| Abstract |
The Onemana area, which lies at the eastern coast of the Coromandel Peninsula of New Zealand, is part of a broad NNE-trending corridor of epithermal Au-Ag deposits that runs from Tui mine in the southwest to Onemana/Ohui in the northeast. Silicified and altered rhyolitic flow banded rocks of the Whitianga Group and minor Coromandel Group andesites outcrop in the area. High-level silicification and quartz-kaolinite-type alteration follows a 3.5 km long and 1.8 km wide NE-SW trending aeromagnetic low/resistivity high which is delimitated to southeast by the NE-striking Whitipirorua fault. Veins and hydrothermal breccias are also confined to this alteration corridor. The paleo-geothermal system was present in the hanging wall of the Whitipirorua fault. At this fault block occurs a 260 m deep body of lacustrine sediments with hydrothermal eruption breccias and, possibly, sinters associated. Extensional fracturing is the main rock failure mechanism and dictates the structural permeability style at Onemana. Extensional veins and hydrothermal breccias strike N-S to NE, which are the main structural trends of the area, and could be linked with faults at depth, where shear failure is favoured. Flow banding exerted some control on fluid flow either through its primary permeability or by controlling fracture formation. The lower tensile strength orthogonal to the flow banding coupled with low overburden stress control fracture aperture, in a process facilitated by hydrostatic to supra-hydrostatic fluid pressure. As a result, vertically and sub-horizontally dipping hydrothermal breccias and veins trending parallel to the rock fabric are formed where the flow banding dips steeply and gently, respectively. The formation of highly silicified rock layers results from fluid circulation along porous rock beds, e.g., a spherulitic rhyolite flow. |