K-AR AND RB-SR AGE STUDIES OF THE METAMORPHISM AND QUARTZ VEIN AU MINERALIZATION ON TERAWHITI HILL, NEAR WELLINGTON, NEW-ZEALAND

K-Ar and Rb-Sr ages have been determined for low-grade Torlesse Terrane metagreywacke sequences (Wellington Greywackes) at Terawhiti Hill near Wellington and the more schistose rocks in a narrow sheared zone containing Au-bearing quartz-reefs. A Rb-Sr whole-rock isochron age of 200+/-7 Ma provides the best age estimate (late Triassic or earliest Jurassic) for regional metamorphism of the Wellington Greywackes, and their initial Sr-87/Sr-86 ratio at this time, 0.70690+/-0.00009 is characteristic of the extensive Torlesse Terrane elsewhere, that is derived from a silicic-continental source. Both K-Ar and Rb-Sr ages indicate a local but intense shearing event at Terawhiti Hill, at 125-130 Ma (early Cretaceous), after earlier metamorphism and uplift of the greywackes and possibly during imbrication of Torlesse rocks on a regional scale. Formation of quartz veins and some deposition of Au in schists and cataclastic rocks is possibly synchronous with this event but Au is more importantly associated with isolated and rare later quartz-reefs formed at 100-110 Ma. From Sr isotope evidence the earlier quartz veins have a local in situ origin but the younger veins have an extraneous, possibly deeper source.