2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 10
Presentation Time: 10:45 AM


SMITH, Steven M., EPPINGER, Robert G., KELLEY, Karen D. and FEY, David L., U.S. Geol Survey, Denver Federal Center, MS 973, Denver, CO 80225, smsmith@usgs.gov

The Pebble deposit, located 320 km southwest of Anchorage Alaska, is an unmined Cu-Au-Mo porphyry deposit that contains one of the largest resources of copper and gold in the world. The deposit is divided into two zones: the Pebble West zone is lower grade ore and covered by 0-50 m of glacial deposits while the Pebble East zone is higher grade ore and subcrops beneath a 300-600 m thick wedge of Late Cretaceous to Eocene volcanic and sedimentary rocks. The Pebble deposit is undisturbed, except for surface drilling, and provides an excellent opportunity to test and refine exploration techniques for discovering covered mineral deposits.

Soil samples were collected along three traverses across the West and East zones of the Pebble deposit and were analyzed by the following commonly-used and proprietary geoanalytical methods: (1) de-ionized water leach; (2) BioLeach; (3) Enzyme Leach; (4) TerraSol leach; (5) Ionic Leach; (6) Mobile Metal Ion (MMI) extraction; (7) cold hydroxylamine hydrochloride leach; (8) sodium pyrophosphate leach; (9) aqua regia partial digestion; (10) 4-acid near-total digestion; (11) sodium peroxide sinter total digestion; plus (12) soil gas hydrocarbon analyses.

The interpretation of this complex multi-element dataset derived from multiple analytical techniques is challenging and ongoing. All of the analytical methods used show anomalous single-element distribution patterns in some soils over the near-surface Pebble West zone and most methods also show possible indications of the deeply buried Pebble East zone. The most significant patterns were seen in Ag, As, Au, Cu, Mo, Re, Sb, Tl, U, and V concentrations; although some of these elements may be related more to the cover rocks than to buried mineralization. The mechanisms for migrating elements from depth into the soil profile are yet to be determined although there is some evidence for ‘leakage’ along fault zones.