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Formation and Eruption of Large Silicic Magma Chambers and Their Potential Genetic Relationship to Carlin-Type Gold Deposits in the Northern Great Basin

Kathryn Watts

kathryn Watts

Project Title: Formation and Eruption of Large Silicic Magma Chambers and Their Potential Genetic Relationship to Carlin-Type Gold Deposits in the Northern Great Basin

Mendenhall Fellow: Kathryn Watts, (650) 329-5308, kwatts@usgs.gov

Duty Station: Menlo Park, CA

Start Date: January 17, 2012

Education: Ph.D., University of Oregon, 2011

Research Advisor: Joseph Colgan, (650) 329-5461, jcolgan@usgs.gov; David John, (650) 329-5424, djohn@usgs.gov; Jorge Vazquez, (650) 329-5240, jvazquez@usgs.gov; Christopher Henry, University of Nevada, Reno, chenry@unr.edu; John Muntean, University of Nevada, Reno, munteanj@unr.edu; Joseph Wooden, Stanford University, jwooden@stanford.edu

Project Description:

Calderas are formed during explosive volcanic eruptions that tap large silicic magma chambers. These eruptions release toxic gases into the atmosphere and blanket thousands of square kilometers with ash, causing severe and sustained consequences to society. Understanding the underlying magmatic processes that drive caldera volcanism is critical for evaluation of potential hazards from active caldera volcanoes. The late Eocene (34 Ma) Caetano caldera in north-central Nevada provides a world-class opportunity to study the inner workings of a large caldera volcano exposed to a paleodepth of >5 km (Fig. 1). High-strain extensional faulting dissected the caldera into a series of titled blocks that preserve a complete record of magmatism from small-volume, precursory eruptions to catastrophic caldera formation and resurgent intrusions (John and others, 2008). In addition to producing one of the most voluminous ignimbrite eruptions in the western U.S. (>1,100 km3 Caetano Tuff), the Caetano caldera is spatially and temporally correlated with formation of large Carlin-type gold deposits that contain more than 15 million ounces of gold (worth over US$20 billion at 2012 prices).

The goal of this project is to develop an integrated model of magma evolution at Caetano caldera, synthesizing new petrologic, geochemical, and geochronologic studies with field studies and geologic mapping (Colgan and others, 2011). Extracaldera silicic dikes and domes that were coeval with gold formation will allow me to assess the potential genetic relationship between Caetano magmatism and Carlin-type gold deposits. Microanalysis of individual crystals will be a main focus of this work. By analyzing successive growth bands in core-rim crystal transects, I will reconstruct records of magma evolution from the earliest stages of formation (cores) to the final stages preceding eruption (rims). Major elements, trace elements, and isotopes of major and accessory crystal phases will be used to determine the timing and role of crystallization, assimilation, and recharge. Crystals in Caetano caldera samples are abundant (~30-60 vol%) and large (up to 10 mm), and thus ideal for microanalytical geochemical studies (Fig. 2). Zircon U/Pb and sanidine Ar/Ar geochronology datasets will provide robust temporal constraints that can be synthesized with major, trace, and isotope geochemistry datasets.

Objectives:

Geologic map of north-central Nevada, emphaisizing Cenozoic geology and structures
Figure 1. View of Mt. Caetano in the Caetano caldera, Nevada. From the base to the summit of Mt. Caetano, >1 km thickness of intracaldera Caetano Tuff is exposed. Pre-caldera lava flows and Paleozoic basement rocks underlie tuff exposures. Photo credit: David John.

View north across Rocky Pass in the northern Toiyabe Range
Figure 2. Photomicrograph of a large plagioclase crystal cluster in the Caetano Carico Lake pluton. The large euhedral crystal in the center exhibits oscillatory zoning. Core-rim crystal transects in crystals such as these can be used to piece together the pre-eruptive magmatic history of the Caetano caldera.

References

John, D.A., Henry, C.D., and Colgan, J.P., 2008, Magmatic and tectonic evolution of the Caetano caldera, north-central Nevada: A tilted mid-Tertiary eruptive center and source of the Caetano Tuff: Geosphere, v. 4, p. 75-106.

Colgan, J.P., Henry, C.D., and John, D.A., 2011, Geologic map of the Caetano caldera, Lander and Eureka counties, Nevada: Nevada Bureau of Mines and Geology Map 174, scale 1:75,000, 10 p.

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