INVESTIGATION OF ”MYSTERY FLUID” AROUND NANO-FRAGMENTS IN BRECCIA FROM RED CONE PEAK, COLORADO

Breccia exposed along the southwest slope of Red Cone Peak in northern Park County, Colorado fill fractures over the top of an igneous-hydrothermal complex, that intruded to within perhaps 750 feet (230 m) of the surface about 40 million years ago. The breccias formed largely in-place through dilation of the country rocks – evidence of multiple periods of brecciation, and some flow of fragments is present, but is not strong. On outcrop, fragments of sillimanite gneiss, quartzite, and quartz monzonite porphyry range to about an inch in diameter, in a matrix of secondary iron oxides. The original matrix cannot be seen, but fortunately, bodies of the breccia were penetrated below the zone of surface oxidation during exploratory drilling of the complex. Back-scattered SEM images of breccia were examined at magnifications to 25,000x, where they reveal a “dust” of country rock and sulfide mineral fragments within an even finer matrix of uncertain origin – the “mystery fluid”. To determine the minimum size of fragments, and to collect data on the composition of the matrix surrounding the fragments, a polished thin section was sectioned using FIBS (Focused Ion Beam Sectioning) in the OSU Campus Electron Optics Facility. TEM study yielded the following: (1) breccia fragments range to less than 50 nm in diameter, and (2) the fluid surrounding the fragments consistently contains rock-forming components (Si, Al, K) plus Fe, Cu, and occasionally Mg, Mo, and Ga. The fluid apparently originated in magmatic emanations that rose into the overlying rocks as they dilated and broke apart – the country rocks may also have contributed chemical components to the mix at this time.