Geochemical Epochs

time intervals, characterized by appearance of deposits of a particular composition or of sediments and rocks enriched with a particular element or whole association of elements. The earth’s geochemical epochs include an iron-ore epoch, connected with the beginning of the Lower Proterozoic (about 2,500 million years ago), and an epoch of lead deposits (1,700-1,400 million years ago).

Each geochemical epoch is characterized by a combination of external data that determines the conditions for the concentration of a particular metal. For example, the intensive global appearance of iron-ore deposits and ferruginous facies—itabirites and ferruginous quartzes—was probably due to the first manifestations of life, and the enrichment of the atmosphere of the earth by free oxygen and the resulting substantial change in the character of the aqueous medium of the World Ocean. Owing to the oxidation of bivalent iron into trivalent, the iron that had been accumulating in the ocean in the form of bicarbonate compounds was precipitated. This process lasted 200-300 million years and created iron-ore formations at Krivoi Rog (USSR), Lake Superior (USA), Minas Gerais (Brazil), and other areas with reserves exceeding all other iron-ore deposits. Lead accumulation is connected with carbonate sediments, characterized by high average lead content. Massive precipitation of the first carbonates and with them, of dispersed lead, belongs to the epoch of lead deposits, when metasomatic processes mobilized the lead of the carbonates and redeposited it in ore beds. The formation of the deposits at Sullivan (Canada), Broken Hill (Australia), and Sawar (India) belongs to this epoch.

There was a gold-ore epoch in the Archean period. The epoch of uranium, vanadium, and nickel accumulation was a result of massive precipitation of those metals at the beginning of the Paleozoic on Eurasian territory. The well-known Carboniferous period of coal-bearing formations was caused by the growth of luxuriant vegetation and its subsequent burial and conversion to coal beds.