2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 7
Presentation Time: 3:05 PM

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


ROSENBERG, Philip E., School of Earth and Environmental Sciences, Washington State Univ, Pullman, WA 99164 and RAMOS, Frank C., Geological Sciences, New Mexico State University, Las Cruces, NM 88003, rosenberg@wsu.edu

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.