Cameron Snow

Timing of Motherlode Au-Mineralization

2010-01-24T13:21:00.000-08:00

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

Cameron A. Snow ^1,2

Dennis K. Bird¹

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

2010-01-17T03:51:00.000-08:00

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.

Link to Article

Franciscan Complex

2010-01-17T03:48:00.000-08:00

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.

Link to Article

Mesozoic Tectonics of California

2010-01-17T03:45:00.000-08:00

Contrasting early and late Mesozoic petrotectonic evolution of northern California

W.G. Ernst
Cameron A. Snow
Hannah H. Scherer
Stanford University, Geological and Environmental Sciences, Building 320, Room 118, Stanford, California 94305-2115, USA

Abstract
Devonian–Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted arc origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170–200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty mélange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic mélange with North Fork petrotectonic affinities. The North Fork–Eastern Hayfork–Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt–Eastern Klamath–Feather River–Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ∼230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks.

In contrast, the Late Jurassic to largely Cretaceous Klamath–Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ∼70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian–Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units.
Link to Article

Jurassic Volcanism in the Sierra Nevada Foothills

2010-01-17T03:42:00.001-08:00

Petrotectonic evolution and melt modeling of the Peñon Blanco arc, central Sierra Nevada foothills, California

Cameron A. Snow
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

Abstract
The Peñon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Peñon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Peñon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Peñon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Peñon Blanco arc against the inboard Calaveras Complex.

New geochemical data from the Peñon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Peñon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Peñon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Peñon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

Link to Article

Geochemical Discrimination

2010-01-17T03:41:00.000-08:00

A reevaluation of tectonic discrimination diagrams and a new probabilistic approach using large geochemical databases: Moving beyond binary and ternary plots

Cameron A. Snow
Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA

Statistically rigorous confidence intervals calculated from analyses of basaltic volcanic samples obtained from two large geochemical databases (PetDB and GEOROC) demonstrate that published binary and ternary discrimination diagrams seldom correctly classify samples from mid-ocean ridges, island arcs, and ocean islands with better than 60% accuracy. The confidence intervals provide a measure of certainty that was previously absent from these diagrams, allowing for a more robust analysis of results. A new probabilistic method for geochemical discrimination is developed using the geochemical databases. The new method is N-dimensional and uses a priori data to construct probability distribution functions from which a posteriori probabilities are generated. Tests of the new method demonstrate single analysis classification success rates for volcanic rocks from island arcs to be 83%, from ocean islands to be 75%, and from mid-ocean ridges to be 76%.

Link to article

Cameron A. Snow

2010-01-16T23:36:00.000-08:00

PO Box 27582
Houston, TX 77227

Phone (Egypt): +20 (012) 042-7209
Email: cameron.snow@gmail.com

EDUCATION
2006 Ph.D., Geology, Stanford University
Thesis: On the Jurassic tectonic evolution of California
Advisor: W. Gary Ernst

2002 M.S., Geology, Utah State University
Thesis: Geology of the Cuesta Ridge ophiolite remnant near San Luis Obispo, California: Evidence for the tectonic setting and origin of the Coast Range Ophiolite
Advisor: John W. Shervais

1999 B.S., Geology, N.C. State University

WORK EXPERIENCE
2009 Geologist III Khalda Petroleum, J.V.
General Duties: Worked as development geologist for an Apache Corporation joint venture with EGPC. Responsibilities included taking lead in mapping in multiple fields, developing known reserves, optimizing field development plans, and building geologic models to determine reservoir distribution and characteristics. Collaborate with national staff with respect to the above duties to maintain and foster relationships.

2008 Petrophysicist II Apache Energia Argentina
General Duties: Served as lead petrophysicist in Argentina office. Performed all operational petrophysics, managed core acquisition and logging programs, lead efforts to identify and develop bypassed opportunities. Worked with project geoscientists and engineers to aid in mapping key reservoir properties.

2006-2008 Petrophysicist II Apache Corporation
General Duties: Determine reservoir storage properties, flow potential, and fluid saturations using well logs, petrophysical core data, seismic data, and production data. Develop geocellular models from multiple data sources, and distribute rock properties using robust geostatistical methods. Consult with project geologists and engineers to optimize field development. Experience includes working in conventional reservoirs, tight gas sands, and shale gas systems. Geographic areas worked include United States (Gulf Coast, Anadarko basin, and onshore Texas), Canada, Argentina, and Australia.

