This paper models the physico-chemical conditions of a Neoarchean to Paleoproterozoic marine basin in which the sedimentary sequence of BIF, Fe and mn ores of the Lake Superior-type formed. The model is based on Eh-pH diagram stability fields for Fe, silica and Mn solubilities (taken from the literature) and on field observations of the lithological sequences. BIF formation took bplace in epicontinental marine basins with free access to the ocean. The main Fe source for BIF formation was ocean enriched with about 6-10 ppm ferrous Fe of hydrothermal geochemical affinity. Land-derived Fe influxes into the BIF-forming basins certainly contrbuted, but the lack of clastic sedimentation precludes estimation of element budgets. The main silica source for formation of chert layers is sea water. If silica was precipitated by evaporation, the silica concentration of the BIF-forming sea must have been close to saturation (15-20 ppm). Biogenic silica concentration from a possible silica undersaturated sea may not be excluded. These inferred BIF-forming conditions fit the global occurrence of Lake Superior-type BIF in general, whereas special sedimentary environments were probably responsible for the formation of highly enriched laminated Fe ore at the Maremane Dome and in the Dishen-Kathu mining district in Griqualand West, and for the Fe-Mn ores in the Kalahari field. Formation of laminated Fe ore in the Maremane Dome and in the Sishen-Kathu areas were restricted to local deeps within the BIF basins, caused by karst collapse in the underlying Campbellrand dolomates. In such deeps, increased pH values relative to the normal BIF-forming sea caused sufficiently increased silica solubility, resulting in the alnost exclusive sedimentation of colloidal Fe precipitates.
In the Kalahari field, the BIF sedimentation pile became silica-depleted when approaching the Mn layers. This was genetically controlled by the increased pH of sea water and increased silica solubility. Under such increased pH conditions, Mn oxides become stable for precipitation. if minimum Mn activity is achieved in the sedimentary basin. The sedimentation sequence of low silica BIF - kutnahoritic BIF - jacobsitic BIF - braunitic Mn ore can be explained, using combined Eh-pH diagrams, as reflecting a precipitation path of increasing redox potential in a pH environment slightly above 9. These conditions were achieved by closing the access of the basin to the open ocean, resulting in the reduction of water level by evaporation and thereby increasing salinity and pH. Precipitation of low silica BIF followed and, in the presence of sufficient Mn activity with increasing Eh in the precipitating water stratum, deposition of the Mn mineral associations occurred.』
Consideration of atmospheric conditions and sea water character during the time of Lake Superior BIF Formatin
Pricipitation of the Fe and silica which form the BIF
The formation of silica-poor laminated Fe ore
Laminated Fe ore in the Griqualand West Supergroup of the northern Cape Province in South Africa
The formation of Mn ores associated with BIF