The ultramafic rocks in the Komati Complex, Barbeton greenstone belt, South Africa have been serpentinized thoroughly and carbonated with variable intensities. Conditions of the serpentinization and carbonation of the ultramafic rocks in the Komati Complex were studied using serpentine phase characterization by Raman spectroscopy, calcite hosted fluid inclusion microthermometry and chlorite geothermometry. Three serpentine phases, lizardite, chrysotile, and antigorite occur in four samples studied with Raman spectroscopy. Antigorite dominates the serpentine mineralogy in two of the samples and other two have large amounts of lizardite and antigorite, chrysotile being a minor constituent in three of the four samples. Abundant antigorite indicates serpentine crystallization above a temperature of ~ 320 °C. Fluid inclusion petrography and microthermometric data revealed four fluid inclusion assemblages (FIA), FIA 1 - FIA 4 in an order from earliest to latest entrapment. The FIAs have homogenization temperatures in the ranges of ~170 - 240 °C (FIA 1), 154 - 163 °C (FIA 2), 149 - 180 °C (FIA 3), 112 - 137 °C (FIA 4). Relatively constant NaCl equivalent salinities in the range of 6.4 - 11 wt-% were recorded for the FIAs 1 and 2, similar salinities were indicated for the FIAs 3 and 4.
Chlorite geothermometry yielded temperatures in the approximate range of 150 - 250 °C. Chlorite crystallization is texturally indicated to be related to the formation of the calcite that hosts the fluid inclusions. The overlap of chlorite geothermometry temperatures and the homogenization temperatures of the earliest fluid inclusions (FIA 1) indicates fluid inclusion entrapment at pressures lower than 200 - 300 bar and at temperatures equal to or slightly above the recorded homogenization temperatures. These pressure and temperature estimations suggest that carbonation occurred in a seafloor environment at moderate hydrothermal conditions. The properties of carbonate hosted fluid inclusions resemble those of fluid inclusions reported from Archean greenstone belts and interpreted as Archean seawater by previous contributions. Calcite hosted fluid inclusions may, thus, represent entrapped Archean seawater.
