Molybdenum is a trace metal commonly used as a marker of paleoredox conditions of ancient depositional settings. Pyrite is an important molybdenum (Mo) host for enriched sedimentary rocks. In this paper we study the relationship between Mo and pyrite in the Bancs Jumeaux Formation, a Jurassic succession in northern France consisting of limestone and pyrite-rich marls. This formation is relatively enriched in Mo compared to other redox-sensitive trace metals. Our approach is grounded on bulk rock chemical analysis and delineation of two contrasting types of pyrite that can be extracted from the rocks: polyframboids and nonframboidal concretionary masses. The morphological characteristics of both morphotypes were studied using scanning electronic microscopy. The polyframboids are richer in Mo than the concretions but are not markedly enriched in other trace metals. This discrepancy in geochemical composition could result from pyrite precipitation at different times during early diagenesis. Our results tend to indicate that the polyframboids would have formed very early in reducing “microniches”, within dominantly dysoxic sediment. This early pyrite precipitation occurred at shallow depth below the sediment-water interface close to the abundant Mo source in overlying oxic seawater (molybdate ions), and would have fostered Mo-capture by the polyframboids in relatively large amounts. The concretions would have formed later during early diagenesis (within the sulfidic zone) under conditions of more limited Mo availability.

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