Structural controls on hydrothermal gold mineralisation above the Hyde-Macraes Shear Zone, Golden Bar pit, Macraes, east Otago

Jones, Philip Hamish.

The Golden Bar pit is part of the Macraes mine, the largest gold mine in New Zealand. The Macraes mine is mainly developed in the late metamorphic Hyde-Macraes Shear Zone. However, the Golden Bar pit was developed in a mineralised zone about 200 m structurally above the Hyde-Macraes Shear Zone. The host rocks are dominated by massive quartzofeldspathic schist (5-10 m layers) with minor intercalations (1-5 m scale) of micaceous schist. All the host schist has disseminated pyrite (micron to mm scale) from pervasive late metamorphic hydrothermal alteration. In addition, there has been more localised (metre scale) relatively intense hydrothermal alteration controlled by shears subparallel to foliation. Gold occurs in three types of mineralised rock: silicified schist, which dominates, fissile black sheared rock, and cross-cutting quartz veins. There are three principal structural zones in the deposit: (a) a strongly sheared micaceous and graphitic zone at the base of the structural sequence; (b) a weakly deformed schist zone cut by relatively flat-lying (S 20° dipping) quartz veins; and (c) a stack of imbricated schist pods and deformed silicified schist confined between two sub-parallel low-angle N (.ea: 30°) shears. These structural zones were juxtaposed during thrust deformation. The lower black shear zone has m-scale folding and overthrust packages, kink folding and foliation stacking with associated striations that imply west-directed thrusting. Striations and imbrication geometry between the upper two shears imply oblique thrusting that was directed towards the west. Post-mineralisation normal faults locally disrupt the mineralised zone. The Golden Bar pit mineralised zone has some of the structural and mineralogical characteristics of the main Hyde-Macraes Shear Zone, and may be genetically related.

vi, 84, [6] leaves : ill., maps ; 30 cm. + 3 charts.

2006Jones_P

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