Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

North Fork. Dings and Robinson (1957) also described the same quartz diorite intrusions
and came to the same conclusion, that collectively the quartz diorite intrusions are older
than the Mount Princeton quartz monzonite.
The quartz monzodiorite is medium to dark gray, very fine to medium grained,
and equigranular (fig. 24). Estimated modes indicate about 10 percent quartz (range 2 to
18 percent), 8 percent alkali feldspar (range 2 to 15 percent), 60 percent plagioclase
(range 55 to 70 percent), 10 percent biotite, 8 percent hornblende (range 12 to 23 percent
combined biotite and hornblende), 2 percent clinopyroxene (range trace to 8 percent), and
2 percent accessory minerals including magnetite, sphene, allanite, apatite, and zircon.
Plagioclase, as subhedral-euhedral, tabular laths has a random orientation. Sample 05-821
has plagioclase with andesine (An 44) composition (Michel-Levy method, 17 grains).
Sample 05-447A has more strongly zoned plagioclase with labradorite (An 55) cores and
oligoclase (An27) rims. On the basis of estimated mineral modes the intrusion is
predominantly quartz-monzodiorite with some quartz diorite and tonalite compositions
(IUGS classification). The presence of zoned plagioclase with labradoritic cores also
suggests quartz monzogabbro affinities. The mineralogy is variable, especially the
amount of quartz and alkali feldspar, and clinopyroxene, which varies from trace to about
8 percent.
The quartz monzodiorite is an approximately 9,000-ft long, N35°E-trending, large
dike-like body, about 500 to 1,800 ft wide, that extends from just off the west boundary
of the Maysville quadrangle, southeast of Lost Mountain (in the Garfield quadrangle),
across upper Lost Creek and into the North Fork valley. It makes relatively good outcrop,
consisting of large spheroidal-weathered blocks. No contacts were observed, but the
outcrop pattern relative to topography suggests a N-NE strike with a moderate dip to the
NW. The grain size decreases from medium grained in the north to fine grained in the
south. In addition, the grain size generally gets finer grained from east to west across the
body. In the south, the western portion of the body (in the Garfield quadrangle) is very
fine grained to almost aphanitic. The overall features suggest the body is chilled along
the northwest contact.
There are three previously published whole-rock analyses of the quartz
monzodiorite intrusion and one new analysis (sample 05-821; table 1) for this study. One
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