Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

eastern contact of the diorite intrusion but cross-cutting relations could not be established.
Neither chilling of the diorite or development of foliations near the float contacts is
obvious. One outcrop near the margin of the intrusion shows minor mineralogical
banding of plagioclase and hornblende that may be weak localized gneissic structure.
This diorite from the Maysville quadrangle is similar to the interior parts of Early
Proterozoic quartz-diorite intrusions in the Buena Vista West (McCalpin and Shannon,
2005) and Buena Vista East quadrangles (Keller and others, 2004). The quartz diorite
intrusions in these two quadrangles are mineralogically and texturally variable and have
finer-grained textures and local development of foliation and gneissic structures near
contacts.
Xgp Gneissic granite and pegmatite (Early Proterozoic) – This unit consists of
abundant sill intrusions of fine-grained granite with pegmatite that are spatially
associated with a sequence of hornblende intermediate gneiss (Xhig). The two units are
intimately interlayered (shown on plate 1 as a Xgp/Xhig fractional unit) and form a large
15,000-ft long and 2,000- to 3,000-ft wide crescent-shaped zone crossing the North Fork
in the northwest part of the Maysville quadrangle. The Xgp/Xhig unit is about 1.5 percent
of the surface area of the quadrangle. The granite and pegmatite concordant sills (about
60 to 75 percent) are interlayered with septum and screens of hornblende intermediate
gneiss (about 25 to 40 percent). The hornblende intermediate gneiss (Xig) is described
separately.
The granite is white and ranges from very fine-grained to fine-grained aplite. It is
composed of quartz, alkali feldspar, and plagioclase with minor (less than one percent)
hornblende and/or biotite and trace fine disseminated magnetite. Slight variations in the
distribution of hornblende and biotite locally produce a faint gneissic layering, and
preferred alignment of mafic minerals creates a weak foliation. Pegmatites are white to
slightly pinkish-white, simple, unzoned bodies consisting of quartz, alkali feldspar,
plagioclase, biotite, and/or muscovite and local magnetite and red garnet. The pegmatites
lack exotic minerals and evidence of prospects.
The granite and pegmatite sills form linear, resistant-rib outcrops and
concentrated block float while the septum of hornblende intermediate gneiss is very
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