New Zealand Journal of Geology and Geophysics abstracts
Gold mineralisation near the Main Divide, upper Wilberforce valley,
Southern Alps, New Zealand
J. A. BECKER
D. CRAW
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
T. HORTON
C. P. CHAMBERLAIN
Department of Earth Sciences
Dartmouth College
Hanover, NH 03755, USA
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 180-253deg.C, and arsenopyrite composition
(28.3-30.8 at.% As) suggest formation temperatures of 250-350deg.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 d18O ranging from 11.1 to
25.6[[perthousand]], and d13C ranging from -12.5 to
-1.1[[perthousand]]. 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.
Keywords gold; Southern Alps; hydrothermal system; fluid
inclusions; stable isotopes; Torlesse Terrane
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
199-215
0028-8306/00/4302-0199 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1733K); (scanned from paper original: notes about this process)
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