Diagenesis and Mineralization of the Neoarchean Bushy Park Lead-Zinc Deposit, Northern Cape Province, South Africa

The Bushy Park Pb-Zn deposit, hosted in unmetamorphosed carbonates of Neoarchean age, displays similarities to Phanerozoic Mississippi Valley-type (MVT) and Irish-type deposits. Mineralization is dated, by radiogenic methods, to Paleoproterozoic time. As such, Bushy Park is one of the oldest mineral deposits of this type in the world. Synsedimentary silicification and dolomitization preserve sedimentary fabrics, including microbial laminates, stromatolites, and oolites. Dolomitization likely was by evaporated seawater, as in Phanerozoic analogs. Structural control on mineralization, particularly solution collapse breccias, is similar to many Phanerozoic MVT and Irish-type deposits. Fluid inclusion data indicate three fluid endmembers involved in mineralization: a high-temperature, moderate-to-high salinity fluid; a low-temperature, moderate-to-high salinity fluid; and a moderate-to-low temperature, low salinity fluid. Saline fluids may have been sourced by evolved, evaporated seawater, and dilute fluids by meteoric and/or normal seawater. The fluids repeatedly mixed during ore and gangue mineral formation. Compositional zoning in gangue dolomite cement indicates that mineralizing fluid chemistry fluctuated over time. Petroleum inclusions and solid bitumen indicate that petroleum (oil) was an important fluid component at Bushy Park. Petroleum may have played a critical role in sulfur availability, addressing the issue of limited oceanic sulfate prior to and during the Great Oxidation Event.