Timing of Motherlode Au-Mineralization

Chronology of gold mineralization in the Sierra Nevada Foothills from ⁴⁰Ar/³⁹Ar dating of mariposite

Cameron A. Snow ^1,2

Dennis K. Bird ¹

James Metcalf ^1,3

Michael McWilliams ^1,4

1.      Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305

2.      Exploration and Production Technology, Apache Corporation, Houston, TX 77056

3.      Department of Earth Sciences, Syracuse University, Syracuse, NY 13244

4.      John de Laeter Centre of Mass Spectrometry, Curtin University, Perth, Australia

ABSTRACT

            Mariposite (Cr-rich muscovite) formed during metasomatic replacement of serpentinite yield ⁴⁰Ar/³⁹Ar ages demonstrating that gold-quartz-carbonate mineralization in the Sierra Nevada Foothills occurred as early as 152 Ma and continued until at least 122 Ma. The earliest gold mineralization is found in the Grass Valley District; mineralization subsequently migrated southward along the Melones Fault Zone, forming the Mother Lode Gold Belt. These ages correspond with a lull in Sierra Nevada plutonism and with periods of ductile deformation along the Melones and related fault zones.  Our data suggest that gold-quartz-carbonate mineralization was not associated with Sierra Nevada plutonism, but instead was related to changes in plate dynamics, and generation and migration of CO₂-rich ore-forming metamorphic fluids. In this model, gold mineralization was produced by heating of the lower crust within the structural mélange of the Sierra Nevada as a response to an increase in dip of the subducting slab and oceanward migration of the saw-tooth thermal structure.

Published in International Geology Review, Vol. 50, 2008, p.1-16.

Mariposa Formation detrital geochronology

Detrital zircon constraints on sediment distribution and provenance of the Mariposa Formation, central Sierra Nevada foothills, California


1 Cameron A Snow
2 W.G Ernst

1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA, and Exploration and Production Technology, Apache Corporation, Houston, Texas 77056, USA
2 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

Abstract
New single-grain detrital zircon U-Pb age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Sierra Nevada foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Bragdon and Baird Formations of the Eastern Klamaths and the Paleozoic miogeocline of Nevada ± sources farther to the east, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval in the Mariposa Formation; and (3) the youngest zircon ages constrain the onset of clastic deposition at 152 ± 1 Ma. Zircon age data also suggest that the local drainage divide migrated westward, resulting in a higher proportion of detritus derived from the Sierra Nevada arc over time. New geologic mapping in the central Sierra Nevada foothills shows that the Mariposa Formation thickens eastward, and that the number of coarse-grained sandstone bodies increases up section. These observations indicate that a topographically low Sierran volcanic arc gradually began to rise, providing increasing amounts of clastic debris to the Mariposa depositional basin.

The Mariposa Formation was deposited in a volcanically active deep-water forearc basin and was subsequently disrupted by Nevadan orogenesis during the Late Jurassic. Inasmuch as it was located in the forearc inboard from the Middle Jurassic Coast Range ophiolite, Nevadan deformation cannot have resulted from arc-continent collision in the Sierra Nevada foothills but instead must have been related to tectonism along the plate margin.

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Franciscan Complex

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California


1. Cameron A. Snow
2. John Wakabayashi
3. W.G. Ernst
4. Joseph L. Wooden

1 Exploration and Production Technology, Apache Corporation, 2000 Post Oak Boulevard, Suite 100, Houston, Texas 77056, USA
2 Department of Earth and Environmental Sciences, California State University, Fresno, California 93740-8039, USA
3 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

Abstract
We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic–Cretaceous grains originating from the Klamath–Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100–160 Ma grains; the upper El Cerrito also contains several grains in the 1200–1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this mélange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component.

Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early–Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area.

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