MULTIPLE FLUID EVENTS ASSOCIATED WITH PRECIOUS AND BASE-METAL VEIN MINERALIZATIONS AT COEUR D’ALENE AG-PB-ZN MINING DISTRICT

Metasediment hosted Ag-rich siderite-quartz-tetrahedrite veins from the Sunshine and Galena mines and an Au-quartz vein from the New Jersey mine in the Coeur d'Alene district, were studied by fluid inclusion, stable isotopic, and bulk crush-leach methods. Results indicate that at least three fluids of distinct P-V-T-X and halogen signatures were responsible for mineral deposition in the veins. Main stage siderite and tetrahedrite in the Ag-rich veins were deposited from warm, CO₂ undersaturated fluid (X_CO2 > 0.4; ~270°C; ~13 eq. wt. % NaCl) of uniform halogen composition (Br/Cl x 10³  1.46 ± 1.8;  I/Cl x 10⁶  40.1 ± 13.6). The discovery of quartz-carbonate veins with identical P-V-T-X properties and halogen ratios which cut across ~142 Ma felsic intrusive rocks indicates that Ag-rich veins are of Mesozoic or Tertiary age. The ultimate origin of the fluid associated with main stage Ag mineralization in the district is unclear. Calculated d¹⁸O_fluid (~9.2 ‰) indicates protracted water to rock interaction at moderate temperatures. Halogen ratios fall close to magmatic values and are broadly similar to those of main stage ore fluids at Slocan Lake and Keno Hill Ag-Pb-Zn districts. Low average d¹³C_fluid (~11.0 ‰) is consistent with carbon derivation from organic matter oxidation in the host metasediments.

             The second fluid event in the Ag-rich veins involves warmer and less saline fluids (~325°C; ~11 eq. wt. % NaCl) of distinct halogen signature (Br/Cl x 10³ 2.1 to 2.8 ;  I/Cl x 10⁶ 50.2 to 85.7) which precipitated small amounts of quartz and siderite. Minor tetrahedrite deposition suggests that this fluid event either represents the latest stage of ore precipitation in the veins or tetrahedrite was re-mobilized from early massive ore.

             The third fluid event is marked by an influx of warm (280 to 320°C), diluted CO₂(N₂, CH₄)-rich fluids in the Ag-rich vein fractures which precipitated flat quartz veinlets at high pressures (1.6 - 1.9 kb). The P-V-T-X properties and halogen ratios of this fluid are similar to those of the fluids associated with the Au-quartz vein (Br/Cl x 10³ 3.2 ± 0.7; I/Cl x 10⁶ from 11.7 to 46.8) which suggests that the two fluids are genetically related. This event is associated with tectonic shortening and metamorphism which continued in the district until mid Tertiary.