New Mexico Geological Society Annual Spring Meeting — Abstracts

Southwestern New Mexico lies within a complex metallogenic province that records multiple episodes of magmatism and mineralization related to both Laramide age Cu-porphyry systems and younger Paleogene Mo-W deposits. This project examines the geologic evolution and mineralization of mineral deposits in the Cookes Peak, Old Hadley, and Macho districts of Luna and Sierra counties to determine whether their carbonate-replacement and volcanic-epithermal systems formed during a single magmatic-hydrothermal event or reflect multiple and or different mineralizing episodes. These districts are particularly important because they host Pb-Zn-Ag replacement bodies and epithermal veins that may represent the distal expressions of porphyry-related magmatism of the Cookes Peak pluton. In addition to Pb, Zn, and Ag, these mining districts are potential sources of critical minerals including Mo, W, REE, Te, and V.

The study focuses on the Eocene to Miocene magmatism at ca. 40–20 Ma which marks inceptions of extension and formation of additional smaller porphyry and epithermal systems. Existing mapping and limited geochronology suggest a possible relationship among the Cookes Peak, Old Hadley, and Macho districts, but the timing and genetic links among these systems remain poorly constrained. The Cookes Peak granodiorite has a reported K-Ar biotite age of 38.8 ± 1.4 Ma (Loring & Loring, 1980) and the
Fluorite Ridge granodiorite has a reported 40Ar/ 9Ar hornblende date of 38.82 ± 0.57 Ma (McLemore,
2001). However, the Old Hadley and many other intrusive units lack direct and modern geochronologic data. Mineralization at Macho has been attributed to middle-Cenozoic magmatism based on field relations rather than radiometric constraints. To address this uncertainty, this study integrates field observations, petrography, mineralogical and geochemical characterization, and 40Ar/39Ar geochronology to better constrain the timing and relationships of magmatism and mineralization.

​New emplacement ages of the Cookes Peak granodiorite are similar to previous dating attempts. A biotite sample from the summit granodiorite block produced an age of 40.1 ± 0.1 Ma, and a hornblende age from below the summit produced an age of 38.66 ± 0.21 Ma. A biotite age for the extrusive Rubio Peak Formation from the Northern part of the Cooke’s peak districts shows an age of 38.95 ± 0.07 Ma.

Age dating and whole rock geochemistry from the Cookes Peak, Old Hadley, and Macho districts will be used to unravel mineralizing episodes and tectonomagmatic evolution of these magmatic-hydrothermal systems. By comparing intrusion ages, hydrothermal alteration, and mineral formation across the three districts, this research will clarify the role of post-Laramide magmatism in concentrating critical minerals and improve genetic models for carbonate-replacement and epithermal deposits in this underexplored region of southwestern New Mexico.

References:

1. Loring, A. K., and R. B. Loring., 1980. K/Ar Ages of Middle Tertiary Igneous Rocks from Southern New Mexico. Isochron/West A Bulletin of Isotopic Geochronology, vol. 28, pp. 17–20, https://geoinfo.nmt.edu/publications/periodicals/isochronwest/28/iw_v28_p17.pdf.
2. McLemore, V.T., Donahue, K., Breese, M., Jackson, M.L., Arbuckle, J. and Jones, G., 2001. Mineral resource assessment of Luna County, New Mexico. New Mexico Bureau of Geology and Mineral Resources Open-File Report OF-459, p.85.