Detrital and Authigenic Gold, in Quartz Gravels, Belle-Brook, Southland.

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Falconer, DM

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Abstract:

Detailed characterisitics of gold from auriferous gravels at Belle-Brook in the Waimumu District of Eastern Southland, have been studied using a combination of optical microscopy, energy dispersive microprobe analysis and scanning electron microscopy techniques.
Two auriferous gravels units crop out at Belle-Brook, the Gore Lignite Measures and the Waimumu Quartz Gravels. Both gravels units are gold bearing, however, the later is anomalously so. The Gore Lignite Measures are characterised by greywacke and schistose derived material and correspondingly variable gold values. In comparison the Waimumu Quartz Gravels are predominently quartzose gravels with an associated authigenic mineral suite which includes abundant marcasite and localised , extremely high, gold values.
The gold at Belle-Brook is classified as either detrital or irregular. It is proposed in this report that the irregular gold is of authigenic origin. Authigenic gold is only found in the Waimumu Quartz Gravels. A number of different morphologies are recognisable within each gold type. Six detrital morphologies are readily indentifiable along with four possible phases of gold mineralisation.
Scanning electron microscopy together with reflected light microscopy has revealed the authigenic nature of much of the irregular gold. The occurrence of octahedral gold crystals, and straight line structures strongly suggest there has been some gold mobility as well as very little transportation since this crystallisation episode. Flat 'faces' within the gold are interpreted as crystalline gold features, rather than features produced by the liberation of quartz crystals.
The occurrence of marcasite, seems to suggest that the gold found at Belle-Brook is found within gravels very conducive to the environment for gold accretion. The oxidation of thiosulphate complexes is most probably responsible for precipitating gold from solution, these complexes are formed from the oxidation of marcasite. It is intended to be shown here that an authigenic origin for at least some of the gold is entirely feasible and in the author's opinion is very likely.

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iii. 110 p. ill. Photos. 30 cm.

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1987Falconer

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

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Falconer, DM, “Detrital and Authigenic Gold, in Quartz Gravels, Belle-Brook, Southland.,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/212.

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