Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block
K. Sugitani et al., Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block, SEDIMENTOL, 45(6), 1998, pp. 1045-1062
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.