Gold mineralisation near the Main Divide, upper Wilberforce valley, Southern Alps, New Zealand

Veins up to 8 m wide fill extensional fractures in Torlesse Terrane metased iments near the Main Divide in the upper Wilberforce valley, Canterbury, Ne w Zealand. The upper Wilberforce veins are part of a prominent 40 km long, NNE-trending swarm of gold-bearing veins formed across the Main Divide duri ng the late Cenozoic rise of the Southern Alps. The veins occur within, and near, a prominent set of faults which constitute the Main Divide Fault Zon e. The veins are irregular in shape due to contrasting host rock properties , and have been only weakly sheared and deformed. Veins cut across greywack e beds and follow irregularly along argillite beds, on the 1-10 m scale. Qu artz dominates vein mineralogy, but albite forms up to 45% of some veins, a nd minor chlorite, pyrite, arsenopyrite, chalcopyrite, and gold occur spora dically, especially in breccias near vein margins. Fluid inclusions in vein quartz homogenise at 180-253 degrees C, and arsenopyrite composition (28.3 -30.8 at.%As) suggest formation temperatures of 250-350 degrees C. elevated arsenic levels (up to 200 ppm above a background of 10 ppm) in some host g reywackes and argillites suggest that hydrothermal activity pervaded host r ocks as well as forming veins, but there is no textural evidence for this f luid flow. Late-stage carbonates in faults adjacent to the quartz veins, bu t which postdate the quarts veins, have delta(18)O ranging from 11.1 to 25. 6 parts per thousand, and delta(13)C ranging from -12.5 to -1.1 parts per t housand. These carbonates were deposited by a mixture of meteoric and crust ally isotopically exchanged fluid as a shallow-level manifestation of the s ame hydrothermal system which deposited the quartz veins. The upper Wilberforce veins structurally and mineralogically resemble some late Cenozoic gold-bearing vein systems in the Mt Cook area, 100 km to the southwest along the Southern Alps.