THE DEPLETIONS AND ENRICHMENTS OF FE IN THE 2.7 GA "MT ROE PALEOSOLS" WERE CAUSED BY REDUCED HYDROTHERMAL FLUIDS

The 2.7 Ga Mt. Roe basalt near Whim Creek, Pilbara, Western Australia, exhibits extensive developments of chlorite and sericite alteration zones. Because of strong depletions of Fe from the sericite zone, it has been interpreted as a paleosol formed under an anoxic atmosphere (Macfarlane et al., 1994; Rye and Holland, 2000; Young et al., 2001). The overprint by hydrothermal alteration, including K addition, has been thought by the previous researchers to have occurred ~2.2 Ga. However, our investigations suggest entirely a different scenario.

The parental rocks of the "paleosol" are comprised of: (a) the foot-wall basalt lava, (2) basaltic tuffs with inter-bedded thin lavas, (3) clastic sediments (mostly sandstone) with inter-bedded thin limestones containing stromatolite-like textures, and (4) the hanging-wall basalt lava, upward in the sequence. The sericite zone is developed mainly in (2) and (3), which is enveloped by chlorite zones in (1) and (4). The sericite zone is characterized by strong depletions of Fe as well as Mg and by enrichments of K, while the chlorite zones are characterized by enrichments of Fe, especially near the contacts with the sericite zone. Thick veins of pyrophyllite-diaspore are abundant in the sericite zone. They are dated as 2.7 Ga based on the SHRIMP age of apatite crystals. Fluid inclusions in diaspore crystals in the veins show the filling temperatures of 140-220 oC and the salinity of 10-15 wt% NaCl equivalent. Laser-Raman analyses indicate the inclusions are rich in methane, but poor in carbon dioxide.

These mineralogical and geochemical data suggest the following scenario for the alteration zones in the Mt. Roe basalt: (1) The Mt. Roe basalt is composed of a series of lavas, tuffs, and sediments accumulated ~2.7 Ga; (2) Large-scale hydrothermal activity occurred essentially concurrent with the volcanic activity; (3) The hydrothermal fluids, which were channeled preferentially through the porous tuff layers, were strongly acidic, reducing, and saline; (4) The hydrothermal fluids leached Fe from the sericite zone and reprecipitated in the chlorite zones. The sericite-chlorite zones of the Mt. Roe basalt are, therefore, not paleosols, but the products of hydrothermal alteration 2.7 Ga ago.