LATE-STAGE MINERALOGY OF THE SEERIE PEGMATITE, SOUTH PLATTE PEGMATITE DISTRICT, JEFFERSON COUNTY, COLORADO

摘要

Pegmatites of the South Platte pegmatite district, located in Jefferson County, Colorado, are hosted by the 1.01 Ga Pikes Peak batholith. These NYF pegmatites are complexly zoned and fall into two groups. The first group, hosted by granite, has large mapable zones of fluorite along with replacement units hosting heavy-rare-earth-element (HREE)-enriched minerals including samarskite-(Y) and samarskite-(Yb). The second group, hosted by quartz monzonite, has less fluorite and HREE-bearing minerals and instead has abundant allanite-(Ce), a light-rare-earth-element (LREE)-enriched phase. The Seerie pegmatite, although hosted by granite, is geographically located several kilometers from other South Platte pegmatites. It displays characteristics of both these two groups, as it contains neither abundant LREE or HREE minerals. Only a few late-stage pods of fluorite, zircon, thorite, and bastnasite-group species have been found. The pegmatite is complexly zoned, with a wall zone composed of a graphic intergrowth of bladed biotite, albite, and potassium feldspar; an intermediate zone that is almost exclusively microcline; and a composite quartz-microcline core. At the core-intermediate zone boundary, euhedral microcline crystals are intergrown with quartz core. The Seerie pegmatite is the only pegmatite in the district that has both an intermediate zone and a composite core. Approximately a ton of topaz with individual crystals to 50 cm in length and 25 cm across were removed from the core margin. This pegmatite is also one of the few in the district with miarolitic cavities, one of which had a 0.3-meter-long quartz crystal.