Geological Modeling and Numerical Simulation of Cold-Water Trap in the Low-Sulfidation Epithermal Sirawai Au-Ag Deposit, Mindanao, Philippines

摘要

For low-sulfidation epithermal deposits, which are globally significant sources of Au and Ag, we propose a new multi-stage ore-deposition mechanism initiated by cold groundwater. This cold-water trap was verified in the Philippine Sirawai Au-Ag deposit through geological modeling using the data of ore minerals, X-ray diffraction-based alteration minerals, and fluid-inclusions, and the numerical simulation of fluid flow using TOUGH2. Gold grades and Ag/Au ratios were especially focused on because they were characterized by various Au ore-deposition mechanisms. Initially, neutral-pH hydrothermal fluids at similar to 270 degrees C ascended through veins and encountered a shallow aquifer. The cooling of rising fluids by cold, shallow groundwater resulted in early-stage mineralization (Au similar to 1 g/t, Ag/Au ratio 4-89). Hydrothermal alteration halos were also generated at this stage by fluid flow from the veins into surrounding permeable zones (i.e., a shallow aquifer). The greatly decreased permeability of these alteration halos formed mushroom-shaped low-permeability alteration halos (LPAHs) around the veins. Middle-stage mineralization (Au similar to 26 g/t, Ag/Au ratio <= 10) occurred by the boiling of fluids when temperatures increased due to low permeability and insulating properties of the LPAHs. As the permeability of the LPAHs decreased further, hydrothermal brecciation occurred preferentially in horizontal brittle zones (i.e., the caps of the LPAH in the shallow aquifer). Late-stage mineralization (Au similar to 2 g/t, Ag/Au ratio similar to 30) then occurred by the boiling of fluids as the pressure decreased. Consequently, the cold-water trap mechanism is an innovative approach that clarifies mineralization in low-sulfidation epithermal Au deposits.