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

Abstract

In the Wakamarina valley, Marlborough, ribbonbanded quartz lodes fill tensional faults in Caples Terranederived, 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 fades psammitic schists on the footwall from stilpnomelanepumpellyite-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 quartzscheelite ± 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 275-330°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.

Wakamarina Quartzite and associated mafic rocks of Pelorus Group, Marlborough: Geochemistry and origins

“…Analyses of 21 samples of Pelorus Group psammitic subschists and schists from the Wakamarina valley and the nearby Rai Valley (Skinner & Brathwaite 1998) confirm their compositional synonymy with Pelorus Group and Caples Terrane rocks as defined by Mortimer (1993). Ratios of incompatible high field strength (HFS) and transition metal elements (e.g., La/Sc-Ti/Zr; Fig.…”

“…Further evidence for this is found by a correlation coefficient (r) comparison between the major and trace element analyses of Wakamarina Quartzite (Skinner & Brathwaite 1998) and the chert chemical dataset of Murray et al (1992a, Adachi et al 1986); and (C) average chert from Costa Rica (after Hein et al 1983). …”

“…Seventeen metabasites including four metagabbros from Wakamarina and the Rai Valley have been analysed (Skinner & Brathwaite 1998). Whereas the HFS and incompatible elements, particularly Nb, La, Ce, Zr, Ti, Y and P, are well documented to be stable during very low grade to low grade metamorphism (Abdel-Rahman & Kumarapeli 1999 and references therein), the LIL elements are easily mobilised and typically have a wide range of concentrations with high standard deviations.…”

“…Two pelitic interbeds (50 and 61.4% SiO 2 anhydrous), two pelitic quartzites (73 and 75.2% SiO 2 anhydrous), and 16 quartzites (85.7-96.1% SiO 2 anhydrous) were analysed by XRF (Skinner & Brathwaite 1998).…”