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

The age and origin of quartz-carbonate veins associated with the Coeur d'Alene (C d'A) Mining District have been evaluated using Sm-Nd, Pb and Sr isotopes on host carbonate minerals (calcite, ankerite, siderite). Microscopic traces of xenotime are present in all carbonate-bearing veins. Sm/Nd results for 23 ankerite/calcite veins and 4 ore-bearing (Pb, Zn, Ag) siderite veins yield linear arrays corresponding to ages of ~1.45 Ga. Laser ablation studies of xenotime yield a 207Pb/206Pb model age of ~1.0 Ga, consistent with previously reported mica Ar/Ar ages. Ankerite/calcite veins with initial 87Sr/86Sr <0.82 may have originated from leaching Belt metasediments. Initial 87Sr/86Sr ratios >0.82 associated with siderite from ore-bearing veins require leaching of older, unexposed pre-Belt rocks that were previously 87Sr enriched at 1.5 Ga. Pb isotope ratios of all minerals lie along a single linear array which appears to represent mixing of C d'A-type Pb, widely accepted to be ~1.45 Ga, with Pb (~1.0 Ga) in xenotime. Enrichment of heavy REE with respect to light REE and extreme Sm/Nd fractionation suggest that REEs were concentrated by U-rich minerals before remobilization.

We propose that: 1) U and REEs were deposited (and later separated from Pb) under euxinic conditions in a shallow marine sea early in the rifting history of the Belt basin (~1.45 Ga) 2) Further rifting resulted in the formation of SEDEX deposits with high concentrations of Pb, Zn and Ag sulfides 3) About 1.0 Ga, REEs, and C d'A-type Pb were deposited by fluids moving along faults and fractures forming xenotime-bearing, quartz-carbonate veins. Where solutions encountered SEDEX deposits Zn, Pb and Ag were mobilized resulting in the mineralization of ore-bearing veins in the C d'A Mining District. 4) Late Cretaceous overprinting caused by regional igneous activity produced Pb metasomatism in some areas within and near the C d'A district and may have resulted in localized remobilization and emplacement of older vein minerals.