Genesis of Gem-Quality Peridot from Sapat, Pakistan: Constraints from Gemmology, Mineralogy, and Geochemistry

Pakistan is recognized as a notable source of gem-quality peridot, which is renowned internationally for its attractive yellowish-green to light greenish-yellow color and large crystal size. This paper elucidates the petrogenesis of Pakistani peridot based on coupled gemological, mineralogical, and geochemical evidence. Pakistani peridot is commonly associated with serpentine, magnetite, and minor talc, occurring within pockets and veins of serpentinized dunite. The most common inclusions identified in peridot include ludwigite, magnetite, serpentine, and brucite. Among these, ludwigite inclusions serve as a significant indicator for geographic origin determination. Pakistani peridot exhibits significant internal compositional heterogeneity, with forsterite (Fo) contents ranging from 88 to 96. Trace element analyses show that Pakistani peridot is enriched in incompatible elements such as boron and lithium. The peridot samples yield δ18O compositions spanning 3.94‰–6.17‰, averaging 5.04 ± 0.63‰. Isotopic signature confirms a mantle-derived signature and precludes significant involvement of crustal-derived fluids. The mineral composition and geochemical characteristics, combined with the geological setting, suggest that the Pakistani gem-quality peridot precipitated from B- and CO2-rich subduction-derived hydrothermal fluids, which migrated along extensional fractures within the highly permeable dunites.