| Zdroj: |
Economic Geology & the Bulletin of the Society of Economic Geologists; May2010, Vol. 105 Issue 3, p655-667, 13p, 1 Black and White Photograph, 1 Diagram, 1 Chart, 7 Graphs |
| Abstrakt: |
Laterites and bauxites are produced in tropical soils by weathering, which enriches iron (of laterites) and alumina (of bauxites)--as well as trace elements such as nickel, gold, phosphorus, and niobium--to ore grade. Laterites and bauxites can be redeposited into sedimentary sequences, and remain as ores if not transported far and diluted with other materials. The age of redeposited laterites and bauxites, and of bauxitic and lateritic paleosols, can be established from the geologic age of overlying rocks, an approach especially effective in paleosols within sequences of isotopically datable volcanic rocks. Lateritic profiles can also be dated by paleomagnetic inclination in special cases in which land masses such as in Australia and India drifted long distances northward during Cenozoic time. In addition, cryptomelane and other K-Mn oxides can be dated by K-Ar and 40Ar-39Ar techniques to obtain multiple ages from different crystals in a single relict paleosol. Compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k.y. duration at 2, 12, 16, 35, 48, 55, 65 and 100 Ma. Such events can also be inferred at times older than 100 Ma from paleolatitudinal distribution of laterites and bauxites, but these are poorly sampled. Laterite and bauxite peaks were coeval with times of global high warmth and precipitation, elevated atmospheric carbon dioxide, oceanic anoxia, exceptional fossil preservation, and mass extinction. These CO2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact, flood basalt volcanism, and methane outbursts. Concentration of bauxite and latente resources, in particular stratigraphic horizons formed during greenhouse crises, suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores. [ABSTRACT FROM AUTHOR] |
