Samples of phosphatic mudstone collected from several measured sections in the Retort Phosphatic Shale Member of the Phosphoria Formation near Divide, Montana, contain an abundance of montmorillonite, locally associated with kaolinite and talc. Of several possible modes of origin, hydrothermal alteration of pre-existing illite to montmorillonite best accords with related geologic phenomena, including the presence of intrusives within and/or adjacent to the formation, the associated hydrothermal alteration of the underlying dolomite to talc, the associated low-grade metamorphism of the phosphatic mudstone itself, the network of calcite and quartz veins within the mudstone, and the association of talc and montmorillonite. The kaolinite, which occurs in very carbonaceous mudstones, may be (1) further hydrothermal alteration of montmorillonite, (2) an authigenic mineral formed in a restricted part of the Retort Basin with acidic bottom conditions, or (3) a detrital mineral.

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