Sunday, April 26, 2015

Classification of Mineral Deposits

Relation to Host Rocks

Host Rocks

There are three types of rocks that host the mineralization namely igneous, sedimentary and metamorphic.
There are three types of rocks that host the mineralization namely igneous, sedimentary and metamorphic.
The examples of igneous rocks are porphyry copper deposits in granite, platinum-palladium-chromium-nickel deposits in dunite, peridotite, gabbro, norite and anorthosite, tantalite, columbite, cassiterite in pegmatite.
Ore deposits can exclusively be formed under sedimentation process like iron ore as Banded Iron Formation (BIF), and Banded Hematite Quartzite (BHQ), zinc-lead deposits in dolomite, copper-gold in quartzite, diamond in conglomerate and limestones. Metamorphic rocks host important ore deposits generated as contact metamorphic aureoles. As garnet, wollastonite, andalusite, and graphite. The metamorphic equivalent of sedimentary and igneous rocks forms large deposits of marble, quartzite, and gneisses and commonly used as building stones and construction materials.

Different from Host
Gold-bearing quartz veins act as an exclusive host for Au and different from the surrounding rocks such as Kolar gold deposit, Karnataka, India.

Host Rocks

Gradational Contact
Are often formed around the vein systems with characteristics disseminated mineral distribution. Bulldog Mountain vein systems, Colorado, show abundance of fine-grained sphalerite and galena, with lesser tetrahedrite and minor chlorite and hematite. The mineralization becomes progressively richer of barite and silver with increasing elevation.

Metal Zoning
occurs in a multiple series of hydrothermal depositional source. Mineralization zoning is characterized by Fe-Ba-Cu-Pb-Ag-Au. Metal zoning is an indication of metal deposition in relative order during primary crystallization or sedimentation. It may be modified by deformation and remobilization at later stage. The common metal zoning is in massive sulfide deposits. e.g. El Guanaco gold-copper in Chile, Zn-Cu-Au-Ag deposits of Scuddles, Golden Grove, Gossan Hill, Western Australia.

Wall Rock Alteration
The mineral deposits formed under epigenetic condition, magmatic intrusion and hydrothermal depositional environments cause changes in mineralogy including formation of new minerals, chemical composition, color and texture of the host rock at the contacts and some distance from the orebody. The size of the alteration halo around the orebody varies from narrow to wider depending on the physical and chemical condition of the process of alteration.
If these alteration halos are identified properly it adds a great value to the mineral exploration in general and particularly for planning drilling targets. The most common form of wall rock alterations are silicification, chloritization, sericitization and serpentinization. Presence of pyrite, siderite, titanium, manganese, potassium, lithium, lead, silver, arsenic, rubidium, barium, calcium, epidote and carbonaceous material is common.

wallrock alteration

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