2002-2006 Research & Teaching Assistant Stanford University
Project: Geology, geochemistry, and geochronology of the Peñon Blanco arc complex, Sierra Nevada Foothills, California.
Courses Taught: Fundamentals of Geology, Geology of Death Valley and Owens Valley, Earth Materials, Geologic Development of CA, and CA Landforms.

2004 Geoscience Intern Apache Corporation
Projects: Determination of subsurface sandstone distribution using well log data in the Anadarko basin in central and western Oklahoma and structural analysis of carbonate reefs using 3-D seismic and well data.

2000-2002 Research & Teaching Assistant Utah State University
Project: Geologic mapping of the Cuesta Ridge ophiolite, California at the 1:12,000 scale to determine the magmatic evolution of the ophiolite.
Courses Taught: Introductory Geology for Scientists and Engineers, Introductory Geology for non-Scientists, Mineralogy and Crystallography, Optical Mineralogy, Igneous and Metamorphic Petrology, and Advanced Igneous Petrology.

2000 Teaching Assistant N.C. State University
Courses Taught: Structural Geology and Geologic Field Camp I & II

SOFTWARE KNOWLEDGE
Petrel
Interactive Petrophysics
Fast RTA
Gas3D
MatLab
Petra
Vertex
SeisWorks

SHORT COURSES
2006 Introduction to Petrophysics and Log Analysis
2007 Introduction to 3D Seismic Survey Design
2007 Applied Subsurface Geological Mapping Methods
2007 Carbonate Reservoir Architecture & Applied Carbonate Sequence Stratigraphy
2007 Mapping Workflow with Introduction to Geocellular Modeling
2007 Geological Evaluation & Interpretation of 3D Seismic using Visualization Techniques
2007 Continental Shallow Marine Clastic Reservoir Systems
2007 Advances in Fracture Diagnostics for Fractured and Horizontal Wells
2008 Recent Depositional & Stratigraphic Analogues for Fluvial and Shallow Marine Reservoirs
2008 Tight Gas Reservoir Completions

PROFESSIONAL AFFILIATIONS
Society of Petrophysicists and Well Log Analysts
Society of Petroleum Engineers
Geological Society of America
American Geophysical Union

PUBLICATIONS
Snow, C.A., Ernst, W.G., Wakabayashi, J., and Wooden, J., 2010, Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California: GSA Bulletin, v. 122, no. 1-2, p. 282-291.

Snow, C.A., and Ernst, W.G., 2008, Detrital Zircon Constraints on Sediment Distribution and Provenance of the Mariposa Formation, central Sierra Nevada Foothills, California, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: GSA Special Paper 438, p. 311-330.

Ernst, W.G., Snow, C.A., and Scherer, H.H., 2008, Contrasting early and late Mesozoic petrotectonic evolution of northern California: GSA Bulletin, v. 120, p. 179-194.

Snow, C.A., Metcalf, J., McWilliams, M., and Bird, D., 2008, Constraints on timing and tectonics of Au-mineralization in the Sierra Nevada Foothills, CA from 40Ar/39Ar radiometric dating of mariposite: International Geology Review, v. 50, no. 6, p. 513-518.

Ernst, W.G., Snow, C.A., and Scherer, H.H., 2008, Mesozoic transpression, transtension, subduction, and metallogenesis in northern and central California: Terra Nova, no. 20, p. 394-413.

Ernst, W.G., Snow, C.A., and Scherer, H.H., 2008, Mesozoic transpression, transtension, subduction, and metallogenesis in northern and central California: GSA Abstracts with Programs, Vol. 40, No.1, p. 46.

Snow, C.A., Wakabayashi, J., and Ernst, W.G., 2007, SHRIMP-based depositional ages of Franciscan Complex metragraywackes , San Francisco Bay Area, western California: GSA Abstracts with Programs, v. 39, no. 6, p. 454.

Snow, C.A., 2007, Petrotectonic evolution and melt modeling of the Peñon Blanco Arc, central Sierra Nevada Foothills, California: GSA Bulletin, v119 p. 1014-1023.

Scherer, H., Snow, C.A., and Ernst, W.G., 2007, Geologic-Petrochemical Comparison of Early Mesozoic Oceanic Terranes: Western Paleozoic and Triassic Belt, Klamath Mountains, and Jura-Triassic Arc Belt, Sierran Foothills, GSA Special Publication 410.

