Fluid flow, basin evolution, and the controls on giant stratiform ore deposits

摘要

The formation of hydrothermal ore deposits requires the transport of large quantities of relatively insoluble metals from a source region, through a transport pathway, to the site of deposition. Very large fluid fluxes are needed, due to the low solubility of the metals, and these fluids must be channelled at the point of deposition through relatively small (deposit scale) and normally impermeable rock volumes. Fracturing a rock mass within a fault zone is the simplest way to create the high permeability needed, which leads to the simple conclusion that fault zones are the major means of transporting hydrothermal fluids from the source region to the site of deposition. To form a large ore deposit, the permeability of the fault zone must be maintained throughout the passage of the mineralising fluid, i.e. the fault zone must be a site of movement at the time the hydrothermal system is active. Permeability is unlikely to be the same everywhere along a fault zone, and will be greatest where damage is greatest (e.g. where the fault bends, branches or steps away from its average orientation) and deformation of the rock is required to accommodate the irregularity. Fluid flow is, therefore, maximised on irregularities in fault zones, which explains the localisation of many major hydrothermal deposits in such irregularities.