Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Tma Mount Aetna quartz monzonite porphyry ring dikes (late Eocene to early
Oligocene) – Consists of a light-gray to light-pinkish-gray, distinctive highly
phenocrystic, coarsely porphyritic rock. The Type 2 ring dikes are characterized by large,
tabular alkali-feldspar phenocrysts to 1.5 inches, tabular plagioclase phenocrysts to 1.0
inch, quartz phenocrysts to 0.3 inch, and smaller hornblende and biotite phenocrysts to
0.2 inch in a medium-grained groundmass.
Modal analyses on the Mount Aetna cauldron ring dikes and resurgent intrusion
were conducted by Shannon (1988). A modal analysis (1866 points on slab stained for
alkali feldspar) on the main Mount Aetna ring dike from just northeast of Tabeguache
Peak in the Mount Antero quadrangle showed 7.6 percent quartz, 17.4 percent alkali
feldspar, 20.2 percent plagioclase, and 7.1 percent hornblende-biotite phenocrysts. Eight
modal analyses of Mount Aetna quartz monzonite, including six samples from the
resurgent intrusion and two samples of ring dikes, show the total phenocryst content to
vary from 52.5 to 78.9 percent and the proportion of phenocrysts to be somewhat
variable. Quartz phenocrysts show the largest variation, from 7.2 to 17.3 percent. The
average of the eight modal analyses gives a total phenocryst content of 62.1 percent with
11.6 percent quartz phenocrysts, 19.0 percent alkali feldspar phenocrysts, 24.8 percent
plagioclase phenocrysts, 5.9 percent biotite-hornblende phenocrysts, and 0.4 percent
accessory magnetite. Thin-section analyses indicate the presence of accessory magnetite,
sphene, apatite, and zircon in all samples and allanite in most. The average modes
indicate the rock is classified as a pheno-granite b on the basis of phenocryst abundances
(IUGS classification).
An approximately 2,200 ft long segment of Mount Aetna quartz monzonite
porphyry ring dike is present in the northwest corner of the Maysville quadrangle. The
dike does not crop out, but concentrated float indicates that it is about 200 to 300 feet
thick. The float trend of the dike is about N38°E and the float pattern relative to
topography (on the Maysville, Garfield and Mount Antero quadrangles) suggests a steep
dip to the northwest. The ring dike in the northwest corner of the Maysville quadrangle is
not the main ring dike along this part of the cauldron structure. This dike is an outer ring
dike that extends for about 2,000 feet northeast in to the Mount Antero quadrangle, and
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