Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. I. Geology, Mineralization, Stable Isotopes, and Implications for Exploration
Titel:
Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. I. Geology, Mineralization, Stable Isotopes, and Implications for Exploration
Auteur:
Gao, Z. Kwak, T.
Verschenen in:
International geology review
Paginering:
Jaargang 37 (1995) nr. 10 pagina's 910-944
Jaar:
1995-10
Inhoud:
Major gold provinces in central Victoria, Australia, are separated by the Heathcote Fault Zone into two zones: the Bendigo-Ballarat Zone (BBZ) and the Melbourne Zone (MZ). The gold-quartz vein deposits are hosted predominantly by reverse fault systems in the Ordovician to Early Devonian turbidites, which have undergone low-grade regional metamorphism as well as deformation. Mineralization in the two zones is distinct, predominantly Au-As in the BBZ and Au-Sb-As in the MZ. In the BBZ, quartz veins are large (up to 5 m wide) and are hosted by Ordovician metasediments. Large deposits are confined to the reverse faults or fold crests in the N-S-elongated narrow domal structures. The sequence of vein genesis is: early narrow, auriferous, laminated veins; massive barren veins; one or more periods of narrow, auriferous, brecciated veins; and late carbonate-quartz veins. Arsenopyrite, pyrite, chalcopyrite, and gold (fineness = 960) are found in the laminated veins, whereas a similar assemblage plus galena and sphalerite is found in the breccia veins. Hydrothermal alteration of the host rocks includes arsenopyrite, sericite, chlorite, pyrite, ankerite and (in the west of the region) biotite, up to at least 60 m from the veins. In the MZ, the Au-Sb quartz veins are small (commonly less than 20 cm), in some places occurring as vein stockworks, hosted by Silurian-Devonian metasediments. The MZ deposits are generally located within broad N-S- and/or E-W-elongated brachydomes. Laminated veins are uncommon and, if occurring, thin, whereas massive veins are absent. Host rocks are moderately or weakly altered by ankerite, pyrite, arsenopyrite, and/or stibnite less than 5 m from the veins. Native gold (fineness = 999), rare aurostibnite, chalcostibnite, and stibnite post-dated or were synchronous with the major vein formation. The ores in the BBZ reflect a metamorphic or possibly magmatic water isotopic signature (δ18O: 5-10‰; δD: -65 to -50‰), whereas in the MZ they bear an evolved meteoric water signature (δ18O: 7-15‰ δD: -99 to -88‰). The δ13C and δ18O values of ankerite and the δ18 in associated vein quartz indicate that they are not in oxygen isotopic equilibrium. In the BBZ, δ34SH2S in the veins has an average ratio of 1.0 ± 3‰ values are lower in the western BBZ (-1.5 ± 2‰) than those in the eastern BBZ (3.0 ± 2‰). Measured δ34S values in the wall rocks at the New Cambrian deposit increase systematically from -16.9 to -18.5‰ at 60 m from the veins to -3.5 to 2.3‰ in the veins, which indicates that ore and wall-rock sulfur originate from different sources. δ13C values in vein carbonates in the BBZ deposits show an enrichment trend of late vein carbonates, whereas δ18O values show a corresponding depletion trend. In the Brunswick deposit (MZ), a narrow spread of δ13C and δ18O values of vein carbonates exists compared to those of the BBZ, the carbonates are slightly enriched in 18O and depleted in 13C toward the later stages, and the carbonates again are not in isotopic equilibrium with the quartz. Calculated δ34S values in the MZ ore fluids average 9 ± 1‰ and suggest a mixed source. The central Victorian turbidites were metamorphosed and folded under greenschist-facies conditions during the Devonian Tabberabberan Orogeny. Auriferous fluids were generated in association with the devolatilization of the Cambrian greenstones in the lower crust during this or possibly the earlier Silurian Benambran Orogeny. Brittle deformation produced dilatant structures, which led to the migration of ore fluids into these structures. Self-sealing of the faults resulted in the build-up of fluid pressure, which in turn caused hydraulic fracturing (= the breccia veins). In the MZ, meteoric waters circulating in the Silurian-Devonian rocks progressively dominated the auriferous fluids. Veins were formed in faults or fracture stockworks during immiscibility induced by cooling and pressure fluctuation. Finally, the BBZ and the MZ were juxtaposed by the westover-east movement along the Heathcote Fault at the end of the regional deformation associated with the Tabberabberan Orogeny.
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