Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 130

Fig. 5.7 Eh-pH diagram
showing stability of iron
minerals (From Garrels and
Christ ( 1965 ))
Fig. 5.8 Model for the formation of iron formations from Klein and Beukes ( 1993 )
oceans were hotter, acidic and they contained abundant dissolved Si and Fe. In the
early Proterozoic, the oxygen content of the atmosphere and the oceans built up,
because or, or in parallel with, the appearance of abundant oxygenic cyanobacteria,
and this change led to the precipitation of Fe and Si oxides. Figure 5.8 shows in
more detail how the process might have worked during chemical or biochemically
aided precipitation in shallow water on continental shelves or intracratonic, often
evaporitic basins. In this model we assume that the iron is derived from hydrother-
mal input implying an exhalative volcanic source, but other models propose a
source via weathering of continental or oceanic crust. Whichever the case, the