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Microprobe Study: Geochemistry of the Phosphoria Formation: Ben Perkins

Project Title: Microprobe Study: Geochemistry of the Phosphoria Formation
Mendenhall Fellow: Robert "Ben" Perkins, (650) 329-5479, rperkins@usgs.gov
Duty Station: Menlo Park
Start Date: January 16, 2001
Education: Ph.D., Environmental Science and Resources: Geology, Portland State University, 2000
Research Advisor: David Z. Piper, (650) 329-5187, dzpiper@usgs.gov 		      Ben Perkins
Project Description: Research is focused on investigating the partitioning of trace elements in phosphate ores from Idaho and Wyoming using microprobe techniques (SEM, electron microprobe, ionprobe). This work has two objectives: to help understand water-rock interactions influencing the concentrations of potentially harmful elements in ground and surface waters and to further our understanding of the origins and occurrence of phosphate deposits.

The first objective is of particular importance given the high concentrations of potentially harmful elements in the Phosphoria Formation (for example, up to 1500 ppm Se and 11,000 ppm V), the extent of phosphate mining in southeast Idaho, and the potential for leaching of these elements from mine-waste rocks. The emphasis of this work to date has been identification of mineral phases hosting these elements, particularly Cd, Cr, Cu, Mo, Ni, Se, V and Zn. Zn, as well as Cd, is hosted primarily in sphalerite. Both sphalerite and pyrite were found in every sample analyzed. The highest detected concentrations of Cu, Ni, and Se to date have been associated with pyrite although they are also present in additional phases, including residual organic material and, in the case of Ni, silicates. Mo and V appear to be associated with sulfides. Cr is hosted in multiple phases and currently we are using XAFS to help identify these.

Research is also being conducted on the distribution and variability of REEs and redox-sensitive elements within individual phosphate pellets and ooids. Intrapelletal zones of trace element enrichment or significant shifts in elemental ratios may be related to changing environmental conditions during growth and early diagenesis of phosphate grains. Such geochemical signatures as well as stable isotope ratios may provide information regarding the larger-scale environment of the Permian Phosphoria Sea. Information gathered from this small-scale study along with bulk chemistry from across the Phosphoria Basin will be utilized to develop a model for the genesis and internal variability of the Phosphoria Formation. An improved understanding of the environmental conditions responsible for this type of deposit will enable us to better predict the occurrence, grade, and trace-element content of similar deposits elsewhere in the world. Such predictive tools are important given our fundamental reliance on phosphate ore for food production, the lack of well-defined phosphate deposits in much of the world, and the potential environmental problems associated with phosphate extraction.

Other aspects of the project include:

  1. Development of standards for microanalytical work. Emphasis here is on the development of homogeneous phosphate reference materials with trace element concentrations comparable to those found in natural sedimentary phosphates. Another aspect is the characterization of existing and newly developed sulfide standards.

  2. Comparison of existing bulk chemistry data from sections spanning the Phosphoria Basin (~ 400 km). Specifically, the examination of variations in geochemistry to help decipher the paleoenvironment of the Phosphoria Sea and to investigate how changing sea levels may have influenced the development of the Phosphoria sediments.

  3. Conducting field and lab work on the Rex Chert Member of the Phosphoria Formation. The Rex Chert unit overlies the main phosphate ore body (Meade Peak Member) that has been the focus of our other studies. It has previously been assumed to be relatively innocuous. However, the interbedded organic-rich shales present in this unit may contain high levels of trace metals that could be released through mining activities, for example, the removal of overburden and its exposure to weathering.

  4. Conducting a detailed geochemical study of the Lakeridge core. This core, collected in the early 1970s from western Wyoming, represents a relatively unweathered profile extending completely through the Phosphoria Formation and into the overlying and underlying formations.

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