Mesozoic mesothermal quartz-gold-scheelite lodes, Wakamarina, Marlborough,New Zealand

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 .