Detrital zircon age studies of Haast Schist in western Otago and Marlborough, New Zealand: constraints on their protolith age, terrane ancestry and Au–W mineralisation

Abstract

Detrital zircon U–Pb ages are reported from the Haast Schist in western Otago and Marlborough, South Island, New Zealand. The majority are from the Aspiring Lithological Association in Otago, which intervenes between Caples Terrane and Rakaia Terrane protoliths. These indicate provenances similar to those in established Jurassic and Late Triassic sandstones of the Waipapa Composite Terrane in the North Island. The populations are dominated (>60%) by Early Jurassic, Triassic and Permian zircons with only small proportions of pre-Permian zircons. In contrast, schists east of the Aspiring Lithological Association have substantial proportions (>20%) of early Paleozoic–Precambrian zircons and are thus assigned to the Triassic–Permian, Rakaia Terrane (Torlesse Composite Terrane), which is extensive across eastern Otago. However, three small areas of anomalous schist, with dominantly (80%) Carboniferous zircons, are considered the oldest level of the Rakaia Terrane protolith. Similar datasets from western Marlborough Schist confirm the correlation with western Otago Schist and are also assigned to the Waipapa Composite Terrane. The new Haast Schist datasets support a suspect terrane model in which both Rakaia and Waipapa Composite terrane protoliths have sediment provenances in eastern Australia and Au–W sources at their subsequent mineralisation sites. KEY POINTS Original protoliths are recognisable within the Haast Schist of New Zealand on the basis of distinctive detrital zircon age patterns. The Aspiring Lithological Association (Otago Schist) of western Otago, is correlated with a similar belt of schistose rocks (Marlborough Schist) in western Marlborough, and both are assigned to a Waipapa Composite Terrane protolith of Jurassic to Late Triassic age. The Jurassic-Late Triassic sandstones within Waipapa Composite Terrane are dominantly of granitoid provenance in eastern Australia. Detrital zircons of Carboniferous age in the Waipapa Composite Terrane support a possible New England Orogen provenance in northeastern NSW and southeastern Queensland, and also indicate possible primary sources for the gold and tungsten in the western Otago Schist and western Marlborough Schist. This result is in marked contrast to eastern Otago Schist where the protolith is Rakaia Terrane of Late Triassic age and whose provenance is in central-eastern Queensland.