Geology and mineral deposits of the northern Big Burro Mountains-Redrock area, Grant County, New Mexico
Authors: Hewitt Charles H. 
Published: March 2023

The northern Big Burro Mountains-Redrock area, part of the Basin and Range province, is in Grant County, southwestern NM. Precambrian metamorphic and igneous rocks underlie the northwest-trending mountains; Pliocene Gila conglomerate and younger, unconsolidated gravels underlie the desert basin southwest of the mountains. North-dipping, Upper Cretaceous Beartooth quartzite and Colorado shale beds and early- to middle-Tertiary quartz latite-rhyolite flows cover the northern end of the mountains. Laramide and post-Laramide stocks and dikes have been intruded into Precambrian, Cretaceous, and early-Tertiary rocks.

The Precambrian metamorphic rocks include an older Bullard Peak series and a younger Ash Creek series, both of which occur as xenoliths and roof pendants in the Burro Mountains batholith. The Bullard Peak series is made up of interlayered quartz-feldspar gneiss, biotite gneiss, mica schists, hornblende gneiss, and amphibolite that were formed by dynamic metamorphism of argillaceous feldspathic sandstone, calcareous argillaceous siltstone, shale, and dolomitic shale. These source sediments were intruded by diabase and leucogranite that were metamorphosed to hornblende gneiss and granite gneiss along with the sediments prior to the deposition and metamorphism of the Ash Creek rocks.

The Ash Creek series was derived from a sequence of shales, quartzose shale, feldspathic argillaceous sandstone, dolomitic sandstone, and siliceous dolomite that were first subjected to mild regional metamorphism, and then to three periods of contact metamorphism successively by anorthosite, diabase, and the Burro Mountains granite. The intrusion of diabase was primarily responsible for the formation of sericite phyllite; andalusite-sericite schist; cordierite, andalusite, and biotite hornfelses; diopside quartzite; and several varieties of serpentine-carbonate rocks.

Intrusion of the Burro Mountains batholith resulted in the development of metasomatic muscovite, microcline, and sillimanite and, by lit-par-lit injection, converted part of the Bullard Peak hornblende gneiss and biotite schist to migmatite. Part of one migmatized hornblende gneiss xenolith contains orbicules. The granite enclosed mixed xenoliths of Ash Creek metasediments and diabase and converted the latter to metadiabase. The complex batholith is made up of several varieties of granite and lesser amounts of tonalite, granodiorite, pegmatite, and aplite.

Bullard Peak rocks have a well-developed bedding foliation; lineation is minor. Local contortions and minor folds appear on northeast-trending major folds. The Ash Creek rocks are poorly foliated and not folded. Northwest- and northeast-striking normal faults of post-Upper Cretaceous age, which are partly controlled by Precambrian structures, are the dominant structural features. Dikes of diabase, andesite, monzonite porphyry, rhyolite and rhyolite porphyry, and basalt of Laramide and post-Laramide age have been localized by northeast- northwest-, west-, and north-trending faults and shears.

Uplift of the Big Burro Mountains by means of faulting along northwesterly faults and along northeasterly cross faults began in Late-Cretaceous/early-Tertiary time. The Gila conglomerate formed by the merging of fanglomerates on the mountain flanks. Continued uplift and a change to an arid climate in late-Tertiary/Quaternary time resulted in sedimentation on the western mountain flank, dissection of Gila conglomerate, and deposition of Bajada and Bolson deposits in the basin to the southwest.

Precambrian metamorphic deposits of serpentinite, asbestos, and magnetite in the Ash Creek rocks are of mineralogical significance only. Laramide and post-Laramide hydrothermal veins related to stocks and dikes of intermediate composition contain fluorite and manganese oxides, as well as minerals containing W, Cu, Pb, Zn, Ag, and U. Over a million dollars in silver and several thousand tons of fluorite have been produced from the area. Significant production in the future is dependent upon the discovery of new ore bodies or extensions of deposits already known.