Genthelvite overgrowths on danalite cores from a pegmatite miarolitic cavity in Cheyenne Canyon, El Paso County, Colorado

A miarolitic cavity in a pegmatite in Cheyenne Canyon, El Paso Co., Colorado has an unusual mineral assemblage that includes epitaxial genthelvite overgrowths on danalite cores. The cavity was lined with white microcline crystals with clear epitaxial albite overgrowths, small clear to slightly smoky quartz crystals, micas completely altered to sericite, and fluorite up to 10 cm in size. In addition to the genthelvite/danalite, accessory columbite-(Fe), ilmenite, and bastnasite-(Ce) are present. Late-stage trivalent iron minerals, generally goethite, coat the feldspar, quartz, and sericite. This mineral assemblage is, in part, the result of the alkaline nature of the melt that leads to the formation of helvine-group minerals rather than beryl. The epitaxial overgrowths of genthelvite on danalite are particularly unusual because these two minerals do not coexist in equilibrium, and thus record changing conditions in the miarolitic cavity. Decreasing fS (sub 2) from the early to late pocket forming stage resulted in the change from danalite to genthelvite crystallization. Increasing fO (sub 2) is indicated by both the cessation of danalite crystallization as well as the change in crystallizing oxide phases, from siderite to hematite to "limonite." Compositional changes in the late stage fluid include a decrease in trace element abundances as well as an increase in Fe/Mn ratios. This increase is indicated by the change from earlier-formed red genthelvite overgrowths with Fe > Mn as compared to the more Fe-depleted late stage beige genthelvite coatings on exposed danalite faces. In the final stages of mineralization, an extremely late-stage fluid may have altered the final mineral assemblage to produce an outer genthelvite rind that is enriched in trace elements. The mineral assemblage in this small pegmatite pocket in Cheyenne Canyon illustrates that variability in fluid composition in small pegmatite systems can be as complex as those in larger pegmatite systems.