GEOCHEMICAL CHARACTERISTICS OF ARCHEAN CHERTS AND OTHER SEDIMENTARY-ROCKS IN THE PILBARA BLOCK, WESTERN AUSTRALIA - EVIDENCE FOR ARCHEAN SEAWATER ENRICHED IN HYDROTHERMALLY-DERIVED IRON AND SILICA

Geochemical and mineralogical characteristics of Archean sedimentary rocks such as cherts, banded iron-formations (BIFs), shales and sandstones collected from the Pilbara Block, Western Australia were studied. Lithologic features of various types of chert occurring within the Pilbara Supergroup are closely related to their chemical compositions. Red and brown bands of banded cherts generally show a higher Fe2O3* (*=total iron as Fe2O3) concentration than do other cherts, whereas grey, greenish-grey and pale yellowish-grey cherts are characteristic of relatively higher concentrations of Al and Ti. White cherts are generally composed almost exclusively Of SiO2. Geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions. Evidence for the hydrothermal origin includes (1) low MnO/Fe2O3* values, (2) low concentrations of heavy metals, (3) positive Euanomalies, (4) low Co/Zn and Ni/Zn values, and (5) high concentrations of Mn and Fe in carbonates. Positive correlations between Fe and heavy metals such as Ni and Zn imply the intermittent precipitation of Fe-oxides (hydroxides) along with these heavy metals in the ancient ocean. Cherts, except for grey, greenish-grey and pale yellowish-grey cherts, are highly depleted in detrital materials. This may have been caused by rapid precipitation of hydrothermally-derived Fe and Si. The Archean "Pilbara Ocean" is believed to have been enriched in Fe and Si derived from hydrothermal activity.