Thermal and fluid flow history of the Lawn Hill Platform, northern Australia: implications for the formation of Zn-Pb deposits

Abstract

Isa Superbasin strata in the Lawn Hill Platform host major base metal sulfide mineralization including the giant Century Zn-Pb deposit. Mineral paragenesis, organic maturation, K-Ar dating and stable isotope studies demonstrate that long-lived structures such as the Termite Range Fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic. K-Ar dating of illites, in combination with other datasets, may identify three thermal events in the Lawn Hill Platform at 1500, 1440-1400 and 1250-1150 Ma. 1500 Ma is a Late Isan Orogeny age recorded only in the south that may reflect exhumation of a provenance region. The 1440-1300 Ma ages are related to fault reactivation and a thermal/?fluid pulse at ~1440-1400 Ma, with subsequent enhanced cooling. The youngest thermal/fluid flow event at 1250-1150 Ma is recorded mainly in the northeast and is probably related to the assembly of Rodinia. An extensive carbonate alteration halo in the footwall of the circa 1575 Ma Century Zn-Pb deposit extends some 15 km along strike. Calculations of fluid oxygen and carbon isotope composition based on model temperatures of 120°C for the ore zone siderites and 180°C for siderites and ankerites in the regional carbonate alteration halo indicate similar ore and alteration fluid compositions (d18O = 3 to 10 per mil; d13C = -7 to -3 per mil). This fluid isotope composition is consistent with highly evolved basinal brines and mixed inorganic and organic carbon sources. The good agreement between maximum temperature estimates from illite crystallinity and organic reflectance and inverse correlation with carbonate oxygen isotopes in northeast Lawn Hill platform boreholes indicate thermal maturation and carbonate formation resulted from interaction with the 1250-1150 Ma fluids. The calculated fluid isotope compositions are consistent with an evolved basinal brine (d18O = 5.1 to 9.4 per mil V- SMOW; d13C = -13.2 to -3.7 per mil V-PDB) that contained a significant organic carbon component. Differences in carbon and oxygen isotope systematics of alteration carbonate are significant to their use with lithogeochemical vectors in mineral exploration in the northern Australian basins.