New Mexico Geological Society Annual Spring Meeting — Abstracts

Understanding the timing of mineralization pulses within southwestern New Mexico mining districts is necessary to determine where and how critical minerals can be targeted for exploration. New Mexico lies at the eastern edge of a well-known metal-bearing province, hosting numerous Laramide and Paleogene mineral deposits. Southwestern NM contains two spatially overlapping mineralization events; the Laramide porphyry copper (±Mo, Au) deposits (80-45 Ma formed during the subduction of the Farallon plate) and Paleogene Mo-W deposits (40-25 Ma formed during basin and range extension). Porphyry deposits are generally large, low-grade deposits containing Cu, Mo, Au sulfides and exhibit characteristic alteration styles. Porphyry mineral systems are centered on the porphyry stock and encompasses an area of hydrothermally altered rock (volumes of 10 to 100km3), that may also include precious metal mineralization involving several other deposit types; polymetallic veins, skarn and carbonate replacement deposits. In contrast to porphyry copper deposits, Mo-W deposits were formed during extensional magmatism, tend to be of smaller scales and lie proximal to porphyry copper deposits. However, all these deposit types have the potential to host various critical minerals, including Cu, Ag, Zn, Bi, Co, Ni, rare earth elements (REE), Re, Te, and W along with hosting significant gold and molybdenum resources. The LHM have two metal producing districts; the Eureka district (northern section) and the Sylvanite district (southern section), the range itself contains numerous plutons, dikes, and lava flows. For the Little Hatchet Mountains (LHM), previous research has also shown confusing and overlapping ages, as well as inaccuracies using the K-Ar method. Now, with new 40Ar/39Ar geochronological data and mineralogical assessments, there is clearer evidence for both Laramide and Paleogene magmatism in Eureka and Sylvanite districts. Unaltered igneous rock samples were sampled for geochemistry and a selected group of those samples were analyzed using 40Ar/39Ar geochronology. The NM Laramide porphyry copper deposits and Paleogene Mo-W deposits have been constrained using current 40Ar/39Ar dating methods into three mineral-producing magmatic pulses: 1) 78-71 Ma Laramide pulse, 2) 59-50 Ma, Laramide pulse, and 3) 40-25 Ma Paleogene Mo-W pulse.

For the Eureka district, four drill core samples from the Oro Southern Silver drill core project were sampled for 40Ar/39Ar geochronology. There is also previously recorded U-Pb dating for the Eureka district as well. All this data has now assisted in a determined general age of mineralization for the Eureka district to be within Laramide Pulse 2. LHM-010 and LHM-013 were the two samples collected for geochronology in the Sylvanite district. These samples were able to produce a preferred age of cooling and now a confirmed general age for the Paleogene pulse of mineralization within the Sylvanite district. The geochronological results show a complex evolution of the LHM due to the diverse development of multiple metal producing districts and different pulses of magmatism. Geochemical results also showed deposits of similar ages share similar mineralogical characteristics, such as Paleogene Mo-W deposits are elevated in Te (36.4ppm in LH710) and are of smaller scales compared to the Laramide porphyry copper deposits.