⁴⁰AR/³⁹AR AGE AND GEOMORPHIC DEVELOPMENT OF HYDROTHERMAL ALTERATION SCARS IN THE RED RIVER VALLEY, NEW MEXICO

Over twenty hydrothernal alteration scars occur in two bands of elevation (3200 – 2900 and 2800 – 2600 m amsl) along the Red River Valley between the towns of Red River and Questa in northern New Mexico. These elevations correspond to hanging valleys on the north facing slopes and sharp changes of relief on the margins of the Red River Valley. Scars are the locus of sporadic mass movements and contribute to water quality degradation of the Red River.

The amphitheater-like scars form over highly fractured areas of extensive quartz-sericite-pyrite ± kaolinite-alunite alteration. Alunite from the Hottentot scar yielded a ⁴⁰Ar/³⁹Ar age of 24.96 ± 0.16 Ma, slightly older than mineralization at the Questa molybdenum deposit. The presence of alunite suggests that hydrothermal alteration of at least this scar is different from that associated with molybdenum mineralization at the Questa Molybdenum mine. This is also reflected in different δ³⁴S_CDT pyrite between the deposit (0.0 per mil) and scars (-1.7 to -11 per mil).

Development of the alteration scars on the landscape began at least 1.85 Ma based on ⁴⁰Ar/³⁹Ar ages of jarosite from ferricrete at high elevation on the margin of the Goat Hill scar. Ferricretes at the base of the lowest elevation scars yield ⁴⁰Ar/³⁹Ar maximum ages as young as 0.34 ± 0.16 Ma. Five other ages within this range mark periods of increased erosion in the Red River Valley. The spectrum of seven ⁴⁰Ar/³⁹Ar ages yielded from different scars generally coincides with geomorphic surface ages established by other workers. These geomorphic and radiometric ages are best correlated to interglacial periods, during which times abundant surface water probably enhanced the oxidation of the pyrite in their host rocks.