Paper No. 5
Presentation Time: 2:15 PM
REE ABUNDANCES OF QUARTZ-CARBONATE VEINS ASSOCIATED WITH THE COEUR d'ALENE MINING DISTRICT, IDAHO AND WESTERN MONTANA
REE analyses of carbonates provide insights into the origin and emplacement of 1.4-1.5 Ga quartz-carbonate veins associated with the Coeur d'Alene Mining District. Ore-bearing (quartz-siderite-sulfide) and ore-barren (quartz-siderite, ankerite or calcite-dominant) veins transect the Coeur d'Alene Mining district from NW to SE within the Lewis and Clark fault zone along the Idaho-Montana border, a distance of ~180 km. Vein hosted trace minerals include ubiquitous xenotime (ore-barren, laser ablation age ~1.42 Ga) and synchysite, which occurs largely in calcites. REEs are concentrated in the carbonate fractions of all veins but siderite-dominant, ore-bearing veins have much lower REE abundances (~6 ppm) than siderite/ankerite-dominant, ore-barren veins (~40 ppm). Carbonates are LREE depleted, HREE enriched, and show significant Sm/Nd fractionations reflecting the effects of mineralogical control and/or complexation, which favor the HREEs. However, some calcites are LREE enriched due to the abundance of synchysite, which concentrates LREEs. Positive Eu anomalies (Eu/Eu*=1.1-3.0) characterize ore-bearing veins whereas ore-barren veins show slightly negative Eu anomalies (0.87±0.13). Wallrocks of all ore-barren veins are LREE-enriched, HREE-depleted, and show negative Eu anomalies (0.60±0.07). Normalization of REE patterns of vein carbonates to those of the adjacent wallrocks results in near zero Eu anomalies. The positive Eu anomalies and initial Nd isotope ratio (0.5083) of the ore-bearing veins are consistent with derivation from Belt basin SEDEX deposits whereas the negative Eu anomalies and initial Nd isotope ratio (0.5108) of ore-barren veins suggest leaching of Belt metasediments. 87Sr/86Sr ratios (average ore-bearing, 1.146; ore-barren, 0.769) are in accord with this interpretation. Successive depositional episodes from fluids derived by leaching Belt metasediments, probably produced the siderite-, ankerite- and calcite-dominant, ore-barren veins as well as the sequential carbonate zoning reported around ore-bearing veins. Thus, ore-bearing veins appear to be older than ore-barren veins and must originate from different sources.