Ore Deposits as Guides to Geologic History of the Earth 论文

摘要

The tectonic evolution of the Earth depends largely on changes in the rates and mechanisms for generation and dissipation of its internal heat. By contrast, the chemical evolution at the Earth's surface and in its fluid envelopes is driven chiefly by solar energy, in part regulated by the bio­ mass. Magma-generating processes and magmatic crystallization are both commonly selective of ore-forming ingredients, as are also systems of hydrothermal circulation driven by magmatic heat or regional meta­ morphism. Solar energy exerts major control on ores accumulated as detrital con­ centrations and as chemical sediments. The colossal energy storage by the biomass through photosynthesis has provided most of the diversity of redox potential that characterizes the Earth's epizonal ore-forming environments. The chemical action of free oxygen is evident in many sediments such as red beds and banded iron formations, but, of course, photosynthesis must also sequester chemically equivalent amounts of organic carbon in other sedimentary rocks. At the time of sediment accumulation or diagenesis, organisms or residual carbon may buffer oxidation potential low enough to stabilize sulfide ores. Furthermore, if either strongly reduced or strongly oxidized rocks become recycled through partial melting, the resulting magmas may also selectively incorporate special metal assemblages. These may later be further concentrated into ores by igneous or hydrothermal processes. So tectonic activity and vol