STRUCTURAL CONTROL OF EPITHERMAL MINERALIZATION: EVIDENCE FROM THE SOVEREIGN-JUBILEE DEPOSIT, HAURAKI GOLDFIELD, NEW ZEALAND

Sovereign-Jubilee is an adularia-sericite Au-Ag deposit in the Miocene to Pliocene Hauraki Goldfield of New Zealand. This work helps constrain crustal-scale tectonic processes associated with the formation of epithermal Au deposits.

Gold-bearing veins at Sovereign-Jubilee strike NE- to NNE, occur in andesitic and dacitic volcanic rocks, and rhyolitic intrusions, and formed 300 to 400 m below the paleo water table. Structural mapping and kinematic analysis indicate a tectonically complex area with at least two generations of veins: (i) the NNE-striking steeply-dipping main vein and associated moderately- to steeply-dipping veins with variable strikes, and (ii) NNE-striking low angle (10-30°) veins. Tectonic brecciation of veins reflects considerable post-vein faulting. Northeast- to NNE-trending normal faults dominate, but E-W trending faults occur locally. The main NNE-trending veins exploited pre-existing faults. Striations and analyses of conjugate faults indicate mostly dip-slip movement with extension in a SE to ESE direction. The thin, simple veins at Sovereign-Jubilee indicate focussed fluid flow in very narrow, purely tensional fractures.

Data from Sovereign-Jubilee are compatible with data from other areas in the Hauraki Goldfield. Northeast- to ESE-trending normal faults with steep dips occur in most deposits, indicating regionally consistent NW-SE extension during mineralization. Late W- to WNW- trending low-angle veins, such as those at Sovereign-Jubilee, are likely still in their original orientation and are not seen elsewhere in the Hauraki Goldfield, indicating local compression in the area. In contrast, some early high-angle veins at nearby Golden Cross were rotated, and now have shallow dips. Throughout the goldfield, structural differences between deposits indicate there are small variations in the overall tectonic framework, allowing localized shortening and tilting, probably due to a combination of regional tectonics and more localized caldera processes. Complex vein patterns reflect opening of a wider range of defect orientations than would be the case at greater depth. Finally, epithermal veins in the Hauraki Goldfield record high volume fluid flow along dip-slip normal faults rather than strike-slip faults, and there is little evidence for simple dilational jogs.