AGE AND ORIGIN OF QUARTZ-CARBONATE VEINS ASSOCIATED WITH THE COEUR D'ALENE MINING DISTRICT, IDAHO AND WESTERN MONTANA: INSIGHTS FROM ISOTOPES AND RARE-EARTH ELEMENTS

Abstract

Sm-Nd, Pb, and Sr isotope and rare-earth element (REE) analyses of quartz-carbonate veins associated with the Coeur d’Alene mining district, Idaho and western Montana, constrain the age and origin of Coeur d’Alene veining. An Sm-Nd isochron age of siderites from ore-bearing veins (1511 ± 45 Ma) and a Pb-Pb isochron age of siderites, ankerites, and calcites from ore-barren veins (1523 ± 41 Ma) are similar to the model age of Coeur d’Alene-type Pb (~1450 Ma) and constrain Mesoproterozoic events in the earliest history of the Belt-Purcell Basin. Xenotime, an accessory mineral found in all veins, is zoned with a core laser ablation age of 1420 ± 90 Ma, confirming vein emplacement early in the diagenetic and/or metamorphic history of the Belt-Purcell Basin, and an overgrowth age of 990 ± 130 Ma results from later, Grenville-age metamorphism. Pb isotope ratios of carbonates from both ore-bearing and ore-barren veins define a single linear array resulting from Mesoproterozoic vein emplacement with an initial Pb isotope ratio similar to that of Coeur d’Alene-type Pb observed in galena, while moderately radiogenic Pb isotope ratios originate from variable amounts of accompanying xenotime. This array is similar to the previously determined “more radiogenic” array of Leach et al. (1998a). Ore-barren veins are characterized by initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5108 and 0.769, respectively, which may be partially inherited from Belt-Purcell Supergroup and/or Archean rocks. In contrast, initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5083 and 1.146 of ore-bearing veins require a source with highly unradiogenic Nd and highly radiogenic Sr, most likely Archean crust. Ore minerals may result from interaction of hydrothermal fluids from Archean sources with potential Belt-Purcell Basin sedimentary exhalative (sedex) deposits. REEs are concentrated in carbonate fractions of veins. Siderite and ankerite are light REE depleted and heavy REE enriched, resulting in significant Sm/Nd fractionation from the dual effects of REE complexation in carbonate fluids and mineralogical control, which in the case of siderite and ankerite favors the heavy REEs. Positive Eu anomalies characterize ore-bearing veins, whereas ore-barren veins retain negative Eu anomalies. Normalization of REE patterns of ore-barren veins to those of adjacent Belt-Purcell wall rocks results in a near-zero anomaly, implying that REEs from Belt-Purcell metasediments were locally scavenged by carbonate-rich fluids responsible for ore-barren veins. In contrast, leaching of Archean gneisses followed by probable interaction with sedex deposits produced fluids that formed ore-bearing deposits. Further faulting and deposition from successive fluxes of carbonate-rich solutions derived in part from leaching Belt-Purcell metasediments generated ore-barren siderite-, ankerite-, and, finally, calcite-dominant veins.