COPPER IN THE SOUTHERN BASIN AND RANGE OF THE UNITED STATES—IS ANYTHING LEFT?

The Southern Basin and Range Province, one of the world’s premier copper provinces, hosts 43 porphyry copper deposits with total identified resources (including past production) of more than 200 million metric tons of copper. The area contains known but as yet undeveloped porphyry copper deposits (such as Resolution, Rosemont), continues to be explored after more than a century of activity, and is covered by regional-scale geologic mapping, stream-sediment geochemical analyses, mineral occurrences databases, and geophysics (magnetics, gravity, remote sensing). Numbers of undiscovered Phanerozoic porphyry copper deposits were estimated using two approaches: subjective expert opinion and geostatistical models. The study area, from southeastern California across southern Arizona into southwestern New Mexico, includes Jurassic arc fragments, part of the Laramide arc that extends into Mexico, and post-subduction(?) Tertiary volcanic rocks and plutons. Four permissive tracts for the occurrence of porphyry copper deposits were delineated primarily on geology and a database of >700 prospects and copper occurrences ranked in terms of relative importance as indicators of porphyry-related mineralization. Areas with alluvial fill >1 km thick (identified from gravity anomaly maps) were excluded. The Laramide tract was subdivided based on differences in levels of exposure indicated by relative amounts of coeval extrusive and intrusive rocks across a prominent geophysical boundary. Subjective expert opinion estimated a mean of 17 undiscovered porphyry copper deposits (64 million metric tons of copper) whereas geostatistical modeling predicted 9 undiscovered deposits. For two tracts, estimates were made before and after consideration of alteration mapping derived from ASTER remote sensing data; estimates that used remote sensing data were higher and less uncertain. Results suggest that the ratio of mean undiscovered to identified copper resources is <1 in the southeastern Laramide tract, ~1 in the northwestern Laramide tract, ~4 in the Jurassic tract, and >50 in the Tertiary tract. The Tertiary, where volumes of exposed extrusive rocks greatly exceed intrusive rocks, may host significant resources under cover.