Petrology of gem peridot from Sapat mafic-ultramafic complex, Kohistan, NW Himalaya

Abstract

A large quantity of peridot has recently been marketed from the Parla Sapat area to the NE of Naran. The peridot occurs in pockets and veins located in shear zones in partially serpentinized dunitic host rocks. The latter constitute basal cumulates of the Kohistan magmatic arc thrust onto the Indian plate during the Early Paleocene. The peridot is associated with serpentine (essentially chrysotile), minor magnetite, and local magnesite and talc. It is mostly yellowish green, takes a good polish, and suitable specimens are faceted into brilliant stones of highest quality. It occurs in euhedral to subhedral crystals, but most stones are broken due to crude methods of recovery. The stones are mostly <3 cm in length, but up to 15 cm long crystals weighing 2 kg have been recovered. The largest faceted stone is reported to measure about 310 ct.

The peridot is pleochroic from pale green (α = Y) to yellowish green or olive in greenish yellow variety (β = Z). Refractive indices (α =1.644 to 1.653, γ=1.682 to 1.684±0.003), density (3.26 to 3.44) and EPMA data suggest that most specimens range in composition from Fo₈₉ to Fo₉₂, but some are more magnesian (up to Fo₉₇). The depth of colour increases from light green to yellowish green with increase in the Fe content, but the greenish yellow colour may partly be related to a higher Ti content. Some peridot contains tiny inclusions of magnetite and hairy to acicular, brownish black ludwigite [(Ti _0.011 Cr_0.041 Fe³⁺_0.947) (Mg_1.085 Fe²⁺ _0.897 Mn _0.004 Ni _0.013) BO₅]. The paragenesis may be related to hydrothermal activity which also introduced CO₂ and B. The undeformed nature of the peridot suggests that the solutions may be related to the late phases of Himalayan metamorphism or leucogranites Of the underlying Indian plate, both of which are Of Eocene age.