Origin and deposition of a graphitic schist-hosted metamorphogenic Au-W deposit, Macraes, East Otago, New Zealand

McKeag, S. A.; Craw, Dave; Norris, R. J.

doi:10.1007/bf00206316

Cited by 74 publications

(27 citation statements)

References 24 publications

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“…Uplift and resultant tensional fracturing would have enabled the release of trapped fluids such as those derived from the breakdown of the hydrous mineral phases chlorite and micas during transition to higher grade metamorphic facies at depth. Such a derivation of the hydrothermal fluids in Otago from underlying higher grade schists has been argued by several authors (e.g., Henley et al 1976;McKeag et al 1989;Craw & Norris 1991;Craw 1992). However, the similar oxygen isotope values obtained for the quartz lodes and the country rocks simply show that the hydrothermal fluids equilibrated with the host schists, and cannot distinguish other fluid sources such as deeply circulating meteoric water.…”

Section: Ages Of Metamorphism Mylonite and Hydrothermal Alterationmentioning

confidence: 73%

“…A distinct Cr enrichment in and about the Otago lode> (McKeag et al 1989;Ashley & Craw 1995), has been related to leaching of Cr from the underlying metabasite-rich chloritic schists (Craw & Norris 1991 ;Craw 1992). Although the schist country rocks at Wakamarina are not enriched in Cr, the lodes themselves are enriched close to Cr-depleteo metabasites in the hanging wall, and metabasites with up to 411^70 ppm Cr are not uncommon in the Marlborough Schist (Johnston 1994;Skinner & Brathwaite 1998) Therefore, potential sources for the Cr-enrichment at Wakamarina are metabasites in underlying schist-hosted fluid reservoirs as well as from the Cr-leached hanging wall metabasites.…”

Section: Metal Deposition and Sourcesmentioning

confidence: 99%

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

Skinner

Brathwaite

1999

New Zealand Journal of Geology and Geophysics

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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.

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Section: Ages Of Metamorphism Mylonite and Hydrothermal Alterationmentioning

confidence: 73%

Section: Metal Deposition and Sourcesmentioning

confidence: 99%

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

Skinner

Brathwaite

1999

New Zealand Journal of Geology and Geophysics

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“…Sr is local!) depleted (Paterson & Rankin 1979) or decreases due to dilution (McKeag et al 1989) (Fig. 3).…”

Section: Comparison To Gold Deposits Elsewhere In the Torlesse Terranementioning

confidence: 97%

“…Fractionation factors are those summarised by Friedman & O'Neil (1977 Table 3). Otago Schist data ranges (hatched fields) derived in a similar way are presented foi comparison: Macraes and Glenorchy are deep-level late metamorphic deposits with a metamorphic fluid signature (McKeag et al 1989), and Shotover veins are shallow-level deposits . Meteoric water has 5…”

Section: Comparison To Gold Deposits Elsewhere In the Torlesse Terranementioning

confidence: 99%

“…Deposits have formed at various depth* during this long history, and therefore have some distinctive characteristics (Craw & Norris 1991). Deposits formed near to the brittle-ductile transition consist of quartz-calcite veins and typically formed from a fluid with very low salinity There is little geochemical signature, other than a distinctive Cr-enrichment anomaly at Macraes and Invincible deposits (McKeag et al 1989;Hay & Craw 1993). Sr is local!)…”

Section: Comparison To Gold Deposits Elsewhere In the Torlesse Terranementioning

confidence: 99%

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Tectonic hydrothermal gold mineralisation in the outboard zone of the Southern Alps, New Zealand

Smith

Craw

Koons

1996

New Zealand Journal of Geology and Geophysics

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Gold-bearing veins cut a belt of low-grade (pumpellyite-actinolite/greenschist facies) schist in the Ben Ohau Range to the east of the Main Divide, in the outboard zone of the Southern Alps continental collisional zone, New Zealand. The schist has been exposed along the currently active Ostler Fault system, which has had c. 5 km of reverse motion since the Pliocene. The veins consist of quartz, ankerite, calcite, chlorite, and pyrite, with minor chalcopyrite and galena. Hydrothermal chlorite contains about 490 ppm Zn, and the gold contains 3-5 wt% Ag. Hydrothermal alteration of host rock is minor apart from Sr enrichment (up to four times background). Fluid inclusions in quartz are aqueous with minor dissolved CO 2 and salts (<4 wt% NaCl equivalent), and homogenise at 236-270°C. The veins formed at 300 ± 20°C and 1000 ± 800 bars fluid pressure, probably under a hydrostatic fluid pressure regime. Oxygen and carbon isotopic data (δ 18 O = +10 to +14; δ 13 C = -6 to -10%o) are similar to data from economic metamorphogenic Au deposits of the nearby Otago Schist, but minor meteoric incursion may have occurred. Isotopic data are also similar to veins formed in the inboard zone of the Southern Alps orogen. The Ben Ohau veins demonstrate that gold can be concentrated in low-grade schists distant from the most active part of the hydrothermal system driven by continental collision.

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Structure and Lithology

Craw

MacKenzie

2016

SpringerBriefs in World Mineral Deposits

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Origin and deposition of a graphitic schist-hosted metamorphogenic Au-W deposit, Macraes, East Otago, New Zealand

Cited by 74 publications

References 24 publications

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

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

Tectonic hydrothermal gold mineralisation in the outboard zone of the Southern Alps, New Zealand

Structure and Lithology

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