Abstract

The Devonian massive sulfide ore deposits of the West Shasta district, California, have produced copper, zinc, gold, silver, and sulfur since 1897. An island-are setting is suggested for the volcanic host-rock sequence beginning with submarine lavas of the Copley Greenstone and succeeded by a thick and elongate accumulation of Balaklala Rhyolite. The rhyolites show a general compositional evolution through time with quartz phenocrysts increasing from zero to 4 mm during the latest stage. Midway through the rhyolite succession, crustal extension occurred along the north-northeasterly volcanic axis, lowering the crust, and generating fracture zones and sea-floor grabens, such as the Eureka system. These depressions exhibit orientations of N 15 degrees -20 degrees E and N 40 degrees -65 degrees E. Numerous massive sulfide ore and pyrite deposits formed along the > 13-km volcanic ridge at sites where submarine hot spring brines precipitated and formed massive sulfide lenses within structural depressions near the top of the lower unit rhyolitic lavas and domes which were altered by heated seawater circulation. Postore rhyolite crystal tuffs of the upper unit of the Balaklala Rhyolite were extruded over the district and thick cumulo domes probably breached sea level and escaped further seawater alteration. Devonian Kennett Formation shales and limestones then lapped onto the flanks of the volcanic pile either in the form of fringing sediments and reefs or as deposits resulting from slumping from a higher elevation as suggested by the black shale association. Folding is moderately severe in the southern part of the district, especially adjacent to the trondhjemitic Mule Mountain stock. However, fold effects are minimal in the northern two-thirds of the district. Postore faulting has progressively downdropped the crust to the north where more stratigraphic section is preserved. A volcanogenic origin for the generation of the massive sulfide deposits is preferred and supportive evidence for this model has come from studies in the West Keystone, Balaklala, and Shasta King mine areas. The similarities with other deposits within the district argue for all of the West Shasta ore deposits sharing a common volcanogenic ancestry.

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