Genesis of the White Cloud and Related Pegmatites, South Platte Area, Jefferson County, Colorado

Abstract

The writer mapped in detail the geology of an area of approximately 12 square miles, including South Platte, Colorado, to ascertain the geologic setting of large, concentrically zoned, rare-earth-bearing pegmatite bodies. These occur within granite of the northeastern portion of the Pikes Peak batholith. The outer batholithic zone of granite contains platy flow structure concentric with the concordant contact between the batholith and metasomatized metamorphic wall rocks, indicating emplacement of a mobile magma in the mesozone. Pegmatite and aplite dike emplacement, as well as development of tensional quartz veins with deuteric wall rock alteration and fluorite veins, mark the late stages of batholith formation. The rare-earth-bearing pegmatite bodies are lens-or pod-shaped, with quartz cores surrounded by four concentric but commonly incomplete zones. These zones are generally: (1) wall zones of graphic granite; (2) outer-intermediate zones of biotite, cyrtolite, and perthite; (3) middle-intermediate zones of giant perthite crystals; and (4) inner-intermediate zones of euhedral fluorite crystals. Compromise surfaces between some minerals are interpreted as evidence of simultaneous growth; crystallization is believed to have proceeded from the pegmatite walls inward following Bowen9s reaction series. Replacement at the White Cloud pegmatite consists of perthite by albite, fluorite by yttrofluorite, yttrofluorite by doverite, quartz and allanite by fluorite, biotite by hematite, quartz by sericite, and gadolinite by an unidentified fluocarbonate of yttrium with an X-ray structure similar to bastnae-site. Several rare-earth minerals have not been identified. Pegmatite emplacement is believed to have taken place by: (1) liquid segregation in a giant miarole, or (2) autointrusion, or (3) a combination thereof in the early stages of batholith crystallization. Paragenesis is interpreted in the light of recent laboratory findings of other workers supporting resurgent boiling and the action of a vapor phase as a transfer medium in replacement.