Is the Mount Isa copper deposit the product of forced brine convection in the footwall of a major reverse fault?

Abstract

Geochemical and mass-balance constraints in conjunction with stable isotope data indicate that the copper deposit of Mount Isa (Australia) formed by mixing of a reduced sulfur-rich fluid from overlying Mount Isa Group metasedimentary rocks with a copper- rich oxidized fluid entering a brecciated contact zone from underlying metabasalts. We have performed numerical simulations to test whether the deposit may have formed by forced fluid convection driven by progressive displacement on the Mount Isa fault zone, indicated by an ∼200 °C offset in metamorphic grade. Results indicate that uplift of ≥1 mm/yr induces regional fluid flow organizing into a stable, permeability-controlled circulation system. Far-field advection through the oxidized metabasalts is superimposed on smaller-scale convection in the metasedimentary rocks beneath the fault. As convection on the large scale gains momentum, the breccia progressively cools, consistent with mineralogic and fluid-inclusion evolution. For realistic uplift rates, the measured silica enrichment in the orebody (≥190 Mt SiO₂) is achieved in ∼1 m.y. Reduced and oxidized fluids mix at the proportions required for high-grade copper-iron sulfide precipitation, aided by rapid oscillations in the influx ratio of the two fluids into the most permeable and vigorously convecting orebody region.