Miocene marine phosphogenesis along the Peruvian coast: origin and sedimentological significance of the Pisco Formation phosphorites

Phosphorites are sedimentary rocks that have a great economic relevance due to their high content in phosphorus, a central ingredient in fertilizers. Along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age, usually concentrated at stratigraphic unconformities. However, the processes leading to their formation are still poorly understood. Therefore, the objective of this study is to investigate the petrographic and chemical composition of these Miocene phosphorite layers, identifying their mineralogical composition, comprehending the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis of P-rich sediments and phosphorite accumulations, and reconstructing the paleoenvironmental conditions that favored their development. Field investigations and sampling were followed by geochemical and textural investigations, carried out by means of optical microscopy, XRD and XRF analyses, SEM, EPMA, ICP-MS, and LA-ICP-MS analyses. Our results indicate that the phosphorite layers are mainly composed of phosphatic clasts showing detrital minerals or fossil fragments at the core and phosphatic coating made of fluorapatite, with minor amounts of terrigenous clasts cemented by dolomite, calcite, Fe-oxides, diagenetic gypsum/anhydrite and baryte. The mineralogical and textural relationships suggest that the phosphorite layers were deposited in a shallow marine environment characterized by high biological productivity and low sedimentation rates. The studied phosphorite beds formed near the sediment–water surface during early transgression phases, at a time when the porewaters were enriched in phosphate due to the presence of organic-rich sediments, as typical of upwelling settings. The concentration of phosphate-coated clasts was favored by sedimentary condensation mechanisms. Overall, this study provides new insights into the genesis of phosphorite layers in the East Pisco Basin and highlights the importance of understanding the complex interplay between authigenic and sedimentological processes in the formation of these economically and geologically significant sedimentary rocks.