Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

samples) indicates they are composed of amphibole (50 percent) and plagioclase (25 to
35 percent), with variable, irregularly distributed quartz (0 to 10 percent and up to 20
percent in some clasts), microcline (0 to 1 percent), biotite (0 to 5 percent in specific
clasts), with accessory sphene, magnetite and apatite, and calc-silicate related epidote that
replaces plagioclase and amphibole. The amphibolite agglomerate tends to have a weak
to moderate, epidote-rich calc-silicate overprint that preferentially replaces the
amphibolite matrix or occurs as irregular veins, bands, and patches. Minor Cu-oxide
associated with epidote calc-silicate suggests the meta-agglomerate locally has weak
copper mineralization (probably disseminated chalcopyrite).
Field relations suggest at least two probable stratigraphic zones of amphibolite
agglomerate, although unrecognized structural complexities could have produced
duplication by isoclinal-like folding. Two east-northeast-trending zones cross the west
edge of the quadrangle on the ridge on the north side of Willow Creek. The south zone is
discontinuous for about 2,000 ft and is about 100 ft to 200 ft thick. Just off the west edge
of the map area the zone is oriented east-west and dips 51° south. To the east the zone is
oriented about N18°W with 20°SW dip, suggesting that the layer is folded. The north
zone is continuous for at least 1,500 ft and ranges from about 200 ft thick on the east
(where truncated by the Proterozoic granodiorite Xgd intrusion) to 600 ft thick off of the
west boundary of the map area. The third continuous zone of amphibolite agglomerate
occurs as a 2,000 ft long and about 200 ft thick, northwest-trending zone about 7,500 ft
noth-northeast of the southwest corner of the quadrangle. The amphibolite agglomerate is
cut by Routt-type granodiorite (Xgd) sills and small irregular dikes of Berthoud-type
granite and pegmatite (YXgp) dikes. On the basis of field observations the amphibolite
agglomerates are pre-metamorphic intraformational breccias that were formed as part of
the protolith stratigraphy. The breccias form continuous, mappable horizons associated
with some amphibolite sequences. The amphibolite agglomerates are potentially good
key marker horizons but they form discontinuous zones in the Maysville quadrangle.
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