21. Innovative applications of the SHRIMP-RG to magmatic-hydrothermal systems

Recent advances in analysis of trace elements and radiogenic isotopes using the USGS-Stanford SHRIMP-RG ion microprobe offer unprecedented opportunities to address fundamental questions concerning mineral deposits formed from magmatic-hydrothermal systems in volcanic arc settings. Magmatic-hydrothermal mineral deposits are a major world source of precious and base metals, and important deposits are present in the western United States. Magmatic-hydrothermal systems are present on many active Cascade volcanoes in the northwestern United States, where resulting rock alteration may increase volcano hazards. Other magmatic-hydrothermal systems in the Great Basin provide geothermal energy, and some of these are depositing ore metals. Fundamental genetic questions regarding the source of metals in relation to evolution of magmas associated with these systems may be investigated through innovative applications of the SHRIMP-RG.

The research offers opportunities to refine and apply techniques that utilize the SHRIMP-RG to address fundamental geochemical, isotopic, and geochronologic questions regarding magmatic-hydrothermal systems and associated mineral deposits in the western United States. Questions might include: (1) the magmatic and early hydrothermal evolution of igneous rocks that are the source of heat and may provide fluids, metals, and ligands to associated mineral deposits; (2) the absolute age and duration of magmatic-hydrothermal systems; and (3) the source(s) of metals, especially Au and Ag, in associated epithermal deposits. Types of analyses might include Pb isotope analysis of ore minerals, including Au and Ag minerals, U-series and U-Pb dating of silica phases and/or carbonate minerals, and U-Pb dating and trace element analysis of igneous and hydrothermal zircons.

The western United States contains many important epithermal Au-Ag deposits related to Cenozoic magmatism. Knowledge of the detailed magmatic-hydrothermal history of genetically related igneous rocks is critical to understanding the genesis of the ores but is often obscured by hydrothermal alteration. Both magmatic and hydrothermal zircons may be present in altered igneous rocks, and trace and rare-earth-element compositions of these zircons may help elucidate the magmatic-hydrothermal history of these rocks (for example, Hoskin, 2005), distinguish mineralized from barren rocks, and constrain processes that lead to ore-forming magmas.

Dating hydrothermal alteration and determining the duration of hydrothermal activity on active Cascades volcanoes and associated geothermal systems is essential for developing genetic models that integrate evolution of volcanic and hydrothermal systems. Dating often has proven difficult, however, using conventional ⁴⁰Ar/³⁹Ar techniques. U-series and U-Pb dating of hydrothermal silica and carbonate phases using the SHRIMP-RG (Maher et al., 2007) offers a potential method for dating alteration and linking it to ore genesis and volcanic hazards and for dating silica sinter, U-rich travertine, and associated carbonate alteration in active and fossil hydrothermal systems. Additional isotopic information (for example, Sr, U, Pb) from dated deposits also can be used to infer past fluid flow rates and to address the hydrologic evolution of these systems.

Understanding the source of precious metals in epithermal Au-Ag deposits is of critical importance in developing better genetic models. A recent reconnaissance Pb isotope study of bulk gold samples from bonanza veins in the northern Great Basin suggests that Au and Ag may be derived from degassing basaltic magma rather than leached from nearby wall rocks, as suggested by stable isotope data and Pb isotope compositions of gangue minerals (Kamenow and others, in review). Application of the SHRIMP-RG to the study of micro-size gold samples permits study of a number of gold-silver deposits and could provide a significant improvement in our understanding of the source of metals and contribute critically important information to genetic models of these deposits.

The Mendenhall Fellow will refine and apply existing techniques within the two-year postdoctoral appointment. The applicant will collaborate with ongoing regional and detailed studies of hydrothermal systems and mineral deposits in the Cascade arc and Great Basin, including studies of hydrothermal alteration on active volcanoes in the Cascade arc. The research advisors have broad experience with these systems and have materials suitable for detailed study using the SHRIMP-RG.

References

Hoskin, P.W.O., 2005, Trace element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills, Australia: GCA, v. 69, p. 637-648.

Kamenow, G.D., Saunders, J.A., and Hames, W.E., in review, Mantle plume magmas as sources for gold in hydrothermal systems, submitted to Economic Geology.

Maher K., Wooden J.W., Paces J.B., and Miller, D.M., 2007, 230Th/U dating of surficial deposits using the ion microprobe (SHRIMP-RG): a microstratigraphic perspective. Quaternary International, v. 66, p. 15-28.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geology, geochemistry, isotope geochemistry, igneous petrology, economic geology

Qualifications: Applicants must meet one of the following qualifications: Research Chemist, Research Geologist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): David John, (650) 329-5424, djohn@usgs.gov; James Rytuba, (650) 329-5418, jrytuba@usgs.gov; Joseph Wooden, (650) 725-9237, jwooden@usgs.gov; Peter Vikre, (775) 784-5597, pvikre@usgs.gov; Katharine Maher, (650) 329-4978, kmaher@usgs.gov; James Saunders (Auburn University), (334) 844-4884, saundja@auburn.edu

Human Resources Office contact: Erica Settlemyer, (916) 278-9383, esettlemyer@usgs.gov

Summary of Opportunities