Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

alluvial deposits appear to contain sand and gravel of commercial quality, on the basis of
natural and artificial exposures. The best potential resources are contained in the alluvial
terraces and flood plain of the South Arkansas River downstream from Maysville. Within
these terraces, the younger alluvial deposits are less weathered and constitute the better
resource (map units Qal, Qpo, Qbo, Qboy, Qboo, Qi). Additional gravel resources exist
in the older terraces (in order of increasing age, map units Qk3, Qk2, Qk1, Qk1, Qn2,
Qna1, Qna, Qn), but the gravels are progressively more decomposed with age and contain
progressively more soil formation products (calcium carbonate and red clay), so are less
suited for most commercial uses.
PEGMATITE DEPOSITS – Abundant pegmatite (YXp) and granite and pegmatite
(YXgp and Xgp) dikes and sills invade the Proterozoic gneisses in the western half of the
Maysville quadrangle. Some relatively large pegmatite bodies (both Xgp and YXp) are
associated with the hornblende intermediate gneiss and amphibolite gneiss in the steep
cliffs on the north and south sides of the North Fork. Some of the pegmatites are
associated with fine-grained granite gneiss with weak foliation and faint gneiss layering.
Because of the metamorphic fabrics in the associated granite they are considered to be
related to the Routt Plutonic Suite. Other large pegmatite bodies are considered to be
related to the Berthoud Plutonic Suite although the distinctions are commonly
ambiguous. The pegmatites are simple and unzoned, consisting predominantly of quartz
and alkali feldspar with minor biotite, muscovite, garnet, and magnetite. The remote
location prohibits future exploitation.
The Proterozoic gneisses in the southwest part of the Maysville quadrangle are
intruded by swarms of Berthoud-type granite and pegmatite dikes and sills. The
pegmatites tend to be elongated bodies that are both concordant and discordant to the
enclosing gneisses. They are commonly up to 800 to 1,200 ft long and range from tens of
feet to 200 feet wide. They are simple, unzoned pegmatites consisting predominantly of
quartz, alkali feldspar, and plagioclase. Minor minerals include biotite, muscovite, garnet,
epidote, magnetite, zoisite, thulite (mangenian zoisite), rhodochrosite, and tourmaline.
Kouther (1969) noted the presence of beryl and trace scheelite in pegmatites in the
adjacent Mount Quray quadrangle.
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