“PRESSURE-COOKER” BRECCIATION AT RED CONE, COLORADO

A model has been developed for the formation of recently-discovered intrusion breccias in an area of felsic intrusion and hydrothermal activity near Red Cone peak in northern Park County, Colorado. The breccias formed over a vertical distance of more than 1000 feet (300 m), and to within 750 feet (230 m) of the Eocene Flattop peneplain, remnants of which were mapped in the area by Lovering (1935). Irregular bodies of breccia are present in surface outcrop and in three cores drilled during the summer of 2002. The bodies range from <1 cm to more than 50 cm in thickness, and are present throughout a roughly 1 sq mi (2.6 sq km) region of altered and mineralized rocks on the southwest slope of Red Cone peak – forming an irregularly-spaced reticulate network of bodies that developed over the cupola of a crystallizing composite stock. In outcrop, breccia fragments are supported by comminuted rock debris, whereas in core the matrix is igneous porphyry – fragments identified are quartzite, sillimanite gneiss, and pegmatite. With few exceptions, breccia bodies appear as single phase intrusions, perhaps representing a ‘blast’ release of pressure. Thin sections often show subtle rounding of fragments within a matrix that overall is quite angular.

We believe the breccias – rock debris and igneous matrix – filled the plumbing system as pressure was released episodically. The igneous-hydrothermal system apparently behaved like a ‘pressure cooker’ – overpressures released in short bursts, keeping the contents largely confined within the system and allowing liquids to accumulate and steep, perhaps to concentrate significant mineralization in the upper part of the complex.

The breccia bodies provide a snap-shot of high level (distal) igneous-hydrothermal processes that often is eroded away or blown away as systems evolve. The presence of the brecciation, together with multiple phases of felsic porphyry, and widespread metal anomalies within altered rocks provide enticing clues to mineralization near Red Cone peak.