From Skinner and Porter, 1987
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:
- Geological processes operating due to energy released at plate boundaries control the process of mineral deposition.
- Mineral deposits form in particular tectonic settings which are governed by plate tectonics.
- Reconstruction of fragmented continents can provide a useful basis for exploration of new mineral deposits.
Several requirements must be fulfilled in any tectonic setting for the production and accumulation of minerals. A number of tectonic settings meet these requirements:
I. DEPOSITS AT OCEANIC RIDGES (DIVERGENT PLATE MARGINS)
- Hydrothermal activity at the ridges gives rise to a) Sulfide deposits and b) Metalliferous sediments on the flanks of ridges. Important metallic deposits formed are Fe, Zn, Cu, Pb, Au and Ag. In the Red Sea metalliferous sediments containing Fe, Zn and Cu are being deposited.
- Mn oxide deposits are important at some ridges Eg the TAG Hydrothermal field on the Atlantic Ridge.
- Ultramafic rocks in ophiolites containing asbestos, chromite and nickel ores. These are generally accessible in Phanerozoic orogenic belts to which sites they have been transported due to plate movement.
- Podiform chromite deposits associated with serpentinized ultramafic rocks.
- Cyprus Type massive sulfide deposits (Cu-Fe rich) are also associated with ophiolites and represent hydrothermal deposits formed at ocean ridges.
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:
- Base metals (Cu, Pb, Zn, Mo).
- Precious metals (Pt, Au, Ag).
- Other metals (Sn, W, Sb, Hg). (Red Bed uranium deposits are also associated with convergent boundaries Eg SW United States).
Zoning of mineral deposits forming at convergent margins is apparent Eg in the Andes, going from west to east, the various zones encountered are:
a) contact metasomatic Fe- deposits;
b) Cu-Ag and Ag veins;
c) Porphypy Cu-Mo deposits;
d) Pb-Zn-Ag veins and contact metasomatic deposits; and
e) Sn 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.
Potential 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:
- Deposits generally related with oceanic ridges (ophiolites).
- Those associated with convergent plate margins.
- Mineral deposits associated with cratonic assemblages.
- Deposits associated with continental rifts.
- Deposits genetically related to collision zones are hydrocarbons which may accumulate in foreland basins associated with such zones, Eg the Persian Gulf SW of the Zagros Suture in Iran.
IV. DEPOSITS IN CRATONIC RIFT SYSTEMS
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.
- Granites intruded at this stage have associated Sn and fluorite deposits.
- Evaporites accumulate in the rifts during the more advanced stages, with Pb-Zn-Ag deposits in limestones forming during the early and middle stages of rifting. These are followed by oceanic metalliferous deposits.
- Aulacogens are characterized by the presence of fluorite, barite, carbonatites (with Nb, P, REE, U, Th etc) and Sn-bearing granites.
- Geothermal fields occur along the rifts due to the upwelling of the asthenosphere.
- Carbonatites (unusual igneous rock rich in calcite and other carbonate minerals which are considered to be mantle derived), kimberlites, and alkaline granites within or adjacent to rifts provide a major source of metallic and other minerals.
V. DEPOSITS IN CRATONIC BASINS
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.
by: S. Farooq Department of Geology Aligarh Muslim University, Aligarh - 202 002 (India)