and isotopiealy most akin to modern evolved basin or low-grade metamorphic brines. The timing and fluid-tracer observations in conjunction with thermodynamic mass transfer modeling permit the following correlation between district-scale alteration processes and the mine-scale hydrothermal events associated with copper deposition at Mount Isa.District-scale epidote-sphene alteration of metabasalts temporally overlapped with the peak of greenschist facies metamorphism and deformation and is widely distributed in flow-top breccias and as halos around synmetamorphic quartz-calcite veins. It is associated with calcic brines with 12 to 38 wt percent CaCl 2 + NaCl equiv, and thermodynamic modeling suggests that the observed mineralogy and bulk mass changes can be explained by focusing of metabasalt-equilibrated fluids and by reaction with similar rocks at somewhat higher temperature. Similar timing and fluid characteristics are associated with dolomitic alteration at the Mount Isa mine, where mixing of metabasalt-derived calcic brine with metamorphic CO 2 from the metasediments led to massive dolomite precipitation.Kilometer-scale fracture zones associated with carbonate-Fe oxide alteration (calcite + albite + magnetite + or - hematite + or - biotite + or - chlorite) cut all penetrative foliations in the metabasalts east of Mount Isa. They are locally associated with uranium mineralization and are characterized by medium-salinity NaCl-rich fluid inclusions (8-93 wt % NaCl equiv). Carbonate-Fe oxide alteration is interpreted to mark channelways of infiltration by oxidized (sulfate-bearing) brines from outside the metabasalt sequence, possibly from an overlying evaporitic cover sequence. Fluid characteristics, evidence for extreme copper mobility, and timing criteria of alteration relative to deformation suggest a temporal, and possibly genetic, correlation with the main copper and silica deposition stage at the Mount Isa mine.Mg chlorite-rutile alteration is restricted to metabasalts immediately below the copper deposit and the vicinity of the Mount Isa fault zone, where higher grade amphibolite facies metabasalts were upthrust against the greenschist facies metabasalts and metasediments now hosting the copper ore bodies. Mg chlorite-rutile alteration of metabasalts, and copper mineralization in the adjacent reduced sediments, can both be explained by fluid focusing along an actively extending contact zone between the oxidized metabasalts and the reducedmetasediments. The brines focused into this contact zone were initially oxidized as a result of infiltration through the carbonate-Fe oxide fracture zones and less altered metabasalts, and became reduced by reaction with the metasediments. The unusual geochemical and isotopic (low 13 C and high 34 S) composition of Mg chlorite-rutile alteration can be explained as a result of back flow of reduced fluids from the metasediments into the metabasalts. Although Mg chlorite-rutile alteration of metabasalts probably liberated some of the copper now present in the orebodies, this alteration may be a relatively localized effect associated with the site of ore deposition rather than an essential sourcing mechanism for the copper.The regional feature of greatest significance to ore formation at Mount Isa, and to exploration for similar high-grade copper deposits elsewhere, is not the somewhat elevated initial copper content of the metabasalts nor their locally significant copper depletions, but their moderately high oxidation state evident from the ubiquitous occurrence of Fe-rich epidote and the large-scale Fe oxide-altered fracture zones. Oxidation of the basalts (or any other silicate rocks) favored high copper solubility along extensive transport paths and provided the essential chemical contrast to the depositional environment, which was anomalously reduced and sulfide rich as a result of an earlier process of sedimentary to diagenetic Pb-Zn-Fe-S mineralization.

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