Skarn zonation and fluid evolution in the Groundhog mine, Central mining district

The Groundhog mine contains one of the largest and best exposed Zn-Pb-Cu-Ag skarn deposits in the United States. Skarn is formed in Carboniferous limestones patially associated with a nearly vertical swarm of Early Tertiary granodiorite porphyry dikes. Metal ratios, min-eralogy, composition, and fluid inclusion characteristics of the skarn are systematically zoned from northeast o southwest along the>3-km strike length of the system and relative to the original dike-limestone contact. Zn/Cu, Zn/Ag, Pb/Cu, Pb/Zn, and Pb/Ag ratios all increase toward the distal southwest end of the mine where highest Zn, Pb, and total metal grades also occur. Endoskarn is zoned in terms of the relative proportions and manganese content of epidote and chlorite. In the Northeast zone, skarn is zoned away from dikes in the sequence: garnet to pyroxene to pyroxenoid to marble. In the Central skarn zone, garnet is absent and pyroxene begins the zonation sequence at the dike contact. In the Southwest zone, pyroxene is less abundant and pyroxenoid locally begins the zonation sequence. Sphalerite-galena-pyrite-quartz-carbonate are present in all skarn zones and in carbonate replacement mantos and chimney