CONTROLS ON VOLCANISM ASSOCIATED WITH THE INTRUSION OF THE REDWELL AND MOUNT EMMONS STOCKS, CRESTED BUTTE, COLORADO: IMPLICATIONS FOR CLIMAX-TYPE PORPHYRY MO EXPLORATION

The Miocene Mount Emmons complex (Crested Butte, Colorado) is a Climax-type porphyry Mo system that contains the Redwell and Mount Emmons stocks, which are ~600‒1000 m apart but connected at depth. The complex also contains a volcanic breccia pipe that emanates from the top of the Redwell stock and likely breached the paleo-surface, but the Mount Emmons stock lacks a volcanic pipe. Three Climax-type deposits have preserved breccia units that are thought to have breached the paleo-surface: the Mount Emmons complex contains two stocks and one breccia pipe, the Cave Peak deposit in western Texas contains three stocks and three breccia pipes, and the Pine Grove deposit in Utah contains one stock and one breccia pipe.

Because the stocks at Mount Emmons are part of the same magmatic parent event, we can investigate local causes of volcanism without needing to compare deposits from different geologic settings or exposed at different erosional levels. Intrusive paleo-depth differences of the top of the stocks and the number of magma recharge events into the bottom of the two stocks are not likely controls on the occurrence of volcanism. The presence of intersecting faults above the Redwell stock may have promoted development of the associated volcanism whereas these were lacking at the Mount Emmons stock.

This has important exploration implications because even though the Mo mineralization associated with the Redwell stock was discovered first due to the outcropping breccia, its two Mo orebodies contain 37% less Mo compared to the single Mo orebody associated with the Mount Emmons stock. In a magmatic arc environment, venting to surface during porphyry emplacement inhibits the formation of porphyry Cu orebodies due to the release of fluid and volatiles from an otherwise fertile magmatic-hydrothermal system; fluid that was channeled toward the surface but never breached the surface may promote porphyry Cu orebodies. This research suggests the same may hold true for Climax-type Mo deposits formed in extensional to transitional environments, and that for stocks with associated breccias that breached the paleo-surface, higher tonnage orebodies may be nearby (within 1 km), around stocks with no obvious volcanic vents. This is particularly important because Climax-type deposits are typified by multiple intrusive bodies.