Geology of the Summit Mountains and vicinity, Grant County, New Mexico, and Greenlee County, Arizona

The Summit Mountains and surrounding region is on the Arizona-New Mexico state line about 25 km northeast of Duncan, Arizona and includes the Steeple Rock and Duncan mining districts. Bedrock in this region consists of a layered volcanic pile of mid-Tertiary calc-alkaline andesites, basaltic andesites, dacites, rhyolite flows and associated volcaniclastic sedimentary rocks. A few thin ashflow tuffs are intercalated with the andesites as are lenticular bodies of flow-banded rhyolite intrusive-extrusive complexes. The volcanics are intruded by shallow sub-volcanic glassy, flow-banded rhyolite plugs and dikes, by intrusive breccias, and by hypabyssal stocks and dikes of granite porphyry and microdiorite. Mid-Tertiary to Recent basin fill sediments and alluvium bound the study area on the west. The principal structures in the region are northwest-trending high angle normal and reverse faults and shear zones. Northeast-trending structures are also important and lesser fault and shear zones trend north and east. Many of the ancestral faults are probably Precambrian or early Paleozoic in age; most faults show evidence of recurrent movement, possibly into the present. Some evidence of right lateral strike-slip fault movement was noted. The possibility of a cauldron in the study area is considered to be weak. Ore deposits in the region are epithermal veins of gold, silver. copper, lead and zinc, and fluorite. Large parts of the volcanic pile have been affected by solfataric alteration. The locations of the ore deposits and altered areas are controlled by fault and fracture intersections. The alteration and mineralization are believed to result from localized deuteric alteration in a sulfur-rich volcanic sequence.Earth and Atmospheric Sciences, Department o