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Publication authorized by the Director, U. S. Geological Survey.

Abstract

The final stage in the magmatic development of a composite alkalic stock of late middle Eocene age in the Bearpaw Mountains, Montana, is marked by the formation of a volcanic neck of hyperalkalic, subsilicic, porphyritic cancrinite syenite. Extensive alteration of the syenite produced seriate, calcite, sulfide minerals, barite, and zeolites, particularly in the central part, or throat, of the volcanic neck.

The sericitized syenite plug and nearby rocks were then severely fractured, and these fractures were filled by pegmatites, veins, and vein-dikes up to 50 feet thick and composed essentially of different proportions of potassic orthoclase, biotite, calcite, pyrrhotite, and pyrite. Accessory minerals include apatite, aegirite, zircon, uranium-rich pyrochlore, barite, rare-earth carbonates, and ilmenite. These silicate-carbonate-sulfide deposits petrologically are within the families of rocks designated by Brogger as carbonatites and silicocarbonatites.

Different mineral assemblages in the sericitized syenite and in the nonseri-citized carbonatites-silicocarbonatites as well as petrographic and chemical variations of rocks and minerals in the different assemblages pose problems of physical state, chemical composition, and geochemical history. The evidence indicates that early crystallization of silicate minerals in the magma yielded an aqueous residual fluid that became progressively enriched in CO2, S, F, P, Ca, Fe, Mg, Si, Al, K, and Na. Anionic complexing is a likely mechanism for transporting metals in fluids relatively richer in CO2 and H2S than normally envisioned for hydrothermal solutions. Fractionation of these fluids accounts for the wide variety of mineral composition observed. Maximum concentration of cations in these fluids remains a problem for experimental inquiry.

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