Synchronous advanced argillic alteration and deformation in a shear zone-hosted magmatic hydrothermal Au-Ag deposit at the Temora (Gidginbung) mine, New South Wales, Australia

Paragenetic relations indicate that advanced argillic alteration at the Temora mine and the adjacent Dam-Lilydale prospect is coeval with movement along shear zones which host each system. The interaction between movement along the host structure and developing alteration zones of contrasting rheology controlled the Bow of hydrothermal fluid and resultant overall system geometry Neither of these systems represents older, typical epithermal or porphyry-related systems that were subsequently deformed. However, advanced argillic alteration at both prospects cuts across the peripheral parts of older porphyry-style alteration systems, with paragenetic relations analogous to the Butte and Dizon districts. Each system is characterized by an early phase of massive or breccia-textured, quartz-dominated advanced argillic alteration, cut by foliated mica-rich, quartz-absent advanced argillic alteration. Phase relations indicate that early quartz-saturated advanced argillic alteration developed at temperatures less than or equal to 275 degrees C, whereas later quartz-absent advanced argillic alteration developed at temperatures between 275 degrees and 340 degrees C. Most types of advanced argillic alteration are barren, but background 0.2 to 1 ppm Au grades are locally associated with silica-pyrite alteration. In the Temora mine, economic Au grades are associated with barite-sulfide veins that are generally confined to the silica-pyrite core of the earlier quartz-rich advanced argillic alteration. Cu-Au mineralization at the Dam prospect is associated with sericite-clay-chlorite alteration inferred to be the deeper level equivalent of quartz-absent advanced argillic alteration in the Temora mine. Ore-bearing veins in the Temora mine predate this quartz-absent alteration and were deformed and partly remobilized, The restricted distribution of ore-bearing veins in the silica-pyrite core of the Temora mine reflects the brittle rheology and susceptibility of this rock type to failure during movement along the host structures. O and D/H isotope and fluid inclusion data indicate a dominantly magmatic fluid focused through the host shear zone, possibly at depths in excess of approximately 4.5 km. Mineral stability relations and sulfur isotope data indicate magnetic sulfur input. Shear zone-hosted systems are inferred to represent a largely unrecognized style of advanced argillic alteration developed at slightly greater depths than alteration associated with epithermal and shallower porphyry-related systems. Intersection of the host shear zones with either a crystallizing intrusion or immediately adjacent country rocks is postulated to account for the magmatic isotopic signature and high sulfidation alteration geochemistry. Transient internal pressure drops within the host structures may have aided separation of a magmatic volatile phase at depths below which this would normally occur. Rapid upward movement of this volatile phase within the host structure during hydraulic reequilibration, with subsequent condensation and disproportionation of SO2 resulted in development of advanced argillic alteration over vertical extents in excess of 350 m, possibly several kilometers above the causative intrusion.