Pb-Sr-Nd isotopes in surficial materials at the Pebble Porphyry Cu-Au-Mo Deposit, Southwestern Alaska: can the mineralizing fingerprint be detected through cover?

Robert A. Ayuso; Karen D. Kelley; Robert G. Eppinger; Francesca Forni

doi:10.2113/econgeo.108.3.543

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Pb-Sr-Nd isotopes in surficial materials at the Pebble Porphyry Cu-Au-Mo Deposit, Southwestern Alaska: can the mineralizing fingerprint be detected through cover?

Robert A. Ayuso·Karen D. Kelley·Robert G. Eppinger·Francesca Forni

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Abstract

The Cretaceous Pebble porphyry Cu-Au-Mo deposit is covered by tundra and glacigenic sediments. Pb-Sr-Nd measurements were done on sediments and soils to establish baseline conditions prior to the onset of mining operations and contribute to the development of exploration methods for concealed base metal deposits of this type. Pebble rocks have a moderate range for 206 Pb/ 204 Pb = 18.574 to 18.874, 207 Pb/ 204 Pb = 15.484 to 15.526, and 208, Pb/ 204 Pb = 38.053 to 38.266. Mineralized granodiorite shows a modest spread in 87 Sr/ 86 Sr (0.704354–0.707621) and 143 Nd/ 144 Nd (0.512639–0.512750). Age-corrected (89 Ma) values for the granodiorite yield relatively unradiogenic Pb (e.g., 207 Pb/ 204 Pb <15.52), low values of 87 Sr/ 86 Sr, and positive values of ɛNd (1.00–4.52) that attest to a major contribution of mantle-derived source rocks. Pond sediments and soils have similar Pb isotope signatures and 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values that resemble the mineralized granodiorites. Glacial events have obscured the recognition of isotope signatures of mineralized rocks in the sediments and soils. Baseline radiogenic isotope compositions, prior to the onset of mining operations, reflect natural erosion, transport and deposition of heterogeneous till sheets that included debris from barren rocks, mineralized granodiorite and sulfides from the Pebble deposit, and other country rocks that pre- and postdate the mineralization events. Isotopic variations suggest that natural weathering of the deposit is generally reflected in these surficial materials. The isotope data provide geochemical constraints to glimpse through the extensive cover and together with other geochemical observations provide a vector to concealed mineralized rocks genetically linked with the Pebble deposit.

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Source-reported bounding extent: 59.866667° to 59.95° latitude; -155.333333° to -155.233333° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Robert A. Ayuso, Karen D. Kelley, Robert G. Eppinger, Francesca Forni. 2013-03-07. Pb-Sr-Nd isotopes in surficial materials at the Pebble Porphyry Cu-Au-Mo Deposit, Southwestern Alaska: can the mineralizing fingerprint be detected through cover?. https://doi.org/10.2113/econgeo.108.3.543

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