Abstract

Fluid inclusions, geochronology, and field studies show that the world-class base- and precious-metal veins of the Coeur d'Alene district, Idaho and Montana, are the product of deformation coupled with regional metamorphism of the Belt basin around 850 Ma. Fluid-inclusion data show that the mineralizing solutions were complex CO2-CH4-CnHm-N2-H2O-NaCl fluids that changed in composition from CH4-CnHm-rich fluids in the older Zn-rich veins to CO2-rich fluids in the younger Ag-rich veins. Ore deposition occurred at about 250 to 350 °C and at pressures in excess of 1 kbar.

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