RS-11 Rare Earth Pegmatites of the South Platte District, Colorado


Location, maps, zonation, and mineral constituents of rare earth-rich pegmatites of the South Platte District, Jefferson County. 131 pages. 8 tables. 14 figures. 31 plates. Digital PDF download. RS-11D

From the abstract:

The South Platte district in Jefferson County, Colorado, consists of a distinctive cluster of well-zoned, complex, rare-earth-, fluorine-, iron-rich pegmatites underlain chiefly by consanguineous granitic rocks of the Pikes Peak batholith. The district lies within the northern end of the Pikes Peak batholith near its contact with Precambrian gneisses and schists. The district is characterized by over 50 large, complex, concentrically zoned, nearly vertical pegmatites, which all occur within the pluton rather than marginal or exterior to it.

Whole-rock neutron activation analysis of the granitic rocks reveals that the parental magma was anomalously rare-earth rich and that the bulk of the granitic magma apparently retained most of its original rare-earth content. This original chemical nature of the magma appears to have been the most important factor in the formation of the rare earth-rich deposits. The large concentrations of these elements found in the pegmatites are due chiefly to original concentration with fractionation becoming an effective agent only in the volatile-rich, residual pegmatitic fluids.

The district has been a major source of feldspar in the past and has also become a major supplier of terrazzo-stone quartz. In addition, small amounts of fluorite and rare-earth minerals (chiefly Y-group-rich) have been obtained.