TECTONIC CONTROLS ON HEAT AND MASS TRANSFER IN EXTENSIONAL VOLCANIC PROVINCES: IMPLICATIONS FOR EPITHERMAL ORE LOCALISATION

Extensional volcanic provinces can be categorized into several different types according to crustal-scale architecture (wide, narrow), obliquity of opening, and plate tectonic context (subduction-related versus continental break-up/incipient mid-ocean ridge). In all cases, mechanical failure on new, or more often inherited, faults and shear zones partitions the crust into discrete segments that are defined by arrays of sub-parallel and predominantly extensional faults and intervening transfer zones. Such segmentation controls basin development and influences permeability throughout the crust, at least to depths where strain is localised on faults or shear zones. Northern New Zealand provides a useful case study for exploring the role of crustal segmentation on heat and mass transfer, two essential ingredients in the epithermal ore-forming environment. Hot waters endowed with gold and silver have been pumped through the crust of northern New Zealand since at least the late Miocene, in close association with the lateral migration of the Australian-Pacific convergent plate margin and its attendant arc systems. Evidence for the passage of ore-quality metal-bearing fluids is recorded in the epithermal vein-style mineralisation of the Hauraki goldfield, Coromandel Volcanic Zone, and within geothermal systems of the Taupo Volcanic Zone by the presence of gold on back pressure plates in production wells, disseminated gold in drill core, and gold in samples of deep fluids. Over the entire lifetime of hydrothermal activity, more than 70 plumes of hot water have been localised, perhaps 10-to-20 or so active at a time, for tens to hundreds of thousands of years, in quasi-predictable positions controlled by deeper-seated magmatic foci, across a region that spans more than 10,000 km² in areal extent. Temporal affinities of epithermal deposits and prospects, and geothermal systems, tend to cluster spatially along northeast structural trends, parallel to the length of the migrating arc. However, the highest-flux geothermal systems, and arguably the richest epithermal deposits, are localised on transfer zones.