The mesothermal quartz-scheelite lodes of the Wakamarina valley are one of
twelve known mined or prospected gold-bearing areas in the Marlborough Schist.
Recorded production was 556 kg gold, a little silver, and c. 390 t scheelite.
134 m of the 1800 m long Golden Bar reef are accessible in the No 2 level of the
Golden Bar mine. The reef is a typical multi-banded mesothermal vein, mostly c.
1.75 m wide, enclosing narrow horses of schist country rock. Scheelite is
sparsely distributed, and is more common in small veinlets in the hanging wall
schist. The country rocks are textural zone I-1m schists of prehnite-pumpellyite
to pumpellyite-actinolite metamorphic facies. Their geochemistry suggests they
are part of the Caples Terrane, and of calc-alkaline volcanogenic arc origin.
The associated Wakamarina Quartzite is strongly recumbent folded, and of the
same metamorphic grade, with abundant pumpellyite, stilpnomelane, and garnet,
presumably spessartine. Although it has geochemical signatures of a deep sea
sediment, it is depleted in Cr and Cu, and greatly enhanced in K and Ti, when
compared with a standard chert. It also has trace element characteristics of a
calc-alkaline arc-related sediment. However, associated greenschist has
MORB-like incompatible element chemistry.
The Golden Bar reef lies within a steeply dipping, normal to transtensional
dip-slip fault adjacent to a shallow-dipping mylonite shear zone, which, at
least on the footwall side, is 040 m wide. The shear foliation has been severely
dragged down to nearly parallel the Golden Bar reef within 1 m of the quartz
lode. Preliminary structural analysis suggests that the shear zone is
extensional. X-ray diffraction analyses of the wall rocks show a progressive
depletion of amphibole, stilpnomelane, and pumpellyite towards the reef, and
enhanced illite/muscovite and Kfeldspar adjacent to the lode, along with
possible graphitic carbon within 10m. Two zones of intense quartz veining occur,
one adjacent to the hanging wall for 12 m and one from 40 m outwards. Quartz
vein systems in the country rock are complex and span from pre-schistosity to
post-mylonite. They include multiple generations of quartz, actinolite,
stilpnomelane, pumpellyite, albite, adularia, chlorites, illite/muscovite,
calcite, and scheelite. Metal analyses across the Golden Bar reef showed gold
values of 0.13.1 ppm, and low tungsten values, consistent with the observed
sporadic scheelite occurrences. As and Sb are at background levels, but enhanced
Cr (150 - 430 ppm) relative to the schists (30 - 41 ppm) suggests that the
source hydrothermal fluids passed through and leached mafic to ultramafic
rocks.