PROCESSES THAT FORM WORLD-CLASS ORE DEPOSITS AND THEIR APPLICATION IN MINERAL-RESOURCE ASSESSMENTS

World-class ore deposits are the largest 10% of a given deposit type as ordered by metal content (Singer, 1995). These deposits dominate the supply of many metals and are, therefore, of great importance. To better understand how these deposits formed, why they grew to such sizes, and to enhance the ability of the U.S. Geological Survey to forecast at regional, national, and global scales the occurrence of as-yet undiscovered world-class deposits, the USGS has initiated a new project to address these issues. Using geology, geophysics, geochemistry, and geochronology, the project is studying epizonal hydrothermal base- and precious-metal-bearing deposit types. The science strategy is to (1) treat deposits as a product of coupled thermal, mechanical, chemical, and hydraulic phenomena, (2) compare and contrast attributes of world-class and sub-world-class deposits, (3) document the coupled mechanical, chemical, and thermal evolution of ore bodies, and (4) develop criteria and methods to apply process understanding in mineral-resource assessments. Basic science questions include the following: (a) What roles do subduction-boundary zone dynamics play in magmatic-hydrothermal ore formation? This includes studies of subduction-boundary zone tectonic segmentation, and how synhydrothermal strain accommodation and strain partitioning affect the localization and economic productivity of hydrothermal systems. (b) How do regional zones of alteration (e.g., argillic/advanced argillic) form and relate genetically to circumscribed, but spatially smaller sites of later ore deposit formation? (c) What are the size and shape of ore-forming systems and what are the size and shape of ore bodies within the systems? (d) What constraints do observed district- and deposit-scale geometries set on regional and local stress conditions and strain accommodation? (e) What roles do fracture density and fracture connectivity play in compartmentalizing fluid flow and forming ore bodies? We present interpretations of new geophysical and geological data from porphyry Cu and epithermal Au-Ag districts in the western U.S. that provide clues towards answering some of these science questions, including districts in Arizona, Nevada, California, and Washington.