THE ROLE OF GIS IN THE GLOBAL MINERAL-RESOURCE ASSESSMENT—A PORPHYRY COPPER EXAMPLE

A global assessment for copper, platinum-group elements, and potash was completed by U.S. Geological Survey (USGS) and partner organizations in 2013. A worldwide assessment of copper in undiscovered porphyry deposits was a major part of this project. This global porphyry copper assessment was based on mineral deposit models using the USGS form of mineral resource assessment. Minimum data requirements for this type of assessment include (1) 1:1,000,000 scale geologic maps or larger; (2) deposit, prospect and mineral occurrences databases; and (3) grade and tonnage models. Where available, geophysical and other data can be used to outline the subsurface extent of permissive rocks.

Geographic information systems (GIS) provide users with tools to analyze, visualize and interpret many data types. Map unit polygons from digital geologic maps were extracted and classified as permissive or non-permissive for porphyry copper mineralization. Once permissive units were identified and grouped by age, an interim map of permissive rocks was made. Extensions of map boundaries under younger cover rocks were edited based on geophysical or structural data or by applying digital buffers. Then, existing deposit and prospect databases and other available information were used to refine the preliminary permissive tract. Preliminary areas were aggregated, smoothed and edited to define the final permissive tract. Tract areas were calculated in a local equal-area projection to assure accurate areas. After permissive tracts were finalized and known resources identified, undiscovered porphyry copper deposit estimates were produced by an assessment team and combined with grade and tonnage models to create the probabilistic assessment of undiscovered porphyry copper resources. GIS products include shapefiles and/or geodatabases of permissive tracts attributed with tract name, age, grade and tonnage model used, areas and probabilistic undiscovered deposit estimates, point locations of deposit and prospects attributed with age, commodity, location and comments and joined data tables of assessment results.