Abstract

We describe the complex structure in the highest (now mined-out) levels of the world-class Martha Hill gold-silver deposit, Coromandel peninsula, which is hosted in late Miocene andesitic rocks. A northeast-striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast-striking gold-silver lodes up to 30 m thick. Gold production was entirely from quartz veins. The mineralization is embedded in the following geological history: (1) deposition of the host volcanic rocks, (2) initial tectonic fracturing, (3) intrusion of clastic dikes and sills, (4) start of hydrothermal alteration, (5) main phase of faulting, (6) main phase of veining, (7) erosion and deep oxidation, (8–11) deposition of further volcanic material, including prominent rhyolites, and formation of present topography. Veins occupied fractures but also followed a network of preexisting, mostly normal faults with a small to medium amount of displacement. Faulting involved northwest-southeast, north-south, and east-west extension, implying complex 3-D strain. Clastic dikes of carbonaceous sandstone intruded during the faulting phase. Fault rocks were mostly brittle cataclasites or breccias, but some clay-rich zones displayed ductile structures. Many veins overprinted faults, with a change from shear-mode to opening-mode deformation. Stockwork veins formed a mesh composed of two orthogonal sets: one with northeast-southwest– and northwest-southeast–striking veins, including the dominant mineralized lodes, and the other with north-south– and east-west–striking veins. Although northwest-striking veins were earliest, during the main vein phase, opening oscillated between all the vein directions, with the two main lodes in the open pit area, the Martha and the Welcome, capturing most of the strain. A variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid. A special concentration of structural features facilitated focusing of fluid flow to produce this world-class deposit, which contrasts with the less well endowed vein systems around it. The overall tectonic control was dominated by northwest-southeast extension and dip-slip deformation. The 3-D strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north-northwest and northeast trends associated with migration of a subduction zone past the Coromandel peninsula. This demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales.

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