Sedimentary mineral deposits and the evolution of Earth's near-surface environments

The nature of sedimentary mineral deposits has evolved during Earth's history in concert with changes in the oxidation (redox) state of the ocean-atmosphere system, biological evolution, and the growing importance of geologically young accumulations of ore-grade material. There is now strong evidence that the atmosphere and the oceans were anoxic, or essentially anoxic, before 2400 Ma. Banded iron formations (BIF) and the detrital uranium ores formed prior to 2400 Ma are consistent with such a state. The period between 2400-2000 Ma is called by some people the Great Oxidation Event. Its ores bear unmistakable marks of the presence of atmospheric O (sub 2) . Shallow-water BIF from this period tend to be oxidized, hydrothermal uranium ores took the place of the earlier detrital uranium ores, the concentration of the redox-sensitive elements in carbonaceous shales began to increase, and phosphorites made their first appearance. Between 1800 and 800 Ma the Earth system seems to have been remarkably stable. Sedimentary ore deposits of this period were influenced by the presence of O (sub 2) . BIF, sedimentary manganese, and phosphorites disappeared approximately 1800 Ma, but sedimentary exhalative (SEDEX) deposits and unconformity-type uranium deposits flourished, and nonsulfide zinc deposits put in an appearance. The period between 800 Ma and the end of the Proterozoic at 540 Ma was as turbulent or more so than the Palaeoproterozoic. BIF returned, as did sedimentary manganese deposits and phosphorites. A further rise in the O (sub 2) content of the atmosphere and an increase in the sulfate concentration of seawater during this period brought the composition of the atmosphere and of seawater close to their present redox state.