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Ore Geology Reviews

Hydrothermal controls on iron and lead mineralization on the farms Leeuwbosch and Cornwall, Thabazimbi District, South Africa

 

The Paleoproterozoic Malmani Subgroup and Penge Formation (Transvaal Supergroup) exposed on The Farms Leeuwbosch and Cornwall north of Thabazimbi (Limpopo Province, South Africa) host hydrothermal Pb-Ag-Cu-Zn and high-grade hematite iron ore deposits which include the historic Leeuwbosch lead mine. Based on ore petrography, fluid inclusion and stable isotope analyses and geochemical modelling, the structurally controlled, stratabound Leeuwbosch Pb-Ag-Cu-Zn-Fe deposits formed from high salinity NaCl-CaCl2-rich basinal brines with total salinities from 18 to 24 weight % NaCl equivalent which are similar to those found in Mississippi-Valley-Type (MVT) Pb-Zn deposits. The minimum formation temperatures (homogenization temperatures) for the lead deposits range between 120 and 185 °C and the estimated formation conditions are 200 °C at 1 kbar pressure. Stable isotope analyses of gangue siderite and calcite indicate that the deposits formed from base metal-enriched brines derived from recycled seawater with δ13Cvs. PDB from 0.46 to 3.17‰ and δ18Ovs. SMOW from -2.95 to -1.79‰. Hydrothermal transport of lead and silver as 〖"PbCl" 〗_"3" ^"-" and 〖"AgCl" 〗_"2" ^"-" respectively took place at mildly acidic pH and elevated fO2 in the 〖"SO" 〗_"4" ^2"-" predominance field. Extensive host rock interaction neutralized and reduced the ore fluid to precipitate galena hosting Ag with chalcopyrite and sphalerite. Fluid inclusions in gangue quartz and calcite in the extensive structurally controlled and pervasive stratabound hydrothermal iron ore deposits on the Farms Leeuwbosch and Cornwall, chiefly consisting of high-grade hematite with rare relict magnetite, show a wide compositional variability due to episodic fluid mixing and successive hydrothermal activity in the presence of several distinct fluid end members. Total salinities of fluid inclusions in quartz from the hematite deposits range from 9.2 to 39.9 wt% NaCl eq. with highly variable proportions of NaCl and CaCl2 and minimum formation temperatures are between 100 and 190 °C. The estimated formation conditions for the iron ore deposits are 175 °C and 1 kbar. Stable isotopes of gangue calcite indicate that diagenetic fluids with δ13Cvs. PDB from -3.33 to -2.09 and δ18Ovs. SMOW from 2.52 to 5.60‰ and a second fluid with δ13Cvs. PDB of -7.45 and δ13Cvs. PDB of 17.45 were controlling the formation of the hematite deposits. In agreement with oxidized conditions during hematite formation and characteristic host rock alteration patterns traced by carbon and oxygen isotopes, the second low-salinity fluid was derived from the overlying Waterberg Group sandstones and played an important role in the formation of the iron ore deposits, in addition to high salinity diagenetic brines. Iron was leached from the BIFs of the Penge Formation, transported as FeCl+ under mildly acidic conditions and likely deposited in response to the oxidation of the iron-bearing fluids. The lead and iron ore deposits formed by combinations of structural controls which influenced the regional fluid flow patterns and fluid compositions, and fluid-rock interaction with reactive carbonate lithologies of the Malmani Subgroup which acted as a sink for metals transported in acidic hydrothermal solutions.

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