and the Tectonic Cycle
Till the end of the 18th century, geologists had a static concept of the earth, i.e. they believed that it was created exactly as we see it today. Since the beginning of the 19th century, the new way of looking at the earth included the recognition that constant changes take place as geological forces modify the surface and interior of the earth. The geological cycle as deduced by Hutton postulates that the geological environment in any place undergos a cyclic change through a number of stages as outlined below:
1. Erosion and planing down of mountains - The Weathering Stage.It must be considered that since ores are rocks, they are likely to be intimate parts of the environment in which they occur. This is supported by the evidence obtained from the examination of a wide variety of ores and the geological environment in which they occur. It is confirmed that most ore deposits indeed appear to be related fundamentally to their environment. To be more specific, ores are not merely parts of their environment, each one of them is a characteristic part of a particular type of environment. This realization leads to another important principle --the geological environment existing in any segment of the earth's crust or surface is not static -- it is constantly changing. In most cases the changes involved are progressive and systematic, that is to say that the environmental metamorphosis follows an evolutionary pattern. This systematic change is referred to as CRUSTAL EVOLUTION. Most of these changing environments are those that have certain ore deposits of their own particular characteristic kind.
It follows from the above that any given pattern of evolution of the crust is accompanied by a parallel pattern of evolution of the ore types.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)