The structure and geochemistry of gold mineralisation in the Mt. Todd goldfield, Pine Creek Inlier, Northern Territory
Creators: name: Hein, KAA
Publisher: name: University Of Tasmania
Year: 2023

description: The Mt Todd goldfield is located approximately 40 km northwest of the township of Katherine, in the Northern Territory, Australia. The goldfield is host to several discrete ore bodies that strike north-northeast within a broad northeasterly trending corridor of gold mineralisation. The largest ore body occurs at the Batman mine and constitutes an identified mineral resource of 92.9 million tonnes at 1.4 git Au. The goldfield lies in the southern region of the Central Domain of the Pine Creek Inlier and is hosted by a Palaeoproterozoic sequence of rocks termed the Burrell Creek Formation. The formation is dominated by greywackes, siltstones, sandstones and shales that exhibit sedimentary features akin to those of a river-dominant delta front to pro-delta environment. The formation is conformably overlain by volcanodastic and volcanolithic sediments of the Tallis Formation (-1890 Ma). Three deformation events are recognised, viz. , 0 1, 0 2 and 0 3. The earliest deformation, 0 1, is characterised by close to tight, northeast to northerly to northwesterly trending asymmetric folds (F1), a continuous axial planar cleavage (S1), and numerous strike-slip faults. The deformation is associated with the development of buck quartz veins and was preceded by the emplacement of the Yenberrie Leucogranite of the Cullen Batholith (1835-1820 Ma) which contact metamorphosed the sediments of the Burrell Creek Formation to hornblendehornfels facies (H1 ) , with the development of cordierite porphyroblasts (type C1). D1 was coincident with peak regional metamorphism to greenschist facies. 0 2 is associated with westerly trending open folds (F2) , a spaced disjunctive to fracture cleavage (S2) in transaction to the folds, and strike-slip and normal faults. The deformation is associated with the development of quartz-tourmaline and gold-bearing quartz-sulphide veins and lodes. It was preceded by the emplacement of the Tennysons Leucogranite which contact metamorphosed the Yenberrie Leucogranite of the Cullen Batholith and the sediments of the Burrell Creek Formation to hornblende-hornfels facies (H2), with the development of cordierite porphyroblasts (type C2) . D3 is characterised by the reactivation of strike-slip faults (mostly sinistral), a steeply dipping Type S3-C type foliation, and mesoscopic en echelon folds (F3) that trend oblique to the faults in a left stepping (sinistral) array. The faults offset 01 and 0 2 structures. Calcite-base metal veins and epithermal style quartz veins may have formed during this deformation. The age of emplacement of the leucogranites, D1 and D2 is constrained by the age of emplacement of the Cullen Batholith at 1835-1820 Ma. D1 and D2 are correlated with deformation during the Maud Creek Event (-1850 Ma), while D3 is correlated with deformation during the Shoobridge Event (-1780 Ma). Mineralisation in the goldfield is associated with quartz-sulphide veins and lodes that comprise pyrrhotite, chalcopyrite, arsenopyrite, pyrite, marcasite, loellingite, with minor galena, sphalerite, bismuth-sulphosalts, ISS CuFeS minerals, and gold. Gold is associated with bismuth and CH4-rich hypersaline fluid inclusions (29-33 wt% NaCl eq) in trails which cross-cut early quartz and sulphide. The general vein/lode alteration assemblage comprises quartz, biotite, muscovite, chlorite, sericite, rutile, accessory tourmaline, chalcopyrite, pyrrhotite, pyrite or marcasite. The quartz-sulphide veins and lodes are typified by extension concomitant to hydrothermal fracturing and/or normal faulting. Across the goldfield, the veins and lodes exhibit a remarkable similarity with respect to strike geometry, morphology, vein forming process and isotopic composition. They are located within the thermal aureole of the Tennysons Leucogranite and strike north-northeast within a corridor that trends northeasterly above a basement strike-slip fault. They are preferentially hosted in competent rock types such as greywackes and siltstones. Detailed structural and petrographic analyses has indicated that the veins and lodes were generated at the same time by hydrothermal activity during retrograde (contact) metamorphism associated with cooling of the Tennysons Leucogranite, and early in D2, prior to the development of the regional S2 fabric. Integrated petrologic, isotopic (sulphur and oxygen) and fluid inclusion studies suggest that the hydrothermal fluids were chiefly magmatic-metamorphic in origin. The fluids were enriched in metal and sulphur derived either as magmatic metal or sulphur (from the Tennysons Leucogranite), or scavenged during fluid-rock reactions from sources in the sedimentary pile, the Yenberrie Leucogranite, or from sulphide-bearing greisens in the Yenberrie Leucogranite. Fluid evolution was controlled by retrograde reactions which accompanied rehydration of the country rock and T ennysons Leucogranite, at low fluid-rock ratios. The hydrothermal fluid was reduced in nature and exhibited a tendency to lower fO2 and fS2 concomitant to an overall decrease in temperature and salinity for a given pH. ˜í¬•34S values of 4.5%o to 13.3%0 indicate that the source for sulphur was mixed (magmatic-sedimentary). Initial precipitation of crack-seal fibre quartz from a silica-saturated brine occurred during early retrograde metamorphism by progressive fracturing and sealing of the host rocks. The subsequent precipitation of quartz and tourmaline coincided with a significant vein dilation event and accompanied decompressive effervescence and boiling of a surface-derived hydrothermal fluid at a minimum pressure of 180-225 bars or a minimum depth of 1.8 to 2.6 kilometres. Fluid salinity ranged from 1 to 20 wt% NaCl eq: this fluid interacted with a mixed magmatic-metamorphic fluid. The precipitation of a silicate-sulphide-carbonate assemblage accompanied an influx of hot (~ 400¬¨‚àûC), acidic (pH ~ 4.0 at 330¬¨‚àûC), hypersaline brine (30-50% NaCl-CaCl2). This metal and sulphur enriched brine was mixed magmatic-metamorphic in origin. The principal cause of metal deposition was fluid unmixing at a shallow crustal level (minimum depth of 1.8 to 2.6 km) during a decrease in temperature from approximately 370¬¨‚àûC to 240¬¨‚àûC, a decrease in salinity from 30-50% NaCl-CaCl2 to 29-33 wt% NaCl eq, and vertical transport of the brine along joints, fractures and faults. Gold precipitation occurred late in the development of the silicate-sulphide-carbonate assemblage from a hot (~ 250¬¨‚àûC), acidic (pH of 2.3-4.9 at 250¬¨‚àûC), hypersaline brine (29-33 wt% NaCl eq), and accompanied the precipitation of bismuth and the entrapment of CH4-rich fluid inclusions. A genetic model for mineralisation in the goldfield invokes sinistral reactivation of a northeasterly trending basement strike-slip fault under the influence of effective tensile stresses caused by cooling of the Tennysons Leucogranite. The reactivation of the fault caused brittle failure in the upper crust and/or dilation of existing north-northeasterly trending faults, fractures and joints in competent rock types. The generation of dilatant structures, coupled with a sudden reduction in pressure facilitated channelization of fluid flow into the upper crust, probably away from convection cells juxtaposed about the cooling leucogranite. Rising fluids decompressed causing phase separation with mineral precipitation. Throttling of the conduit or fluid pathways resulted in over-pressuring of the fluid, this giving way to hydrothermal fracturing and an enhanced permeability.