207-5 The Neoarchean Carbonate Hosted Lead-Zinc Deposit at Bushy Park, Northern Cape Province, South Africa

Session: Honoring the Late Professor Mohamed Abdelsalam: Outstanding Researcher, Generous Colleague, Legendary Mentor, and Ambassador for the Geosciences In Africa

Presenting Author:

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Abstract:

Neoarchaean carbonates (2.59–2.52 Ga) at the Bushy Park Pb-Zn prospect belong to the Campbellrand Subgroup (Transvaal Supergroup) in the Griqualand West basin, South Africa. They are among the oldest unmetamorphosed platform carbonates in the world exhibiting meter-scale cycles consisting of subtidal mudstones shoaling upwards into shallow subtidal oolites, overlain by stromatolites, microbial laminates, and brecciated subaerial surfaces. These cycles organize into a stratigraphic hierarchy consisting of eighty-six 5th and sixteen 4th order parasequences, which in turn organize into two 3rd order sequences. Although an autocyclic origin cannot be ruled out, the stratigraphic sequences studied here are very similar to those recognized in Phanerozoic carbonate sections controlled by Milankovitch orbital forcing. Syngenetic and early diagenetic dolomitization and silicification played an important role in preserving sedimentary fabrics in these rocks. We speculate that early diagenetic dolomitization was facilitated by evaporated seawater during cyclic sedimentation, as occurred in Phanerozoic dolomite analogs. These strata host hydrothermal lead-zinc deposits at Bushy Park and the nearby Pering mine. Structural controls on mineralization, particularly solution collapse breccias, display similarity with Phanerozoic Mississippi Valley and Irish-type deposits. Ore minerals include sphalerite, which is dominant, followed by galena and minor chalcopyrite. Minor Ag, Ni, Sb, and Sn mineralization has been recognized at Bushy Park and Pering by other researchers. Open space-filling gangue cements include dolomite, calcite, quartz, fluorite, and hydrothermal clay minerals as well as both early and late diagenetic pyrite. Age of mineralization is likely Paleoproterozic based on radiometric dating of hydrothermal clays (2.22–2.05 Ga) and timing of petroleum migration. This roughly coincides with Paleoprotozoic tectonic activity in the Transvaal. Fluid inclusion homogenization temperatures, measured in gangue and ore minerals, range from 70° to 230°C and salinities up to 30 weight % NaCl equivalent. These data indicate three fluid endmembers that mixed repeatedly during ore formation. Earlier studies indicate that saline fluid may have been sourced by evolved, evaporated seawater. Petroleum inclusions, in quartz and dolomite cements, and solid bitumen, observed throughout the mineral paragenesis, indicate that petroleum was an important fluid component at Bushy Park. The presence of petroleum may have played an important role in sulfur availability for mineralization, addressing the issue of limited oceanic sulfate prior to and during the Great Oxidation Event.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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