MINERALOGY AND PROVENANCE OF CLAY MINERALS IN MIAROLITIC CAVITIES OF THE PIKES PEAK BATHOLITH, COLORADO

Samples from 105 miarolitic cavities throughout the Pikes Peak batholith (PPB) were analyzed for clay minerals by X-ray diffraction. The PPB, in central Colorado, is a Precambrian, anorogenic, epizonal intrusive with numerous plutons of sodic or potassic affinity. Miarolitic pegmatites are concentrated in and around these late plutons.

Clays within the miarolitic cavities show a varied mineralogy, including smectite, illite and kaolinite; only samples showing a dominant phase were tabulated and further analyzed. Soil samples were also analyzed to differentiate soil and authigenic cavity clay minerals. Provenance can be determined in part by evaluating physical characteristics of the crystallites, e.g., cavity clays were found to show lognormal or nearly lognormal crystal size distributions (CSDs), in contrast to those found in nearby soils. Consequently, regional occurrences of miarolitic cavity clays can be assessed.

Samples from northern areas of the LGR (both coarse- and medium-grained granite) and from Sugarloaf Mountain near Wigwam Creek show mostly high-expandable (e.g., > ~65%) illite/smectites with lognormal or nearly lognormal CSDs. These clays are presumed to originate within the miarolitic cavities, in contrast to soil illite/smectites which consistently show a comparatively lower expandability.

Kaolinite is commonly noted in miarolitic cavities from other late-stage potassic plutons. In contrast, only one sample from the LGR (of 39 total) shows principally kaolinite, and most of these samples showed approximately lognormal CSDs. The relative absence of kaolinite in PPB soils suggests that it is authigenic when found as a predominant constituent within miarolitic cavities. A loose correlation can be made between cavity mineralogy and the presence of kaolinite, with kaolinite-bearing cavities often noted to contain topaz.

Illite is also common in miarolitic cavities throughout the LGR complex, as well as in the surrounding soils. Cavity and soil illites can be differentiated based on a plot of the mean of the logs of their sizes (a) and the size variance (b²) that shows two distinctly separate populations. Cavity illite CSDs deviating from a lognormal shape are presumed to reflect varying degrees of cavity weathering and consequent infiltration of soil illites.