COUPLED DEFORMATION, CHEMICAL TRANSPORT, AND HEAT TRANSPORT IN EPITHERMAL BONANZA-ORE FORMATION

Epithermal bonanza ores form in banded veins as discrete bodies within spatially more extensive banded vein networks. Banding has been shown to result from periodic oscillatory behavior of hydrothermal fluids in response to localized dilatation of fractures. For hydrothermal systems close to vapor saturation, dilatation and decompression boiling lead to conductive and diffusive heat transfer across a vein-wallrock interface. The magnitude of the heat transfer effect is a function of fracture density and connectivity, with close-spaced fracture meshes showing the greatest effect. Thus, concentrations of dissolved components relative to the ambient state and heat transport are coupled directly to dilation of a vein-fracture and phenomena (e.g., heat transport) in the fracture mesh feeding into it.

The two attributes, spatially extensive epithermal-style banding yet discrete bodies of bonanza ores within them, allow the inference to be drawn that compartmentalization of flow is necessary for bonanza ore formation. Three styles of synhydrothermal deformation illustrate mechanisms for achieving conditions that favor the complex oscillations, heat transfer, and chemical feedback mechanisms necessary for bonanza-grade ore. At Mahd adh Dhahab, Saudi Arabia, compartmentalized flow occurred in a tensile-shear mesh within a strike-slip pull-apart. In the Comstock Lode, Nevada, USA, subeconomic veins on sidewall normal faults in a strike-slip extensional duplex were deformed and flow compartmentalized and bonanzas formed during concurrent fault-motion inversion and cross-basin faulting. At Round Mountain, Nevada, USA, hydrothermal brecciation and subhorizontal fracture dilation followed the recession of a lake thereby compartmentalizing mass flux that previously had been dispersed on several sheeted sets of high-angle fractures. Very low-grade gold ores are associated with the period of dispersed flow whereas bonanza grades are associated with the focused flow. An important implication from the examples is that deformation leads to fracture networks and permeability that facilitate the feedbacks between all of the coupled phenomena that result in bonanza-grade ore formation.