The White Devil Gold Deposit, Tennant Creek, Northern Territory, Australia
Phung T. Nguyen, S. A. Booth, R. A. Both, P. R. James, 1989. "The White Devil Gold Deposit, Tennant Creek, Northern Territory, Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The White Devil gold mine is 33 km northwest of Tennant Creek, in the Northern Territory, Australia. Production began in August 1987, and as of 30 June 1988 the measured and indicated resource totaled 343,000 metric tons at 20.6 g/metric ton Au, with a further 73,000 metric tons having been mined to that date. The full extent of the deposit is yet to be defined.
Well-bedded siliciclastic sedimentary rocks of the lower Proterozoic Warramunga Group, which host the mineralization, have undergone two main deformations. The early ductile deformation (D1) was a moderate deformation, which produced upright east-west-trending open-close folds, with a regular plunge of 40° to 50° toward 245°. The late semiductile to brittle deformation (D2) was a progressive deformation, which was associated with at least three closely spaced events, that is, the intrusion of a set of quartz-feldspar porphyry dikes, an early east-west shearing associated with the emplacement of hydrothermal magnetite-rich bodies (ironstones), and a progressive shearing associated with the mineralization. The shearing produced a slickenside-growth fiber lineation (L2), that suggests a vertical-oblique displacement, with the south block eastwardly uplifted and the north block westwardly downthrown.
The magnetite veins commonly have a brecciated texture, and en echelon arrays of sygmoidal tension microfractures are extensively developed. The geometric forms of these microfractures and the internally crystallizing vein fibers indicate formation during a progressive and incremental deformation. Gold-bismuth-copper mineralization was emplaced during the late stage of this progressive deformation, and the ore minerals are mainly concentrated in the tension fractures, replacing quartz and chlorite fibers. Continued progressive shearing was the latest event in the second deformation, when it displaced the porphyries and caused the brecciation of the ironstones.
Gold-bismuth-copper mineralization is mainly confined to the Main Zone and Deeps Zone orebodies, which are associated with magnetite-rich bodies in the hinge region of an anticlinal F1 fold. The mineralogical composition of the ore is relatively simple and consists of gold, chalcopyrite, bismuthinite, bismuth, pyrite, marcasite, and molybdenite associated with magnetite, chlorite, quartz, and hematite with minor carbonate and talc. Ore grades are quite variable over 1-m lengths of drill core, with Bi up to 15 percent, Cu up to 8 percent, and Au up to 1,000 g/metric ton. The primary zone in the Main Zone orebody averages 0.5 to 0.8 percent Cu and 17.6 g/metric ton Au and the Deeps Zone orebody 0.1 percent Cu and 25.2 g/metric ton Au. Gold is generally fine grained and not visible in hand specimens, except in very rich sections of the orebodies.
Textural relationships of the minerals and studies on fluid inclusions in quartz demonstrate that there were two distinct phases of hydrothermal fluid involved in the formation of the deposit. Magnetite was formed from a fluid of relatively high temperature (approximately 350°C) and high salinity (probably CaCl2-NaCl), whereas later gold-bismuth-copper mineralization was formed from a fluid of lower temperature (approx. 300°C) and lower salinity. The origin of the solutions is uncertain, but a preliminary sulfur isotope study suggests a magmatic source for the sulfur. The close association of the ore and magnetite suggests that the magnetite was an important factor in controlling ore deposition.