![Sample our Earth Sciences journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5930/TOC-Subject-Sample-2021-Earth-Sciences.jpg"})
Abstract
Veins up to 8 m wide fill extensional fractures in Torlesse Terrane metasediments near the Main Divide in the upper Wilberforce valley, Canterbury, New 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 during 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 Zone. The veins are irregular in shape due to contrasting host rock properties, and have been only weakly sheared and deformed. Veins cut across greywacke beds and follow irregularly along argillite beds, on the 1–10 m scale. Quartz dominates vein mineralogy, but albite forms up to 45% of some veins, and minor chlorite, pyrite, arsenopyrite, chalcopyrite, and gold occur sporadically, especially in breccias near vein margins. Fluid inclusions in vein quartz homogenise at 180–253°C, and arsenopyrite composition (28.3–30.8 at.% As) suggest formation temperatures of 250–350°C. Elevated arsenic levels (up to 200 ppm above a background of 10 ppm) in some host greywackes and argillites suggest that hydrothermal activity pervaded host rocks as well as forming veins, but there is no textural evidence for this fluid flow. Late‐stage carbonates in faults adjacent to the quartz veins, but which postdate the quartz veins, have δ18O ranging from 11.1 to 25.6‰, and δ13C ranging from ‐12.5 to ‐1.1‰. These carbonates were deposited by a mixture of meteoric and crustally isotopically exchanged fluid as a shallow‐level manifestation of the same hydrothermal system which deposited the quartz veins.
The upper Wilberforce veins structurally and mineral‐ogically resemble some late Cenozoic gold‐bearing vein systems in the Mt Cook area, 100 km to the southwest along the Southern Alps.
