152 Economic Geology (Posters)

Tuesday, 29 October 2002
1:30 PM - 5:30 PM

INTERESTING OCURRENCES OF BRECCIA NEAR WEBSTER PASS, COLORADO

Paper number: 152-3
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The Alligator Breccia lies within a one square mile area of felsic intrusion and hydrothermal activity that crops out on the southwest slope of Red Cone peak in northern Park County, Colorado. From a distance, the breccia looks like a typical pipe of milled rock fragments that is stained by iron oxides and stands out in relief from the surrounding slope. Close-up, however, the body consists of small (1 inch to 2 feet thick) veins of comminuted Precambrian rock that anastomose through Precambrian quartzite and sillimanite gneiss. The fragments are angular and range from sub-microscopic to more than an inch in diameter. The vein brecciation is concentrated in a 100' x 300' (30 x 90 m) body, but it also occurs in smaller, less well-defined bodies across a half-mile region that may mark the geometry of underlying source intrusion(s). An outcrop of fresh "Lincoln-type" porphyry 3000 feet NW of the Alligator Breccia yielded a K-Ar date of 40 m.y. Within the region of brecciation, Precambrian rocks and porphyry have been pervasively pyritized, argillized, silicified, and sericitized, and four samples of oxidized breccia average more than 30 ppb Au; 0.2 ppm Ag; 25 ppm Pb, Zn, As, and Mo; 125 ppm Cu; and 650 ppm F plus 10% Fe and 1% S.

The widespread distribution and reticulate character of the breccia bodies may mean that they are the distal phase of brecciation at depth. Exposures lie between 11,400 and 12,100 feet elevation, and mountain peaks in the vicinity range from 12,500 to 13,200 feet above sea level. If the latter are remnants of the Oligocene erosion surface that was forming at this time in central Colorado, it could mean that the breccia intruded to within perhaps 1000 feet of the paleo-surface.

The presence of breccias and felsic porphyry, together with widespread trace element anomalies within altered rocks provide enticing clues to mineralization in the Webster Pass region.