High Sr/Y Magmas Reflect Arc Maturity, High Magmatic Water Content, and Porphyry Cu ± Mo ± Au Potential: Examples from the Tethyan Arcs of Central and Eastern Iran and Western Pakistan

• DOI: 10.2113/econgeo.107.2.295
• Publication Date: 2012-02-15T21:44:16Z
• Authors: Jeremy P. Richards; Terry Spell; Esmaeil Rameh; Abdul Razique; Tim Fletcher
• Keywords: Back-arc basin; Island arc; Volcanic arc; Phenocryst

The temporal and geochemical evolution of arc magmatism that culminated in porphyry Cu ± Mo Au deposit formation has been studied three separate Neo-Tethyan systems central eastern Iran, western Pakistan. Porphyry Cu-Au deposits the Lut block Iran formed middle Eocene at end a period extensive Paleocene-Eocene volcanism; Cu-Mo Kerman belt Miocene voluminous Eocene-Oligocene Chagai Pakistan four pulses during Eocene, early Miocene, middle-late late Miocene-Pliocene, after prolonged began Late Cretaceous (and is still active).

In each region, porphyry-related magmas are more geochemically evolved hydrous (as evidenced by presence hornblende phenocrysts) than preceding volcanic rocks. We suggest this reflects maturation magmatic system over tens millions years, leading to generation evolved, volatile-rich later stages arc’s history. High water contents prerequisite for shallow crustal emplacement subsequent potentially ore-forming subvolcanic magmatic-hydrothermal systems.

It thus suggested fertility within given terrane can be assessed observing relative timing plutonic suites (later prospective), noting common or biotite phenocrysts (indicating high contents), through lithogeochemical fingerprinting fractionation processes (relatively Sr/Y La/Yb ratios, Eun/Eu* ratios ≈1, indicating abundant suppression plagioclase crystallization magmas).