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 uncomformably. 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. The 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 northeast yielding. WNW-trending open folds were subsequently formed. Volcanism began 30 m.y. later 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.
Two types of ore deposits are present. Contact metasomatic copper mineralization occurs adjacent to monzonite porphyry intrusive rocks at the Chapo, Sumertime, and Apache mines. Mineralization is characterized by pods and dissemination of chalcopyrite and secondary copper minerals in a calc-silicate gangue. Quartz-sulfide veins containing Pb, Zn, and Cu minerals are zoned peripheral to the center axis of the Apache stock. Their distribution appears to be related to possible couldron structures. Pervasive silification and quartz veining in the Mojado Formation coincides with a magnetic anomaly in secs. 12 and 13, T.29 S.,R. 14 W. South of the Apache mine, in sec. 13, T. 28 S., R. 14 W. and in adjacent areas, volcanic rocks of the Chapo Formation are propylitically altered and contain up to several percent pyrite
