MINERALIZATION ALONG CONVERGENT AND COLLISIONAL PLATE BOUNDARY ENVIRONMENTS

 I. Deposits at Convergent Boundaries:

• In convergent plate boundary environments mineralization takes place in the principal arcs as well as in the inner side of the principal arc.

• Principal arcs are linear belts of volcanoplutonic rocks that occur above subducting lithospheric slabs.

• Mineralization, particularly of Cu, Fe, Mo, Au and Ag are closely associated with calc-alkaline magmatism in such zones.

• 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:

 i) Base metals (Cu, Pb, Zn, Mo).

ii) Precious metals (Pt, Au, Ag).

iii) Other metals (Sn, W, Sb, Hg).

 (Red Bed uranium deposits are also associated with convergent boundaries Eg SW United States).

• Mineralization is also noted on the inner side of the principal arcs.

• Examples of these are the contact metasomatic (skarn) deposits of Zn, Pb, Ag in eastern Peru, and on the eastern side of the Sierra Madre ranges in Mexico.

• These deposits are related to subduction in the western United States during Cretaceous to mid-Tertiary.

• The tin-tungsten belts of Bolivia, East Indian Archipelago and South China are thought to be located along similar tectonic settings.

• 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.  Fig. 1 shows a compressional arc system and related metallogeny.

• 'Tensional arcs' represented by the Japanese arc-system are thought to be suitable for the deposition of Kuroko-type deposits (skarn deposits are also common here).

• 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.  Fig. 2 shows an extensional arc system and related metallogeny.

• Potential geothermal fields also occur along convergent margins.

II. Deposits at Collision Boundaries:

• In the collisional settings, important tectonic zones are the hinterland margins, suture zones, foreland thrust belts and foreland basins.

• In the obducted ophiolites in the suture zone, stratiform exhalative Cu-Fe sulfide deposits occur at Cyprus and Newfoundland, Canada (Ordovician), and podiform chronite deposite at Semail, Oman (Cretaceous).

• In the foreland thrust belts, Sn-W mineralization occurrs in the S-type granites in SW England and probably in the Central Himalaya.

• In the foreland molasse basins, uranium mineralization is reported from the Siwaliks in India and Pakistan.  Fig. 3 shows a collisional tectonic environment and related metallogeny.

• Most of the deposits that occur in collisional zones may actually have formed in diverse tectonic settings and have merely been transported to the collision zones.

• Consequently, a variety of metallic deposits are abundant here:

1. Deposits generally related with oceanic ridges (ophiolites).

2. Those associated with convergent plate margins.

3. Mineral deposits associated with cratonic assemblages.

4. Deposits associated with continental rifts.

5. 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.