OXYGEN, CARBON, AND STRONTIUM ISOTOPE GEOCHEMISTRY OF THE SUNSHINE MINE, COEUR-DALENE MINING DISTRICT, IDAHO

The Pb-Ag ores of the Sunshine mine, located in tile Coeur d'Alene min ing district of northern Idaho, occur within steeply dipping, tabular, west-northwest trending veins of siderite gangue, that crosscut the o verturned northern limb of the Big Creek anticline. The delta(13)C(PDB ) (9.9 to -6.4 parts per thousand) and delta(18)O(SMOW) (13.1-17.7 par ts per thousand) values of the siderites vary in a proportion of about 1:2, but siderites with delta(18)O greater than or equal to 16 exhibi t a slope close to the 1:4 trend expected for a system with variable t emperature, whereas no correlation is evident for siderites with delta (18)O < 16. The delta(18)O values of crosscutting quartz veins (13.4-1 5.5) and the surrounding wall rocks (10.7-15.5) are several per mil to o low to be in isotopic equilibrium with tl-le siderite at the tempera ture of similar to 325 degrees C determined from fluid inclusion studi es. At this temperature the calculated delta(18)O values for the preci pitating 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 fluid s reflect their exchange with the high O-18 metasedimentary rocks of t he Belt Super group in higher temperature fluid source regions. Detail ed sample traverses across two 1- to 3-m-mide siderite veins show that il)some veins are composite, containing materials deposited at differ ent times or in different places which were juxtaposed by subsequent, vein-parallel faulting; (2) siderite in structurally simple veins is m ost enriched in C-13 and O-18 adjacent to the walls, resulting in U-sh aped spatial trends; and (3) the delta(18)O values of metasedimentary wall rocks are substantially lower than those of the siderite in the v eins and increase adjacent to them. These small-scale variations accou nt 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 wer e hotter and considerably higher in O-18 than the fluids in equilibriu m with adjacent wall rock. The progressive O-18 decrease from the marg ins to the centers of the siderite veins, further decreasing into the early crosscutting quartz veins, probably represents a progressive inc rease in the temperature of deposition but could also represent variat ions in fluid source in an evolving system. The dispersion of delta(13 )C values for samples with delta(18)O < 16 per mil may be the result o f late-stage explosive pressure release and CO2 effervescence during f luid decompression, when fissure filling was nearly complete. The stru ctural simplicity of the veins and the primary hydrothermal character of tile stable isotope variations argue against postdepositional defor mation and metamorphism. Stable isotope and strontium isotope data sup port the interpretation that ore deposition occurred during the Late C retaceous or early Tertiary as the result of metamorphic hydrothermal processes associated with regional compression and the formation of ma jor granitic batholiths.