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Circular 177—Geology and geothermal waters of Lightning Dock region, Animas Valley and Pyramid Mountains, Hidalgo County, New Mexico

By W. E. Elston, E. G. Deal, and M. J. Logsdon, 1983, 44 pp., 7 tables, 26 figs., 1 sheet.

Covers the geology and geochemistry of Pyramid Peak, Swallow Fork Peak, Table Top Mountain, and South Pyramid Peak 7½-min quadrangles, which include the Lightning Dock Known Geothermal Resources Area (KGRA). This circular covers the geology of the Pyramid Peak, Swallow Fork Peak, Table Top Mountain, and South Pyramid Peak 7½-min quadrangles, which include the Lightning Dock KGRA. Hot wells (70–115.5ºC) seem to be structurally controlled by intersections of the ring-fracture zone of an Oligocene ash-flow tuff cauldron (Muir cauldron), a Miocene-to-Holocene north-trending basin-and-range fault (Animas Valley fault), and a northeast-trending lineament that appears to control anomalously heated underground waters and Pliocene-Pleistocene basalt cones in the San Bernardino, San Simon, and Animas Valley. The Muir cauldron, approximately 20 km in diameter, collapsed in two stages, each associated with the eruption and collapse, not to the modern geothermal system. Contrary to previous reports, no silicic volcanic rocks younger than basin-and-range faulting are known; unconformities beneath rhyolite ring-fracture domes are caused by Oligocene caldera collapse, not by basin-and-range faulting. The Animas Valley is the site of widespread post-20 Ma travertine deposits and near-surface veins of calcite, fluorite, and/or psilomelane, controlled by north- or northwest-trending basin-and-range faults. The fluoride-bearing waters of the Lightning Dock KGRA may be a late stage of this hydrothermal activity. Distribution of Pliocene-Pleistocene basalt suggests that deep-seated basalt near the solidus may be the ultimate heat source.

In 1948, boiling water at 115.5ºC and a rock described as hot rhyolite were encountered at 26.5 mi in a water well being drilled in the Animas Valley, Hidalgo County, New Mexico. Since then, several more wells have been drilled. Their locations, shown on the U.S. Geological Survey Swallow Fork Peak 7½-min quadrangle, are in the southwest corner of New Mexico, approximately 25 km southwest of Lordsburg. The nearest settlement is Cotton City, about 7.5 km to the southwest. The well site is directly behind the house of the present owner. It can be reached by a graded road from NM-338, 4.5 km to the west. The Animas Valley is typical of the Basin and Range province. Its interior drainage leads to two playa lakes, remnants of Pleistocene Lake Animas which once was up to 20 m deep. The valley is approximately 18 km wide; the hot wells are 5 km from its eastern edge. To the west, the valley is bordered by the Peloncillo Mountains and to the east by the Pyramid and Animas Mountains. Compared to other ranges in the region, the Pyramid Mountains are low and deeply eroded; their highest point, Pyramid Peak, elevation 1,830 m, stands approximately 566 m above the floor of the nearest playa. Annual precipitation at Lordsburg is about 240 mm and natural vegetation is sparse.

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