The Ruby Range is one of a number of uplifted blocks
in southwestern Montana and east-central Idaho with ancient metamorphic rocks
The uplifted block of the
Ruby Range has been stripped of younger strata, wholly or in part, to expose a
Precambrian core that consists mainly of crystalline and metamorphic rocks of
Archean-age.
The Archaen-age rocks
have been identified to occur in three northeast-trending belts with a general
age progression of the oldest rocks to the southeast and youngest in the northwest.
The youngest belt has been designated the “Cherry Creek Group” and the
middle belt the “Dillon Granite Gneiss.”
Pegmatites are abundant in the area, and the graphite deposits are
often associated with the pegmatite bodies which range in age from Late-Archean to
Early- to Middle-Proterozoic age.
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Regional Geologic Map of the Ruby Graphite
Vicinity
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Project Area Geologic Map of the Ruby Graphite
Holdings
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The Cherry Creek Group is an alternating sequence of metamorphosed
sedimentary layers consisting of seven or more layers of marble alternating with
gneiss, schist and quartzite, and combinations and variations of these rock types.
Bedding and foliation of the metamorphic layers strike northeast and
dip from 40° to 60° to the northwest. All lithologies can be intensely folded
and crinkled on a small scale, but the overall structure of the layers is fairly planar
from a regional viewpoint.
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Diabase dikes transect all metamorphic structures in a northwesterly
direction. The dikes follow or intruded into existing or concomitant faults.
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