NEAR-SURFACE SECONDARY GOLD MOBILITY AND GRAIN-SIZE ENHANCEMENT, BAREWOOD MINE, EAST OTAGO, NEW-ZEALAND

Primary gold at Barewood, east Otago, New Zealand, occurs in quartz veins in the Otago Schist. The gold typically occurs as 1-3-mu-m grains enclosed in arsenopyrite, pyrite, or chalcopyrite. Secondary gold mobility occurred during formation of a zone of kaolinitisation (up to 20 m deep) in the Otago Schist immediately beneath an early Tertiary unconformity. Oxidation of the sulphides released the gold and allowed local migration, possibly as thiosulphate or reduced sulphur complexes. Silver was slightly more soluble than gold, and secondary gold has higher atomic fineness (930) than primary gold (870). Secondary gold is coarse grained (up to 1 mm), and was deposited in fractures, shears, and cavities in quartz veins and associated sheared schist. The secondary gold was precipitated with goethite and kaolinite, and locally with scorodite and/or pharmacosiderite. Dissolution, migration, and redeposition of gold is due to fluid composition evolution from local moderately reduced, sulphur rich, acid conditions at the site of decomposition of sulphides, to highly oxidised neutral pH conditions which prevailed elsewhere. There is no evidence for enrichment of gold in the alteration zone. Grain-size enhancement of gold by secondary migration near the unconformity was important for formation of alluvial gold deposits in Otago when the unconformity was uplifted and eroded in the late Tertiary.