1. What are mineral deposits? Describe five ways by which a mineral deposit can form.
A mineral deposit is a natural concentration of economically valuable minerals. The minerals in a mineral deposit are not carbonaceous; i.e., they do not contain electron-rich (chemically reduced) carbon. Coal is an example of a carbonaceous deposit. Coal is a fossil fuel rather than a mineral deposit.
A mineral deposit need not be valuable enough that it could be mined profitably. A mineral deposit which meets this profitability criterion is considered to be an ore deposit. All ore deposits therefore are mineral deposits but the reverse is not necessarily true.
The value of most mineral deposits does not depend as much upon the mineralogy of the deposit as upon the chemical composition of the deposit, i.e., the concentration of valuable elements within the deposit. However, the value of gem deposits is directly linked to mineralogy rather than chemical composition.
Mineral deposits form by a wide variety of processes. All of these processes involve the concentration of one or more chemical elements from a previous host in which the elements were not as concentrated. Most elemental concentration depends upon the chemical properties of the concentrated element but the physical property of density controls the formation of placer mineral deposits and some ultramafic layered deposits.
The most famous placer deposits are those of gold. The density of pure gold is 19.3 g/cm3 but the density of placer nuggets is a little less than this because gold invariably is alloyed with some silver. The density of pure silver is 10.5 g/cm3 .
Before becoming physically concentrated within streams, specks of gold-silver alloy are disseminated within a hostrock which may be of any rock type. No matter what the hostrock may be, the gold-silver nuggets are at least five times denser than the hostrock and so readily become separated on the basis of density. Density separation from clasts of the hostrock occurs when both the gold-silver specks and hostrock clasts are eroded by a stream.