Carbonate-Hosted Deposits
Carbonate-hosted deposits are bodies of sphalerite, galena and iron sulphide (pyrite or marcasite) in limestone or dolostone. These deposits are also called Mississippi Valley-type deposits, since that part of North America has many of them. The deposits are generally believed to be formed when mineral-laden fluids travel through fractures or pore spaces in the rock. Chemical conditions in the carbonate rocks cause metals to precipitate from the fluids, to be deposited on fractures and in openings in the rock.
The deposits usually have a tabular shape on a large scale, but in detail, because they form in areas where the rock is fractured, broken or caved, they can have an irregular shape.
Red-Bed Copper: Stratiform sedimentary deposits, also known as “red-bed” deposits, are fine-grained disseminations of base metal sulphide, sometimes with lead, zinc, cobalt and silver minerals or native copper. The host rocks are normally shales or sandstones. The minerals appear to have precipitated in the pore spaces of the sedimentary rocks from fluids circulating in the rock.
Geologists’ interpretations differ on whether this happened at the time the host rocks were deposited or later on, but both schools of thought agree that the minerals were deposited when the fluids reached a “chemical trap”, a place where the chemistry of the rock changed in a way that made it impossible for the metals to remain in solution. The large copper deposits of Zambia and the Democratic Republic of Congo, formerly Zaire, are of this type.
There are also sandstone-hosted lead-zinc deposits similar in form to stratiform copper deposits, but usually without iron sulphide minerals.
Coal, Potash, Salt and Gypsum: These minerals occur in bedded in deposits in sedimentary rocks. They are not ores; rather they are extracted and used as is. Coal forms from decayed plant mineral, hence the name “fossil fuel”. Potash, salt and gypsum are left over when seawater evaporates in shallow basins. All these deposits have a tabular form.
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