Geospatial and statistical techniques were used to apply Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) remote sensing data as a new method to map porphyry copper mineral resource potential in the southwestern United States. Quantitative mineral-resource assessments for undiscovered porphyry copper deposits in four permissive tracts were conducted using ASTER remote sensing, geochemistry, gravity and magnetic, lithologic, and deposit and prospects data. Permissive tracts are discrete geographic areas that have potential for hosting ore deposits of a particular type and for which estimates of numbers of undiscovered deposits are made. All permissive tract data were compiled in a geographic information system (GIS) and USGS scientisits applied new geospatial-geostatistical techniques to form the basis of new quantitative methods for analysis and visualization of tract data.
In previous assessment studies applying ASTER alteration mapping, sites that may be associated with porphyry copper mineralization (based on a visual assessment of remotely sensed alteration types, patterns, and lithology) were represented as point locations on a map. A more accurate, automated method of compiling geometric properties and evaluating hydrothermal alteration sites using alteration areas (polygons) was developed from a regional alteration map of the southwestern U.S. consisting of 247 ASTER scenes. Alteration density of argillic, phyllic, and propylitic units based on a 1-km-diameter circle around each pixel was mapped using a low-pass filter. Alteration polygons were compiled from rock units that typically host porphyry copper deposits that contained alteration densities greater than 19%. Physical characteristics of each polygon were recorded and then ranked. Polygon scores were classified and color coded on maps in three groups: low (0–4), moderate (5–7), and high (8–22). In addition, alteration polygons that were not associated with known deposits or prospects were identified to signify an area that had potentially not been explored. The classified ASTER alteration polygons were particularly effective for showing areas of favorable alteration for porphyry copper deposits on regional-scale tract maps. Although the study area is one of the most thoroughly explored porphyry copper districts in the world, with 43 known Phanerozoic deposits, this assessment indicates that there are 14, or possibly 17 (using the ASTER data), additional porphyry copper deposits likely to be present. In addition, the ASTER polygon dataset showed two areas that have a high probability of containing porphyry copper deposits that have not been extensively explored using conventional means.
ASTER hydrothermal alteration map and outlines of alteration polygons southwest of Tucson, Arizona. Colors of alteration polygons indicate rank based on geological, geophysical, and geochemical properties within each polygon and their proximity to other polygons. Higher rank indicates more favorable properties within the polygon for the presence of porphyry copper deposits (altered rocks in mines omitted).