Geology of the Apache No. 2 Mining District, Hidalgo County, New Mexico
2 map files are included in Additional Files.
The Apache Hills, 10 km south of Hachita, New Mexico are a WNW-trending series of low hills, approximately 12 km long.
The rocks range in age from Paleozoic to Holocene. Limestone and sandstone of the Lower Cretaceous U-Bar and Mojado Formations are overlain by a thrust plate of Paleozoic limestone. The Oligocene Chapo Formation (new name) overlies the thrust plate unconformably. The Formation is over 1,700 m thick and ranges in composition from rhyolite to basalt. The sedimentary and volcanic rocks have been intruded by several igneous rock types, the most prominent of which is a stock of quartz monzonite porphyry, elongated WNW. '!he stock was emplaced at shallow depth and has been dated at 27 m.y.
The first post-Mojado structural event was the formation of thrust faults with attendant drag folds. Orientation of drag folds and fracture patterns in the footwall rocks suggest northeastward yielding. WNW-trending open folds were subsequently formed. Volcanism began 30 m.y. ago with the extrusion of quartz latite flows and culminated 3 m.y. later with intrusion of a quartz monzonite porphyry stock. Cauldron formation is suggested by the large volume of silicic flows, resurgent nature of the subvolcanic stock, the alignment of rhyolite dikes and plugs along possible cauldron margins, and possible moat deposits. Movement on high-angle faults post-date volcanic rocks.