Mississippi Valley-type deposits: Products of brine expulsion by eustatically induced hydrocarbon generation? An example from northwestern Australia

Abstract

Mechanisms for fluid flow such as compactive dewatering, gravity-driven hydrologic flow, and seismic pumping are not geologically plausible for Mississippi Valley-type (MVT) mineralization in the Lennard Shelf, Canning Basin, northwestern Australia. Sulfides at Cadjebut and the other Lennard Shelf MVT deposits precipitated from overpressured, hydrocarbon-rich brines. The deposits lie below a major mid-Carboniferous unconformity that is linked to a global sea-level fall. Independent evidence at Cadjebut also constrains the time of mineralization to the mid-Carboniferous. A sea-level fall at this time could have raised the sediment-water interface above the thermocline, thereby increasing the temperature of the basin by [minus]16[degree]C. Model calculations show that a rise in basin temperature would induce sufficient hydrocarbon gas maturation to expel 1005 km[sup 3] of brine. This brine volume would be more than sufficient to transport and deposit the metal budget of the known MVT deposits on the Lennard Shelf. The existence worldwide of MVT deposits below unconformities suggests that sea-level-induced hydrocarbon generation could have assisted MVT mineralization in other provinces. 33 refs., 2 figs.