Gold, tungsten, and antimony mineralisation at Barewood, east Otago, New Zealand

Author:

MacKenzie, Douglas James.

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Abstract:

The historic Barewood Mining Area comprises an important gold and tungsten deposit in east Otago, New Zealand. Numerous gold and scheelite-bearing quartz vein systems are hosted within several well defined subparallel normal faults which crosscut the regional flatlying schistosity at approximately 50°-70°. The faults are traceable for several kilometres and strike northwest subparallel to the predominant lineation direction and the axes of regional macroscopic folds. The faults lie on the lower limb of a large macroscopic recumbent fold and dip subperpendicular to its axial surface.
Correlation of the Barewood faults with regional northwest trending normal faults suggests formation in the mid to late Cretaceous during the later stages of schist uplift. A complex history of fault movement is recorded by quartz vein textures and slickenside lineations. Vein textures are indicative of a mesothermal mineralising environment significantly above the brittle-ductile transition. Mineralisation occurred within quartzofeldspathic sediments derived from the Torlesse Terrane and are classified Textural Zone IV.
Hydrothermal alteration extends no more than a few metres from individual veins and is characterised by silicification and kaolinisation of altered host rock. Geochemical XRF analyses reveal significant additions of Si02, K20, Ba and Rb with the dilution of most other major and trace elements. Gold, tungsten, arsenic and antimony are the main elements indicative of mineralisation and are thought to have been transported in the mineralising fluid.
Quartz vein textures record a succession of brecciation and mineral deposition events. Gold, scheelite and sulphide distribution is concentrated in zones of repeated fracturing where textural overprinting is highest. Native Au commonly occurs as free blebs in quartz veins. Fe-sulphides ± Au are associated with replacement textures in altered schist inclusions. Enhanced permeability created by repeated fracturing of wall rock and pre-existing vein material is thought to have contributed significantly to fluid-rock interaction and thus the concentration and deposition of metals.
Arsenopyrite geothermometry, combined with fluid inclusion temperature and density determinations, yields an estimate of 325°± 60°C and 2 ± 1 kbars for the conditions of Barewood mineralisation. A mineralising fluid of low apparent salinity (1-2 wt.% equiv. NaCl) is indicated by fluid inclusion studies. The carbon and oxygen isotope composition of mineralised vein carbonate from Barewood and its calculated fluid composition suggest equilibration with metamorphic rocks, and is consistent with a metamorphic source for the mineralising fluid.
Stibnite-bearing quartz veins at the Hindon Antimony Mine are hosted in a series of subparallel moderately dipping reverse faults and steeply dipping extensional fractures. The fault zone comprises a complex thrust system at low angle to S2, and dips 30°- 40° SE. Significant postmetamorphic deformation of host schist, evidenced by local steepening of schistosity, 25°- 60° SE, is confined to the immediate mine area. Stibnite is the predominant sulphide at the Antimony Mine and the main indicator of mineralisation. The most significant mineral deposition occurred in zones of localised extension within the thrust system and in late stage crosscutting extension veins. Vein textures are quite different from those observed at Barewood and are more suggestive of an epithermal environment. Fluid inclusion studies indicate a mineralising fluid of significantly higher salinity than Barewood (3.5-5.0 wt.% equiv. NaCl) and a
minimum mineralising temperature of approximately 200°C.

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Thesis description:

viii, 173 p. (some folded) : ill. (some col.), maps ; 30 cm.

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OU geology Identifier:

1990MacKenzie

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Location (WKT, WGS84):

POLYGON ((170.363343925000095 -45.677738555999952,170.363342507000084 -45.677770029999976,170.363324848000047 -45.678233217999946,170.203418739000085 -45.674492338999983,170.203431676000037 -45.674155054999972,170.203435572000103 -45.674006607999956,170.203446572000075 -45.673709789999975,170.203448940000044 -45.67366033199994,170.2036999500001 -45.666270803999964,170.203728444000035 -45.665407246999962,170.202522899000087 -45.665378877999956,170.178030688000035 -45.664799768999956,170.142849419000072 -45.600745822999954,170.208853985000019 -45.602407127999982,170.337510127000087 -45.605532020999931,170.366386611000053 -45.606215769999949,170.36594673500008 -45.616639051999982,170.365552884000067 -45.625965109999981,170.363364384000079 -45.677284429999986,170.363357902000075 -45.677428311999961,170.363343925000095 -45.677738555999952))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/1990MacKenzie.pdf

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Citation

MacKenzie, Douglas James., “Gold, tungsten, and antimony mineralisation at Barewood, east Otago, New Zealand,” Otago Geology Theses, accessed September 24, 2017, http://theses.otagogeology.org.nz/items/show/620.