Geology and geochemistry of the mid-Tertiary alkaline to calc-alkaline intrusions in the northern Hueco Mountains and adjacent areas, McGregor Range, southern Otero County, New Mexico
— Virginia T. McLemore

Two lithologic and chemically distinct groups of igneous rocks are found in the McGregor Range, southern Otero County; syenite to trachyte and quartz monzonite and rhyolite. Alkaline and metaluminous syenite to trachyte stocks, sills, and dikes intruded Paleozoic sedi­mentary rocks in the northern Hueco Mountains. Calc-alkaline and metaluminous quartz monzonite is exposed in the eastern Three Buttes area of the McGregor Range. Drill holes in the Hueco geothermal field at Davis Dome, south of Three Buttes, encountered caIc-alkaline and peraluminous to metaluminous rhyolite in the subsurface. In the Hueco Mountains, the igneous rocks are 34.5-34.7 Ma and are similar in age and chemical composition to those in the Cornudas Mountains. The Three Buttes pluton may be similar in age, however poor results from geochronological dating (36.7±9.3 Ma), are of limited geochronological use. Geochemical compositions of the igneous rocks suggest that the magma probably originated within the lithospheric mantle and acquired a significant enrichment in lithophile-group elements during its movement through the crust. These data are consistent with either a continental rift or a subduction-related back-arc extension setting. Only small, discontinuous skulls and jasperoids that are low in precious and base metals are found adjacent to some intrusions, thereby making the mineral-resource potential low.