Fluid-rock interaction in the White Rock Stock contact metamorphic aureole, Gunnison County, Colorado

Petrologic observations and thermodynamic data can be used to determine the temperatures, pressures, and fluid chemical compositions of metamorphic rocks.; Metamorphism of the Mississippian Leadville Formation in the contact aureole of the White Rock Stock, Gunnison County, Colorado has produced a number of calc-silicate assemblages and reaction textures. The pressure of metamorphism was estimated at 1.1 kbar or less. The peak metamorphic temperatures have been calculated from calcite-dolomite solvus thermometry.; The prograde mineralogy of the White Rock Stock contact aureole is consistent with internal buffering of fluid conditions by the rock in a closed system.; Oscillatory behavior in the optical birefringence of pyroxene crystals from a skarn correlates with variations in the Si + Mg and Al + Fe 3+ content of the pyroxenes. A new model is proposed whereby the zoning is caused by reaction-heating-hydrofracture cycles. The CO₂ released by the diopside-forming reaction is heated and expands until the fluid pressure exceeds the lithostatic pressure plus the tensile strength of the rock. Higher fluid pressures favor smaller molar volume pyroxene components, Ca-tschermakite and esseneite, whereas lower pressures favor a more diopsidic pyroxene.; Wollastonite formed under two very different sets of conditions in the contact aureole. Wollastonite formation in a white marble close to the intrusive contact took place at high temperatures (550–630°C) in a H ₂O-CO₂ fluid. Wollastonite formed further from the contact at a temperature of 300°C in the presence of a H₂O-CH ₄ fluid. The low temperature wollastonite is characterized by large curved, radiating crystals and finely disseminated graphite.; The contact aureole contains distinctive blue and white marbles. The color of the blue marbles is imparted by partially graphitized organic matter. At temperatures above 380°C at 1kbar graphite breaks down in the presence of H₂O. The distribution of blue and white marbles in the contact aureole is therefore controlled by temperature.; Fe-oxide and Fe-sulfide minerals constitute no more than 0.4% of the total rock by volume. These mineral assemblages evolve from pyrrhotite and pyrite in equilibrium with the graphitic phase at low temperatures to hematite and pyrite at high temperatures. The distribution of these minerals is consistent with the internal buffering of redox conditions.