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Abstract
The Rise & Shine Shear Zone is a c. 50 m thick zone of hydrothermally altered schist that has been sheared subparallel to the pervasive foliation during hydrothermal alteration. The shear zone is locally auriferous, and has some structural similarities to the Hyde‐Macraes Shear Zone that hosts the currently active Macraes mine. Shearing was accompanied by recrystallisation of metamorphic phengite to hydrothermal muscovite, alteration of metamorphic titanite to rutile, and partial decomposition of metamorphic albite and calcite. Metamorphic chlorite has been almost completely decomposed to form hydrothermal ankerite (ferroan dolomite and magnesian siderite), and this alteration reaction yielded minor quartz for silicification of sheared schist and localised formation of millimetre‐scale quartz veinlets. Hydrothermal ankerite occurs principally in fractures in quartz‐feldspar segregations adjacent to micaceous shears. Chalcopyrite and sphalerite accompany this ankerite, but there has been no Cu or Zn enrichment. Gold occurs in sheared schist that contains elevated As (up to 10000 ppm) in arsenopyrite. Hydrothermal arsenopyrite and pyrite have replaced some schist and have been controlled by foliation‐parallel shear development. Antimony enrichment (up to 8 ppm) mimics As enrichment in altered schist. There has been no Sr enrichment associated with ankeritic carbonation of the schist like that observed in some nearby Otago Schist auriferous vein systems.
The shear‐related alteration is postdated by an entirely different style of alteration that accompanied upright folding and vein formation. This later alteration stage was dominated by formation of hydrothermal albite, chlorite, and calcite. Hence, the later alteration reactions were essentially reversals of those of the earlier stage. Differences in alteration assemblages are a result of differences in fluid composition during alteration, but the reasons for this, and the exact fluid composition parameters, remain unconstrained.
