Thesis: Geology and geochemistry of Au-Ag-mineralizations in the
Virginia City Mining District (VCMD), Montana, U.S.A.
Complete report (PDF-file in English, 2.6 MB)
Map (PDF-file, 1.4 MB)
Virginia City Mining District (VCMD) in southwestern Montana was
one of the richest placer deposits in the USA This study is attempting
to answer the yet unresolved question on the nature of mineralization
of the VCMD. Based on detailed geologic mapping of the surrounding
host rocks this study reveals geological, structural, petrological
and geochemical information on the Lucas/Atlas and South Bachelor
gold-silver system. A geodynamic interpretation of the emplacement,
protolith and tectonic setting of the gneisses/amphibolites and
the gold bearing quartz veins in the VCMD has been made and a
descriptive genetic model for the gold mineralization in the VCMD
has been achieved.
of unaltered wall rock gneiss in Lucas/Atlas and South Bachelor
areas is dominantly „granitic“ with or without magnetite (Aggm or
Agg, respectively) which reached upper amphibolite to lower granulite
facies. Minor host rocks are represented by „tonalitic“ gneisses
and hornblende-plagioclase gneisses and amphibolites, especially
in the vicinity of the Kearsarge mine. Numerous granitic pegmatites
and several aplite dikes transect wall rock foliation and are in
turn crosscut by the vein system (and the related alteration halo).
No large intrusive bodies have been observed in the district, though
an aplite/alaskite intrusive close to the Easton/Pacific mines in
the western part of the district could be interpreted as the top
region of a possible deeper seated intrusive body.
and discrimination of country rocks of the Lucas/Atlas and South
Bachelor areas suggest protoliths of dominantly volcanic origin.
This suite of rocks possibly have been generated by subduction-like
processes. Field observations, however, leave no doubt that the
volcanics are intercalated with sedimentary rocks especially to
the west of Alder Gulch. Therefore, they must have been deposited
in the same volcanic arc tectonic setting and accreted by collisional
though not far-travelled tectonics.
and three metamorphic events could be defined in the host rocks.
The first two deformational events produced ductile isoclinal folds
(at 2.7-2.9 Ga, upper amphibolite to lower granulite facies) and
open folds (at 1.6-1.9 Ga, upper amphibolite facies). Greenschist
facies assemblages accompanied brittle deformation during Sinian
(Late Proterozoic) and Laramide (Late Cretaceous-Early Tertiary)
uplift and Tertiary extension.
mineralization in the VCMD can be assigned to the second uplift
of metamorphic core complexes in south-western Montana due to complex
interaction of paired fault systems between thrust belt structures
and northwest-trending left-lateral strike-slip faults and development
of positive flower structures during Laramide crustal shortening.
This may have been accompanied by unexposed coeval intrusions similar
to the Tobacco Root Batholith although they are not necessarily
needed to explain mineralizations. Veins could have been emplaced
in an en echelon array between major left-lateral strike-slip faults
as subsidiary shear fractures and Riedel shears which developed
at a small angle (roughly 10-20°) synthetic to the main strike-slip
mineralization in the VCMD is considered to be a low-sulphide and
meso- to epithermal epigenetic deposit in which deposition of precious
metals coincides with the formation of high temperature potassic
alteration halos. Superimposed textures such as later carbonate
brecciation and fracture fillings are common, as well as argillic
alteration in other parts of the district. They indicate multistage
hydrothermal activity during repeated reactivation of earlier vein
structures possibly due to renewed uplift, popped-up or tilted parts
of earlier mineralization or due to telescoping.
rocks may be considered as the primary source of gold that was deposited
syngenetically, redistributed subsequently, and concentrated to
ores in favourable structures.