Cretaceous tungsten-tin mineralisation in the Tin Range, Stewart Island, New Zealand

Abstract

The discovery of placer tin in southern Stewart Island caused New Zealand's only known tin-rush. The source of at least some of the placer tin is greisenised granodiorite and metasedimentary rocks on the crest of the Tin Range. There are three mineralised lithologies: a sericite-quartz greisen, a quartz-topaz greisen, and a gahnite-schist greisen. However, very little cassiterite (SnO2) is observed and instead wolframite ((Fe,Mn)WO4) and gahnite (ZnAl2O4) are common. U-Pb dating of host early Paleozoic Pegasus Group metasediments indicate the area experienced metamorphism at ∼117 Ma (monazite), with granodiorite emplacement at 107.9 ± 2.2 Ma (zircon). U-Pb isotope and trace element analysis of xenotime in the mineralised zone reveals three distinct growth phases: one at 107.5 ± 1.8 Ma that overlaps with granodiorite emplacement, and then 94.2 ± 0.9 Ma and 91.6 ± 0.6 Ma. Although it is not clear precisely which growth phase correlates with mineralisation, it must have been a mid-Cretaceous event and we correlate it with the ∼108 Ma Tin Range Granodiorite. The window of mineralisation, ∼ 108-92 Ma, means that this event was likely associated with the initiation of thinning of the Zealandia lithosphere as a precursor to separation from Australia and Antarctica.