Simmons and Heinrich reported in 1980 that the South Platte district, 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 major petrologic units are:
1) schists and gneisses of the Idaho Springs Formation,
2) Pikes Peak granite,
3) Pikes Peak quartz monzonite,
4) aplite dikes,
5) xenolithic inclusions of biotite gneiss, granitic gneiss, and associated aplite,
6) pegmatites,
7) quartzite dikes, and
8) high-level Tertiary gravel.
Normative analysis indicates that the granite and quartz monzonite formed from liquids of essentially similar composition. Plots of Ab/An and Ab/Q, ratios for water-saturated melts indicate that the granite crystallized under lo wer PH 2 o conditions than the quartz monzonite. Loss of water vapor at the margin of the batholith is believed to have resulted in marginal crystallization of a one-feldspar, hypersolvus granite whose early solidification acted as an armor to confine water in the interior part of the pluton. Hence the remaining interior magma, with its higher water vapor content, crystallized at lower temperatures as a subsolvus two-feldspar quartz monzonite.
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. In shape and external structure, two end-member types are distinguishable: 1) vertical pipe-like bodies with essentially circular plans, and 2) vertical to steeply dipping, thick ellipsoidal lenses. Pegmatites of the central group are restricted to elevations between 6800 and 7800 feet within the granite, forming a clearly defined level or "niveau" of pegmatites. The concentric internal zonation consists of the following generalized, idealized sequence:
1) very poorly developed border zone (usually absent),
2) wall zone of biotite graphic granite,
3) outer-intermediate zone of giant biotite crystals (rarely present),
4) intermediate zone of microcline-perthite,
5) core-margin zone of green fluorite,
6) large core massive quartz or quartz-microline, and
7} secondary replacement units superimposed on the primary zonal sequence and containing albite, fluorite, rare-earth minerals, and hematite.
The core-margin green fluorite is believed to be primary. The bulk composition of an average South Platte pegmatite consists of 42 percent microcline, 10 percent quartz, and 2 percent biotite. Fractional crystallization of the Pikes Peak magma resulted in water-enriched liquid segregates of residual fluids which became the parent fluid from which the pegmatites crystallized.
Whole-rock neutron activation analysis of the granitic rocks reveals that the parental magma was anomalously rare-earth rich. Calculations show that volumetrically the pegmatites are enriched in rare earths as compared with the grantic rocks. The wall and intermediate zones of the pegmatites are strongly depleted in rare earths indicating that within the pegmatites these elements were partitioned into the residual pegmatitic fluids, from which the
replacement units formed. The original chemical nature of the magma appears to have been the most important factor in the formation of the rare earth-rich deposits. The bulk of the granitic magma apparently retained most of its original rare-earth content. 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 recently has 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.
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