Stable isotope and chemical studies of the genesis of ore deposits
Mineral resource assessments for important deposit types must be based on an understanding of the fundamental processes that occur during their genesis. Stable isotopes when combined with geologic and other geochemical studies are exceptionally powerful tools in understanding the processes during genesis ore deposits. Such knowledge is essential for the development of scientifically defensible strategies for assessing the Nation's mineral resources. Alunite, for example, has proven to be an exceptionally important mineral in the regional assessment of mineral deposits. Indeed some of the largest recent discoveries of gold and copper in the world can be attributed to the fact that alunite can be recognized by remote sensing methods such as ASTER. There are different types of alunite that can be recognized by stable isotope methods and which are known to be associated with different kinds of mineral deposits. However, the significance of coarse banded alunites such as occur at Alunite Ridge in south central Utah are poorly understood. In addition the chemical characteristics of the various types of alunite and their possible significance as indicators of mineralization are poorly understood.
Our objective is to conduct focused multidisciplinary, stable isotope and chemical studies that are designed to answer specific questions about critical gaps in knowledge of processes during the genesis of ore deposits. The topics selected for study are closely coordinated with the goals of several major projects in the Mineral Resources Program (MRP) and the new knowledge gained is expected to impact the direction of these and future projects. A current critical question is what does the occurrence of coarse grained banded alunite such as occurs at Alunite Ridge in south central Utah tell us about the mineral potential of an area? Likewise, what does the occurrence of coarse banded jarosite such as occurs at the Gilbert district in west central Nevada tells us about the mineral potential of an area? Finally we need to have a better understanding of the connection between alunite chemical and isotopic signatures as indicators of crustal and magmatic evolution and the formation of mineral deposits as most occurrences of alunite do not appear to have associated deposits.
Statement of Work
Alunite has proven to be an exceptionally valuable mineral for the regional identification of porphyry and high sulfidation type mineralization, especially given the fact that it can be indentified by remote sensing imaging. Indeed, some of the largest discoveries of Cu-Au deposits in the last 15 years have been related to the indentification and understanding of the genetic implications of various types of alunite. This year we will expand efforts to correlate the stable isotope geochemistry with data on the trace element geochemistry to further enhance alunite as an assessment mineral.
The addition of new argon ages on vein alunite at Alunite Ridge has given us the geochronological framework to write the final paper for the coarse-banded magmatic steam alunite at Marysvale and their significance as indicators of magmatic hydrothermal processes. This area is currently undergoing renewed exploration interest and may be an important Climax type Mo prospect.
Work will continue on the banded hydrothermal jarosite from the Gilbert district with comparisions to magmatic steam alunite and its significance as an indicator of mineralization in the area.
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- Diehl S.F., Hofstra A.H., Koenig A.E., Emsbo P., Christiansen W., and Johnson C., 2010, Hydrothermal zebra dolomite in the Great Basin--- Attributes and relation to Paleozoic stratigraphy, tectonics, and ore deposits: Geosphere, v. 6, pp. 663-690.
- Goldfarb R.J., Marsh E.E., Hart C.J.R., Mair J.L., Miller M.L., and Johnson C.A., 2010, Geology and origin of epigenetic lode gold deposits, Tintina Gold Province, Alaska and Yukon: U.S. Geological Survey Scientific Investigations Report 2007-5289, Chapter A, pp. A1- A18.
- Johnson, C.A., Taylor, C.D., Leventhal, J.S., and Freitag, Katja, 2010, Geochemistry of metasedimentary rocks in the hanging wall of the Greens Creek massive sulfide deposit and of shales elsewhere on Admiralty Island: U.S. Geological Survey Professional Paper 1763, pp. 163-186.
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- Slack, J.F., Causey, J.D., Eppinger, R.G., Gray, J.E., Johnson, C.A., Lund, K.I., and Schulz, K.J., 2010, Co-Cu-Au deposits in metasedimentary rocks—A preliminary report: U.S. Geological Survey Open-File Report 2010-1212, 13 p.
- Taylor, C.D., Premo, W.R., and Johnson, C.A., 2010, Sulfur and lead isotope characteristics of the Greens Creek polymetallic massive sulfide deposit, Admiralty Island, southeastern Alaska: U.S. Geological Survey Professional Paper 1763, pp. 241-286.
- Taylor, C.D., and Johnson, C.A., 2010, Introduction and overview of the U.S. Geological Survey-Kennecott Greens Creek Mining Company cooperative applied research project at the Greens Creek mine: U.S. Geological Survey Professional Paper 1763, pp. 3-12.