Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Nevada foothills province, California

R.D. Taylor, Richard Goldfarb, T. Monecke, I.R. Fletcher, M.A. Cosca, N.M. Kelly

    Research output: Contribution to journalArticle

    12 Citations (Scopus)

    Abstract

    © 2015 Society of Economic Geologists, Inc. The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6-161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (∼800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (
    Original languageEnglish
    Pages (from-to)1313-1337
    JournalEconomic Geology
    Volume110
    Issue number5
    Publication statusPublished - Aug 2015

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    Geochronology
    Gold deposits
    geochronology
    grasses
    veins
    Gold
    valleys
    phosphates
    gold
    Phosphates
    deposits
    granodiorite
    phosphate
    grass
    xenotime
    valley
    Rocks
    magmatism
    rocks
    Wall rock

    Cite this

    @article{6965129389df4b91802a7b77f26db1d7,
    title = "Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Nevada foothills province, California",
    abstract = "{\circledC} 2015 Society of Economic Geologists, Inc. The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70{\%} of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6-161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (∼800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (",
    author = "R.D. Taylor and Richard Goldfarb and T. Monecke and I.R. Fletcher and M.A. Cosca and N.M. Kelly",
    year = "2015",
    month = "8",
    language = "English",
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    Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Nevada foothills province, California. / Taylor, R.D.; Goldfarb, Richard; Monecke, T.; Fletcher, I.R.; Cosca, M.A.; Kelly, N.M.

    In: Economic Geology, Vol. 110, No. 5, 08.2015, p. 1313-1337.

    Research output: Contribution to journalArticle

    TY - JOUR

    T1 - Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Nevada foothills province, California

    AU - Taylor, R.D.

    AU - Goldfarb, Richard

    AU - Monecke, T.

    AU - Fletcher, I.R.

    AU - Cosca, M.A.

    AU - Kelly, N.M.

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    N2 - © 2015 Society of Economic Geologists, Inc. The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6-161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (∼800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (

    AB - © 2015 Society of Economic Geologists, Inc. The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6-161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (∼800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (

    M3 - Article

    VL - 110

    SP - 1313

    EP - 1337

    JO - Economic Geology

    JF - Economic Geology

    SN - 0361-0128

    IS - 5

    ER -