New Zealand Journal of Geology and Geophysics abstracts
Mesozoic mesothermal quartz-gold-scheelite lodes, Wakamarina, Marlborough, New
Zealand
D. N. B. SKINNER1,2
R. L. BRATHWAITE1
1Institute of Geological & Nuclear Sciences Ltd
2GeoGnostics
P.O. Box 30 368
Lower Hutt, New Zealand
email: d.skinner@gns.cri.nz
Abstract In the Wakamarina valley, Marlborough, ribbon-banded
quartz lodes fill tensional faults in Caples Terrane-derived, Marlborough
Schist and Wakamarina Quartzite. The largest of the lodes, the Golden Bar, has
a strike length of at least 1.8 km and an average width of 2 m. It fills a
steeply dipping normal fault which separates pumpellyite-actinolite facies
psammitic schists on the footwall from stilpnomelane-pumpellyite-spessartine
quartzite (Wakamarina Quartzite) and metabasites on the hanging wall. The lode
consists of bands of buck quartz separated by thin chlorite-sericite-rich
laminae that have been split off the wall-rock schist.
Five deformation events are recognised. The first two are linked with early
Jurassic metamorphism, the younger event producing the earliest scheelite,
whereas in early Mid Jurassic (illite/sericite K-Ar age of c. 175 Ma), the
third event produced late metamorphic transtensional mylonites with minor
quartz-scheelite veining. As uplift of the schist belt continued through the
brittle-ductile transition zone, the fourth deformation created tensional
faults across the mylonites which localised deposition of the
quartz-scheelite +/- gold lodes (Golden Bar and Alfords). Internal lode
structures indicate that internal pulsed fluid inflow and ductile vein
deformation continued during this deformation. The fifth deformation, again
with minor quartz-scheelite veining, created late brittle fracturing and
compressional deformation of the lodes and country rocks.
Fluid P-T conditions are probably comparable to those in similar mesothermal
quartz-scheelite lodes at Glenorchy in Otago (1.5-3.5 kbar and 275-330deg.C),
but co-existing albite and calcic plagioclase in Alfords lode implies that
fluid temperatures at Wakamarina may have been locally higher. An extensive
zone of sericitic hydrothermal alteration characterised by sericite and
K-feldspar (sericite K-Ar age of c. 140 Ma -- earliest Cretaceous) and loss of
pumpellyite, stilpnomelane, and actinolitic amphibole surrounds the Golden Bar
lode. The potassic enrichment, the stability of albite and chlorite in both
wall rocks and the lodes, and a low sulphide content indicate that the fluids
at Wakamarina had a higher aK+/aH+ than in
the Otago lodes, and a more neutral pH with a low capacity to carry gold in
solution.
Keywords Marlborough Schist; Wakamarina Quartzite;
metabasites; mesothermal quartz lodes; Golden Bar lode; Alfords lode; gold;
scheelite; metamorphism; mylonite; geochemistry; hydrothermal alteration; K-Ar
dating
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
335-348
0028-8306/99/4203-0335 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (4440K); (scanned from paper original: notes about this process)
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