SULFUR ISOTOPES IN A METAMORPHOGENIC GOLD DEPOSIT, MACRAES MINE, OTAGO SCHIST, NEW-ZEALAND

The Macraes gold quartz vein deposit, New Zealand, is located in a she ar zone cutting greenschist facies metasediments of the Otago Schist. The deposit has been interpreted as being metamorphogenic in origin as there is no evidence for coeval magmatic activity in the Otago Schist orogen. The immediate host rock at the Macraes deposit is pyritic (de lta(34)S = -2.7 to -1.3 parts per thousand) and locally weakly graphit ic schist, a rare rock type in the Otago Schist. Sulphur isotope analy ses of pyrite and arsenopyrite extracts from auriferous veins and wall -rock schist provide a similar narrow range in delta(34)S values, from -3.0 to -1.0 parts per thousand (n = 9). The lack of isotopically dep leted sulphur, the narrow range in values, and the replacement sulphid e textures help discount a primary bacteriogenic origin for host-rock sulphide. Sulphide in both veins and wall rock is of hydrothermal orig in. Sulphides in metasediments and metavolcanics elsewhere in the Otag o Schist have delta(34)S in the narrow range -6 to +6 parts per thousa nd. The sulphur isotope data are consistent with origin of hydrotherma l sulphur within the metamorphic pile but provide no constraint on spe cific rock types which contributed the sulphur to the hydrothermal flu id. The study demonstrates that sulphur isotopic signatures near zero per mil can arise without any direct magmatic input into the mineralis ation process.