Gold-Bismuth-Copper mineralisation in the Tennant Creek district, Northern Territory, Australia

Gold, bismuth and copper mineralisation at Tennant Creek occurs in transgressive magnetite- and hematite-rich lodes within the Carraman Formation of the Lower Proterozoic Warramunga Group. Rocks of the Warramunga Group are dominantly felsic greywackes and shales with features indicative of turbidity current deposition. These are interbedded with massive pyroclastic rocks, rhyolitic lavas, preconsolidation slump breccias and minor lenses of banded iron formation. The magnetite-hematite lodes (locally referred to as ironstones) have an ellipsoidal to pipe-like shape commonly flattened in the direction of the regional east-west cleavage. They are typically localised in small anticlinal structures within the greywacke-shale turbidites adjacent to thin lenses of hematite-chlorite-calcite bearing banded iron formations or hematite-rich shales. A smaller number of mineralised ironstones have replaced the preconsolidation slump breccia horizons within the felsic sediment pile. Silicate, oxide and carbonate gangue minerals within the lode structures are grouped into a series of compositionally distinct zones which exhibit sharp contacts against one another and the enclosing country rocks , Massive magnetite (>80%) and Fe-Mg chlorite (<20%) commonly constitute the core of the mineralised lode and are surrounded by vari ous umbrella-shaped zones. These may be: talc-magnet.ite; dolomite; chloritised sediments, as at J¬∑uno Mine, or quartz-hematite; hematitemagnetite; hematite-chlorite; chloritised sediments, as at Gecko Mine. The magnesium content of the chlorites (dominantly ripidolites) increases from the base, to the top, of the lode structures. The chemical and mineralogical characteristics of these 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. A zone of intense chloritisation extends below each of the orebodies and constitutes what is thought to be a channel of hydrothermal alteration. This investigation deals with the structure, mineralogical constitution, mineral zoning, textures, and origin of the lode rocks in the three largest operating mines in the goldfield, namely the Juno, Gecko and Warrego deposits. At the Juno and Warrego mines, gold, bismuth and copper mineralisation has been shown to occur in three overlapping zones within the magnetite-rich lodes e Gold is concentrated at depth, and is overlain above by an umbrella shaped zone rich in bismuth sulphosalts. Chalcopyrite is concentrated at the top of the lode structures enveloping the bismuth zone. At Gecko (anomaly 2), bismuth and copper show a similar vertical zonation but gold is completely lacking. Within the bismuth zone at Juno, the sulphur/selenium and bismuth/ lead ratios of the bismuth sulphosalts increase from its inner edge (overlapping the gold zone) to its outer edge (overlapping the copper zone). The most common bismuth sulphosalt at Juno is junite, a new mineral, unique to Juno, which has been shown by microprobe analysis to have the formula Bi8PbJCu2 (S,Se) 16 , containing 3.8 to 11.6 wt.% selenium. Junite is easily distinguished from other lead-bismuth sulphosalts by its characteristic x-ray powder pattern. The second most abundant bismuth sulphosalt has a composition close to Bi10Pbg(S,Se) 23 and may be equivalent to the mineral wittite , previously reported from Falun, Sweden by Johansson in 1924 . Other selenium bearing sulphosalts at Juno include heyrovskyite and members of the aikinite-bismuthinite series. Colloform textures occur throughout all the lode structures in the goldfield, strongly indicating that replacement of the sediment host rocks to form the ironstone bodies was achieved by the processes of dispersive metasomatism. Replacement of this type involves the gradual hydrolysis Mg 2+ 1' .n t h e of the host sediments and permits ready exchange of Fe 2+ and hydrothermal solutions with si4+, Al3+, Na+ and K+ in the hydrolysed sediment matrix. Most of the magnetite in the central lode zones replaced needle-shaped a - FeO(OH) and S - Fe203.H20 crystal forms which probably developed from ageing of ferric hydroxide gels. Thermodynamic considerations of gangue mineral stabilities in hydrothermal solutions of the type which may have caused mineralisation at Tennant Creek, suggests that the solutions were initially acidic in nature and capable of 1 each1‚. ng the I base catl.' ons I Fe 2+ and Mg 2+ (p 1 us ore metals) from the Carraman sediments at depth. There is evidence of leaching of this type in the lowest parts of the Juno hydrothermal channel. The process of metasomatic lode formation was most probably initiated by interaction of rising, chloride-rich, hydrothermal solutions with the calcite bearing banded_ iron formation or with particularly porous horizons containing abundant pore fluids (e.g., preconsolidation slump breccias) leading to an increase in solution pH and f02 which caused the deposition of amorphous, hydrated, ferric oxides. Under the influence of increasing solution p~the gangue minerals at Juno, were deposited in the order:- iron-rich chlorite at depth, followed by hematite, magnetite, talc and dolomite, to form a well zoned lode structur e . The progressive drop in f02 associated with the deposition of magnetite probably resulted in the increase in sulphur/selenium and sulphur/metal ratios of sulphides passing up the Juno lode structure, and may have also lead to the zonal distribution of gold, bismuth and copper . Sulphur isotope studies at Juno lend support to this proposal. The 16o;18o and 12c;13c ratios in dolomites from the outer envelope zone at The 32 S/34 S ratlio of syngenetic sulphides in the tuffaceous greywackes and vein sulphides in the hydrothermal channel, below the orebody, supports a proposal that the sediments of the Lower Carraman Formation provided the source for the sulphur. These sediments also contain sufficient trace quantities of gold, bismuth and copper to constitute a source for the ore components. Connate wate.rs (pore water and interlayer water) released from the argillaceous sediments in the vicinity of granitic and rhyolitic porphyry intrusions provides the most probable source for the hydrothermal solutions. Such solutions moved upwards continually leaching iron, magnesium and ore elements from the sediments in their path and were eventually channelled into low pressure anticlinal sites and deposited their metal load in favourable structural-lithological traps.