Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Many of the calc-silicate gneiss zones have been prospected and most of the
larger mines (for example the Bon Ton mine) in the southwest quadrant of the Maysville
quadrangle are related to zinc, copper, and lead mineralization that is associated with
strong calc-silicate zones. The origin of the calc-silicate zones and the associated
mineralization is a complex issue and multiple hypotheses have been suggested
(Heinrich, 1981 and Sheridan and Raymond, 1984) (see Economic Geology section).
Oxide and sulfide minerals can locally be important constituents of the calcsilicate
assemblage. Oxides include magnetite, which most often occurs as concentrated,
massive bands or horizons (up to 2-ft thick) interlayered with the calc-silicate and
amphibolite layers or as disseminated grains in the calc-silicate layers. Oxides also
include the unusual, locally abundant mineral gahnite (Zn-spinel), traces of specular
hematite, and Cu-oxides. Some of the mineralized calc-silicate zones contain two spinel
phases (gahnite and magnetite) that appear to be mutually stable. Sulfides include
sphalerite, chalcopyrite, pyrite, galena, and covellite. Mineralized calc-silicates at the
Bon Ton mine contain both gahnite and sphalerite. Reaction rims of muscovite occur on
gahnite when in contact with sphalerite, suggesting disequilibrium relations.
Possible protoliths for calc-silicate gneiss include limey clastic sedimentary rocks,
possibly siltstones and shales, and strong calc-silicate zones may be related to impure
limestone or marble layers. The magnetite-rich and massive magnetite layers are possibly
replacements of relatively pure limestone or marble layers. No Proterozoic limestone or
marble horizons have been identified in the Maysville quadrangle. Olson (1983)
described a 3- to 6-ft thick marble layer that is along a northeast-trending zone with calcsilicates
on the west side of Fooses Creek in the Palone Peak quadrangle, which is just
outside the southwest corner of the Maysville quadrangle.
Xhfg Hornblende felsic gneiss (Early Proterozoic) – Minor hornblende felsic gneiss
zones are associated with areas of weak to moderate calc-silicate development scattered
in the Proterozoic terrane in the southwest part of the Maysville quadrangle. They are
associated with hornblende intermediate gneiss (Xhig) and locally interlayered with
amphibolite gneiss (Xag). The largest area of hornblende felsic gneiss and associated
weak calc-silicate gneiss (Xhfg/Xcs fractional unit) is present between Willow and Green
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