Integrated study of the deformed phosphorite dolomite of the Early Cambrian Sirban formation in the Lesser Himalayas, Pakistan

Abstract

Phosphorite deposits in tectonically active regions present complex mineralogical and geochemical signatures due to post-depositional alteration. This study investigates the highly deformed Cambrian age phosphorite dolomite unit within the Sirban Formation of the Lesser Himalayas, Northern Pakistan, to understand its genesis, deformation history, and economic potential. Using integrated field observations, petrography, SEM–EDS, XRD, XRF, and ICP-MS analyses, the study documents the effects of intense brittle deformation along the Garlanian and Tarnawai thrust faults. Numerous quartz veins and iron oxide staining suggest active post-depositional hydrothermal fluid flow. The mineral assemblage is dominated by apatite, dolomite, quartz (micro- to cryptocrystalline), pyrite, hematite, magnetite, and chalcedony, with textural evidence of multiphase phosphate formation. SEM imaging confirmed chert and chalcedony overprints, while EDS and XRF analyses revealed high Ca and P concentrations indicative of apatite. XRD confirmed both hydroxyl-apatite and fluorapatite phases. Geochemical proxies including Co/Zn ratios (> 0.5) and V/(V + Ni) suggest a dominantly reducing, hydrothermal depositional environment, although a negative Ce anomaly (δCe = 0.68) implies oxidizing conditions during early phosphogenesis. Rare earth element analysis (ΣREEs + Y = 503.16 µg/g) showed strong light (L)REE enrichment, with economic evaluations using REE_def and K_outl (avg. 0.96) highlighting its potential as a critical REE resource. The deposit’s average P₂O₅ content of 28.33% classifies it as furnace grade, with beneficiation potential to upgrade it to acid grade. These findings support the Sirban Formation as a promising target for future phosphate and REE exploitation in the region.