Geology of the Tanami gold mine, Northern Territory
Creators: name: Tunks, AJ
Publisher: name: University Of Tasmania
Year: 2023

description: The Tanami Gold Mine (TGM) is situated 600km NW of Alice Springs in the Paleoproterozoic 'Granites-Tanami Inlier' of Northern Australia. The deposit, which was discovered in 1904 and has been mined intermittently since, is one of several gold-only deposits that occur in the inlier. The host rocks to gold mineralisation at the TGM are a sequence of northwest-dipping, interbedded tholeiitic pillow-basalts and volcaniclastic-sedimentary rocks. The sediments were deposited by mass-flow mechanisms in a below wave-base subaqueous environment. The wholerock geochemistry of the basalts is similar to that of rift tholeiites, consistent with an intracontinental tectonic setting at the time of basalt eruption. The presence of hematite and high-grade metamorphic detritus in the sedimentary lithologies is also consistent with an intracontinental tectonic setting. The deformational history of the Tanami area involved two sub-orthogonal episodes of folding that generated northeast-trending Fl folds and northwest-trending F2 folds. Interference between the two fold generations has created dome and basin interference patterns. Illite-crystallinity measurements of sandstones and siltstones from the TGM indic~te diagenetic temperatures, probably less than 250¬¨‚àûC. The metamorphic grade, intensity and number of deformations at the TGM are less than at the 'Granites Gold Mine and elsewhere in the inlier. The host rocks to the mineralisation at the TGM (Black Peak Formation) are therefore interpreted to be younger than at the Granites (Ditjiedoonkuna Suite). Intracontinental rifting during the Palaeoproterozoic Leichardt rifting event (1810-1740Ma) created a small rift basin into which the Black Peak Formation was unconformably deposited onto a Archaean/Paleoproterozoic metamorphic basement. The Granites-Tanami Inlier is intruded by at least two distinct granite suites; the Mt Winnecke Suite (1830-1815Ma) and the Gregory Suite (1800-1790Ma). The granites in both suites are dominantly reduced, I-type granites that are enriched in incompatible elements. Gold mineralisation at the TGM is hosted within a complex sinistral wrench-fault array and associated veins and alteration halos. The main mineralised faults trend approximately NS and dip steeply east. Subsidiary structures trend at 030¬¨‚àû and 070¬¨‚àû and dip southeast. Economic gold mineralisation occurs within quartz-carbonate veins and in the surrounding sericite + quartz +pyrite¬¨¬± carbonate alteration halos. High-grade southeast-plunging, oreshoots are present where the mineralised fault trends intersect. Detailed structural studies indicate that the main mineralising event post-dated the bulk of F1 shortening, and was synchronous with the emplacement of felsic dykes into the mine sequence. Stress inversion calculations, based on fault striation populations, have revealed that at the time of the Au mineralising event, ˜ìvâ1 was sub-horizontal and SE-NW directed with ˜ìvâ2 subvertical. This contrasts with the pre-mineralisation deformation which occurred under a similarly directed ˜ìvâ1, but with ˜ìvâ3 sub-vertical. Flipping of the stress axes has allowed for the formation of steeply-dipping faults that were effective fluid focussing zones during the mineralisation event. A range of internally deformed vein textures and the presence of crack-seal and extension veins are evidence for cyclic fault rupturing caused by variations in the fluid pressure, shear stress and permeability of the fault zone. Mass balance calculations undertaken on the sericitic alteration assemblages that are spatially associated with Au mineralisation indicate addition of K, and volatiles (mainly C02 and S), and leaching of Si and Na, which caused minor volume loss during the metasomatic event. Fluid inclusion studies have revealed the presence of high-temperature (300¬¨‚àûC), low-salinity (5 wt.%) fluids with low C02 contents. Sulfur and oxygen isotope data are consistent with a hybrid magmatic/contact-metamorphic origin for the ore-forming solutions, which are inferred to be related to granite emplacement. During migration through the footwall the fluids, partly re-equilibrated with wallrocks to acquire their characteristic isotopic and geochemical compositions of ˜í¬•34S‚Äöv¢v†12‚ÄövÑ‚àû, ˜í¬•18˜ívº‚Äöv¢v†10‚ÄövÑ‚àû, K/Rb ‚Äöv¢v† 330. Mineralising solutions were weakly.acidic (pH ‚Äöv¢v† 5), reduced (S04‚Äöv¢v†/H2S ‚Äöv¢v† 0.001) and had a high ˜í¬£S content (0.006 molal); Gold was predominantly transported as AuHSO, although Au(HS)2 may have also been important, particularly if the pH or ˜í¬£S was higher than estimated. Gold deposition most likely occurred due to H1S loss associated with sulfidation reactions as magnetite and hematite, present in the wall rocks, were altered to pyrite. Phase separation occurring due to fluid pressure drops in dilational fault zones may have also been locally important for gold deposition. The ultimate source of gold at the TGM remains unclear. Two possibilities are suggested: i) gold was magmatically sourced, and was partitioned into an exsolved magmatic-hydrothermal fluid during magma crystallisation, or ii) gold was present as detrital gold in the contact aureole of the granite and was scavenged and remobilised by magmatic and/or contact metamorphic hydrothermal fluids. In the second scenario, Au mineralisation in the underlying metamorphic basement may have provided a source of detrital gold in the sedimentary lithologies of the Black Peak Formation.