CAMPBELL, A.R., HEIZLER, M.T., DUNBAR, N.W., Geoscience Department and New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM 87801.

The 300 km3 Capitan pluton located in central New Mexico hosts small REE-bearing zones of mineralization which contain high temperature (up to 600 C), high salinity (up to 80 wt.%) fluid inclusions. The hydrothermal veins contain quartz, fluorite, adularia plus a number of REE-bearing phases. Based on the microthermometry 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 magmatic/hydrothermal 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 of the inclusions allows determination of their ages of entrapment using the 40Ar/39Ar technique. Furthermore, the presence of hydrothermal potassic feldspar (adularia) in the veins allows independent determination of the 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 28.30.1 Ma. The CPU-2 adularia age spectrum is suggestive of minor argon loss, with the last 25% of 39Ar released giving an isochron age of 29.00.3 Ma. The isochrons define a trapped 40Ar/36Ar component which is essentially atmospheric (~298) for both samples. Hydrothermal quartz, co-precipitated with the adularia from three prospects, CMX-1, MTE and FN, was also analyzed. All three samples show saddle-shaped Ar age spectra indicative of excess Ar (non- atmospheric 40Ar trapped during sample formation). The minimum steps of the saddle (between temperatures of 800 and 1300 C) form the flattest part of the spectra, but do not attain ages equal to the adularia age. Isochron regressions of these portions of the spectra yield ages of 28.30.1, 27.73.5 and 38.09.3 Ma for FN, MTE and CMX-1, respectively, with 40Ar/36Ar compositions of 3282, 3185, and 30415. We interpret these intercepts to represent the 40Ar/36Ar composition of the fluid that deposited these samples. The Cl/K ratio varies sympathetically with the saddle-shaped age spectra suggesting a correlation between Cl and excess argon. The excess argon to Cl ratio (40Ar*E/Cl) is relatively uniform (~5 x 10-8 for MTE and CMX and 2 x 10-7 for FN). This relationship allows correction for the excess argon associated with the Cl, and yields an age consistent with the adularia, but is imprecise because of the very low K content of the samples. The saddle form of the age spectra for the quartz samples represents different degassing behavior of the fluid phase relative to the daughter minerals or solid inclusions within the crystals. The gas generated between 800-1300 C is dominated by the high K daughter and/or solid phases which approach the known age of mineralization, whereas the fluid phase of inclusions appears to degas at lower and higher temperatures in the experiment.

The 40Ar/39Ar areas of hydrothermal quartz from the Capitan pluton closely match those determined from hydrothermal adularia, suggesting that this technique yielded true depositional ages. However, the Ar release spectra were complicated because of the trapped Ar 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 hydrothermal/mineralizing systems.

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