GLOBAL MINERAL RESOURCE ASSESSMENT: CHALLENGES AND OPPORTUNITIES FOR DEVELOPING AND REFINING ASSESSMENT METHODS

The U.S. Geological Survey, in collaboration with scientists from more than 20 countries, conducted a global mineral resource assessment for copper and potash to provide science for decisionmaking on issues of long-term resource supply, land use-, and environmental planning. Permissive areas (tracts) for undiscovered deposits were delineated at a scale of 1:1M; updated databases of known deposits and significant prospects were prepared; and amounts of undiscovered resources were estimated. Each deposit type posed challenges and opportunities for developing and refining assessment methods. Tracts were based on fundamental geologic features: subduction-related magmatic arcs for porphyry copper deposits; sedimentary basins with carbonaceous, pyritic, or petroleum-bearing beds proximal to source rocks (red beds, former red beds, flood basalts) for sediment-hosted copper deposits; and evaporite-bearing sedimentary basins with halite for potash. Permissive igneous rocks were selected from maps and projected under shallow cover to define arc-related tracts. For sandstone-hosted copper deposits, tracts were based on selection of appropriate stratigraphic units. Statistically significant regional differences in grade-tonnage distributions required basin-specific models for assessment. Reduced-facies sediment-hosted copper deposits occur in few basins worldwide, deposit sizes among basins span orders of magnitude, and single deposits represent a mineral inventory defined for part of a laterally-extensive mineralized bed, requiring a customized, basin-scale assessment approach. For potash, subsurface data and new deposit models guide identification of halokinetic and bedded salt that may contain potash. A custom resource simulation was developed for non-halokinetic (bedded) potash tracts; initial tonnages were determined based on deposit geometry and modified using basin-specific data on grade, thickness, mineralogy, and geologic loss. Probabilistic estimates of numbers of undiscovered deposits combined with appropriate grade-tonnage models in Monte Carlo simulations or custom simulations were used to estimate undiscovered resources for all deposit types. Results are released as online reports with a GIS (shapefiles of permissive tracts and deposits and prospects).