Early Archaean hydrothermal fluids; a study of inclusions from the reversible reaction .4 Ga Buck Ridge Chert, Barberton Greenstone Belt, South Africa

Fluid inclusions have been studied from cavities and breccias in one of the oldest well-preserved, relatively low-grade metamorphosed sedimentary sequences on Earth. The fluid inclusion-bearing samples are from the uppermost part of the early Archaean ( reversible reaction .4 Ga) Buck Ridge volcano-sedimentary complex in the Barberton Greenstone Belt (South Africa), which was affected by hydrothermal alteration during and immediately after its deposition. In banded, silicified sandy sediments this alteration is characterised, among other features, by quartz-cemented hydrothermal breccias, layer-bound stockworks of cm-size quartz veins and quartz-filled cavities below stacked pebbles. Quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two-phase aqueous or mixed H sub(2)O-CO sub(2) fluid. The aqueous inclusions are divided into three different groups, based on their microthermometric characteristics: type H sub(A) (relatively high salinity), type M sub(A) (intermediate salinity) and type L sub(A) (relatively low salinity). Optical and microthermometric characteristics of the fourth type of inclusions, the mixed H sub(2)O-CO sub(2) fluid (type C) indicate that it was homogeneous at the time of trapping, with only some minor changes occurring after trapping. Type H sub(A) 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 CO sub(2)-H sub(2)O 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 H sub(A)), resulting in a dilution trend from H sub(A) to M sub(A) to L sub(A) 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.