Geology of the Modoc Pb-Ag-Zn District, Inyo County, California
1986-05
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Geology of the Modoc Pb-Ag-Zn District, Inyo County, California
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1986-05
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Thesis or Dissertation
Abstract
The Modoc District, in Central Inyo County, California is underlain by a 1600 meter sequence of Devonian to Permian marine carbonate rocks. These have been intruded by a quartz monzonite stock of Jurassic age and a swarm of andesite porphyry dikes of Cretaceous (?) age. Late Cenozoic olivine basalt flows and sedimentary deposits unconformably overlie the older rocks. Three distinct zones of contact metamorphism are developed within the carbonate strata as an aureole around the quartz monzonite stock. These zones are divided on the basis of their characteristic mineral assemblages and consist of: a) an albite-epidote hornfels facies zone (2.5 km to 700 meters away from the intrusive contact), b) a hornblende-hornfels facies zone (approximately 700 to 30 meters away), and c) a pyroxene-hornfels facies zone (30 meters away up to the contact itself). Structurally controlled Pb-Ag-Zn ore of the district is in marble of Devonian and Mississippian ages; whereas, silty limestone of Pennsylvanian and Permian ages appear unfavorable for ore development. The ore bodies are small but high grade. Argentiferous galena with exsolution blebs of tetrahedrite are the principal primary ore minerals. Secondary ore is manganese-rich, and contains cerussite, hemimorphite, cerargyrite and relict galena. High silver grades are encountered throughout many of the deposits with the primary source of the silver being within galena as Ag2s, and within tetrahedrite as a solid solution between tetrahedrite-freibergite series minerals. Dolomitization of the marble, formation of manganese minerals, and the occasional addition of silica, are the common wall rock alterations associated with ore mineralization; this alteration is not widespread. It is envisioned that hydrothermal solutions of connate/meteoric origin were set into convective circulation by the heat generated from the quartz-monzonite stock. These water-rich saline solutions leached metals from the country rock, carrying them in chloride complexes. As these hydrothermal solutions migrated upward, their temperature and salinity decreased, and their pH increased, thus precipitating out the metals in structurally prepared zones and mineralizing the Devonian and Mississippian marble of the district.
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Daniel Lee England in partial fulfillment of the requirements for the degree of Master of Science, May 1986. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.
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