A descriptive model for silver-lead-zinc veins in clastic metasedimentary terranes

Abstract

A comparative study of selected geologic features from six classic districts, the Kokanee Range and Keno Hill, Canada; Coeur d'Alne, United States; Freiberg and the Harz Moun- tains, Germany; and Pibram, Czechoslovakia, demonstrates that silver-lead-zinc veins in clastic metasedimentary terranes form a distinct class of mineral deposits which can be typi- fied by a descriptive model. These silver-lead-zinc veins have characteristic metal ratios and are comprised of galena, sphalerite, and a diverse suite of silver and sulfosalt minerals in a gangue of siderite, quartz, or calcite. The veins are typically enclosed by spatially restricted phyllic alteration. Silver-lead-zinc veins occur in various tectonic settings. The veins are hosted by monotonous sequences ofclastic rocks intruded by gabbroic to granitic plutons and metamorphosed to at least the greenschist facies. The veins are late features in the tectonic evolution of an orogen, commonly occur near a crustal-scale fault, and are not genetically related to felsic intrusions. Mineralization occurs at temperatures near 250 o to 300oC from dilute to saline fluids at an average depth of 6 km. Precipitation results from district-scale mixing of up to three distinct fluids and localized boiling. These fluids include a deep-seated hydrothermal fluid, an upper crustal fluid of ultimate meteoric origin, and a late-stage meteoric-dominated fluid. Sulfur is derived from the local country rocks, whereas carbon is derived from organic and/or deep- seated sources. Pb is mainly derived from the local upper crustal country rocks.