A ground-water mixing model for the origin of the Imini manganese deposit (Cretaceous) of Morocco

Abstract

The Imini deposit, just south of the Atlas Mountains of Morocco, consists of three beds of manganese oxide ore in a slightly deformed dolomite unit about 10 m thick, of Cenomanian-Turonian (Late Cretaceous) age. The dolomite host is sugary textured, massive, and permeable; dolomite occurs as nearly euhedral rhombs. Each ore bed is about a meter thick and consists almost entirely of intergrowths of pyrolusite and hollandite-family minerals (BaO up to 11% and PbO up to 28%) with a flattened geode structure. Overlying the ore beds are solution collapse breccias with dolomite clasts, janggunite (Mn,Fe oxide) and dolomite matrix and pyrolusite-hollandite-calcite cement. The ore, solution-collapse breccia, and coarsely recrystallized pure dolomite are coextensive--a zone only 400 to 1,000 m wide but 25 km long and elongate east-northeast-west-southwest.Diagenetic features of the deposit are: recrystallized texture of the dolomite, flattened geode structure of the ore, solution-collapse breccia and associated draped laminae of the insoluble residues, fossil replacement by Mn oxides, and zoned chert nodules with inclusions. A mineralogic paragenesis shown independently in each of these features from older to younger is (1) precursor carbonates, (2) dolomite-janggunite, and (3) hollandite-pyrolusite-chert-calcite.Diagenesis probably occurred during the Late Cretaceous in a mixing zone of fresh and saline ground water, as similar diagenetic sequences without manganese deposits are widely reported to have formed in such environments. Carbon and oxygen isotope compositions of carbonates and oxygen isotope compositions of chert show that an increasing component of fresh meteoric ground water was present as diagenesis proceeded. Falling sea level allowed the mixing zone to occupy various positions within the carbonate unit and convert the primary calcareous carbonate rocks to dolomite with chert nodules.Anoxic seawater is known both worldwide and from the local depositional basin (Atlas gulf) in the Cenomanian-Turonian. This is believed to be the key to manganese deposition, because anoxic conditions greatly enhance manganese solubility. Two specific versions of the mixing-zone model are presented for manganese deposition; one in which a primary sedimentary manganese precipitate formed along margins of the anoxic basin and was recrystallized during diagenesis in the mixing zone, and the other in which anoxic saline ground water brought manganese into the mixing zone, where it precipitated.