Siliceous complexes and silicification dynamics in the Maastrichtian–Eocene phosphate series of Boucraa, Morocco

Abstract

ABSTRACT Chert-bearing phosphate successions are globally widespread and provide a robust framework for investigating the interplay between silica, redox conditions, and phosphogenesis. The Upper Cretaceous–Paleogene BouCraa phosphate deposit in southern Morocco exemplifies this framework, featuring two distinct silica-rich complexes in a carbonate-poor stratigraphic succession. Siliceous lithofacies from three representative sections were analyzed using optical petrography, X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), inductively coupled plasma–optical emission spectroscopy (ICP-OES), and inductively coupled plasma–mass spectrometry (ICP-MS). The results reveal two types of siliceous facies, detrital and nondetrital, characterized by variable P2O5 contents. XRD patterns are dominated by chalcedony, quartz and opal-CT, and carbonate fluorapatite (CFA). Silica is mainly of biogenic origin, sourced from the early dissolution of diatoms in a restricted marine environment. The geochemical signature of the silicification and phosphogenesis environments appear to be the same, marked by suboxic to ferruginous conditions, as evidenced by the general enrichment of redox-sensitive trace elements (e.g., Mo, V, U). Two groups of REE signatures were distinguished: the first group, associated with clay-rich and low-phosphate samples, shows moderate Ce anomalies and δCe values indicative of suboxic conditions, probably reflecting limited circulation and water-mass turnover. In contrast, the dominant, more phosphate-rich group shows pronounced negative Ce and δCe anomalies and HREE-enriched profiles, similar to those of seawater, indicating oxic depositional conditions. These oxygenation episodes were associated with higher bottom-water energy and episodic reworking. Such conditions exerted a fundamental control on silicification, which proceeded in two stages: an early phase of matrix replacement under reducing interstitial conditions, locally intensified by evaporative concentration, followed by a late-stage cementation associated with circulation of phreatic fluid. This study introduces a multiphase silicification model linked to phosphogenesis in a redox-stratified epicontinental setting. The proposed framework is applicable to other siliceous phosphate systems and offers a valuable tool for guiding the exploration of economically significant phosphorite deposits.