Ore and gangue minerals
Primary - hypogene
Secondary - supergene
Many ore minerals are found in tabular or lenticular bodies known as veins. Veins form when minerals are deposited along preexisting cracks or fissures. The fluids carrying the vein minerals in solution are often under pressure and may enlarge the fissure or vein.
The shape of the fissure may be straight, with few side passages. This might happen in a jointed granite, or other rock with high structural integrity. Such fissures are often narrow with respect to their vertical and horizontal extents. Interlacing fissures are often found in less competent rocks, like schist or slate, that are easily fractured. In soluble rocks the fractures will be irregular due to differential dissolution of the wall rock.
Mineralization usually occurs from one wall of the fissure to the other wall, as a solid mass. This produces sharp, well-defined boundaries between the wall rock and the vein. In other cases, the solution interacts with wall rock, partially replacing wall rock minerals with vein minerals. The boundary is then not sharply defined. Replacement deposits are formed when most of the vein mineral-ization occurs in the wall rock. Sometimes the middle of the vein may remain open, as a cavity called a vug. The crystallized minerals often grow inward toward the center of the vugs.
Most vein mineralization occurs from hydrothermal solutions. Hydrothermal solutions may be formed when magmas lose some of their volatile components in the form of aqueous solutions. They may also form when ground water comes in contact with, or near, a magma body. The result is often a solution above the critical point of water, and under significant pressure. Sometimes, especially with heated ground water, the temperatures may be lower. Vein mineralization is generally divided into three temperature ranges, as follows:
Low temperature 50 - 150 C
Intermediate temperature 150 - 400 C
High temperature 400 - 600 C
Vein minerals may be classified as primary or hypogene minerals. These will form in the main part of the vein. Hypogene deposits often include sulfide minerals, which are susceptible to alteration under low-temperature, high oxygen conditions. Such alteration produces secondary, or supergene, mineralization. The resulting minerals are often oxides or carbonates (if the meteoritic waters causing alteration are rich in dissolved bicarbonate or carbonate anions). The meteoritic waters circulate through the upper layers of the vein, completely dissolving the primary minerals. This produces a barren region. As the altered meteoritic waters move deeper, they redeposit some of the dissolved minerals in a secondary enrichment zone. Supergene enrichment frequently produces rich ores.
If the deposit is rich enough, we can call it an ore. The material must be discovered, mined, processed, and transported to a buyer for a cost that enables the seller to make a profit. Minerals which are not ore minerals are called gangue. The price of the metal will often determine weather certain deposits are ores or not.
© 2000 by David L. Warburton
Last revised: March 27, 2000