Abstract Gold, bismuth, and copper mineralization at Tennant Creek is hosted by magnetite-hematite replacement bodies in lower Proterozoic sediments of the Warramunga Group. The sediments have been folded about east-west axes, are characterized by a pervasive axial-plane slaty cleavage, and are intruded by pre- and postfolding granites. Marked structural and stratigraphic control yields lines of lode (ironstones) that can be traced for distances of up to 40 km. Ironstone lodes are restricted to the magnetite-rich Black Eye Member of the Carraman Formation and concentrate adjacent to argillaceous banded iron-formations. They are aligned parallel to the regional axial-plane cleavage, commonly lying in the cores of third-order folds, especially in areas of fold hinge plunge reversal. Faulting and shearing parallel to the cleavage may also play a role in the localization of some lodes. Gold, bismuth, and copper mineralization and associated alteration form a late-stage overprint on the magnetite lodes, with gold typically concentrated toward the footwall of the ironstone or at its margins in distinct pods associated with chlorite and muscovite. Copper and bismuth mineralization occurs in overlapping zones around these pods, and this zonation is complemented by gangue mineralogy, and trace element and sulfur isotope zonation patterns. A model for the formation of the ironstone lode involves the movement of hot connate brines into developing fold axes during regional deformation of the Warramunga Group. The fluids in equilibrium with the magnetite in the sedimentary pile reacted with more oxidized horizons (e.g., hematite shales), resulting in the deposition of hematite (or a hydrated precursor) that was subsequently converted to magnetite as equilibrium was restored. Economic mineralization is associated with faulting and fracturing of the ironstone lodes and introduction of hot, saline, relatively reduced and sulfur-bearing solutions. Reaction of these solutions with chlorite in the lodes resulted in its replacement by muscovite with a consequent increase in pH and reduction in f O2 of the fluid. This reaction is likely to have controlled gold, bismuth, and copper deposition. The relative availability of sulfur, metals, and fluid between ironstone lodes is thought to be responsible for the spectrum from unmineralized to copper- and gold-rich ironstone lodes.

Abstract 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 (D 1 ) 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 (D 2 ) 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 (L 2 ), 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 F 1 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 CaCl 2 -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.