Abstract

The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event.

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