Abstract

The western succession of the Mount Isa basin in northwest Queensland hosts four supergiant Zn-Pb ± Cu deposits and numerous smaller Cu and Zn-Pb deposits. Mineralization is primarily hosted in carbonaceous and calcareous shales and siltstones belonging to the 1670 to 1575 Ma Isa superbasin, but little is known about the source of metals that formed these deposits. The underlying clastic and volcanic successions belonging to the 1800 to 1750 Ma Leichhardt superbasin and the 1735 to 1690 Ma Calvert superbasin are potential metal source rocks and host a variety of diagenetic minerals that preserve geochemical information about the evolution of brines in the basin.

Quartz overgrowths and pressure solution features formed during shallow burial in all clastic lithologic units but are particularly common in the well-sorted, marine-dominated units that became aquitards due to the porosity-occluding diagenetic cement. Microthermometry on fluid inclusions in the quartz overgrowths indicates formation between 100° and 174°C from a low-salinity, 2.7 to 9.1 wt percent NaCl equiv fluid (fluid 1). These data together with sequence stratigraphic mapping, basin reconstruction, and stable isotope values from the quartz overgrowths show that the diagenetic aquitards formed at <5-km depth. Illite and chlorite formed locally in some of the diagenetic aquitards and preserve δ 18Ofluid and δ Dfluid values of 4.5 ± 4.2 and −34 ± 14 per mil, respectively, indicating evolution from a seawater-dominated source. Silicate dissolution and the widespread formation of diagenetic illite and chlorite occurred late, during deep burial diagenesis and primarily in the proximal fluvial lithologic units. These units are recognized as diagenetic aquifers and they occur adjacent to and within the Eastern Creek and Fiery Creek Volcanics where metals could have been sourced. Basin reconstruction shows that the diagenetic aquifers formed at depths between 5 and 10 km. Illite and chlorite extracted from the diagenetic aquifers have distinct δ 18Ofluid and δ Dfluid values of 4.5 ± 2.8 and −63 ± 11 per mil, respectively, indicating evolution from a meteoric fluid with a variable marine contribution. These isotopic values cannot be differentiated from published isotopic values of fluid inclusion water in quartz-dolomite-chalcopyrite veins at Mount Isa or sphalerite and illite from the Century Zn deposit and the Zn lodes from the Burketown mineral field. This suggests that the diagenetic aquifers were likely source rocks for metals in the deposits in the Mount Isa basin. In contrast to the phyllosilicates in the diagenetic aquifers, regional dolomitic grainstones and dolomudstones in the Lawn Hill platform precipitated from fluids with δ 18Ofluid between −2.6 and 1.1 per mil and δ 13Cfluid between −8.6 and −3.9 per mil. This suggests that these units did not contribute to the ore-forming brines.

Quartz veins formed during the later diagenetic history in the Mount Isa basin from a low-salinity brine, between 2.7 and 10.4 wt percent NaCl equiv (fluid 2). Oxygen isotope geothermometry on quartz-hematite pairs indicates that these veins formed at approximately 230°C. Crosscutting relationships reveal that a subsequent generation of quartz veins host fluid inclusions with a distinctly saline brine, with compositions between 11.9 and 23.2 wt percent NaCl equiv (fluid 3), indistinguishable from fluid inclusion compositions recorded in sphalerite from the Century and Walford Creek Zn deposits in the Lawn Hill platform and the quartz-dolomite-chalcopyrite veins at Mount Isa. Some of these quartz veins formed at ca. 400°C, based on quartz-hematite geothermometry, but fluid inclusion homogenization temperatures between 86° and 260°C suggest that another set of quartz veins, also containing high-salinity fluid inclusions, is preserved in the basin but formed at lower temperatures. Irrespective of formation temperature or timing, it is noteworthy that quartz veins hosting high-salinity fluid inclusions have δ 18Ofluid and δ Dfluid values that are indistinguishable from those recorded by (1) illite and chlorite in the diagenetic aquifers, (2) fluid inclusion water in sphalerite and synore quartz and illite in the Zn deposits in the Lawn Hill platform, and (3) various alteration minerals from the Mount Isa Cu deposit. Collectively, this suggests that the quartz veins represent fluids that migrated along faults from the diagenetic aquifers during late diagenesis to form the low-temperature Zn-Pb deposits between 1650 and 1575 Ma and later during the Isan orogeny to form the high-temperature Cu deposits.

Metamorphic quartz-hematite ± feldspar ± chlorite veins exist in the basin and formed between 325° and 450°C during the Isan orogeny from a brine having salinities between 12.6 and 23.2 wt percent NaCl equiv (fluid 4). These veins have distinct δ 18Ofluid and δ Dfluid values of 11.8 ± 2.0 and 30 ± 2 per mil, respectively, consistent with formation from fluids derived from graywackes and arkoses during greenschist facies metamorphism. The last recognized fluid identified in the basin (fluid 5) is only found in secondary fluid inclusions that form trails across earlier formed quartz veins. This fluid was trapped after the Isan orogeny, has a low salinity, between 0.0 and 8.1 wt percent NaCl equiv, records temperatures between 131° and 256°C, and is indistinguishable from postore fluids that have been reported in the Mount Isa Cu deposit and the Zn lodes in the Burketown mineral field.

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