Silica precipitation triggered by clastic sedimentation in the Archean: New petrographic evidence from cherts of the Kromberg type section, South Africa

• Chert layers are found at the top of Bouma sequences in the Kromberg Fm., Barberton Greenstone Belt, South Africa.
• Field, petrographic and geochemical evidence indicates that these are primary cherts.
• The chemical composition is dominated by minor detrital components.
• Flocculation on clay particles triggered the sedimentation of cherts.
• We developed criteria to distinguish between primary and replacement cherts.

The Kromberg Formation (ca. 3432 Ma) in the Barberton Greenstone Belt, South Africa, contains well-preserved chert beds at the tops of turbidite deposits. At the interface, siltstone, which consist of K-feldspar, K-mica, microquartz with minor lithic fragments and heavy minerals, grades into chert, which consists of microquartz and minor K-mica (<15%). K-feldspars show preserved twins typical of microcline, orthoclase and sanidine. Based on the heterogeneity of the clastic fraction (i.e. shape, size, nature), the lack of in situ metasomatic features (i.e. crystal overgrowths, silica replacement) and the continuity of the siliceous matrix through the siltstone-to-chert transition, we argue that (1) the clastic particles are detrital, (2) some were altered and metasomatized at their source, (3) in situ metasomatism was limited to minor seritization of K-feldspars, and (4) the silica is of primary origin and precipitated from ambient marine fluids. Our petrographic observations reinforce the model advocated by Rouchon and Orberger (2008) and Rouchon et al. (2009) for chert deposited in clastic-rich setting and we favor a formation of both the siltstones and cherts as chemico-clastic sediments. We argue for the contemporaneous deposition of clastic grains from turbiditity currents and precipitation of silica on phyllosilicate reactive surfaces, both in the water column and at the sediment–water interface. As the rate of clastic sedimentation declined, the accumulation of silica flocs on suspended phyllosilicates first accompanied, then replaced the deposition of detrital grains, to form a siliceous ooze at the seafloor. Contrary to current interpretations for detritus-rich cherts, which invoke a secondary origin via Si- and K-metasomatism, we propose that the present model prevailed in a variety of Archean settings where fine-grained sediments were deposited. The composition of both the siltstone and chert reflects mainly the environment in which they formed. They are interpreted as mixtures of two main components: (1) silica, which contains extremely low concentrations of trace elements and contributes only SiO2 to the bulk composition, and (2) another phase that dominates the trace element composition. Here, K-mica and K-feldspar control the chemical signal and reflect a felsic source to the turbidites (i.e. Hooggenoeg dacites and volcaniclastics).