Ore-bearing quartz-carbonate veins of the Coeur d'Alene mining district yield 87Sr/86Sr ratios of 0.74 to >1.60 for low-Rb/Sr, carbonate gangue minerals, similar to current ranges measured in Middle Proterozoic, high-Rb/Sr rocks of the Belt Supergroup. Stable-isotope and fluid-inclusion studies establish a genetic relationship between vein formation and metamorphic-hydrothermal systems of the region. These extraordinary 87Sr/86Sr ratios require accumulation of radiogenic 87Sr in a high Rb/Sr system over an extended period prior to incorporation of Sr into the veins by hydrothermal processes. Evaluation of the age and composition of potential sources of highly radiogenic Sr indicates that the ore-bearing veins of the Coeur d'Alene district formed within the last 200 Ma from components scavenged from sedimentary and metasedimentary rocks of the Belt Supergroup, the primary host-rocks of the district. These results are consistent with a Cretaceous or Early Tertiary age for these veins. Pb-Zn deposits that yield Pb isotope, K-Ar, and Ar-Ar results indicative of a Proterozoic age probably formed during deposition or diagenesis of the Belt Supergroup at 1350-1500 Ma, possibly as Sullivan-type syngenetic deposits. K-Ar and Rb-Sr apparent ages and 6l80 values of Belt Supergroup rocks decrease southward from the Coeur d'Alene district toward the Idaho batholith, normal to the trends of metamorphic isograds, fold axes, foliation, and the major reverse faults of the district. Isoclinal folding, thrust faulting, high-temperature metamorphism, granitic plutonism, and regional-scale metamorphic-hydrothermal activity is documented in the region between 140 Ma and 45 Ma, but no similar combination of events is recognized for Late Proterozoic time. Combined with Sr P results from the veins, the evidence strongly favors formation of the ore-bearing carbonate veins of the district by fluids related to a complex metamorphic-hydrothermal system during Cretaceous and/or early Tertiary time. Proterozoic Pb-Zn deposits were probably deformed, remobilized along younger structures, and incorporated into the younger hydrothermal deposits during this event.

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Beltian rocks in the Coeur d' Alene district of Idaho, lying within the Lewis and Clark line, display a complex history of deformation and metamorphic fabrics. The earliest fabric is a near-bedding-parallel schistosity (assigned to Dl). Upright folds of northerly trend lie north of the district; comparable folds partly overturned trend WNW within the district, subparallel to the Osburn fault. The folds are assigned to D2a.Metamorphic cleavage (assigned to D2b) trends generally WNW but cuts across the folds in various ways. It shows major differences in two principal areas. In a three-mile-wide zone subparallel to and immediately north of the Osburn fault, cleavage is axially symmetrical about an approximately 40°-NWplunging stretching lineation. Outside that zone, the cleavage is dominantly planar, dips steeply to the SSW, and contains a strong dip-line stretching lineation.Semi-brittle faults and veins (assigned respectively to D2c and D2d) cut the cleavage and are in part axially symmetrical about a steep, SSW-plunging axis, paralleled by ore-shoot axes and slickenlines. Veins are concentrated in several zones or mineral belts symmetrically arrayed across the three-mile-wide zone of cleavage. Post-ore normal faults are axially symmetrical about a near-vertical axis. Brittle faulting in and near the Osburn fault zone cuts all the other structures.

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