Age and Origin of Base- And Precious-Metal Veins of the Coeur d'ALENE Mining District, Idaho

Abstract

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.

Geochemistry of Magnetite from Hydrothermal Ore Deposits and Host Rocks of the Mesoproterozoic Belt Supergroup, United States

“…However, compositional differences between burial metamorphic Belt Supergroup magnetite and hydrothermal magnetite in the Coeur d'Alene district are subtle, which is consistent with derivation of hydrothermal fluids from metamorphic devolatilization of Belt Supergroup metasedimentary rocks (Constantopoulos, 1994;Leach et al, 1998;Rosenberg and Larson, 2000;Fleck et al, 2002). Corresponding oxygen isotope results indicate that magnetite in the Coeur d'Alene district formed at cooler temperatures than magnetite from other settings.…”

“…Two deposit types have been recognized in the Coeur d'Alene district. The Coeur d'Alene veins formed from a complex metamorphic-hydrothermal system with mineralizing fluids that were derived from the Belt Supergroup by metamorphic devolatilization and were channeled along major faults and fractures (Constantopoulos, 1994;Leach et al, 1998;Rosenberg and Larson, 2000;Fleck et al, 2002). Aqueous solution-mineral equilibria for tetrahedrite-bearing assemblages from Ag-Pb-Zn ore deposits from the district confirm these temperatures (Sack and Lichtner, 2009).…”

Verification of a laboratory-based dilation model for in situ conditions using continuum models

“…This region is the home to several significant lead, zinc, and silver deposits. Mineralization in the region typically occurs in the form of galenaesphalerite and tetrahedrite veins in a sequence of Proterozoic rocks belonging to the Belt Supergroup (Fleck et al, 2002). To further illustrate the applicability of the proposed model for brittle rock dilatancy, data from two mine shafts constructed in the region were used for the purposes of back analysis.…”

Heavy Metal–Mineral Associations in Coeur d’Alene River Sediments: A Synchrotron-Based Analysis

“…The history of this district as well as the type of ore deposits and mineralogy has been summarized by Leach and others (Leach et al 1985;Rosenberg and Larson 2000;Fleck et al 2002;Mauk and White 2004;Panneerselvam et al 2006 (Leach et al 1985). Fe minerals including siderite, magnetite, pyrite, pyrrhotite, goethite, hematite, and ferrihydrite have been reported in sediments of LCdA and in the upstream mining district (Farrand and Harsanyi 1997;Cummings et al 2000;Toevs et al 2006).…”

“…Analyses of the sediment and water in the Coeur d'Alene system have been examined by others focusing on geology and geochemistry (Grieco 1981;Farrand and Harsanyi 1997;Rosenberg and Larson 2000;Fleck et al 2002;Mauk and White 2004), as well as surface and subsurface sediment contamination and benthic transport (Maxfield et al 1974a; Reece et al 1978;Horowitz et al 1992Horowitz et al , 1995aHorowitz 1993;Woods and Beckwith 1997;Harrington et al 1998a, b;La Force et al 1998, 1999Winowiecki 2002;Kuwabara et al 2003;Toevs et al 2006). Contamination of lateral lakes adjacent to the delta (Harrington et al 1998a;Sprenke et al 2000;Bostick et al 2001), CdAR and upstream tributary contamination (Maxfield et al 1974b;Reece et al 1978;Paulson 1997;Farag et al 1998Farag et al , 2000La Force et al 1998;Balistrieri et al 2003;Box et al 2005), and downstream of LCdA (Grosbois et al 2001) have also been studied.…”