APPLICATION OF GIS TO ANOMALY IDENTIFICATION IN THE BONANZA CALDERA, SAN JUAN VOLCANIC FIELD, COLORADO

A Geographic Information System has been used to examine geology and mineralization in the Bonanza caldera of SW Colorado. Volcanism and development of the caldera occurred between 35 and 25 m.y.b.p. Stages II, III, and VI of the caldron cycle of Smith and Bailey (1968) can be recognized at Bonanza: pyroclastic eruption, caldera collapse, and ring intrusion magmatism. Evidence of mineralization exists in soil samples that encompass the caldera (1566 samples at a grid spacing of 0.25 mi), and in 75 samples of mineralized rock along the east margin of the caldera. Molybdenum, copper, lead, zinc, silver, and gold are of particular interest, as they provide clues to vein occurrences, and perhaps to large scale “porphyry-type” mineralization beneath the veins. Elements of related interest include iron (pyrite is ubiquitous to vein and porphyry metal systems), and fluorine (a “pathfinder” to molybdenum). The elevation of each soil sample was recorded when the samples were collected.

The GIS at Bonanza includes 15 geologic units that range in age from Precambrian to Quaternary – 12 of the units are Tertiary age and relate directly to development of the caldera. GIS efforts focused on the spatial distributions of elements with respect to the rock units as the caldera evolved. Regionally, evidence of mineralization is particularly strong over the northeast margin of the caldera. There is an apparent “gap” in ring intrusion magmatism here, but a plot of the elevation data reveals a topographic high at this location (perhaps related to silicification of the underlying rocks); and there also is a regional high in soil iron here (broad scale pyritization of rocks?). Concentrations of copper, lead, and zinc are high in the NE, but molybdenum and silver are less well defined there. Most of the samples that contain anomalous gold are from the NE, but the highest fluorine concentrations occur along the SE margin of the caldera.