New Zealand Journal of Geology and Geophysics abstracts
Structure and fluid migration in a late Cenozoic duplex system forming the
Main Divide in the central Southern Alps, New Zealand
SIMON C. COX*
DAVE CRAW
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
C. PAGE CHAMBERLAIN
Department of Earth Sciences
Dartmouth College
Hanover, NH 03755
U.S.A.
*Present address: Etheridge, Henley & Williams Pty Ltd., P.O.
Box 250, West Deakin, ACT 2600, Australia.
Abstract The Alpine Schist immediately west of the Main
Divide of the Southern Alps is a west-dipping duplex system consisting of an
imbricated stack of rock slabs, each c. 250-1000 m thick. The
imbricated stack has low grade, little deformed pumpellyite-actinolite facies
semischists at the base, overlain by progressively higher grade and more
deformed schists. The structurally highest slab mapped consists of
multiply-deformed biotite zone schist. The duplex lies on the hanging wall of
the northwest-dipping Main Divide Fault Zone, which separates semischist from
structurally underlying greywacke. Rock slabs are internally disrupted by
faults subparallel to layering, and consist of lozenge-shaped blocks of
10-100 m. Fault zones separating rock slabs consist of <50 cm
thick zones of weakly lithified cataclasite and soft gouge. Slickensides on
fault surfaces plunge northwest. The duplex formed during late Cenozoic rise of
the Southern Alps.
Four generations of postmetamorphic veins cut rock slabs of the duplex system.
The earliest veins contain quartz, chlorite, calcite, and sulphides and lie
subparallel to foliation in the higher grade slabs only. A distinctive set of
open-space-filling fissure veins in a lower greenschist facies slab contains
euhedral quartz, adularia, bladed calcite, and chlorite. The whole duplex
system is cut by steeply dipping shears and fractures containing quartz and
ankerite infilling and breccia cementation. Late stage iron
oxy-hydroxide-coated fractures, generally steeply dipping, cut all rock slabs.
Pyrite and, locally, arsenopyrite accompanies quartz and ankerite veins in
extensional sites of steeply dipping late stage fractures, and gold contents up
to 850 ppb occur. This gold mineralisation occurred within
c. 1 km of the surface and had epithermal affinities.
Hydrothermal fluids mineralising the various fractures range in temperature
between 350deg.C and near-ambient temperatures. The quartz-adularia-calcite
veins formed at 0.5-2.5 km depth before duplex formation, and later veins
formed at shallower levels after duplex formation. Vein carbonates have
d18O(SMOW) between 11.4 and 26.1[[perthousand]], and
d13C(PDB) between -0.6 and -11.4[[perthousand]]. Calculated isotopic
ratios for the mineralising fluids show little evidence for meteoric origin.
The fluid may be meteoric water which has undergone isotopic exchange with host
rocks, metamorphic water, or a mixture of these, and mineralisation occurred
during rise of the fluids from depth.
Keywords Southern Alps; structure; duplex; veins; fluid
inclusions; isotopes; gold
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 359-373
0028-8306/97/4003-0359 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (4343K); (scanned from paper original: notes about this process)
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