Mo‐bearing granodiorite porphyry plutons of the Early Cretaceous Separation Point Suite, west Nelson, New Zealand

Abstract

Abstract In west Nelson a 130 km long belt containing granodiorite stocks with associated subeconomic stockwork Mo mineralisation is correlated with the I‐type Separation Point Suite and Western Fiordland Orthogneiss, both of Early Cretaceous age. Trace element characteristics of relatively unaltered samples of these Mo‐granite stocks, and the Separation Point Batholith, indicate an arc setting for both groups of rocks, which predominantly represent partial melts of plagioclase‐free oceanic slab of basaltic composition. The Separation Point Suite (including the Mo‐granites) and Western Fiordland Orthogneiss suite chemistry is alkali‐calcic due to high Na. However, high Na, Al, Sr (c. 1000 ppm at 70% SiO2) and TiO2, and low K2O, Rb, Nb, and F, suggests comparison with granitoids similar to those that host porphyry copper deposits rather than alkaline, Climax‐type igneous rocks, indicating grades of Mo mineralisation are likely to be low. The granodiorite plutons are associated with voluminous diorite‐granite magmatism that occurred close to the end of long‐lived (Permian to mid‐Cretaceous) subduction in the New Zealand segment of the Gondwana margin.