Abstract
The Golden Bar gold mine pit near Macraes, east Otago, was developed in Textural Zone (TZ) 3 of the Otago Schist, at least 100 m structurally up-section from the Hyde-Macraes Shear Zone. This study tests the hypothesis that the Golden Bar mineralised zone was an extension of the major Hyde-Macraes Shear Zone mineralisation system. The Golden Bar rocks have been extensively, but subtly, hydro-thermally altered. Principal hydrothermal alteration effects are replacement of titanite by rutile, replacement of epidote by siderite and kaolinite, and addition of pyrite. Mineralised (gold-bearing) rocks within this hydrothermal alteration zone are volumetrically minor and include micaceous schist with disseminated pyrite and arsenopyrite, silicified micaceous schist, and quartz veins. The structurally lowest mineralised rocks occur in a well-defined foliation-parallel zone of black sheared rocks with abundant hydrothermal graphite and sulfide minerals. The top of the mineralised zone is defined by a duplex shear zone, c. 10 m thick, containing abundant silicified and sulfide-bearing schist that has been deformed into an imbricate stack. Between the lower shear and the upper duplex zone, shallow-dipping thick (metre scale) mineralised extensional quartz veins cut across schist foliation and extend for up to 60 m laterally. The Golden Bar mineralised rocks were emplaced during contractional deformation and this resulted in WNW-directed thrusting that controlled the formation of ore-grade zones. The deformation and mineralisation style at Golden Bar is essentially identical to that of the Hyde-Macraes Shear Zone that structurally underlies the Golden Bar zone. This inferred relationship between the Hyde-Macraes Shear Zone and Golden Bar mineralisation extends the overall structural thickness of this gold-bearing deformation zone to >250 m.