Abstract

The Pb-Ag ores of the Sunshine mine, located in the Coeur d'Alene mining district of northern Idaho, occur within steeply dipping, tabular, west-northwest-trending veins of siderite gangue, that crosscut the overturned northern limb of the Big Creek anticline. The delta 13 C PDB ( 9 .9 to -6.4%) and delta 18 O SMOW (13.1- 17.7%) values of the siderites vary in a proportion of about 1:2, but siderites with delta 18 O > or = 16 exhibit a slope close to the 1:4 trend expected for a system with variable temperature, whereas no correlation is evident for siderites with delta 18 O < 16. The delta 18 O values of crosscutting quartz veins (13.4-15.5) and the surrounding wall rocks (10.7-15.5) are several per mil too low to be in isotopic equilibrium with the siderite at the temperature of approximately 325 degrees C determined from fluid inclusion studies. At this temperature the calculated delta 18 O values for the precipitating fluids range from 7.8 to 12.4 for the siderite and 6.9 to 8.9 for the quartz. The high delta 18 O values calculated for these fluids reflect their exchange with the high 18 O metasedimentary rocks of the Belt Supergroup in higher temperature fluid source regions. Detailed sample traverses across two 1- to 3-m-wide siderite veins show that (1) some veins are composite, containing materials deposited at different times or in different places which were juxtaposed by subsequent vein-parallel faulting; (2) siderite in structurally simple veins is most enriched in 13 C and 18 O adjacent to the walls, resulting in U-shaped spatial trends; and (3) the delta ) 18 O values of metasedimentary wallrocks are substantially lower than those of the siderite in the veins and increase adjacent to them. These small-scale variations account for the lack of a simple relationship between delta 18 O and depth in the vein systems. These data also indicate that the vein fluids were hotter and considerably higher in 18 O than the fluids in equilibrium with adjacent wall rock. The progressive 18 O decrease from the margins to the centers of the siderite veins, further decreasing into the early crosscutting quartz veins, probably represents a progressive increase in the temperature of deposition but could also represent variations in fluid source in an evolving system. The dispersion of delta 12 C values for samples with delta 18 O < 16 per mil may be the result of late-stage explosive pressure release and CO 2 effervescence during fluid decompression, when fissure filling was nearly complete. The structural simplicity of the veins and the primary/hydrothermal character of the stable isotope variations argue against postdepositional deformation and metamorphism. Stable isotope and strontium isotope data support the interpretation that ore deposition occurred during the Late Cretaceous or early Tertiary as the result of metamorphic hydrothermal processes associated with regional compression and the formation of major granitic batholiths.

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