Abstract

Earlier structural studies indicate that the dolomitic and siliceous breccias and contained copper mineralization at Mount Isa (Queensland, Australia) formed during regional deformation and greenschist facies metamorphism of their host mid-Proterozoic metasediments. The metasomatic breccia body (locally called "silica dolomite") is zoned, with an outer halo of sparry dolomitic alteration replacing finely laminated dolomitic-pyritic metasediments. The core of the dolomitic breccia next to the underlying altered greenstones is overprinted by siliceous breccia containing the bulk of the chalcopyrite ore.Fluid inclusions in quartz and dolomite were studied by microthermometry, micro-Raman spectrometry, and semiquantitative electron microprobe analysis of inclusion salts. Two types of aqueous inclusions are restricted to dolomitic breccia: CaCl 2 -rich (group 1) with a cation ratio Na/Ca/K/Mg nearly equal 30:10:3:1 and salinity ( approximately 25 wt %) similar to recent Salton Sea geothermal brines; and low-salinity with 10 to 20 mole percent CO 2 (CO 2 -rich, group 2). These two aqueous inclusion types are intimately associated in the same crystals and have probably both been trapped during dolomitic alteration.Siliceous alteration superimposed on dolomitic breccia contains NaCl-rich fluid inclusions (group 3: Na/Ca/K/Mg nearly equal 300:10:30:1, minor CH 4 ). Microstructures suggest that earlier higher salinity (10-20 wt %) and later lower salinity (4-9 wt %) variants of this NaCl-rich fluid bracket the main stage of chalcopyrite introduction. There are no low-density vapor inclusions indicative of any stage of fluid boiling. Rare high-density CH 4 (+ or -CO 2 )-rich carbonic inclusions are associated with NaCl-rich aqueous inclusions in a few samples. Secondary inclusions of a highly calcic low-temperature brine (group 4) occur along late healed cracks and postdate all major stages of mineral precipitation.The C, O, and H isotope compositions of dolomites, quartz, and hydrous silicates and of fluid inclusion extracts from samples of the siliceous alteration zone were imposed by the externally derived, weakly CH 4 + CO 2 -bearing, NaCl-rich fluid at high fluid/rock ratios and over a small range of temperatures. This fluid may have originated as a highly evolved basin brine or an evaporite-derived metamorphic fluid. The early CaCl 2 -rich fluid may have had an origin similar to the NaCl-rich ore fluid, but its different delta D and the mass balance constraints arising from its chemical composition suggest that it has not been derived from the NaCl-rich fluid by progressive rock interaction within the zoned alteration system. Similarly, an independent source is likely for the CO 2 -rich fluid.The absolute pressure-temperature ranges for the two alteration processes seem to be slightly different, but it is uncertain whether the quartz-chalcopyrite mineralization occurred at slightly lower temperature or higher pressure compared with the dolomitic alteration. The latter is confined to 270 degrees to 350 degrees C and 700 to 1,500 bars by isochore and mineral stability data.The following working hypothesis for the syntectonic and synmetamorphic fluid-rock interaction and copper ore formation at Mount Isa is proposed. In an initial stage, dolomitic alteration occurred by chemical interaction between the Urquhart Shale and moderate amounts of two fluids of different chemical and hydrological origin, represented by the CaCl 2 -rich and the CO 2 -rich fluid inclusions. Fluid mixing may have been aided by extension on the limb of a regional fold which temporarily lowered fluid pressures, thus leading to hydraulic brecciation and fluid influx from two fluid regimes, above and below. The CO 2 -rich fluid probably represents the local metamorphic fluid of the Urquhart Shale, whereas the CaCl 2 -rich brine was introduced from the underlying altered greenstones. The initial stage of brecciation and dolomitic alteration was followed by the main stage of silicification and copper introduction by the much more copious NaCl-rich fluid, probably at near-lithostatic fluid pressure. Quartz and chalcopyrite precipitation occurred by a combination of slight cooling, a pH increase, and possibly an increase in sulfur activity, as the initially reduced, acid, and possibly S-deficient, NaCl-rich fluids reacted with the previously dolomitized pyritic and dolomitic Urquhart Shale.

First Page Preview

First page PDF preview
You do not currently have access to this article.