Hydrothermal history of the Mary Kathleen Fold Belt, Mt Isa Block, Queensland

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Abstract
In the Mary Kathleen Fold Belt, in the centre of the Proterozoic Mt Isa Block, four phases of deformation‐related hydrothermal activity are recognized that occurred over a time range from at least 1750 Ma to 1100 Ma. Intense metasomatism occurred during all four phases due to the involvement of highly saline fluids, with the apparent NaCl source being the evaporitic Corella Formation rocks. In all phases, the scale over which the hydrothermal systems operated was ≥ 1 km, with commensurate time integrated fluid fluxes of the order of 104 m3/m2 or more. Phase 1 involved the development of convection cells and fault‐ and shear‐zone fluid pathways related to emplacement of intrusions in and above a major crustal subhorizontal extensional shear zone. Gold mineralization and some U‐REE mobilization occurred at this time. Intrusive rocks and contact aureoles formed discrete kilometre‐scale bodies within the Corella Formation during Phase 1. As a consequence, fluid flow during Phases 2 and 3 (regional deformation and amphibolite‐facies metamorphism) was localized around the boundaries of very competent Phase 1 intrusions and contact metamorphic aureoles. Fluids infiltrating the rocks during this phase were complex NaCl‐CaCl2‐KCl‐H2O‐CO2 brines that may have undergone phase separation. This fluid flow culminated in the formation of widespread calcite vein systems (with copper) and the Mary Kathleen U‐REE vein style orebody, but was sufficiently localized by the pre‐existing structural heterogeneity that many parts of the Corella Formation did not experience significant fluid flow at this time. Phase 4 post‐metamorphic oxidizing alteration was localized around major cross‐cutting faults. In all phases, the oxygen and carbon in the fluid is dominated by igneous or ‘average‐crustal’ oxygen isotopic values, suggesting that devolatilization of the Corella Formation metacarbonates did not provide the bulk of the fluid. However, if the high fluid salinities were due to evaporite dissolution from the Corella Formation, this requires selective or non‐equilibrium evaporite dissolution during the early phases of hydrothermal activity. During the later stages, when all the salt was taken up in metamorphic scapolite, the high salinities may have been derived by equilibration of retrograde fluid with scapolitic rocks.