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M.Sc. 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, 2.48 MB)

Geological Map (PDF-file, 4.60 MB)

The 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.

Composition 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.

Classification 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.

Five deformational 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.

Structurally, 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 fault.

The gold-silver 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.

Volcano-sedimentary rocks may be considered as the primary source of gold that was deposited syngenetically, redistributed subsequently, and concentrated to ores in favourable structures.


 

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