Gold and scheelite mineralization at Round Hill, eastern Otago, New Zealand

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McKeag, SA

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Abstract:

The Round Hill Deposit is a major gold and tungsten resource in eastern Otago. The deposit formed in and is deformed by a complex shear zone which strikes northwesterly and dips shallowly to the northeast. The ore 'shoot' is developed nearly perpendicular to both the strike and the inferred direction of movement along the shear. Gold, tungsten and minor antimony occur in epigenetic reef complexes consisting of quartz reefs and silicified replacement bodies. The ore bodies are developed in semi-ductile flexures, folds and breccias within the shear zone. The mineralization is hosted by a metamorphosed turbidite sequence of Torlesse affinity. The host lithologies are quartzo-feldspathic schists of greenschist facies chlorite textural zone 4, of the Haast Schist.
Fluid inclusion homogenization temperatures used in conjunction with an arsenopyrite geothermometer indicate that mineralization occurred at 325° to 375°C and 3±1 kbars. Freezing point measurements of fluid inclusions indicate that near pure aqueous fluids possibly containing small quantities of CO2 are responsible for mineralization. Gold mineralization occurs as blebs of native gold in pyrite and arsenopyrite within concordant replacement bodies. Within the quartz reefs, gold is commonly found as the native element unassociated with sulphides. Tungsten occurs as scheelite in small discordant 'pull apart' veins and fracture films which have developed within the more psammitic lithologies during a later, more brittle deformation regime. A spatial and morphologic separation exists between the gold and scheelite mineralizations suggesting that two separate fluids or an evolving fluid are responsible for mineralization at Round Hill. Potassic alteration is associated with the auriferous replacement bodies whilst carbonate alteration is associated scheelite mineralization. Carbonate isotopes suggest a metamorphic fluid source for mineralization. REE data from altered and mineralized zones are inconsistent with the igneous complexes in the Haast Schist. Gold deposition is believed to have involved a complex series of redox reactions between the ore-bearing fluids and the graphitic wall rocks. Scheelite deposition probably occurred as the result of a decrease in temperature.
Other gold occurrences in eastern Otago containing stibnite, scheelite and/or base metal sulphides are developed in steeply dipping faults or fractures. Fluid inclusion studies of these deposits indicate lower temperatures and higher salinities than at Round Hill and provide evidence of H2O-CO2 immiscibility. These steeply dipping deposits are believed to have formed from metamorphic fluids similar to, but more evolved than, those at Round Hill. These late fluids have undergone large chemical changes due to reaction progress and unmixing.

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xii., 190 p., Maps (folded in pocket).Ill., 30 cm.

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1987McKeag

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POLYGON ((170.369451499159538 -45.395087654174347,170.375199963391168 -45.303764783617822,170.487931695333288 -45.307373266884525,170.483963017433922 -45.396954705116713,170.369451499159538 -45.395087654174347))

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http://download.otagogeology.org.nz/temp/Abstracts/1987McKeag.pdf

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Citation

McKeag, SA, “Gold and scheelite mineralization at Round Hill, eastern Otago, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/215.

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