U-Pb geochronology and geochemistry of molybdenum-bearing granodiorite porphyry at Copperstain Creek, west Nelson, New Zealand

A molybdenite-mineralised granodiorite porphyry at Copperstain Creek has a U-Pb zircon age of 133.5 ± 0.2 Ma. This is older than the Early Cretaceous (126-105 Ma) Separation Point Suite, with which other molybdenum-bearing granodiorite porphyries in west Nelson have been correlated on the basis of similar compositions and K-Ar ages. The Copperstain Creek granodiorite differs compositionally from the high Na and Sr/Y rocks of the Separation Point Suite in having medium Na and low Sr/Y. Oxygen isotope δ 18 O magma values of ≤10‰ for the Copperstain Creek granodiorite and granites from the Separation Point Suite indicate that both are I-type granites. However, the Copperstain Creek granodiorite porphyry has a minimum δ 18 O magma value of 9.4‰, which indicates a magma with a crustal component, whereas the Separation Point Suite granites have values (5.4-7.9‰) which are very primitive ("mantle-like"). These new U-Pb age and oxygen isotope data indicate that the Copperstain Creek granodiorite is not related to the Separation Point Suite granitoids.