NEW CONSTRAINTS ON THE THERMAL EVOLUTION OF THE MOUNT ISA MINERALIZATION SYSTEM

George Fisher mine is the northernmost occurrence of syndiagenetic, Mount Isa-style Zn-Pb-Ag hosted by the Proterozoic Urquhart Shale. George Fisher is distinguished from Mount Isa and Hilton mines by a scarcity of Cu, despite restricted, syntectonic development of alteration assemblages attributed to that event at the site. Temperature gradients manifested by alteration assemblages accompanying temporally separate Zn-Pb-Ag and Cu mineralization systems are preserved at George Fisher. Comparison with Mount Isa reveals thermal heterogeneity between mine environments, which is attributed to variable syntectonic hydrothermal fluid flow responsible for Cu mineralization.

A maximum temperature of 200⁰C was determined for Zn-Pb-Ag mineralization from migrated bitumens, that occur with sphalerite, as infill in intergranular porosity in mudstone intervals. Bitumen grains further display mesophase textures formed as a result of thermal overprinting (~200⁰C) after migrabitumen emplacement. Previous assumptions that the Urquhart Shale was regionally metamorphosed to Greenschist facies north of Mount Isa, are not sustained.

The assemblage greenalite+quartz formed synchronous with Cu mineralization at George Fisher and constrains maximum temperatures during mineralization to 250-300⁰C at low XCO₂. In contrast, alteration assemblages at Mount Isa mine attributed to the Cu mineralization event are calcite+phlogopite and calcite+biotite (330⁰C and 350⁰C at low XCO₂. Absence of calcite+phlogopite and calcite+biotite at George Fisher further constrains the maximum temperature of Cu mineralization at the site to less than 330⁰C.

Temperature estimates for George Fisher demonstrate the maximum temperature of syndiagenetic Zn-Pb-Ag mineralization was 200⁰C, 50-100⁰C cooler than syntectonic Cu mineralization. In addition, Cu mineralization at George Fisher was some 30-100⁰C cooler than that at Mount Isa mine. Thermal gradients between mines is attributed to changes in temperature, and volume of syntectonic hydrothermal fluid flow rather than regional metamorphic processes. Sparsity of Cu, yet development of syntectonic alteration assemblages, at George Fisher suggests syntectonic hydrothermal fluids channelled to the site were too cool to transport economic quantities of Cu.