Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block

The >3.0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Range s about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified elastic rock), carbonaceous black chert and c arbonate-rich laminated chert. Although the cherts have undergone postdepos itional silica enrichment, such as cementation and metasomatic silicificati on, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other prim ary to early diagenetic components were carbonates, sulphates (gypsum and a nhydrite) and organic matter. Although these mineral associations, on the w hole, correspond to those of modern marine evaporites, they are different f rom modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). Thi s feature is a possible manifestation of peculiar physicochemical condition s in the water mass from which the chemical sediments were precipitated; co mpared with modern ocean waters, the concentrations of Fe and Si were signi ficantly higher and the pH value might have been lower. These conditions co uld be obtained by contributions of Fe- and Si-enriched hydrothermal soluti ons and continental run-off to the site of deposition Grey cherts contain detrital quartz and altered Fe-Ti oxides and were forme d in a period of input of terrigenous detrital materials. They are characte rized by higher concentrations of TiO2, Al2O3, Cr, Ni, Zn, Rb and Zr compar ed with the other types of chert and by very low (< 4)Al2O3/TiO2 values. Th ese features are attributed to the supply of terrigenous detrital materials that contain abundant Fe-Ti oxides (ilmenite and titanomagnetite) and fine TiO2 particles. Such detrital materials might have been formed by extensiv e chemical alteration of source rocks and residual enrichment of Ti relativ e to Al.