Marble - Colorado Geological Survey

ock unit known as the Molas Formation. This
formation, which has been mapped as being up
to 50 feet thick southwest of the area, is missing
in the Aspen-Conundrum area. Where lower
Belden rocks are sufficiently exposed due to
contact metamorphism, as at the Conundrum
Creek property recrystallization and deformation
have destroyed necessary indicators for determination
of a lower Belden contact.
The Belden Formation is of importance economically,
as this is the zone of marble being
considered in this report.
IGNEOUS ROCKS
Overview
Laramide and Middle Tertiary age igneous
rocks have been emplaced throughout the Elk
Mountains in the form of dikes, sills, lacoliths,
and plutons. Laramide age intrusive rocks of
both mafic and felsic affinity occur predominantly
as sills. Laramide igneous rocks described
thus far in the Aspen area include hornblende
quartz diorite, quartz porphyry, aplite,
and aplite porphyry. All of these intrusive rocks
have been recognized in the Conundrum Creek
area. None of these rock types were mapped in
the vicinity of the Colorado Highland Marble
quarry property by CGS. Middle Tertiary age
(Oligocene) calc-alkalic igneous rocks comprise
a number of large plutons and associated dikes,
sills, and lacoliths. The major rock types are
granodiorite, granodiorite porphyry, and hornblende
porphyry. The Oligocene plutons have
been emplaced along a regional zone of weakness
(the Elk Range Thrust Fault) with which
they are generally concordant. The plutons are
also concordant with bedding in gross aspect,
but occasionally cross-cut strata. Figure 3
shows the outcrop pattern, and some structural
relationships of the Oligocene plutons. The
White Rock pluton is of particular interest
because of extensive areas of associated contact
metamorphism which have produced the
marble and hornfels of the Conundrum valley
(Bryant, 1979).
White Rock Pluton
The White Rock pluton outcrops in a roughly
circular pattern in the southeastern part of the
Elk Mountains. Rocks of the pluton make up
the jagged peaks at the head of Castle Creek,
part of Conundrum Peak, the ridge north of
Hayden Peak, Keefe Peak, and the crest of the
Elk Mountains at the head of East Maroon
Creek. Major exposures of the pluton in the
study area are the north ridge of Hayden Peak,
a large mass on hill 12,146 between Cataract
and Conundrum Creeks, and along the plane of
the Elk Range Thrust which cuts the western
side of the Vermont Placer on the west side of
Conundrum Creek. Just north of the Vermont
Placer claim, rocks of the White Rock pluton
form the prominent cliffs on the west side of
the valley.
Rocks of the White Rock pluton are granodioritic.
The granodiorite is white to gray rock
composed mostly of plagioclase feldspar, with
lesser amounts of quartz and potassic feldspar.
Accessory minerals are biotite, hornblende,
pyroxene, and magnetite. The pluton displays
chemical variation with small areas of more
mafic phases occurring at or near margins of
the pluton.
Country rocks at the contacts of the pluton
lack chilled margins and are generally not deformed
due to intrusion. An exception is rocks
of the Belden Formation which are highly deformed
near contact with White Rock plutonic
rocks. Country rocks have been subjected to
contact metamorphism, especially in areas
where the intrusive contact is discordant.
Exposures of the White Rock pluton south of
the Colorado-Highland Marble Quarry contain
substantial quantities of pyrite which is generally
disseminated, but can occur as micro-fracture
fillings or in large masses up to four inches
in diameter (Figure 4). Where sulfides occur
in the pluton, alteration is most pronounced.
Fluid inclusion studies conducted by J.T. Nash
and C.G. Cunningham, Jr. show this to be postmagmatic
alteration which most likely occurred
a million to a few million years after emplacement
of the pluton. Also associated with this
late stage alteration are mineralized fracture
zones containing quartz and/or carbonate, and
pyrite. Some of these zones are reported to
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