Modelling of structural and lithological controls on mobility of fluids and gold in orogenic belts, New Zealand
Phaedra Upton, Dave Craw, 2014. "Modelling of structural and lithological controls on mobility of fluids and gold in orogenic belts, New Zealand", Gold-Transporting Hydrothermal Fluids in the Earth’s Crust, P. S. Garofalo, J. R. Ridley
Download citation file:
Numerical models have been run to evaluate the key parameters that affect fluid flow and gold mineralization at a range of scales, from the full thickness of the crust to the mineral grain scale. These models are constrained with real examples of orogenic gold in southern New Zealand. Large scale modelling shows that differences in crustal strength and thickness affect the locus, scale, and rate of crustal fluid flow and hence influence gold deposition. The most vigorous hydrothermal activity and gold mineralization occurs in narrow zones with maximum uplift close to a major crustal boundary. Relatively strong middle crust develops deformation-driven permeability and fluid flow in a broad diffuse zone. Deformation of randomly inhomogeneous rocks under mid-crustal conditions results in horizontal fabric and horizontal shear zones, and these can control fluid flow locally. The addition of graphite to flat shear zones causes an increase in deformation-induced permeability, and this can encourage further graphite deposition in a feedback effect. Fluid flow in these flat shears is very slow (mm/year) compared to fluid flow in fracture-controlled permeability at shallow crustal levels. Rock strength inhomogeneities in mid-crustal shear zones can result in localized decrease in differential stress, facilitating the switching of orientations of principal stress axes and the formation of steeply-dipping quartz veins.
Figures & Tables
Gold-Transporting Hydrothermal Fluids in the Earth’s Crust
Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposits types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid-inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures; a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world; several comprehensive case studies of different types of gold deposits and their fluids from USA, Brazil, Egypt, Slovakia and Bulgaria; and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.