Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Trp Rhyolite porphyry dikes (early to late Oligocene) – At least four rhyolite
porphyry dikes are present in the northwest quadrant of the Maysville quadrangle (plate
1). They cut Early Proterozoic gneisses (Xhig, Xag and Xgp) and granodiorite (Xgdf).
The rhyolite porphyry dikes range from whitish to light pinkish gray and are generally
massive. They have porphyro-aphanitic texture with variable, moderate to high
phenocryst contents (30 to 45 percent). Quartz phenocrysts (12 to 15 percent) are
generally euhedral (1 to 3 mm) and commonly have a slight smoky color. Alkali-feldspar
phenocrysts (7 to 12 percent) are subhedral to euhedral (2 to 4 mm) and plagioclase
phenocrysts (8 to 15 percent) are generally subhedral (1 to 3 mm)). Small biotite
phenocrysts (1 to 3 percent) are typically intergrown with, or partly replaced by
muscovite. The dikes are characterized by trace to 2 percent accessory orangeish garnet
and trace to 1 percent fine disseminated magnetite. Orangeish garnet appears to be a
primary accessory mineral disseminated in the rhyolite but also locally occurs as fine
coatings on fracture surfaces suggesting some is late magmatic or deuteric.
Two rhyolite porphyry dikes occur in a northeast-trending set subparallel to
aphyric rhyolite (Tr), fine-grained granite (Tcf) and quartz latite porphyry hybrid (Tqlp)
dikes. The main dike occurs in three segments, suggesting a continuous or nearly
continuous dike for about 11,500 ft across the Maysville quadrangle and continuing for
another 6,500 ft in the Mount Antero quadrangle. The central portion of this dike, on the
ridges north and south of Squaw Creek (fig. 20), is composite consisting of both rhyolite
porphyry (Trp) and fine-grained granite (Tcf). Outcrops of the dike show thicknesses
ranging from 10 to 35 ft, and some concentrated, large block float zones are up to 150 ft
wide. Exposed contacts indicate that the dike strikes from N2° to 37°E with dips from
35° to 68°NW. The overall orientation is about N25°E with a 50°NW dip.
The second northeast-trending rhyolite porphyry dike occurs as an almost
continuous float zone for about 6,000 ft along the range front associated with the Sawatch
fault zone north of Squaw Creek. Similar rhyolite porphyry float on the Blank Mine adit
dump, about 7,000 ft southwest, suggests that a rhyolite porphyry dike was also emplaced
along the Sawatch fault zone in that area. North of Squaw Creek, two outcrops of the
rhyolite porphyry are present but no contacts are exposed. A rib of silicified rhyolite
porphyry breccia at the northern Maysville map boundary trends N45°E and is the same
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