SOME CONTROLS ON OXIDATION STATE VARIATION OF OXIDE CU-AU SYSTEMS

Some oxide Cu-Au systems, but not all, exhibit evidence of variable oxidation states, mainly through variations in the abundance of oxidation-sensitive phases (pyrrhotite, pyrite, magnetite, siderite, hematite, sulfates). Variation may be spatially systematic, such as at Olympic Dam (Stuart Shelf), some Tennant Creek-style examples, Mt Elliott (Cloncurry terrane), and the Fennoscandian Bidjovagge deposit. Mixing is one means of accounting for these relationships, as indicated by many previous workers.

In other cases mineralogical variation is not well-zoned, and large apparent fluctuations in fO2 can be inferred over 10's of cm's. Local fluid reduction by pre-existing rock reductants could account for some of these variations. For instance, where large early-stage magnetite masses occurred, as at Starra and Osborne in the Cloncurry terrane, interaction between later CO2-SO4-dominated fluids, and magnetite, can be modelled at 300° C to have produced CH4-H2S-bearing reduced fluids. The sluggish kinetics of carbon isotope systems below 300° C are hypothesized to have allowed the isotopic signatures of these local gradients to be preserved in carbonate minerals. This was likely a two-step process, in which the reduced CH4 was re-oxidised by remixing with unmodified ore fluid. Very light carbon values in ore-related carbonates at Starra and Osborne support this, and cannot be easily explained by other carbon sources.

Evolution of high temperature SO2 is another potential agent of fO2 variation in appropriate systems. Sulfate-sulfide isotope pairs at Monakoff, Cloncurry terrane, record a decisive reduction in H2S/SO4 from ~ 0.25 at 450° C to ~ 1 at ~ 350° C, and require a probable magmatic S source at this site (del 34S ~ 3 permil). This may be a mesothermal analogy to processes that characterize high sulfidation epithermal systems, with the disassociation of magmatic SO2 to SO4 at T ~ 400° C releasing new H2S.