Oxygen isotopic evidence for the origin and evolution of a scheelite ore-forming fluid, Glenorchy, New Zealand

Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschistmetapelite-metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8-10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed during Mesozoic deformation was deformed into upright, open, kilometrescale folds, with some foliation-parallel boudinage, associated with the Miocene inception of the Alpine Fault. The resultant north to northeast trending folds plunge gently southwards. Deformation associated with Alpine Fault movement was only minor in the Alpine Schist greenschist facies and resulted in passive differential uplift of deeper parts of the schist pile in the west.

Section: Upper Balfourmentioning

confidence: 81%

Structures within greenschist facies Alpine Schist, central Southern Alps, New Zealand

Craw

Rattenbury

Johnstone

1994

“…In the past, isotopic studies have been carried out on a regional scale (Devereux 1968;Paterson 1982;Wickham & Taylor 1987), with a focus on changes in whole rock and vein isotope values between outcrops. As such, these studies provide little insight into the mechanisms that produce the observed changes in isotopic signature.…”

Section: Introductionmentioning

confidence: 99%

Penetration of deformation‐driven meteoric water into ductile rocks: Isotopic and model observations from the Southern Alps, New Zealand

Upton

Koons

Chamberlain

1995

A stable isotope laser fluorination probe was used to examine oxygen isotope zoning within foliationparallel quartz veins of two Alpine Fault mylonite samples and two Alpine Schist samples, all of amphibolite facies. The technique used has a spatial resolution of c. 1 mm, and we found variation of δ 18 O values within individual samples and between schist and mylonites. δ18 O values from individual mylonite quartz veins showed a slight variation of up to 1‰. By contrast, there was up to 2.4‰ variation within veins from outside the fault zone.Equilibrium fluid δ 18 O values were calculated for aqueous fluids from which the quartz precipitated. The calculated values range from 5 to 13‰. They suggest the involvement of a near-surface fluid just beneath the brittleductile transition which is depleted in 18 O relative to the fluid calculated to be in equilibrium with the local metamorphic assemblage.The geochemical evidence of penetration of fluids depleted in 18 O into ductile rocks constrains models of the mechanics of fluid flow. Ductile rocks are impermeable to fluids driven solely by thermal and topographic gradients. Simple numerical experiments illustrate the need for coupling of deformation and fluid flow to allow access of meteoric water into ductile material.

“…The Wakamarina lodes most closely resemble those of the Glenorchy field of northwest Otago: (1) in being hosted by a normal fault system within Caples Terrane schists, (2) by the presence of scheelite with minor gold, and (3) by comparable structure of the lodes (Paterson 1982(Paterson , 1986. The host schists at Glenorchy are at a higher metamorphic grade of t.z.…”

Section: Lode Structure and Pt Conditionsmentioning

confidence: 99%

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

Skinner

Brathwaite

1999

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.