PORPHYRY SYSTEM ALTERATION AT QUESTA NEW MEXICO, MAPPED USING AIRBORNE VISIBLE-INFRARED IMAGING SPECTROMETER DATA

Alteration minerals mapped over the molybdenum porphyry system at Questa New Mexico define an extensive east-west mineralized zone along the southern rim of the Latir Caldera. Previous geologic mapping identified a series of post-caldera stocks along this zone that introduced the molybdenum. These stocks are postulated by others to be cupolas of an underlying batholith.

Analysis of Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) data has identified and mapped a series of hydrothermal clays, micas, carbonates, and sulfates. Later supergene weathering produced iron-hydroxides, and various sulfates that were also mapped. These mapped minerals were used to infer alteration assemblages typical of porphyry systems: quartz-sericite-pyrite (QSP), argillic, and propylitic. One assemblage was not characterized using this data; while potassic alteration is present in outcrop, its potassium bearing minerals do not contain identifiable features within the wavelength interval of the AVIRIS data.

The inferred alteration assemblages show a zonation surrounding the mineralizing stocks, grading from QSP near the stocks, outward to argillic, then propylitic. Several areas also contain small anomalies of clays and sulfates that may represent late stage brecciation. Regions of molybdenum deposition and post-mineralization brecciation are associated with these alteration patterns, and relate the three-dimensional position of the deposits with the present ground surface.