New Mexico Geological Society Annual Spring Meeting — Abstracts

The 300 km³ Cllpitan pluton located in central New Mexico hosts small REE-bearing zones ofmineralization which contain high temperature (up to 600°C), high salinity (up to 80 wt.%) fluid inclusions. The hydrothennal veins contain quartz, fluorite, adularia phis a number of REE-bearing phases. Based on the microthennometry of fluid inclusions, stable isotope composition of host minerals and spatial distribution of veins in the outer carapace of the pluton, this mineralization has been interpreted to be the result of magmatic fluids. In contrast to many magmaticlhydrothermal systems, there are few inclusions produced by late-stage meteoric fluids. The primary, high-temperature inclusions contain daughter minerals of halite, sylvite and other phases, and the high-K content ofthe inclusions allows detennination their ages of entrapment using the ⁴⁰Ar/³⁹Ar technique. Furthermore, the presence of hydrothermal potassic feldspar (adularia) in the veins allows independent determination ofthe age of mineralization.

Two samples of adularia, from prospects MTE and CPU-2 were analyzed with the ⁴⁰Ar/³⁹Ar incremental heating technique. MTE reveals a flat age spectrum and an isochron age of 28.2±.09 Ma. The CPU-2 adularia age spectrum is suggestive ofminor argon loss, and gives an isochron age of 28.0±.09 Ma. The isochrons define a trapped ⁴⁰Ar/³⁹Ar component which is essentially atmospheric (~298) for both samples. Four samples of hydrothermal vein quartz were analyzed from the Mm, CMX, FN, and CPU prospects. Two types of degassing behavior were noted during the incremental heating runs. MTE and CMX-1 yielded isochron ages of 26.5±4 Ma and 38.6±9.5 Ma respectively. Both samples yielded Cl/K ratios of about 5 and K/Ca ratios of 1.8 and 1.9. FN and CPU-2 yielded isochron ages of 28.8±.09 Ma and 29.5±.07 Ma respectively. For these two samples the CI/K ratios were very low and the K/Ca ratios increased relative to MTE and CMX-1.

The Cl/K and K/Ca ratios of MTE and CMX-1 are very similar to the elemental ratios determined for the inclusion fluid determined by the bulk crush-leach technique. Therefore, we conciude that dates from these two samples are from K trapped within the inclusions. In contrast, the CI/K and K/Ca ratios from FN and CPU-2 are more similar to those from the vein adularia. For these samples we believe that small (as of yet unseen) crystals of adularia trapped within the quartz are responsible for the age spectra. Although FN and CPU-2 do not represent direct dating of fluid inclusions, they do provide a mechanism for obtaining spectra from unaltered vein adularia.

The ⁴⁰Ar/³⁹Ar ages of hydrotherrnal quartz from the Capitan pluton closely match those detennined from hydrothermal adularia, suggesting that this technique yielded true depositional ages. However, the Ar release spectra were complicated because of the trapped AI component, and were therefore not straightforward to interpret. Recognizing and correcting for the excess argon associated with Cl, and use of standard isochron techniques allows determination of the fluid inclusion age and thus mineralization age. Further work on Capitan pluton samples may help us understand the Ar systematics within hydrothemlal/mineralizing systems.