Geology of the Gallinas Mountains mining district, Lincoln County, New Mexico — Summary

— Virginia T. McLemore, New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801

Introduction

Rare earth elements (REE) are one group of critical minerals and are essential in most of our electronic devices, such as cell phones, laptops, I-pods, computer chips, wind turbines, hybrid/electric cars, etc. (Committee on Critical Mineral Impacts of the U.S. Economy, 2008; Long et al., 2010; McLemore, 2011). Other technologies are being developed like solar panels, water purification, desalination, magnetic refrigeration, and even more efficient light bulbs that require REE and other critical elements in their manufacture. Most of the current world production of REE comes from China, but the U.S. has significant REE resources that could be developed in the future. Some of these REE resources are located in New Mexico, but they have not been important exploration targets in the past because demand has been met elsewhere. However, with the projected increase in demand and potential lack of available REE production from China, the New Mexico deposits are being re-examined for their potential, and several areas are undergoing current exploration.

The REE-Th-U veins and breccia pipes found in the Gallinas Mountains, central New Mexico requires additional study. A small amount of bastnaesite, a REE mineral, was recovered during processing for fluorite from the Gallinas Mountains. Estimated resources amount to at least 537,000 short tons of 2.95% total REE (not NI-43-101 compliant; Schreiner, 1993). Furthermore, geochemical and geochronological studies have been performed recently in the Gallinas Mountains (Williams-Jones et al., 2000; McLemore, 2010; Vance, 2013; Robison, 2017) but none included detailed mapping. New geologic mapping, proposed here, will help to address some of the issues presented in these studies as well as to thoroughly characterize the igneous and mineral system in the Gallinas Mountains.

The Gallinas Mountains mining district is one of several mining districts associated with alkaline igneous rocks in the Lincoln County porphyry belt in central New Mexico. The Lincoln County porphyry belt (LCPB) is part of the North American Cordilleran alkaline-igneous belt (Fig. 1) and, in New Mexico, the North American Cordilleran alkaline-igneous belt extends from the Sangre de Cristo Mountains near Raton, southward to the Cornudas Mountains, east of El Paso, Texas (McLemore, 2018a). Significant mineral production, especially gold, has come from deposits spatially associated with Tertiary alkaline-igneous rocks in the New Mexico alkaline-igneous belt (McLemore, 2001, 2015). These mineral deposits in New Mexico have been referred to as Great Plains Margin (GPM) deposits by North and McLemore (1986, 1988) and McLemore (1996, 2001, 2015). Alternative classifications by other workers include Au-Ag-Te veins (Cox and Bagby, 1986; Bliss et al., 1992; Kelley, 1995; Kelley et al., 1995, 1998), alkalic-gold or alkaline-igneous related gold deposits (Fulp and Woodward, 1991a; Thompson, 1991; Bonham, 1988; Mutschler et al., 1985, 1991; Richards, 1995), porphyry gold deposits, and Rocky Mountain gold province.

The Gallinas Mountains area is currently being re-mapped and sampled by the NMBGMR and results reported here are preliminary. The purpose of this project is to geologically re-map the Gallinas Mountains, central New Mexico, with emphasis on the structural and igneous components. Additional more detailed geologic mapping will focused on defining the extent of the alteration and mineralization/alteration zonation. Highly altered areas could be indicative of higher grades of mineralization. Additional geochemical studies of igneous rocks, mineralization, mineral chemistry, and alteration will aid in a better understanding of the systematics of igneous intrusion and mineralization in the Gallinas Mountains, leading to a better understanding of the igneous and mineral systems and the relationships between the two. Most of the Gallinas mining district is within U.S. Forest Service jurisdiction and this work will provide information needed for land use decisions, including exploration, potential mining activities, and abandoned mine lands (AML) reclamation.Ultimately, this project will provide a better understanding of the relationship of the Gallinas Mountains district with magmatic activity and mineralization in the North American Cordilleran alkaline-igneous belt.

