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Project work for Mineral Systems of the Ancestral and Modern Cenozoic Cascades Arcs and central California Coast Ranges, Western USA

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This project will construct a comprehensive metallogenic model for the Cenozoic Cascades arcs and late Cenozoic magmatism in the central California Coast Ranges and refine genetic mineral deposit models, especially for epithermal deposits that are abundant in these areas. The initial phase of the project will be the development of geologic, geochemical, geophysical, geochronologic, and mineral deposit databases for the southern Cascades arc in northeast California and western Nevada. Once completed, these databases will augment similar databases compiled for the Humboldt River Basin BLM assessment project (northern Nevada), as well as the soon-to-be completed Resources and Hazards of Hydrothermal Systems in Cascades Volcanoes project which pertains to the Western Cascades arc in Washington, Oregon, and northernmost California. Database compilation will identify high-priority data gaps, such as for the Cascades arc in northwest Nevada and northeast California, and selected new data will be collected to fill these gaps. In addition, detailed studies of selected fossil and active hydrothermal systems, both mineralized and un mineralized, will be conducted to improve mineral deposit models. The comprehensive data synthesis and interpretation phase of the project will result in products that specifically address land-use planning and scientific needs. The project will be undertaken with collaboration of the Nevada Bureau of Mines and Geology, the University of Nevada, Reno, the Great Basin Center for Geothermal Energy, and other Federal, State, and Academic organizations.

Task 1 - Metallogeny of the southern Cascades arc

Mineral deposits in continental-margin volcanic arcs are the major source of the worlds copper, gold, silver and other important byproduct trace metals. Magmatic-hydrothermal systems form episodically throughout the evolution of volcanic arcs including the waning stage of arc volcanism. The southern Cascade arc in California and Nevada, and geochemically similar, coeval volcanic rocks in the California Coast Range provide an arc setting in which to understand the major magmatic and tectonic controls on the development of ore deposits throughout the evolution of the arc and its demise as a result of establishment of a transform plate boundary. This task seeks to better characterize known mineral deposits and areas of hydrothermal alteration in the southern Cascades arc and define the magmatic and tectonic controls of ore deposit generation in the arc. Research on the metallogeny of the southern Cascade arc will be combined with previous MRP studies of the northern (Western) Cascade arc in Oregon and Washington in order to create an integrated metallogenic model for the entire Cenozoic Cascade arc. This research will provide knowledge about mineral deposit generation in a continental arc setting that will be more broadly applicable to magmatic arcs worldwide. This task also contributes to the development of quantitative models for both hydrothermal alteration and mineralization in the near surface environment and the interior parts of stratovolcanoes and flow-dome fields.

• Field, geochemical, geophysical, and geochronologic studies will continue to delineate the volcanic centers and hydrothermal systems in the Bodie Hills, CA-NV. The age, character, and distribution of 11 major volcanic centers will be delineated and the relationship of associated hydrothermal systems and ore deposits will be established. The significance of large areas of hydrothermal alteration as defined by ASTER and AVIRIS data will be interpreted. A detailed aeromagnetic study of the Bodie Hills will be used to interpret the subsurface extent of the volcanic centers and intrusions and the extent of hydrothermal alteration. A detailed gravity survey previously acquired will be completed and used to evaluate the geophysical character of the volcanic centers and the 3-d structure of the arc. The evolution of each of the centers will provide the basis for documenting the evolution of the ancestral arc magmatism to extensional related volcanism in the period from 15 to 5 Ma.
• Detailed field studies of epithermal deposits and associated large areas of hydrothermal alteration will be completed in the Monitor, Paramount, and Bodie districts, California, and Aurora district in Nevada. Hydrothermal systems outside known districts will also be studied and include Hg and S deposits in Cinnabar Canyon, and alunite alteration along the Walker River. The age of the ore deposits and hydrothermal alteration will be documented through Ar/Ar dating of alteration minerals and volcanic rocks. The lateral extent of the hydrothermal systems will be delineated by mapping and ASTER data. Areas of focused fluid upwelling within the hydrothermal system will be mapped based on the presence of sinter, hydrothermal eruption craters and pools, and vein systems. The geochemistry and character of each hydrothermal system will be summarized and described within the context of the volcanic center that host the hydrothermal system.
• Structural studies within each of hydrothermal systems will be carried out to document the stress field during evolution of the arc. Geophysical magnetic studies will document the nature and magnitude of the various blocks within the arc segment and the structural studies will be evaluated and interpreted with respect to the amount rotation that has occurred in this part of the arc.
• Geochronologic and radiogenic isotope studies of the Warner Range and associated arc rocks to the southwest will be integrated with geochemical and geologic studies to provide a basis for characterizing the nature and evolution of Oligocene to late Miocene ancestral arc magmatism in NE California. These studies are being done in collaboration with Stanford University, Carleton University, Nevada Bureau of Mines and Geology, and UCSC.
• Digital compilation of geologic, geochronologic, geochemical, and mineral occurrence data by mining district on the northern and southern Cascades arc and time equivalent volcanic fields in the Coast Range will be completed. An interim publication will summarize the present knowledge on the geology, geochemistry, structure, mineral deposits and geophysical anomalies associated with the ancestral and modern arc. This report will also document fundamental questions about arc volcanism and related ore deposits that are being studied by the project.
• The research and data bases will be utilized to create a synthesis of regional tectonics, geology, and geophysics of the southern Cascades arc. Quantitative analysis of the relationship between geophysical anomalies, magmatic centers, areas of hydrothermal alteration, and mineral deposits will be carried out for the important mining districts and large areas of hydrothermal alteration that have not been mined.
• The compilation of geochemistry of igneous rocks and interpretation of petrochemical trends during evolution of the arc will be completed and results from the southern arc will be compared to the northern Cascade arc.

Task 2 - Young hydrothermal systems

Epithermal mineral deposits and weakly mineralized hydrothermal systems in the diverse and complex geologic settings of the Cenozoic Cascades arc, northwestern Great Basin and in the central Coast Ranges, California, are considered the products of magmatic, amagmatic (extensional, non-magmatically heated), and mixed hydrothermal systems. In addition, several types of magmatic hydrothermal systems that develop different types of epithermal mineral deposits are present in these areas. Identification of the origin of many of the epithermal deposits remains uncertain. The criteria used to discriminate among the alternate origins are poorly constrained and commonly limited to empirical observations. The implications of this uncertainty to reliable mineral resource assessments are great, because assessments need to apply appropriate occurrence and grade and tonnage models.

• Define criteria to verify the importance of the three proposed types of hydrothermal system (magmatic, amagmatic, and mixed) on the genesis and characteristics of epithermal mineral deposits resulting from these hydrothermal systems, and to distinguish types and characteristics of epithermal deposits formed from different types of magmatic hydrothermal systems.
• Identify differences in the geologic settings and hydrothermal systems between barren and mineralized terranes. Test the potential of the identified parameters to aid in focusing mineral resource assessments.
• Refine genetic models of epithermal precious and base metal deposits particularly as related to extent and grade of metal accumulations that may result.
• Contribute to the understanding of resource potential to land management agencies.

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