Zonation of hydrothermal minerals at the Juno Mine, Tennant Creek Goldfield, central Australia

Abstract

Gold, bismuth, and copper mineralization at Tennant Creek occurs in transgressive magnetite- and hematite-rich masses within a pile of felsic sedimentary and pyroclastic rocks of lower Proterozoic age. The magnetite-hematite bodies (locally referred to as ironstones) have an ellipsoidal to pipelike shape commonly flattened in the direction of the regional east-west cleavage. Chlorite-rich pipes of hydrothermal alteration extend vertically below the orebodies. The Juno mine is a small but extremely rich deposit situated in the southeastern section of the goldfield, and in 1972-1973 was the major Australian producer of gold, bismuth, and selenium. Both the gangue minerals and ore minerals are vertically zoned within the Juno lode. Massive magnetite (>80%) and chlorite (<20%) constitute the core of the lode, which is enclosed above by a zone consisting of talc-magnetite with minor pyrite. A thin dolomite-rich zone separates the talc-magnetite zone from the enclosing chloritized country rocks. Gold is concentrated in the center of the magnetite-chlorite body and is surrounded above by an umbrella-shaped zone rich in bismuth sulfosalts. Chalcopyrite is concentrated at the top of the lode structure above the bismuth zone. The total sulfur content and sulfur/selenium ratio of the sulfides and sulfosalts increase from the gold zone upward and outward to the copper zone. In addition, the Fe/(Fe + Mg) and Al/Si ratios of the associated chlorites decrease upward from the base of the lode to its outer edge. The chemical and mineralogical characteristics of the gangue and ore mineral zones indicate contemporaneous formation and growth, resulting from the flow of hydrothermal solutions which reacted with the host rocks and suffered continual and systematic changes in chemistry. Precipitation of iron oxides during the replacement of the sediments to form the lode was accompanied by a gradual and continual decline in fo2 of the hydrothermal solution. Thermodynamic studies indicate that this trend of decreasing fo2 (accompanied by increasing pH and decreasing temperature) can adequately explain (a) the increase in sulfur/selenium ratios of sulfides passing up the lode, (b) the vertical zonation of gold, bismuth, and copper, and (c) the increasing trend of [delta]34S within the magnetite-rich portion of the lode. A primary magmatic source for the sulfur appears unlikely as the trend of isotopic fractionation indicates that [delta]34S in the solution was near +20 per mil. Formation waters (evolved sea water and interlayer water) released from argillaceous sediments in the vicinity of granitic and rhyolitic porphyry intrusions subsequent to a period of active felsic volcanism provide the most probable source for the hydrothermal solutions.