Early Archaean hydrothermal fluids; a study of inclusions from the ∼ 3.4 Ga Buck Ridge Chert, Barberton Greenstone Belt, South Africa

Type HA and type C fluids appear as immiscible fluids in their host crystal, most probably resulting from unmixing of the hydrothermal fluid. Phase relations in the CO2-H2O system show that the unmixing must have occurred at a relatively low pressure of ∼ 100 bars. This conclusion is in line with the geological context of the samples, which indicates a shallow environment of deposition for the sediments, (almost) contemporaneous hydrothermal activity, and a very low metamorphic grade. However, it is contrary to the first impression given by the homogeneous character of type C inclusions, which suggests much higher P-T conditions at the time of trapping. The aqueous component of type C fluids may have progressively mixed with high-salinity aqueous fluids (type HA), resulting in a dilution trend from HA to MA to LA fluids. The hydrothermal fluids from the Buck Ridge Chert in the Barberton Greenstone Belt are strikingly similar to those encountered in present-day systems. They show minor differences with older (ca. 3.8 Ga) fluids preserved in hydrothermally altered rocks from the Isua Greenstone Belt in Greenland.