Metamorphogenic Au‐W veins and regional tectonics: Mineralisation throughout the uplift history of the Haast Schist, New Zealand

Abstract

Gold‐scheelite (Au‐W) vein mineralisation in the Mesozoic Haast Schist (Otago, Alpine, and Marlborough Schist belts) resulted from metamorphic dewatering and fluid channelling periodically throughout the >100 Ma tectonic history of the schist belt. The earliest metal mobilisation occurred during synmetamorphic vein formation and was both accompanied and succeeded by ductile deformation. Structural style of deformation within vein zones became progressively more brittle as mineralisation accompanied Mesozoic‐Tertiary isostatic and extension‐related uplift of the Otago Schist. Rapid tectonic uplift in the Late Cenozoic resulted in mineralisation in the narrow belt of Alpine Schist along the Alpine Fault. The wide range of structural styles of vein systems seen in the Haast Schist represents different erosion levels exposed in tectonic zones. Geothermal gradients during Otago Schist mineralisation appear to have been close to normal, whereas mineralisation in the Southern Alps took place under anomalously high gradients resulting from rapid uplift. Metals and fluid were mainly derived from the Aspiring Terrane which underlies much of the exposed Haast Schist collision zone. Although Au is enriched in sulphide‐rich layers in metavolcanic horizons, and W is enriched in cherts, neither of these enriched rock types is volumetrically significant in the Aspiring Terrane. Hence, they are unlikely to provide any more than local sources for Au‐W veins. Au and W were derived by leaching of trace amounts from all rock types. The chlorite‐rich Aspiring Terrane had the potential to generate the large volumes of fluid required for metal leaching.