Waiting for the snow to melt…
The stock at Mount Sopris is one of several Tertiary igneous bodies exposed along the crest of the Elk Mountains in central Colorado. Two stages of intrusive activity are evident: a first stage characterized by quiet rise of a tonalite magma, and a second stage of forceful intrusion of a contaminated granodiorite magma. The stock was intruded into folded and faulted Permo-Pennsylvanian sediments, and extensive contact breccias, numerous inclusions, and notable variations in mineralogy suggest incorporation of considerable volumes of these rocks by stoping and assimilation. Thermal metamorphism of xenoliths resulted in the formation of hornfels. Metamorphism of shales and siltstones adjacent to the stock produced hard green hornfels, and sandstones and conglomerates were converted into quartzites. Deuteric silica- and alkali-rich residual solutions greatly modified the rock and produced sericitization of the feldspars, intense albitization, and graphic and myrmekitic intergrowths. Late hydrothermal activity produced minor bodies of nickel-bearing magnetite in the contact zone. It is suggested that the intrusives of the Elk Mountains are genetically related to the Princeton batholith and are cupolas projecting above a northwesterly extension of that body.
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