ARCHEAN HYDROTHERMAL ALTERATION AND METAMORPHISM OF THE PILBARA BLOCK, WESTERN AUSTRALIA

The North Pole dome affects in an essentially undeformed section of greenstones overlain by the siliciclastic Gorge Creek Group which is in turn unconformably overlain by the basalts of the Fortescue Group. The dome is cored by the North Pole adamellite that intrudes the Warawoona Group that contains an earlier formed, intensely silicified zone of hydrothermally-altered pillow lavas capped by a chert-barite zone. The latter contains some of the earliest forms inferred to be bacterial mat structures. Above the barite horizon, is a chert/greenstone succession that gives way to a monotonous pile of pillow lavas with lessor amounts of chert truncated by the unconformities. The results are: 1) The greenstone silicate spread (6< d¹⁸O < 12; mean value +8.8) is similar to that for greenstone carbonate fraction (7.5 < d¹⁸O < 13.5; mean value 10.8). 2) The carbonate-greenstone silicate fractionation » +2 (70 pairs) is consistent with greenschist conditions. 3) Chert interlayered with greenstone gives an average d¹⁸O value of 13.8 (50 analyses), but the distribution is bimodal with peaks at 12.5 and 15 per mil (heavier values found where chert layers are more abundant). 4) The d¹⁸O values of barite, stratiform and crosscutting, remarkably overlap the range for modern seawater sulfate » +9.6 per mil. 5) Hydrogen isotope values for pillow lavas span the range –33 < dD< -84. 6) Greenstone carbonate d¹³C values exhibit a mean value of –0.8 ± 1.2 per mil with a distribution also similar to secondary carbonate values for Phanerozoic basalt. Whole rock d¹⁸O values for the Gorge Creek Group sandstones cluster near » +10 per mil consistent with quartz derived from Archean granitoids. Chert clast d¹⁸O values within the Gorge Creek Group overlap with those of Pilbara chert constraining the timing of metamorphism. With the exception of the cherts, all of the measured isotopic ratios are similar to comparable Phanerozoic rocks suggesting that the respective stable isotopic cycles were all established by 3.5 Gyr. The low ¹⁸O Pilbara cherts do not reflect isotopic equilibrium with the ancient ocean.