New Mexico Geological Society Annual Spring Meeting
April 7, 2017

Abstract
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40Ar/39Ar Geochronology of Magmatism and Alteration in the Gallinas Mountains With Implications for Rare Earth Mineralization

Alanna Robison1, William McIntosh2 and Virgil W. Lueth2

1Dept. of Earth and Environmental Sciences, New Mexico Tech, Socorro, NM, 87801, alanna.robison@student.nmt.edu
2New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM, 87801

The five alkaline laccolithic intrusions of Gallinas Mountains, along the border of Lincoln and Torrance Counties, New Mexico, have only one age date that was established using the K-Ar dating method (29.9 Ma). This single age is not sufficient to establish the relative timing of all magmatic and the associated alteration events. The geologic contacts between intrusive units are indefinite and few cross-cutting relationships were noted by previous mappers. This project dated four the of five intrusions and associated alteration events using the 40Ar/39Ar dating method. These age determinations are used to relate the age of magmatism to rare earth mineralization that occurs in the area. The oldest and smallest intrusion is porphyritic andesite, which has an age of 38.74 ± 0.058 Ma. It is more similar in age and appearance to the Chupadera dikes to the west. The porphyritic latite of Cougar Mountain has an age of 28.178 ± 0.040 Ma. A porphyritic trachyte, which produced an age range from 27.66 ± 0.18 to 29.232 ± 0.097 Ma, is the largest intrusion and hosts some of the rare earth element (REE) mineralization. An equigranular syenite phase is found as dispersed lenses within the trachyte and produced the range of ages from 26.51 ± 0.15 to 29.77 ± 0.16 Ma. It is mineralogically similar but texturally different than the more abundant porphyritic trachyte. The porphyritic rhyolite that comprises Gallina Peak has no apparent cross-cutting relationship with the other intrusions. It contains fluorite porphyroclasts, variable degrees of vesiculation from top to bottom, but could not be dated because of severe alteration. Based on mineralogy, geochemistry, and the ages of the other intrusions, the rhyolite body is speculated to be the youngest magmatic intrusion. The most intriguing feature of the Gallinas is the large-scale fenitization. This alteration is most abundant at the northern boundries of the trachyte. Dating three different fenite occurrences units has revealed that fenitization may span a period of time (~27 Ma to ~30 Ma). Stair-stepping spectra with integrated ages around 29 Ma suggest a period of hydrothermal alteration after emplacement and/or the potential for excess argon in the analysis. Adularia veins cross-cut the trachyte and the iron replacement skarnwhich was related to the onset of the trachyte magmatism by previous workers. The adularia has a stair-stepping spectra and age range similar to the fenites. REE-fluorite breccias cross-cut both the trachyte and Permian sedimentary host rocks. One breccia pipe contained clasts that resemble the iron skarn, suggesting that the skarn mineralization occurred prior to the REE breccias. The similarity of ages between the fenites, adularia veining, and trachyte alteration suggests that most of the alteration in the area probably occurred during or shortly after the emplacement of the trachyte. The undated fluorite-bearing rhyolite unit may have some genetic relationship to the breccia pipes.

pp. 61

2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM