THE BEGINNING OF INTRUSION BRECCIATION – ROCK BURST ORIGIN OF FRAGMENTS

Explorationists usually see well-developed heterolithic breccias that may be hundreds of meters in diameter, extending perhaps 1000’s of meters vertically, and often overprinted by subsequent intrusion and hydrothermal alteration. Breccia bodies exposed on Red Cone Peak in northern Park County, Colorado, by contrast, are small and irregular, and fill fractures over the top of an igneous-hydrothermal complex. Mapping has shown that the bodies formed to within 230 meters of the paleosurface.

Backscattered scanning electron microscope images at magnifications to 25,000x reveal a “dust” of rock and sulfide fragments within a matrix of finer fragments and “frozen fluid” from the underlying complex. Fragments of country rock and sulfide are log-normally distributed, increasing in frequency with decreasing size, with the smallest dimensions observed at about 200 nanometers. Pyrite fragments are shard-like in appearance, with aspect ratios (major:minor axes) averaging 2.33, whereas country rock fragments appear more tabular (ratios averaging 2.38) – data from a 1000x image. Both types of fragments are angular to subangular (AGI indices=1.5-2.5), and they exhibit little evidence of milling, although flow textures occasionally are seen in the images.

We have characterized the breccias at Red Cone as “pressure cooker breccias.” The fragments apparently formed in millions upon millions of micro-rock bursts. Country rocks above the evolving igneous system swelled to the point of fracturing (pressure cooker ready to release), the pressure dropped precipitously as fractures began to form, and fragments of all sizes literally were “sucked” off the walls and moved short distances as the pressure bled away – evidence of multiple events is present in some specimens. We believe the Red Cone breccias document an initial stage of breccia formation, and it is from such humble beginnings that larger heterolithic bodies develop.