PYROPHYLLITE - DIASPORE VEINS IN THE 2.7 GA PALEOSOLS AT MOUNT ROE, PILBARA, AUSTRALIA: NEW EVIDENCE FOR THE OXIC ARCHEAN ATMOSPHERE

The extensively developed sericite-chlorite alteration zones in the 2.7 Ga Mt. Roe basalt near Whim Creek have been cited by previous investigators (e.g., Macfarlane et al., 1994; Rye and Holland, I 998) as some of the best examples of paleosols that formed under a reducing atmosphere because of major losses of Fe from the sericite zone. Our investigations, however, have revealed a more complicated history of the Mt. Roe “paleosols”: the chloritic zone represents the soils formed under an oxic atmosphere, as indicated by the increased Fe3+/Ti ratios compared to the parental basalt lavas; the chemical characteristics of the overlying sericite zone, including the loss of Fe, were created by hydrothermal alteration of basaltic tuffs and inter bedded sediments.

Veins (up to 2 m thick and > 1 km long), composed of pyrophyllite, diaspore, rutile, apatite and sericite, were discovered in the sericite zone. Other-types of veins, containing sericite, monazite, cristobalite and quartz, are also common. Fluid inclusions in quartz show filling temperatures of ca. 115 °C and evidence of boiling; those in diaspore show 120-140 °C filling temperatures. Some inclusions are rich in CH4, but always poor in CO2. The sulfide contents are generally low in the veins, except in the lower parts. Apatite crystals, characteristics in the upper parts of the veins, are sulfate-rich. These data suggest that very reduced hydrothermal solutions, rich in H2S and CH4, invaded the tuff zone and leached Fe and other elements from the tuffs; the fluids mixed with O2-rich groundwater to produce sulfuric acids, which further leached various elements from the tuffs to form Al- and Ti-enriched minerals. Such processes were essentially identical to those responsible for the “high sulfidation type alteration” associated with many epithermal deposits of Phanerozoic age. Our finding adds new type of evidence to the growing list of evidence for the development of an oxic atmosphere prior to 2.5 Ga.