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

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 δ 18 O ranging from 11.1 to 25.6‰, and δ 13 C 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 mineralogically resemble some late Cenozoic gold-bearing vein systems in the Mt Cook area, 100 km to the southwest along the Southern Alps.

Surface run‐off from mineralised road aggregate, Puhipuhi, Northland, New Zealand

“…The chemistry of these greywackes is almost indistinguishable from the greywacke of the northern North Island (Mortimer 1995). Arsenic contents of these greywackes are typically between 4 and 10 ppm with some outliers up to 30 ppm (XRF analyses; Becker et al 2000). Cu and Pb are generally 20 + 10 ppm and Zn is typically 50 + 30 ppm (XRF analyses; Becker et al 2000).…”

“…Arsenic contents of these greywackes are typically between 4 and 10 ppm with some outliers up to 30 ppm (XRF analyses; Becker et al 2000). Cu and Pb are generally 20 + 10 ppm and Zn is typically 50 + 30 ppm (XRF analyses; Becker et al 2000). Few Hg analyses have been done on South Island greywackes, so a set of analyses for recoverable Hg and As were obtained for this study.…”

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

“…The Fiddes vein system (Table 1; Fig. 1) occurs near the Main Divide, with numerous other similar veins, in greywacke host rocks that have been extensively disrupted by the active Main Divide Fault Zone (Becker et al 2000). Veins have little alteration on their margins other than minor silicification at the centimetre scale and minor brecciation near vein margins.…”

“…The Alpine Schist, however, has been variably overprinted by younger deformation and dynamothermal metamorphism that postdates the youngest metamorphism (c. 110 Ma) that affected Becker et al (2000) the Otago Schist. The nature and timing of these younger metamorphic events are not well understood.…”

“…Post-Miocene uplift along the Alpine Fault has stacked steeply dipping metamorphic zones in the Alpine Schist between the Main Divide and the fault. The auriferous vein swarm cuts across these stacked metamorphic zones, from greywacke in the south to biotite zone schist in the north (Becker et al 2000). The veins fill extensional sites in younger fault-related fractures, possibly associated with superimposition of Hope Fault structures on the older Alpine Fault structures ).…”