Crustal Imaging and Characterization Team

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Processes of World-Class Ore Deposit Formation

Project Objectives

The USGS Mineral Resources Program seeks to increase knowledge about ore-deposit genesis, use this knowledge to improve methodologies for mineral-resource assessment, and apply these methodologies at different spatial scales from local and national to global. The principal goal of this project was to enhance the ability of USGS scientists to forecast the likelihood of as-yet undiscovered world-class mineral deposits at different spatial scales, with particular emphasis on epizonal base- and precious-metal hydrothermal deposit types typically found in subduction-boundary zones. Of highest priority were porphyry Cu, polymetallic veins and replacements, and epithermal Au-Ag-base metal systems. To work towards the principal goal, the project focused on the following interrelated objectives:

1. What are the regional tectonic and structural systematics of convergent plate margins?;
2. How do the tectonics and structural geology of subduction-boundary zones change in time and space?;
3. What tectonic and structural environments most effectively focus coupled fluid flow and deformation?
4. What is the nature and pattern of fracturing in different parts of fault and intrusive rock systems?
5. What understanding may be gained about feedback mechanisms between fluid flow, ore formation, and deformation using, for example, stable isotope geochemistry and chemical patterns in veins and minerals?
6. What, at different spatial scales, are the tectonic and structural geologic settings of world-class and sub-world class ore deposits?
7. What are the similarities and differences between world-class and sub-world-class ore deposits?

Relevance & Impact

The USGS is currently conducting a global assessment of undiscovered mineral resources including copper and gold. At least 71% of the known Cu resource occurs in association with igneous rocks. World-class deposits of Cu account for 84% of the total known Cu resource (Singer, 1995). Thus, a greater understanding about how porphyry Cu deposits are localized and form will significantly improve mineral-resource assessments and assist minerals exploration. Approximately 54% of all non-placer Au is clearly related to igneous rocks (Singer, 1995). If deposits spatially associated but of uncertain genetic relation to igneous rocks (e.g., Mother Lode, Carlin) are included, then 99% of all non-placer Au is related to igneous rocks. Significant Au resources occur in epithermal and porphyry Cu type deposits. Therefore, knowing how and why world-class epizonal ore deposits in subduction-boundary zones form will contribute greatly to the development of assessment methodologies that improve quantitative forecasts of undiscovered world-class deposits. Potential benefits to the USGS Mineral Resources Program include providing knowledge that;

1. can assist a greater number of USGS scientists to carry out quantitative mineral-resource assessments;
2. aids in the quantitative delineation of tracts of land permissive for the occurrence of specific deposit types;
3. allows the subdivision of permissive tracts into regions of differing favorableness; and,
4. allows the prediction of where, within permissive tracts, world-class deposits can occur and where sub-world-class deposits are more likely.

Project Chief:

Products

Reports Delivered

• Anderson, R.E., and Berger, B.R., 2005, A regionally significant revision of Neogene tectonic history, northern Wassuk and Singatse ranges, Lyon and Mineral counties, Nevada, Geological Society of Nevada.
• Berger, B.R., Phillips, J.D., Anderson, R.E., and Tingley, J.V., 2005, Plate-boundary transverse deformation zones and their roles in localization of mineralization in the Virginia City, Goldfield, and Silver Star mining districts, Nevada, Geological Society of Nevada.
• in Rhoden, H.N., Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, pp. 237-267
• in Rhoden, H.N., Seininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, pp. 269-281.

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