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

Abstract

This circular covers the geology of the Pyramid Peak, Swallow Fork Peak, Table Top Mountain, and South Pyramid Peak 7 1/2-min quadrangles, which include the Lightning Dock KGRA. Hot wells (70-115.5 °C) 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 Valleys. The Muir cauldron, approximately 20 km in diameter, collapsed in two stages, each associated with the eruption of a rhyolite ash-flow-tuff sheet and of ringfracture domes. Most of the hydrothermal alteration of the Lightning Dock KGRA is related to the first stage of 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 m.y. 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 deepseated basalt near the solidus may be the ultimate heat source.