SM-ND AGE OF QUARTZ-CARBONATE VEINS ASSOCIATED WITH THE COEUR D'ALENE MINING DISTRICT, IDAHO

Estimates of the age of the Coeur d'Alene mineralization fall into two distinct groups: (1) Proterozoic, based on Pb isotope studies (Leach et al., 1998) and (2) Late Cretaceous, based on Sr isotope data (Fleck et al., 2002). The earlier age is supported by ⁴⁰Ar/³⁹Ar plateau ages of approximately 1000 Ma for fluorphlogopite in ankerite-bearing veins (Rosenberg and Larson, 2001). Fluorphlogopite is closely associated with or entirely enclosed within ankerite suggesting contemporaneous crystallization consistent with the Fe²⁺-F Avoidance Principle.

A study of Sm-Nd isotopes in vein carbonates has been initiated to resolve this problem. Preliminary data lie along a ~1000 Ma isochron (R²=0.97, n=12) and are characterized by extreme Sm enrichments (Sm/Nd=0.2-1.1) and ¹⁴³Nd/¹⁴⁴Nd ratios that range from 0.512-0.519. These ratios require long residence times after Sm enrichment and are consistent with a Proterozoic age. Initial ⁸⁷Sr/⁸⁶Sr ratios between 0.62 and 0.82, observed for most deposits, could have been derived by leaching of Belt meta-sediments at ~1000 Ma but samples with higher initial ⁸⁷Sr/⁸⁶Sr ratios, particularly those from the “Ag-belt” (⁸⁷Sr/⁸⁶Sr >1), must include a component that was leached from older, unexposed, pre-Belt rocks that were already ⁸⁷Sr enriched a billion years ago. Early Proterozoic biotite-quartz monzonite gneisses from the Shuswap complex in British Columbia, for example, had ⁸⁷Sr/⁸⁶Sr ratios >1.0 at 1000 Ma. Alternatively, the apparent 1000 Ma isochron may represent a mixing line between fluids generated from (1) an ancient Sm-enriched source with high ¹⁴³Nd/¹⁴⁴Nd and (2) a less radiogenic source. Although ⁴⁰Ar/³⁹Ar ages support a 1000 Ma age, these fluids could have mixed and been involved in a late Cretaceous veining event.

The enrichment of heavy REE with respect to light REE and the strong fractionation of Sm with respect to Nd in many samples suggest that REE were concentrated in U minerals before remobilization and deposition in the vein carbonates.