Tectonic Controls on the Epithermal Ore-forming Environment, Northern New Zealand

摘要

Northern New Zealand provides a useful case study for exploring the relationship between tectonic segmentation of the crust and the epithermal ore-forming environment. Here, extensional deformation over areal extents much greater than the thickness of the brittle layer has resulted in development of rift segments comprising arrays of normal dip-slip faults and dike intrusions, which are linked along strike by transfer zones localized on deep-seated basement faults oriented oblique to the predominant rift fabric. Hot waters endowed with precious metals have been pumped through the crust via hydrothermal convection since, at least, the late Miocene, in close association with the lateral migration of the Australian-Pacific plate boundary, its attendant arc systems and the rift basins. Evidence for the passage of gold- and silver-bearing fluids is recorded in the epithermal vein-style mineralization 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 time period concerned, more than 70 unique mineralizing environments have been localized, perhaps 10 to 20 active at any one time, in semi-predictable positions controlled by deep, underlying magmatic foci, across a region that spans more than 10,000 km~2 in areal extent. Such environments tend to align spatially with the rift basins that parallel the migrating arc. Young silicic vents, high-flux geothermal systems, and rich epithermal deposits are, also, localized on transfer zones.