MICROSCOPIC CLUES TO THE ORIGIN OF BRECCIAS AT RED CONE, COLORADO

Bodies of intrusive breccia along the southwest slope of Red Cone Peak in northern Park County, Colorado present a rare opportunity to understand how such bodies form. Intrusive breccias normally are large and heterolithic and thus difficult to study, but the Red Cone breccias are simple “auto-lithic” phases that developed above an evolving igneous-hydrothermal complex. We have termed the bodies auto-lithic because fragments within individual outcrops appear to have been derived from the adjacent country rocks, although the latter may vary considerably from outcrop to outcrop. In outcrop, the Red Cone breccias range from less than 0.5 to perhaps 20 inches in thickness (<1 cm to 50 cm). Thin section and polished surface studies provide tantalizing clues to the origin of the breccias, including evidence of the moment when brecciation began. Fragments are angular to sub-angular (AGI indices 1.5-2.5), they range to <1 mm in diameter, and there is little evidence either of extensive fluid flow, or of multiple episodes of brecciation – fragments came from the enclosing rocks and show little evidence of milling. Pyrite and quartz exhibit dilation fractures that formed prior to brecciation. A thin section from 853 feet in drill hole WP #4 (1823‘ below the paleosurface) shows quartz “coming apart” and beginning to rotate as the breccia began to form, but the process stopped almost as soon as it started. Apparently, the country rock swelled in response to increased fluid pressure, fractured (crackle-brecciated), and then began to brake apart explosively. Brecciation ceased, however, as pressure bled away within fractures that were choked with fragments – the breccia system acting as a natural pressure cooker for the underlying igneous-hydrothermal complex.