Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Creeks about 6,000 ft west of the Bon Ton mine. Smaller areas of hornblende felsic
gneiss and calc-silicate gneiss are present in the north-northwest-trending amphibolite
gneiss and calc-silicate gneiss belt between Green Creek and the South Arkansas River.
Hornblende felsic gneisses (Xhfg) are light gray, tan or brown, commonly with
weak orangeish to reddish Fe-oxide staining. They are very fine- to fine-grained, dense,
hard rocks with a siliceous hornfels character. They are generally unfoliated and massive
with faint, fine-scale (fractions of inch to inch) gneissic layering produced by minor
variations in the distribution of hornblende, biotite, and magnetite. Hornblende felsic
gneisses are distinctly more felsic than associated hornblende intermediate and
amphibolite gneisses. They are composed of about 65 to 70 percent plagioclase, 20
percent quartz, less than 10 percent mafic minerals, including about 5 to 7 percent
hornblende and 1 to 2 percent biotite, about 0.5 percent magnetite, and trace accessory
apatite and allanite. On the basis of estimated mineral modes they have daciticgranodioritic
compositions.
A weak calc-silicate overprint is common and has a preferential development of
fine-grained epidote in specific thin layers or irregular patches. Veins of fine-grained
epidote are locally abundant. Minor very fine-grained, disseminated pyrite is related to
calc-silicate areas. Weak calc-silicate zones associated with hornblende felsic gneiss
form broad zones up to about 2,000 ft long and 300 ft thick that are not in a spatial zonal
relationship to the areas of moderate to strong calc-silicates and mineralization. Possible
protolith for hornblende felsic gneiss may be fine-grained, carbonate-bearing, siliceous
sedimentary rocks.
Xq Quartzite-metachert (Early Proterozoic) – Quartzite-metachert is a very
distinctive but minor rock type in the Proterozoic terrane in the southwest part of the
Maysville quadrangle. It is light to dark gray or rarely black, generally very fine- to finegrained,
quartz-magnetite-hematite rock. Individual beds of quartzite-metachert range
from about 3 to 25 ft thick and hundreds of feet up to 1,000 ft long. They are usually
massive but locally have fine-scale (fraction of inch) remnant bedding laminations
produced by variations in iron oxides. The quartzite is completely recrystallized and
contains a trace to about 3 percent magnetite or hematite. Quartzite-metachert beds are
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