18-31. Geochronology and isotopic mapping of Alaska’s geologic and metallogenic domains

Alaska constitutes 1/6th of the land mass of the Nation and is known to contain significant mineral wealth. The state is underlain by a complex framework of accreted geologic terranes that have disparate histories and geologic environments of formation. Their global origins and interactions prior to accretion are not completely understood, and discrete terrane boundaries are also difficult to identify and study because they are: 1) structurally complex, 2) commonly disrupted by major strike-slip faulting, 3) overprinted by extensive magmatism associated with the active southern Alaska margin, 4) poorly exposed, and (or) 5) overlapped and obscured by younger sedimentary basins throughout Alaska. Many different styles of mineral deposits are known or suspected to occur throughout the state because of the diversity of the geologic terranes and tectonic environments that host them. Older syn- and epigenetic mineral systems such as VMS, SEDEX, MVT, and possible Carlin-style deposits are hosted in Paleozoic strata together with remnants of pre-Mesozoic magmatic arcs and associated magmatic-hydrothermal systems. These systems are overprinted by younger Mesozoic and Cenozoic magmatic-hydrothermal and orogenic systems that formed in association with a long-lived convergent margin. Each of these mineral systems has associated critical mineral endowments, but the possible controls that the regional geologic framework of Alaska might have on mineral deposit formation, distribution, and critical mineral enrichment is not completely understood.

Studies from other mineralized regions suggest that crustal age, structural architecture, and geodynamic history have profound influences on metallogenic fertility. However, the cratonic heritage and crustal architecture of Alaska and the northwestern Cordillera are not well constrained because of incomplete exposure and a lack of systematic investigation using modern analytical techniques. To address this challenge, we seek a Mendenhall Fellow with experience in isotope geochemistry, economic geology, and(or) regional tectonics to aid in determining the crustal age, geochemical characteristics, and architecture of Alaska’s geologic framework and major metallogenic systems (i.e. porphyry, orogenic gold, and intrusion-related gold). The Fellow will ideally investigate possible linkages between terrane architecture and trace and critical metal endowments in one or more regions of the state. Radiogenic, geochemical, and(or) stable isotope systems in bedrock or sediment samples can help to characterize the age, isotopic, and geochemical signatures of discrete terranes and to interpret the tectonic affinity and geologic evolution of their crustal domains. Analyzing mineral chronometers and the isotopic and geochemical signatures of minerals from within discrete terranes and, where possible, along and across terrane boundaries is an essential step in delineating regions with distinct isotopic and geochemical signatures. In areas with poor bedrock exposure, samples of modern and recent detrital sediment can be used as proxies to assess the isotopic and geochemical signature of defined catchments and inferred underlying bedrock. These signatures provide important clues about the age and tectonic affinity of underlying cratonic components, with implications for the processes of crust formation and associated metal endowment. When compared with known characteristics of other terranes and cratons worldwide, isotopic and geochemical signatures also provide key constraints on terrane origins and subsequent interactions. Ideally these characterizations will define robust linkages between the terrane architecture, tectonic evolution, and mineral systems of Alaska and will inform assessments of regional metallogenic potential.

Proposals may focus on any relevant geochemical and(or) isotopic system and approach. Fieldwork for collection of new, targeted samples may be conducted in coordination with active USGS or State field-based research projects. Samples may also be obtained from USGS and State archives. The Fellow will have access to multidisciplinary expertise and scientific facilities at the USGS Alaska Science Center in Anchorage, Alaska, and the Plasma Laboratory (laser ablation ICP-MS) at the Geology, Geophysics, and Geochemistry Science Center in Denver, Colorado. The Fellow may also use other USGS laboratories (e.g., Stanford-USGS SHRIMP-RG) or collaborative academic laboratories to conduct the research. Collaboration with other USGS, State, or university geoscientists is encouraged. 

Interested applicants are strongly encouraged to contact the Research Advisors early in the application process to discuss project ideas.

Proposed Duty Station: Anchorage, AK

Areas of PhD: Geology, geochemistry, economic geology or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet the qualifications for: Research Geologist, Research Chemist. 

(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.)

Human Resources Office Contact: Audrey Tsujita, 916-278-9395, atsujita@usgs.gov