Snow, C.A., and Scherer, H.H., 2006, Terranes of the western Sierra Nevada Foothills metamorphic belt, California: A critical review, International Geology Review, v. 48, p. 46-62.

Snow, C.A., 2006, A re-evaluation of tectonic discrimination diagrams and a new probabilistic approach using large geochemical databases: Moving beyond binary and ternary plots, JGR Solid Earth, v. 111, B06206, doi:10.1029/2005JB003799.

Shragge, J.C., and Snow, C.A., 2006, Bayesian geochemical discrimination of mafic volcanic rocks: American Journal of Science, v. 306, p. 191-209.

Snow, C.A., and Ernst, W.G., 2006, Detrital zircon constraints on sediment distribution and provenance of the Mariposa Formation, central Sierra Nevada Foothills, CA: GSA Abstracts with Programs, v. 38, no. 5, p. 36.

Ernst, W.G., Snow, C.A., and Scherer, H.H., 2006, Contrasting petrotectonic evolution of northern California accretionary terranes: GSA Abstracts with Programs, V. 38, no. 7, p. 207.

Snow, C.A., and Ernst, W.G., 2005, New Geochemical and Geochronologic Data from the Peñon Blanco Arc and Mariposa Formation, Sierra Nevada Foothills, California: Abstracts with Programs, GSA, v. 37, no. 37.

Snow, C.A. and Shragge, J.C., 2005, Two New Methods for Geochemical Discrimination of Mafic-Intermediate Volcanic Rocks: A Proof of Concept Study using data from the Sierra Nevada Foothills and Klamath Mountains, California: EOS Transactions AGU, 86(52) Fall Meeting Supplement, Abstract IN21A-1170.

Shragge, J.C., and Snow, C.A., 2005, Two New Methods for Geochemical Discrimination of Mafic-Intermediate Volcanic Rocks: Moving Beyond binary and ternary diagrams systems: EOS Transactions AGU, 86(52) Fall Meeting Supplement, Abstract IN21A-1169.

Shervais, J.W., Kimbrough, D.L., Renne, P., Hanan, B.B., Murchey, B., Snow, C.A., Zoglman-Schuman, M.M., and Beaman, J., 2004, Multi-stage origin of the Coast Range Ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites: International Geology Review, v. 46, p. 289-315.

Snow, C.A., 2004, Implications of new geochemical and field data from the Peñon Blanco Arc, Sierra Nevada Foothills, California: Abstracts with programs, GSA, v. 36 no. 5, p512.

Snow, C.A., and Shervais, J.W., 2003, Contrasting volcanic styles in the Cuesta Ridge ophiolite remnant; evidence for SSZ formation and ridge collision: Abstracts with programs, GSA, v. 35, no. 4, p79.

Snow, C.A., Geology of the Cuesta Ridge Ophiolite Remnant near San Luis Obispo, California: Evidence for the Tectonic Setting and Origin of the Coast Range Ophiolite, M.S. thesis, Utah State University, 2002, 150p.

Snow, C.A., and Shervais, J.W., 2002, Cuesta Ridge ophiolite; new field and geochemical evidence for origin and evolution of the Coast Range Ophiolite, California: Abstracts with programs, GSA, v. 34, no. 5, p23.

Snow, C.A., and Shervais, J.W., 2002, Late stage MORB volcanism at the Cuesta Ridge ophiolite remnant: Evidence for ridge collision or back-arc basin spreading?: American Geophysical Union, EOS v. 83, V52A-1277.

Shervais, J.W., and Snow, C.A., 2001, Plagioclase dissolution in the Lunar Magma Ocean: A new model for the origin of Lunar Ferroan Anorthosites and the Rapid Growth of Highlands Crust, in Lunar and Planetary Science XXXIII, Abstract #1029, Lunar and Planetary Institute, Houston (CD-ROM).

PhD Dissertation from Stanford University

2010-01-16T23:17:00.000-08:00

Below is the PDF from my doctoral studies at Stanford University.

On the Jurassic Tectonics of California

MS Thesis from Utah State University

2010-01-16T23:15:00.000-08:00

Below is the PDF of my MS Thesis from Utah State University.

Geology of the Cuesta Ridge Ophiolite Remnant Near San Luis Obispo, California: Evidence for the Tectonic S...