University of Tasmania
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Setting of the Palaeoproterozoic Koongie Park Formation and carbonate-associated base metal mineralisation, at Koongie Park, northwestern Australia

posted on 2023-05-26, 05:36 authored by Orth, K
This study focuses on the Koongie Park Formation (KPF), a deformed and metamorphosed Palaeoproterozoic succession in northwestern Australia. The KPF hosts massive sulfide occurrences collectively known as the Koongie Park Prospects (KPP). The research aims to assess whether or not the KPF contains volcanic-hosted massive sulfide (VHMS) deposits. Whereas Palaeoproterozoic VHMS deposits are plentiful on other continents, there are very few examples of that age in Australia. This thesis provides new information on the facies, facies architecture, depositional environment and the regional setting, augmented by the geochemistry of volcanic rocks. The Onedin prospect (1 Mt @ 11% Zn, 1% Pb, 1% Cu), the second largest of the KPP after Sandiego, has been studied in detail. The alteration halo and the sulfide zones at Onedin are consistent with a VHMS origin for this prospect. Finally, investigations of the carbonate at Onedin have helped unravel the processes occurring during diagenesis, alteration and mineralisation beneath the seafloor. The KPF (1843 ¬¨¬± 2 Ma) has undergone up to 6 deformation events and metamorphism. The earliest deformations (D1 & D2) occurred during the regional Halls Creek Orogeny, with subsequent peak metamorphism accompanying intrusion of the Loadstone Monzogranite (1827¬¨¬± 2 Ma). Metamorphic conditions were variable across the area, ranging from 650¬¨‚àû‚Äö-720¬¨‚àûC at 5 kb in the north near the Loadstone Monzogranite to <450¬¨‚àûC at 5‚Äö-6 kb near Onedin. Later deformation (D3) was associated with the Yampi Orogeny and the King Leopold Orogeny (D4 & D5). Late strike-slip faulting ended with the Phanerozoic Alice Springs Orogeny (D6). Two units are recognised in the KPF. The lower KPF is found in the north and west of the area and is composed of tholeiitic basalt, turbidites, hemipelagic mudstone, chert and some ironstone with minor rhyolite-bearing breccias. The lower KPF is conformably overlain by the upper KPF, comprising more diverse lithofacies. Coarse-grained sediment-gravity-flow deposits are interbedded with turbidites and hemipelagic mudstone. Volcaniclastic, quartz-bearing units occur in the main prospect areas, along with siliceous sandstone and mudstone. These facies are intruded by rhyolite, tholeiitic dolerite and late tholeiitic basalt dykes. The syn-sedimentary rhyolitic sills may represent the root zone of a large volcanic centre or part of a sill-sediment complex. The cooling profile of one 50 m-thick sill is comparable to that of a large-volume rhyolitic lava. The rhyolite is granopyhric near the base, the middle is spherulitic and an upper zone is broadly (1‚Äö-10 mm thick) flow banded. The KPF represents less than ten million years of deposition in a subaqueous setting, which remained below storm wave base. The sedimentary rocks indicate proximity to a continental source, with a change to more intra-basinal detritus during basin evolution. Immobile element geochemistry indicates that the volcanic rocks of the KPF rocks formed in either an extending island arc or rifted continental/continental margin setting. Provenance of the sedimentary facies indicates proximity to continental crust. Regional investigations by other workers suggested a subduction-related tectonic setting, and so the KPF may have accumulated in a continental backarc or forearc basin. Deformation and metamorphism of the Onedin prospect has recrystallised, annealed and locally remobilised massive sulfide minerals. Hydrothermal alteration at Onedin resulted in a low Na footwall and host sequence. The footwall consists of quartz-bearing mudstone composed of chlorite+quartz+white mica. The host sequence is mudstone, sandstone, chert and ironstone, and contains abundant Mg, Si and Ca in carbonate, talc, chlorite, quartz, and white mica. The rhyolitic sill of the hanging wall contains quartz+albite, commonly partially replaced by phyllosilicate minerals. These minerals in the hanging wall indicate that the hydrothermal system was circulating beneath the floor of the basin. This pattern of alteration is similar to VHMS-related alteration patterns in modern and ancient systems. Sulfide at Onedin is hosted in carbonate or chlorite schist. High grade, zinc-rich massive sulfide comprising sphalerite+pyrrhotite+chalcopyrite ¬¨¬±pyrite with minor galena and silver minerals, is hosted in two 10‚Äö-60 m-thick carbonate lenses. Massive, vein and disseminated sulfides are hosted in chlorite schist between and above the carbonate lenses. These sulfide zones contain sphalerite+pyrite+pyrrhotite+chalcopyrite and minor galena. The carbonate-hosted massive sulfide appears to be replacement style, but the deposit lacks the mineralogical zonation characteristic of skarn ore deposits. Given the pattern of hydrothermal alteration, the simplest explanation is that carbonate alteration pre-dated the base metal-bearing fluid. Carbonate caused an increase in the pH of the metal-bearing fluid, thereby precipitating sulfide minerals. The carbonate alteration at Onedin is complex, with 13 different petrographic carbonate types forming eight carbonate facies. The carbonate types record processes operating during diagenesis, hydrothermal alteration, mineralisation, deformation and metamorphism. Delicate arcuate, cuspate and circular structures preserved in the carbonate are interpreted to be bubble-wall glass shards. The carbonate is dolomite and ferroan dolomite. Carbonate associated with sulfides is more iron-rich with some ankerite. Calcite is the main metamorphic carbonate mineral. Carbon isotopes range from ˜í¬•13C(PBD) ‚Äö-4.8 to 2.1‚ÄövÑ‚àû (av. ‚Äö-3.2‚ÄövÑ‚àû) and oxygen isotopes from ˜í¬•18O(SMOW) 6.7 to 8.2‚ÄövÑ‚àû (av. 7.1‚ÄövÑ‚àû). Modelling of the carbon and oxygen isotopes is consistent with the formation of carbonate alteration involving a glassy felsic volcanic sediment with fluid temperatures between 250¬¨‚àû and 350¬¨‚àûC. Modern VHMS deposits occur in both backarc and forearc settings. A deep water environment, with high-level intrusion and extension, as inferred for the KPF, is suitable for the accumulation of VHMS deposits. Tholeiitic basalt as pillow lavas in the lower KPF and as latestage dykes, attest to extension throughout the deposition of the KPF. A-type igneous rocks, such as the rhyolite in the KPF, contain abundant zinc and can be associated with VHMS deposits. There is no evidence for exhalites at Onedin and the best model for the base metal occurrence is as a sub-basin floor replacement VHMS deposit. Local intrusion of rhyolitic sills resulted in lithification of surrounding sediments and elevated temperatures, generating fluid circulation and confining fluid flow beneath a cap. Fluid circulation in porous volcaniclastic sediments and the host sequence resulted in a low abundance of Na and the precipitation of quartz, chlorite, muscovite, carbonate and talc. Glass shards are preserved in some carbonate, indicating hydrothermal input of Ca, Mg and probably CO2. Although carbonate was present as a component of the host sequence, it was significantly upgraded by alteration and provided the chemical trap to deposit base metal sulfide minerals.


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