Sulphide minerals in Early Archean chemical sedimentary rocks of the eastern Pilbara district, Western Australia

The occurrence and paragenesis of sulphide minerals in chemical sedimentary rocks from the McPhee and the Towers Formations of the Warrawoona Group, e astern Pilbara Craton were examined, in order to evaluate the Archean sedim entary environment. The chemical sedimentary facies of both formations are comprised of chert or chert-carbonate units, which are highly depleted in d etrital materials. The cherty rocks are mostly composed of microcrystalline quartz, containing significant types of syndepositional (or diagenetic) su lphide minerals. In particular, the cherty rocks in the Towers Formation (N orth Pole Chert, Marble Bar Chert) include primary sulphide minerals, such as pyrite, chalcopyrite, sphalerite, monoclinic pyrrhotite, pentlandite, ge rsdorffite and millerite. This assemblage and the measured FeS content (8.4 -10.4 mol%) of sphalerite associated with the Fe-sulphide minerals suggest that the cherty rocks were formed under reducing conditions at temperatures below 200 degrees C (about 150 degrees C), and also that the metamorphic t emperature of the rocks was less than similar to 325 degrees C. Furthermore , the virtual absence of detrital materials and the minor element compositi ons imply that the cherty rocks of the eastern Pilbara Block were formed by rapid precipitation from reducing hydrothermal solutions.