• journal_title：Economic Geology
• Contributor：Robert A. Ayuso ; Karen D. Kelley ; Robert G. Eppinger ; Francesca Forni
• Publisher：Society of Economic Geologists
• Date：2013-05-01
• Format：text/html
• Language：en
• Identifier：10.2113/econgeo.108.3.543
• journal_abbrev：Economic Geology
• issn：0361-0128
• volume：108
• issue：3
• firstpage：543
• section：Articles

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 ²⁰⁶Pb/²⁰⁴Pb = 18.574 to 18.874, ²⁰⁷Pb/²⁰⁴Pb = 15.484 to 15.526, and ²⁰⁸Pb/²⁰⁴Pb = 38.053 to 38.266. Mineralized granodiorite shows a modest spread in ⁸⁷Sr/⁸⁶Sr (0.704354–0.707621) and ¹⁴³Nd/¹⁴⁴Nd (0.512639–0.512750). Age-corrected (89 Ma) values for the granodiorite yield relatively unradiogenic Pb (e.g., ²⁰⁷Pb/²⁰⁴Pb <15.52), low values of ⁸⁷Sr/⁸⁶Sr, and positive values of ɛ_Nd (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 ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd 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.