The evolution of magmatism, brecciation, alteration, and mineralization is described for the Mount Leyshon intrusive complex, Queensland, Australia, using P-T-V-X analysis and major to trace element analysis of single fluid inclusions using laser ablation ICP-MS. The ~290 Ma Mount Leyshon intrusive complex is a subvolcanic porphyry-breccia system mined for ~97 metric tons (t) Au (70 Mt at ~1.4 g/t Au) between 1986 and 2000. The Mount Leyshon intrusive complex is characterized by several generations of genetically related breccias and magmatic intrusions. Both alteration and mineralization can be traced to specific intrusive events. The earliest phase of brecciation was followed by widespread biotite-magnetite alteration (stage 1). Fluid inclusions associated with stage 1 were trapped at >350°C and >145 bars and were anomalously K and Fe rich. This stage was followed by the emplacement of rhyolite to dacite stocks and coeval breccia pipes (stage 2). Fluids exsolved from these magmas simultaneously exsolved Na-K-Fe-Mn-rich brine (~60 wt % NaCl equiv) and dilute vapor at ~500 bars (~2-km paleodepth) and >600°C. Pressures likely oscillated between lithostatic and sublithostatic conditions in response to the transient rupture and sealing of intrusions and wall rocks. Hydrothermal fluids infiltrating breccia pore space produced base metal mineralization (stage 3). Stage 3 fluid inclusions contain 2 to 10 wt percent NaCl equiv and were trapped at 300° to 400°C below 500 bars. Stage 3 fluids were depleted in K, Fe, and Mn, and enriched in Ca, Sr, and Ba relative to stage 2 brines. This trend is consistent with the transition from potassic alteration in stage 2 to feldspar-destructive phyllic alteration in stage 3. Cu, Zn, and Pb were conservative elements above 350°C and precipitated nonselectively as sulfides below 350°C in a bulk ratio very similar to that measured in fluid inclusions. Subsequent emplacement of andesite to rhyodacite dikes (stage 4) was marked by Au and additional base metal sulfide mineralization in veins and breccia cavities. Stage 4 fluids are chemically similar to stage 3 but contain several mol percent CO2, perhaps suggesting a more mafic, CO2-charged magma source relative to earlier intrusive pulses. In addition to fluid cooling, the destabilization of Au bisulfide complexes attending phase separation of H2O-CO2-NaCl fluids below 400°C and 400 bars was the likely cause for Au precipitation. Mount Leyshon appears to indicate that CO2 can play a crucial role in the transport of gold-bearing solutions from a magmatic source at depth into much shallower hydrothermal environments.

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