Geology and mineral resources of the Lake Valley area, Sierra County, New Mexico

Abstract

At the request of the Bureau of Land Management, the U.S. Geological Survey assessed the Lake Valley Area of Critical Environmental Concern (ACEC), which includes the historic Lake Valley townsite and silver-manganese mining district, for undiscovered mineral resources. The Lake Valley ACEC is along the southeastern margin of the Black Range of western Sierra County, New Mexico. The Black Range contains eleven mining districts from which silver, lead, zinc, manganese, copper, gold, and tin have been recovered. As part of the study, an area of over 75 square mi (195 square km) in the area of the Lake Valley ACEC was mapped to understand ore controls and to assess mineral-resource potential. Lake Valley mining district is in Mississippian Lake Valley Formation carbonate rocks, but much of the surrounding terrane consists of volcanic rocks on the southeastern edge of the Emory cauldron which formed about 34.9 Ma. Volcanic rocks are part of the Mogollon-Datil volcanic province which include flows, breccias, ash-flow tuffs, and intrusive rhyolites. The Lake Valley mining district is located about 20 mi (32 km) south of the Late Cretaceous Laramide copper-gold porphyry intrusion at Hillsboro. It is also located at the western boundary of the Rio Grande rift basin.The Lake Valey fault is the major structural feature of the study area. Geological and geophysical data suggest the fault is composed of two segments: a southern, northwest-striking segment that may have a pre-Tertiary history and a northerly-striking segment that is part of the Emory cauldron ring fracture. The mining district is bounded by the southern, northwest-striking segment of the fault which may have up to 800 ft (240m) of normal offset. Deposit types identified in the Black Range, and for which we assess mineral potential, are Laramide porphyry, Laramide skarns, Laramide veins, gold placer, carbonate-hosted, volcanic-epithermal, and rhyolite tin; no Rio Grande Rift baritefluorite-galena deposits are known, but they are included in the assessment. The most likely deposits to be discovered in the Lake Valley district are carbonate-hosted silver-manganese deposits. These deposits could be deposited by hydrothermal fluids or be related to intrusion of Oligocene rhyolite or Laramide porphyry. An aeromagnetic high south of the district probably reflects a small felsic intrusion of unknown age. We assess the Lake Valley ACEC as having low to moderate potential for undiscovered carbonate-hosted deposits associated with volcanic rocks or Laramide porphyry plutons, and low potential for gold placer and rhyolite tin deposits.