Geology of the southern San Mateo Mountains, Socorro and Sierra counties, New Mexico
— Virginia T. McLemore

The San Mateo Mountains of Socorro and Sierra Counties, south-central New Mexico, are bounded by the Monticello graben to the west and the Mulligen Gulch and San Marcial basin to the east. The oldest rocks exposed in the southern San Mateo Mountains are Proterozoic and Paleozoic rocks exposed east of the Nogal Canyon fault at Bell Hill; Paleozoic limestones also are found south of Vicks Peak. The Datil Group contains all late Eocene-early Oligocene volcanic strata up to the 31.8 to 29.4 Ma regional hiatus in volcanism and includes the Rock Springs and Red Rock Ranch formations, Hells Mesa Tuff, and La Jencia Tuff. The southern San Mateo Mountains is dominated by the Nogal Canyon caldera (28.4 Ma). The estimated diameter of the caldera is 25 km, and Lynch (2003) estimated the total volume of the Vicks Peak Tuff as 1816 km³. Re-interpretation of past studies and recent re-mapping by the author in the southern San Mateo Mountains has refined understanding of the Nogal Canyon caldera. The Vicks Peak Tuff (28.4 Ma) and associated rhyolite and quartz latite flows and domes erupted from this caldera. The Vicks Peak Tuff is >490 m thick and is locally overlain by >550 m of rhyolite. Stratigraphic relationships indicate that the eruption of the Vicks Peak Tuff was followed by intrusion of the granite of Kelley Canyon and eruption of the rhyolite of Alamosa Canyon, within <0.42 Ma (Lynch, 2003). The Springtime Canyon Formation overlies the Vicks Peak Tuff east of Vicks Peak and consists of rhyolite, quartz latite and latite flows and associated tuffs erupted along the eastern caldera boundary, probably during this time period, but radioisotopic dating is required. Rhyolite dikes and small rhyolite domes that could be related to the caldera were erupted along the southern and northern boundaries of the caldera. The northern boundary of the caldera is partially concealed by the formation of the Mt. Withington caldera and the eruption of the Vicks Peak Tuff and younger rhyolites. The caldera was offset locally by younger normal faults (i.e. Rock Springs, Priest, Indian Peaks, Rhyolite, Dark Canyon, and Bell Mountain faults). The San Jose mining district is within the Nogal Canyon caldera and could be associated with the formation of the caldera. The San Mateo Mountains mining district is south of the Nogal Canyon caldera.