LITHOLOGICAL CONTROLS OF FLUID FLOW DURING METAMORPHISM OF THE MAROON FORMATION IN THE WHITEROCK PLUTON CONTACT AUREOLE, COLORADO

Fluid flow can play a major role in the development of contact aureoles in the upper crust by promoting mass and heat transfer and in turn these can result in hydrothermal ore deposits. Variations in country rock lithology can influence the distribution of fluid flow but also can complicate quantifying the role of fluids. In the Elk Mountains of Colorado, the Tertiary-aged Whiterock Pluton intruded Pennsylvanian-Triassic sedimentary rocks including the Maroon Formation. The Maroon Formation ranges from unmetamorphosed, red siltstones and carbonate-cemented arkosic sandstones north of Cathedral Lake, to diopside-bearing calc-silicates near pluton contacts north of Pearl Pass. Where metamorphosed, the coarse grained rocks are transformed to light-gray to green hornfels and calc-silicates, whereas siltstones commonly retain their deep-red color. Tremolite is present in low-grade rocks, including some red siltstones, and can persist in some diopsidic-pyroxene-bearing samples. Epidote is widespread and scapolite is present locally in the diopside zone. Near contacts with the Whiterock Pluton magnetite is locally interlayered with diopsidic-pyroxene. Magnetite skarns are found elsewhere in the contact aureole. Tremolite and specular hematite commonly decorate fracture surfaces and veins. Fe mobility may be related to bleaching or localized Na-Ca metasomatism. Collectively these observations suggest original porosity and permeability of unmetamorphosed Maroon Formation has affected the geographical extent and grade of metamorphism in the aureole.