“TARGET MAPPING” AT BONANZA, COLORADO: INTEGRATING GEOLOGY, GEOCHEMISTRY, AND MINERALIZATON MODELS IN A CALDERA SETTING

The San Juan volcanic field of SW Colorado is a remnant of the once much larger volcanic field that covered more than 25,000 km2 of the southern Rocky Mountains during middle Tertiary time. Eruption of the Bonanza tuff 27.8 ma preceded caldera formation and development of gold, silver, lead, zinc, and copper mineralization in the Bonanza region.

Geochemical sampling and mineral modeling was done to determine the potential for porphyry-type copper and molybdenum mineralization in the Bonanza region, and to determine if there might also be undiscovered precious-metal mineralization associated with caldera development. Soil samples encompassing the caldera were collected at a grid spacing of 0.25 mi2 (total 1566 samples); and 75 rock-chip samples were collected from veins in the NE and SE parts of the caldera. Both sets of samples were analyzed for Cu, Mo, Pb, Zn, Au, and Ag, plus iron and fluorine – Fe to test for the presence of iron enrichment in soils (perhaps marking the existence of the “pyrite halo” of porphyry mineralization), and fluorine because it is associated with Climax-type porphyry molybdenum mineralization. The locations of felsic intrusions along the margin of the caldera were noted (the “ring-type” intrusions in the cauldron cycle), as were the elevations of soil samples throughout the region (pervasive and widespread silicification accompanies porphyry-type mineralization).

The fifth order trend surface for iron in soil samples identifies a regional high over the NE margin of the caldera, which coincides with a topographic high and a “gap” in felsic intrusions along the caldera margin – these features suggest the presence of pyrite mineralization that coincides with pervasive silicification of rocks, both perhaps related to felsic magmatism at depth – anomalous Cu, Mo, Pb, Zn, Au, and Ag indicate the presence of a (fertile) source somewhere in the region. These features, together with the geology of the caldera are summarized in a “conceptually smoothed” target exploration map of the Bonanza caldera.