Carbonate-Hosted Deposits

Carbonate-hosted deposits are bodies of galena, sphalerite and iron sulphide (marcasite or pyrite) in dolostone or limestone. These deposits also go by the name of Mississippi Valley-type deposits, since that area of the North American continent has many of these deposits. The deposits are most of the time believed to be formed when mineral-laden fluids travel through pore spaces or fractures in the rock. Chemical conditions in the carbonate rock cause the metals to precipitate from the fluids, to be deposited on fractures and in openings in the rock.

The shapes of the deposits usually happen to be tabular on a large scale, but when observing it in detail; they can have an irregular shape due to the fact that they form in areas where the rock is caved, broken or fractured.

Red-Bed Copper
Stratiform sedimentary deposits which are also known as “red-bed deposits” are very fine-grained disseminations of base metal sulphides, most of the time iron and copper sulphides, sometimes they carry silver minerals, cobalt, zinc and lead or with native copper. The host rocks are most of the time sandstones and shales. It appears to be that the minerals have precipitated in the pore spaces of the sedimentary rocks from fluids circulating in the rock.

The interpretations of different geologists are not the same as to whether this occured at the time the host rocks were deposited or after that time, but both schools of thought agree that the minerals were deposited when the fluids reached a “chemical trap”, an area where the chemistry of the rock changed in a way that made it impossible for the metals to remain in solution. The great deposits of copper found in Zambia and the Democratic Republic of Congo, formerly known as Zaire, are of this type of formation.

There are also sandstone-hosted deposits of zinc and lead which are similar in form to stratiform copper deposits, but most of the time they do not have any iron sulphide minerals.

Sedimentary Uranium Deposits
Shale-hosted uranium deposits and sandstone hosted uranium deposits are precisely what their names indicate them to be, in other words, uranium ninerals which are found in a shale or sandstone host rock. The sedimentary rock is what usually disseminates the uranium minerals.

The sandstone-hosted types seem to have been formed in a similar manner to the stratiform copper deposits, with circulating fluids carrying uranium through the rock and depositing it when they come to a chemical trap. The deposits of uranium in Niger and in the southwestern United States are sandstone hosted.

Uranium in these sorts of settings has the possibility of happening through chemical removal and erosion of traces of uranium from other rocks and concentration in the basin where the shales were being formed.

Each one of the sediment hosted deposits which were described previously – carbonate lead-zinc, red-bed copper and uranium, can be deformed by folding and defaulting later in time. However, due to the fact that they form in stable sedimentary basins where there is usually less tectonic activity, they stay undeformed more often than the exhalative types.