Punctuated Evolution of a Large Epithermal Province: The Hauraki Goldfield, New Zealand

Abstract

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 north-south-trending belt. These deposits have produced approximately 320,000 kg Au and 1.5 Mkg Ag and formed from Miocene to Pliocene subaerial hydrothermal systems. The goldfield has been divided into three provinces (northern, eastern, and southern), based on the host rocks and geologic setting of the deposits (Christie et al., 2007). In the northern province of the goldfield, adularia from Paritu yields a single 40Ar/39Ar plateau date of 16.32 ± 0.13 Ma, and adularia from Opitonui yields a preferred 40Ar/39Ar age of 13.15 ± 0.03 Ma. Two Re-Os dates of molybdenite from porphyry-style mineralization at Ohio Creek overlap within error and yield dates of 11.87 ± 0.06 and 11.97± 0.08 Ma; geologic relationships suggest that this is the likely age of mineralization in the nearby Thames epithermal deposits. In the eastern province, adularia from the Ohui deposit gives a preferred 40Ar/39Ar age of 8.29 ± 0.25 Ma, adularia from the Broken Hills deposit gives a preferred 40Ar/39Ar age of 7.12 ± 0.02 Ma, and adularia from the Wharekirauponga prospect yields a preferred 40Ar/39Ar age of 6.32 ± 0.12 Ma. In the southern province, adularia from quartz veins at the Maratoto deposit provide a preferred 40Ar/39Ar age of 6.41 ± 0.04 Ma, and adularia from a quartz vein at the Sovereign deposit yields a preferred 40Ar/39Ar age of 6.70 ± 0.16 Ma. Two dates from vein adularia at the world-class Martha deposit overlap within error, and we interpret a preferred age for the deposit of 6.16 ± 0.06 Ma. Two samples of molybdenite from veins in the Martha deposit yield discrete Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma. Adularia from one quartz vein from the Favona deposit yields a 40Ar/39Ar date of 6.05 ± 0.04 Ma. Host rock and vein adularia from the Karangahake deposit yield 40Ar/39Ar plateau dates that range from 6.90 ± 0.20 to 5.71 ± 0.13 Ma, which may reflect more than one stage of mineralization or protracted fluid flow. Adularia from veins at the Waiorongomai deposit yields a preferred 40Ar/39Ar age of 5.71 ± 0.03 Ma, and adularia from a vein at the Eliza deposit yields a preferred age of 4.47 ± 0.06 Ma. The southernmost deposit in the Hauraki goldfield, Muirs Reef, has adularia in quartz veins that yield 40Ar/39Ar plateaus dates of 2.12 ± 0.11 to 1.78 ± 0.16 Ma. Combined with previous work, these results indicate that mineralization in the Hauraki goldfield ranges from 16.3 Ma in the north to 2 Ma in the south, and clusters into two distinct groups that correlate with location, volcanic stratigraphy, and mineralization style. The first group, from ~16.3 to ~10.8 Ma contains epithermal veins, including bonanza-style veins, and porphyry-style mineralization that formed in the northern province in an arc that was dominated by andesitic volcanism. The second period of mineralization occurs primarily from 6.9 to 6.0 Ma in the eastern and southern provinces, when precious metals were deposited into thicker colloform-crustiform banded veins that formed in extensional settings in an arc that was erupting bimodal andesite-rhyolite compositions. Therefore, even though volcanism in the Coromandel volcanic zone was active from 18 to 2 Ma, Au-Ag mineralization was focused into two discrete periods of this arc formation, and the style of mineralization changed through time, coinciding with a change in style of volcanism. In addition, while Hauraki goldfield mineralization discontinuously lasted more than 11 m.y., greater than 80 percent of the known gold endowment was deposited in a relatively brief 0.9 Ma window between 6.0 and 6.9 Ma. These changes through time likely reflect, at least in part, reorganization of the Miocene Northland and Colville volcanic arcs in the New Zealand region of the southwest Pacific.