Ore Deposits and Plate Tectonics
Plate tectonics, like crustal evolution, provides a basis for understanding the distribution and origin of mineral and energy deposits. The relationship of plate tectonics and mineral deposits is significant on three counts:
I. DEPOSITS AT OCEANIC RIDGES (DIVERGENT PLATE MARGINS)
Several types of mineral deposits appear to show a genetic relationship to either the hot mantle plume itself or the tracks it produces.II. DEPOSITS AT CONVERGENT PLATE MARGINS
Metallic deposits are commonly found at both continental and arc convergent plate margins. Along the Circum-Pacific Belt major metallic deposits occur in western North and South America, Japan, Philippines, New Zealand and Indonesia. More than half of the world's supply of copper comes from the Porphyry Copper Deposits of this region. Important deposits associated with present and former convergent margins are:
a) contact metasomatic Fe- deposits;These zones are caused due to progressive liberation of metals from the descending slab, with Sn coming from a depth of 300 Km. The metals are derived from some combination of the descending slab and the overlying mantle wedge. They move upwards in magmas or fluids and are concentrated in late hydrothermal and magmatic fluids.
Petroleum occurs in the back-arc basins in arc convergent margins where organic matter is trapped and there is a lack of free circulation so that its oxidation is prevented. Geothermal heat facilitates conversion of organic matter to petroleum, and accompanying deformation forms traps for accumulation of petroleum.
geothermal fields also occur along convergent margins.
III. DEPOSITS AT COLLISION BOUNDARIES
A variety of local tectonic settings exist along collision plate margins. Most of the deposits that occur here have formed in diverse tectonic settings and have been transported to the collision zones. Consequently, a variety of metallic deposits are abundant here:
Regional uplift and doming usually result when a continent comes to rest over a hotspot and huge volumes of magma rise to the surface. Extensional failure of the lithopheric crust may occur with continued doming, triggering the development of a triple junction - a three armed continental rift system. Typically, one arm of the rift fails remaining a fissure in the crust known as an aulacogen, while the remaining two arms open to form an oceanic basin. The prevalence of three armed rifts is revealed by reassembling the continents surrounding the Atlantic Ocean to their positions before Pangea split up. In most cases two of the arms were incorporated into the Atlantic, while the third remained as a blind rift extending into the continent.
Rifting follows crustal doming in response to hot-spot activity in the mantle.
Marginal and intracontinental cratonic basins provide a favourable setting for accumulation of organic matter. During the opening of a cratonic rift, seawater moves into the basin and evaporation exceeds inflow, giving rise to the formation of evaporites. The environment is characterized by restricted circulation and hence organic matter is preserved leading to the accumulation of petroleum. With continued rifting, circulation becomes unrestricted and deposition of evaporites and organic matter ceases.
High geothermal gradients beneath the opening rift and increase in pressure due to burial by sediments facilitates the conversion of organic matter to petroleum. In the final stages of the opening of the basin, the salt beds may begin to rise as salt domes forming traps for oil and gas. Oil and gas may also be trapped in structural and stratigraphic traps as they move up due to increasing temperature and pressure, Eg the Red Sea.
This speculation is lent support by the fact that around the Atlantic there is a close geographic and geologic relationship between hydrocarbons and salt accumulations.
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Department of Geology
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