VOLCANIC AND TECTONIC EVOLUTION OF THE COROMANDEL VOLCANIC ZONE, AND ITS INFLUENCE ON THE STYLE OF EPITHERMAL MINERALISATION IN THE HAURAKI GOLDFIELD, NEW ZEALAND

The Hauraki Goldfield in the Coromandel Volcanic Zone contains approximately 50 adularia-sericite epithermal Au-Ag deposits in a 200 km long by 40 km wide N-S trending belt. These deposits have produced approximately 316,000 kg Au and 1.5 million kg Ag, and formed from hydrothermal systems associated with Miocene to Pliocene subaerial volcanism. High resolution ⁴⁰Ar/³⁹Ar dates of vein and wall rock adularia from twelve epithermal deposits and one porphyry copper prospect help constrain the ages of mineralisation in the Hauraki Goldfield. Andesitic volcanism occurred in the northern goldfield from c. 18 to 11 Ma during SW-directed subduction of South Fiji Basin crust, which formed the Northland Volcanic Arc. Epithermal orebodies in the northern goldfield at Thames and Coromandel have bonanza grades (>30 ppm Au), and form relatively narrow veins. Adularia samples from the northern goldfield yield ⁴⁰Ar/³⁹Ar plateau dates that range from 16.3 to 13.1 Ma, and are consistent with previous K/Ar dates that indicate mineralization in the northern goldfield formed before 10 Ma. The southern goldfield contains predominantly andesitic and rhyolitic volcanic rocks that presumably formed during WNW-directed subduction of Pacific crust that formed the Colville Arc. Epithermal orebodies in the southern goldfield typically form thick (1 to 5 m wide) crustiform banded veins that contain less than 15 ppm Au, and yield ⁴⁰Ar/³⁹Ar plateau dates that range from 7.1 to 5.7 Ma. Taken together, these results indicate that bonanza-grade veins formed early in the lifetime of the Coromandel Volcanic Zone, and we speculate that narrow vein widths and bonanza grades may reflect a compressional environment. A 70° clockwise shift in the orientation of the subduction zone preceded the shift to andesite-rhyolite volcanism, and after a significant (c. 3 My) period of quiescence, epithermal deposits formed in extensional environments. Over 80 percent of the Au from the Hauraki Goldfield has been recovered from the southern goldfield, even though this mineralization interval represents less than ten percent of the volcanic history of the Coromandel Volcanic Zone. Although speculative, this may indicate that epithermal orebodies are most likely to form in extensional environments, and/or in areas of high heat flow that produce silicic volcanism.