Mary Kathleen metamorphic-hydrothermal uranium - rare-earth element deposit: Ore genesis and numerical model of coupled deformation and fluid flow

The Mary Kathleen U-REE orebody of the Proterozoic Mt Isa Block was the product of chemical and physical interaction between regional metamorphic/hydrothermal fluids and preexisting calcic skarns. The deposit provides excellent examples of mechanical control on ore localisation and of the complexity of ores in rocks with protracted thermal histories. Host skarns were produced by contact metasomatism around the 1740 Ma Burstall Granite, whereas the allanite-uraninite ore formed under amphibolite-facies conditions, late during the D₂ phase of the ca 1550-1500 Ma Isan orogeny. Observations of ore geometry are consistent with previous geochronologic data demonstrating a large time gap between skarn formation and ore genesis. Numerical modelling of coupled deformation and fluid flow suggests that veins at the core of ore shoots may have formed as tensile or shear fractures during coupling of the competent skarn host with the late-D₂ Mary Kathleen Shear Zone, allowing a change of orientation of ore shoots with distance from the shear zone. Mineral chemistry and petrographic observations suggest the possible role of a redox control on chemical localisation of ore by conversion of Fe²⁺-rich clinopyroxene-rich skarn host to Fe³⁺-rich secondary garnet 'skarn' and uraninite-allanite ore. Alternately, fluid pressure drops as a consequence of fracturing of the host skarn may have triggered fluid unmixing, or fluid mixing, leading to ore precipitation. Available data do not allow clear definition of the ultimate source of the U and REE, nor of the specific chemical ore-forming mechanism. However, regional constraints, previous Sm-Nd modelling, and our numerical models suggest a combination from proximal skarn hosts and from distal sources accessed by flow of metamorphic and/or late tectonic igneous-derived fluids. The deposit has some similarities with ironstone-hosted Cu-Au ± U deposits found in the nearby Cloncurry Belt.