Being a part of the inorganic matter which constitutes this planet, most of the minerals or their decomposition products are widely distributed in earth’s crust. It is the concentration of these minerals at a particular spot, in quantities sufficient to be exploited economically, which gives rise to a mineral deposit.
The formation of these deposits is essentially a very slow biological or geo-chemical process of concentration of mineral material which takes millions of years to develop an economically viable mineral deposit. These geo-chemical and biological processes may briefly be summed up as follows:
1. Concentration of Minerals during Cooling of Molten Rock Materials:
During the cooling of molten rock material, which is a complex collection of a number of substances, different minerals crystallize out at different temperatures which may settle out in various bands or layers. A number of Chromites deposits are formed in this way. Some deposits are formed as a result of sublimation. The process involves change of state from solid to gaseous and vice-versa, without intervening liquid phase. A number of deposits of sculpture are formed in this manner.
Associated with the molten rock material are heated and super-heated waters which rise in the cracks and crevices of earth’s crust from deeper interior. These carry a number of dissolved materials which are deposited in the cracks within earth’s crust as they cool down. These waters may dissolve and carry away a number of materials at the same time, depositing them elsewhere. Australian deposits of Quartz and Gold, the copper deposits of Lake Superior, USA, and a number of Zinc and Lead deposits are formed in this manner.
2. Formation of Mineral Deposits by Evaporation of Sea-Water:
Mineral deposits may be formed by the heating action of solar radiations which evaporate sea water in shallow basins. The dissolved material is left behind. Deposits of more soluble minerals like Sodium chloride; Gypsum, Chili saltpeter etc. are formed in this manner.
3. Formation of Deposits by Action of Intense Heat and Pressure:
Intense heat and pressure inside earth’s crust causes changes or metamorphosis of rock materials. It is during this process that a number of secondary minerals are developed. Deposits of Asbestos, Talc and Graphite are formed in this way.
4. Concentration of Minerals during Weathering, Transport and Sedimentation:
Weathering, dissolution, transportation and sedimentation – processes which cause formation of sedimentary rocks may result in the concentration of some minerals at certain places. Weathered rock material in solution or suspension is deposited in sufficient amounts to form a mineral deposit as water currents slow down and solubilities, pH, temperatures etc. change. The deposits of Iron- sands of Newzealand are formed in this manner. Aluvial gold deposits, Limonite, cassiterite, Tin- oxide deposits are formed in this way. Materials left behind after weathering which are resistant to further degradation may become an ore or mineral deposit. Deposits of Bauxite are formed in this way.
5. Formation of Mineral Deposits due to Oxidation-Reduction Reaction:
Oxidation- reducation reactions are very important in environmental chemistry which may result in formation of mineral deposits. Copper ions (Cu2+) released from copper minerals near earth’s surface by hydrolytic weathering especially under low pH can be leached down the upper strata. If these ions come in contact with another sulphide mineral like ZnS, the insoluble CuS will be formed while Zn2+ ions thus released shall be carried away. This will cause concentration of copper ore in the upper layers while ZnS shall be deposited elsewhere.
Cu2+ + ZnS ———————- > CuS + Zn2+
(Solution) (Solid) (Solid) (Solution)
6. Formation of Mineral Deposits by Microbial Activity:
Micro-organisms may contribute to the formation of mineral deposits by accumulating some elements, changing environmental conditions, causing oxidation or reduction etc. It is mainly autotrophic bacteria which are involved in mineralization reactions. Sulphate reducing autotrophic bacteria such as Disulphovibrio desulphuricans reduces sulphates to sulphides: