Bulletin-94—Geology and ore Deposits of Eagle Nest area, New Mexico

By K. F. Clark and C. B. Read, 1972, 152 pp., 8 tables, 23 figs., 3 plates, 1 appendix, 1 index, 5 sheets.

A major field work covering stratigraphy, structure, geomorphology, geologic history, and economic geology. In the Eagle Nest area of the Sangre de Cristo Mountains, Precambrian sedimentary and minor igneous lithologies attain a composite thickness of 18,000 ft. Apparently, emplacement of granite during the Elsonian and the formation of migmatites, occurred during the latter stages of regional metamorphism. Foliation is northeasterly.

The crystalline terrane, a positive area in early Paleozoic time, it partly overlain by a 13,000 ft section of Mississippian through Paleocene sediments. Repeated crustal warpings during Mississippian and also in later Triassic and Late Jurassic times formed thin marine and continental deposits, respectively. An aggregate thickness of 711 ft is preserved. In contrast, trough deposition, during the interval between Pennsylvanian and Early Permian accumulated 8,200 ft of marine arkosic and continental red bed sediments. A deepening basin preserved 2,000 ft of Cretaceous marine strata.

The evolution of the southern Rocky Mountains geanticline during Laramide time was accomplished by deep-seated basement uplift along upthrust and flattened underthrust contacts with overlying strata. This orogeny is reflected in 1,000 ft of terrestrial Paleocene deposits. Lower to middle tertiary volcanic units, nearly 3,000 ft thick, contain thin clastics, andesite, latite, and rhyolite. Intrusives include Eocene Rio Hondo hornblende granite, Oligocene quartz diorite porphyry, and early Miocene Questa mine biotite granite. Siliceous intrusives, emplaced along lines of weakness, contributed to uplift by doming.

Early Miocene granites produced widespread and locally intense hydrothermal alteration. Contemporaneously, metallization occurred in the Red River district in fissure veins and disseminations. The Questa open-pit stockwork molybdenum deposit, averaging 0.297% MoS₂, commenced production in 1965. Minor Precambrian quartz-copper mineralization occurs in the Rio Hondo district. In the Elizabethtown-Baldy districts fissure vein gold and contact iron deposits are Oligocene in age. Subsequently placer deposits formed in the Moreno Valley.

Middle to late Tertiary high-angle normal faulting dislocated a mid-Tertiary erosion surface. Structural and topographic relief of Laramide features were enhanced, producing several well-defined topographic, tectonic, and lithologic features: the Taos horst, Red River graben, Moreno Valley and western flank of the Cimarron Range. Late Pliocene to early Pleistocene gravels accumulated in the Moreno Valley, and drainage integration followed. Radially disposed Wisconsin valley glaciation, coupled with fracture patterns, control drainage in uplifted areas.

For many years systematic geologic mapping has been carried out in the Sangre de Cristo Mountain range. In NM much of the range has been mapped, yet some parts received little or no geologic investigation. Although some aspects of the Eagle Nest Area still require more detailed examination, this report presents the general geology as a broad foundation on which future work may be based.

The area exhibits a multitude of geologic features requiring investigation. In particular, the Precambrian terrane has been subdivided; Mississippian, Pennsylvanian, and Permian stratigraphic problems have received special treatment. The structural fabric has been described in detail because of anomalous implications arising from this interpretation. Finally, mineralization in three mining districts, one of which includes a large open-pit mine brought into production in 1965, summarizes the mineral resources.