In the Wakamarina valley Marlborough, ribbon-banded quartz lodes fill tensi
onal faults in Caples Terrane-derived, Marlborough Schist and Wakamarina Qu
artzite. The largest of the lodes, the Golden Bar, has a strike length of a
t least 1.8 km and an average width of 2 m. It fills a steeply dipping norm
al fault which separates pumpellyite-actinolite facies psammitic schists on
the footwall from stilpnomelane-pumpellyite-spessartine quartzite (Wakamar
ina Quartzite) and metabasites on the hanging wall. The lode consists of ba
nds of buck quartz separated by thin chlorite-sericite-rich laminae that ha
ve 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 th
e brittle-ductile transition zone, the fourth deformation created tensional
faults across the mylonites which localised deposition of the quartz-schee
lite +/- gold lodes (Golden Bar and Alfords). Internal lode structures indi
cate that internal pulsed fluid inflow and ductile vein deformation continu
ed during this deformation. The fifth deformation, again with minor quartz-
scheelite veining, created late brittle fracturing and compressional deform
ation of the lodes and country rocks.
Fluid P-T conditions are probably comparable to those in similar mesotherma
l quartz-scheelite lodes at Glenorchy in Otago (1.5-3.5 kbar and 275-330 de
grees C), but co-existing albite and calcic plagioclase in Alfords lode imp
lies that fluid temperatures at Wakamarina may have been locally higher. An
extensive zone of sericitic hydrothermal alteration characterised by seric
ite and K-feldspar (sericite K-Ar age of c. 140 Ma - earliest Cretaceous) a
nd loss of pumpellyite, stilpnomelane, and actinolitic amphibole surrounds
the Golden Bar lode. The potassic enrichment, the stability of albite and c
hlorite in both wall rocks and the lodes, and a low sulphide content indica
te 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
.