Geology, geochemistry, and mineralization of synenites in the Red Hills, southern Caballo Mountains, Sierra County, New Mexico--Preliminary observatons
— Virginia T. McLemore

More than 25 radioactive, brick-red, flat-lying tabular bodies and near-vertical pipes of syenite occur within the Precambrian granite in the Red Hills. They consist of microcline with subordinant quartz, muscovite, hematite/goethite, chlorite, and trace of plagioclase. Compositions range from syenite to quartz syenite to alkali granite. Accessory minerals include apatite, zircon, calcite, fluorite, limonite, magnetite, and barite. Small, discontinuous pockets or zones of uranium and thorium mineralization occur in some syenite bodies and consist of anomalous amounts of spinel, rutile, anatase, thorite, thorogummite, and possible uraninite. These zones are uneconomic because of discontinuity, low grade, small size, and poor market conditions. The syenites are higher in K₂O, Al₂0₃, MgO, U, and Th and lower in Na₂O and SiO₂ than the adjacent granites. They have a high K₂O/Na₂O ratio compared to most syenites. Pervasive hematitization, chloritic alteration, and uranium– thorium mineralization have obscured original mineralogy and textures. Field observations, replacement textures in thin sections, mineralogy, and high K₂O/Na₂O ratios suggest that the syenites are a result of potassium metasomatism of either a syenite intrusive or of the adjacent granite. The syenites may be part of an alkalic magmatic event that affected New Mexico during late Precambrian to Cambrian–Ordovician times; radiometric dating of the Red Hills syenites is needed. Additional geochemical studies, isotopic studies, radiometric dating, and detailed mapping of syenites in the Caballo Mountains and elsewhere in Precambrian terranes in New Mexico are required to understand the origin of these rocks.