Abstract
Tungsten mineralisation within the Otago Schist is hosted by hydrothermal quartz veins that formed close to the brittle-ductile transition zone. The Otago Schist Belt hosts numerous orogenic gold deposits, including the Macraes mine, a number of which also host the tungsten ore mineral scheelite. Despite the strategic importance of tungsten, no commercial mining of scheelite in Otago has occurred since the 1960s, and the Macraes operation does not recover tungsten despite mining tungsten ore. This study examines the spatial and temporal relationships between gold and tungsten from a microscopic up to a deposit scale, focusing on the Macraes deposit and applying the findings to the wider Otago region. Understanding this relationship is essential for modelling and extracting tungsten in Otago, and in the exploration for new deposits.
Macraes’ tungsten is predominantly found within mineralised quartz veins, although a subordinate phase of disseminated scheelite and a remobilised phase are also observed. Ductile microstructures and the cross-cutting relationships observed within the veins suggest that the main phase of tungsten mineralisation occurred early in the development of the deposit. The style of tungsten mineralisation contrasts strongly with that of gold at Macraes, which is disseminated throughout the pervasively altered wall-rock, and which is contemporaneous with both brittle and ductile structures. This relationship is echoed throughout the deposits in Otago: tungsten mineralisation is found within veins that formed at depth within the crust while gold is found associated with both deep and shallower structures.
The extent of tungsten mineralisation was determined in a section of the Macraes mine using Portable XRF (pXRF). This approach identified a 200 m by 80 m pod of tungsten mineralisation coinciding with a jog in a mineralised shear, suggesting that mineralisation is associated with the opening of large-scale dilational sites.
Evidence from the Nd and Sr isotope signatures of the Macraes scheelites suggest that the source mineralisation at Macraes was the schists of the Torlesse Terrane. The data favours a model of a metamorphic source of mineralisation, in which dewatering of the schist under amphibolite facies conditions produces hydrothermal fluid, which leaches metals from the schists as it ascends through the crust.