Geology

The Gallinas Mountainsare in northern Lincoln County where a series of alkaline igneous intrusions, including andesite, porphyritic latite, trachyte/syenite, and rhyolite laccoliths, dikes and plugs, have intruded Permian sedimentary rocks (Perhac, 1970; McLemore, 2010; Vance, 2015; Robison, 2017). The Gallinas Mountains intrusions were emplaced at or just before the beginning of Rio Grande rift extension (27-39 Ma). The oldest intrusion is the porphyritic andesite, which has been dated as 39.74±0.058 Ma (40Ar/39Ar methods, Robison et al., 2017; Robison, 2017). The porphyritic latite is 28.178±0.04 Ma. The trachyte intrusion, the largest in the Gallinas Mountains and hosts much of the REE deposits (McLemore, 2010), is slightly younger at 27.66±0.18 to 29.232±0.097 Ma (40Ar/39Ar methods, Robison et al., 2017; Robison, 2017). The syenite lenses within the trachyte are similar in age as the trachyte (26.51±0.15 to 29.77±0.16 Ma, 40Ar/39Ar methods, Robison et al., 2017; Robison, 2017). Alteration includes brecciation, silicification, chloritization, and fenitization (Griswold, 1959; Woodward and Fulp, 1991b; Schreiner, 1999; McLemore, 2010). Only the trachyte and syenite have been fenitized that has been dated as ~27 to ~30 Ma (40Ar/39Ar methods, Robison et al., 2017; Robison, 2017). Carbonatites are inferred at depth by the presence of fenitization, carbonatiztion of the breccias, presence of REE and similarity of the intrusive rocks and mineralization to areas with carbonatites (McLemore, 2010). Associated igneous rocks in the Gallinas Mountains district are alkalic to alkali-calc (according to Frost and Frost, 2008), predominantly ferroan, metaluminous to peraluminous and plot as A-type granites. The trachyte/syenite and latite samples plot within the within-plate granite (WPA) tectonic field of Pearce et al. (1984; WPG), whereas the rhyolite samples plots within the volcanic-arc granite field (VAG). Trachyte/syenite and latite are probably related, but the rhyolite could be a separate magmatic event (McLemore, 2010). The associated igneous rocks exhibit typical light rare earth elements (REE)-enriched chondrite-normalized REE patterns of alkaline rocks with no europium anomaly.

Mineral deposits

Two types of mineral deposits are found in the Gallinas Mountains and are associated with the trachyte/syenite: Th-REE-fluorite (±U, Nb) epithermal veins and breccia pipes (REE-F veins, Cu-REE-F veins, and REE-F breccia pipes) and iron skarn deposits (McLemore, 2010; 2018). District zonation is defined by Cu-REE-F (±Pb, Zn, Ag) veins that form the center of the district, surrounded by REE-F veins (McLemore, 2010; 2018). The magmatic-hydrothermal breccia pipe deposits form a belt partially surrounding the veins (McLemore, 2010). Iron skarns formed at the top and edge of the trachyte/syenite body and are likely the earliest stage of mineralization. The iron skarns are probably related to the REE-F veins and breccia pipes because they typically contain bastnaesite and fluorite and are similar in trace element geochemistry. A simple paragenesis has been defined by four stages of brecciation and faulting with three stages of fluorite deposition (McLemore, 2010; Vance, 2013). REE minerals were deposited during the 1st and 2nd stage of fluorite deposition. Most fenites are more enriched in REE than unaltered igneous rocks (Schreiner, 1993; McLemore, 2010). The mineralogy is diverse and includes fluorite, quartz, barite, pyrite, iron oxides and accessory bastnaesite, calcite, chalcedony, galena, bornite, chalcocite, pyromorphite, anglesite, chrysocolla, malachite, and azurite and rare agardite (yttrium-arsenic oxide), mimetite, wulfenite, vanadinite, mottramite, and cerusite (Schreiner, 1993; McLemore, 2010). Geothermometric fluid-inclusion studies indicate a temperature of formation of 175-185°C (Perhac and Heinrich, 1964). Trace-element composition of fluorites from the Gallinas Mountains are characterized by relatively flat to LREE-enriched chondrite-normalized REE patterns, with no Eu anomaly (Gagnon et al., 2003). The earliest generation of fluorite is similar to the composition of the trachyte/syenite. The fluorite samples plot in the hydrothermal and pegmatitic field (Gagnon et al., 2003), which is consistent with a magmatic-hydrothermal origin.

Mineral-resource potential

The mineral-resource potential is high with a moderate level of certainty for REE; moderate with a moderate level of certainty for gold, silver, and iron; and low with a moderate level of certainty for copper, molybdenum, uranium, and tellurium in the Gallinas Mountains district (McLemore, 2018b).

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Updated June 24, 2019