Geochemistry of late metamorphic hydrothermal alteration and graphitisation of host rock, Macraes gold mine, Otago Schist, New Zealand

The Macraes gold deposit in the Otago Schist, New Zealand, formed during late metamorphic fluid flow through a lower greenschist facies shear zone. Mineralisation occurred near to the brittle-ductile transition at about 300 °C. Large volumes of host rock in a shear zone up to 120 m thick have been hydrothermally altered by this fluid activity. Most alteration is not structurally controlled apart from proximity to the shear zone. Ductile and brittle microshears traverse the most mineralised rocks and some structural control of fluid flow occurred as well. Fluid flow was slow, similar to that in metamorphic rocks (mm/year) and diffusion through interconnected fluid was a significant chemical process. Localised extensional hydrofractures (m scale) are filled with mineralised quartz. Most alteration of the host rocks was isochemical with respect to the lithophile elements, and mineralised rocks have been variably enriched in As, Au, Sb, W, Mo and Bi, but not Co or Cd. Addition of sulphur has occurred to both host rocks and mineralised rocks, up to 1 wt.% above a background of 0.1 wt.%. Host rock sulphur is mainly pyritic and is not structurally controlled. Mineralised rocks have pyrite and arsenopyrite along microshears. Pyrite, chalcopyrite, sphalerite and galena have formed from sulphidation of silicates with no addition of metals. Graphite has been added to mineralised rocks along microshears, up to 3 wt.% locally, above a background of 0.1 wt.% noncarbonate carbon. Graphite deposition may have occurred as a result of mixing of two fluids, water+methane, and water+carbon dioxide. Graphitisation and sulphidation reactions released low δD water, which accumulated in the slow-moving mineralising fluid. Distinction between this low δD reaction water and meteoric water incursion is difficult.