Invincible Gold Mine, Rees Valley, N.W. Otago, New Zealand

Author:

Hay, Richard M. (Richard Mark), 1966-

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Abstract:

Wakatipu, N.W. Otago, N.Z. The Invincible Au bearing lode occupies a near vertical fault (~060/80E). The Invincible lode fault (I.L.F.) crosscuts the regional west dipping (~50°) schistosity on the eastern limb of the Earnslaw Synform. The host schists are metamorphosed to lowest greenschist fades (chlorite zone, Textural Zone III) and are thought to be part of the metavolcanic-rich Aspiring terrane.
Fault structure suggests that the I.L.F. was initially a west dipping normal fault, at the brittle/ ductile transition zone, before the formation of the Earnslaw Synform in late Oligocene times and most likely in the mid-late Cretaceous. Faulting style later changed to high angle reverse, before the commencement of the Earnslaw Synform formation, whilst still within the brittle/ ductile transition zone. Deformation changed from brittle/ ductile to purely brittle with regional uplift. Rotation of the I.L.F. on the eastern limb of the Earnslaw Synform was associated with purely brittle high angle reverse faulting. Textural evidence of brittle/ ductile deformation in the lode suggests that the mineralisation event occurred whilst the I.L.F. was a normal fault, at the brittle/ ductile transition zone.
Native Au commonly occurs as blebs within quartz. Occurrences of Fe-sulphides ± Au are also common. Au and As are the elements indicative of mineralisation and are thought to have been introduced by the mineralising fluid. Unusually low amounts of scheelite, for an Otago Schist mesothermal deposit in a scheelite rich region, is caused by the relatively low CaO content of the host schists.
Hydrothermal alteration extends for <<1m from the lode, and is characterised by the presence of sulphides. There is no other mineralogical alteration, except the recrystallisation of chlorite. Significant Cr enrichments in the altered schist suggests that the mineralising fluids passed through, or were generated within, Cr-rich rocks such as metavolcanic greenschists of the Aspiring terrane.
The common presence of the metamorphic assemblage quartz/ albite/ muscovite/ chlorite in lode vein material suggests that the prevailing mineralising conditions were similar to the metamorphic conditions that formed the host schist, and that the mineralising fluids were in equilibrium with most host rock minerals. Au transport is thought to have been due to a high fS2 in the mineralising fluid, which was out of equilibrium with the host rock. Fe-sulphides precipitated in the lode zone, which lowered the fS2 in the fluid leading to Au precipitation.
Arsenopyrite geothermometry, combined with fluid inclusion homogenisation temperature and density determinations, resulted in an estimate of 325-375°C and 2.5-4.1 kbars for the formation conditions of the I.L.F. Fluid inclusion studies indicate a mineralising fluid of very low apparent salinity (~l wt% equivalent NaCl).
The Invincible C and O fluid isotopic composition is consistent with a metamorphic source for the mineralising fluid. The fluid isotopic composition is relatively δ13C enriched, due to the relatively low graphite content of the host schist. The δ18O fluid composition suggests that meteoric water was either not involved in mineralisation, or that water/rock interaction has eliminated the meteoric signature.
Invincible and nearby Glenorchy vein systems have different structural styles of deformation and are present at the same erosional level, suggesting that they are of different generations. The Invincible and Macraes lodes have many similarities and most differences can be explained by differences in host rock chemistry. More extensive ductile deformation at Invincible suggests that Invincible was the earliest formed deposit in the Otago Schist.

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Thesis description:

153 p., [3] folded leaves : col. ill., maps ; 30 cm.

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1991Hay

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Location (WKT, WGS84):

POLYGON ((168.429918499477537 -44.738845776927832,168.433071468057932 -44.690383289074227,168.503010979147547 -44.692688043371838,168.497843300004462 -44.742025660596362,168.429918499477537 -44.738845776927832))

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http://download.otagogeology.org.nz/temp/Abstracts/1991Hay.pdf

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Citation

Hay, Richard M. (Richard Mark), 1966-, “Invincible Gold Mine, Rees Valley, N.W. Otago, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/253.

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