The magmatic-hydrothermal transition at Timbarra: implications for the genesis of intrusion-related gold deposits

The Timbarra gold deposit is hosted entirely within the granite core of a zoned pluton and is conformably constrained beneath the fine-grained carapace and internal layers. The ore is characterized by Au-Bi-Te-As-Mo-Sb geochemistry hosted within moderately oxidized I-type granite suggesting that Timbarra is part of the newly recognized intrusion-related gold deposit class. Despite a spatial-temporal link to magma¬tism and a lithophile-enriched ore association characteristic of many granite-related systems, a magmatic ori¬gin for this deposit class is the still subject of much debate. The presence of magmatic-hydrothermal transi¬tion textures including miarolitic cavities, interconnected miarolitic textures, unidirectional solidification textures, pegmatites and vein-dykes combined with melt inclusion-fluid inclusion relationships provide com¬pelling evidence for a magmatic origin for the low salinity COi-rich fluids that formed the deposit. The for¬mation of the Timbarra gold deposit from low salinity C02-rich magmatic fluids at mesozonal levels (5 to 10 km) and 1.3 to 2.6 kbars pressure is consistent with the COi-rich nature of deeper intrusion-related gold, tin¬tungsten and molybdenum systems as well as experimental data. Timbarra represents the first documentation of the magmatic-hydrothermal transition relationships within gold-only systems and has major implications for the future interpretation of fluid sources of such deposits