Magmatic (Cr–Ni–PGE) and secondary/hydrothermal (emerald–peridot–rodingite–nephrite jade) mineralization associated with ultramafic–mafic complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geological history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with ultramafic–mafic plutonic rock occurrences in the western and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of platinum group elements (PGE) and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite and nephrite jade) associated with ultramafic–mafic complexes of the two suture zones. Chromite with high Cr# (68–85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni sulfides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the Indus Suture Zone, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89–92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al and possibly Na. In view of the undeformed nature of the emerald and peridot, and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene–Early Miocene age. The metasomatizing fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or related to 23–26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe 2+ /(Mg 2+ + Fe 2+ ) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in the Dargai ophiolite could have formed by the action of seawater channelled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca-, Al- and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.