Volcanic lithofacies and massive sulfide mineralization, East Shasta district, California



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The East Shasta base-metal district is within a marine, Permo-Triassic, calc-alkaline, intermediate-to-felsic volcanic sequence, overlain by mudstone. The volcanic rocks have undergone hydrothermal alteration and are now keratophyres and quartz keratophyres. Conformable Kuroko-type massive sulfide lenses occur at the contact of the rhyolite and mudstone. The orebodies consist of sphalerite, chalcopyrite, galena, and tetrahedrite-tennantite, with pyrite, barite, calcite, dolomite, and quartz as gangue minerals. Most of the ore has banded or massive texture; a small percentage forms a conglomerate in which subangular banded and massive sulfide fragments are supported by a fine-grained sulfide matrix. Detailed mapping of volcanic depositional facies indicates the presence of a felsic volcanic center characterized by columnar jointed rhyolite and coarse pyroclastics with columnar fragments grading laterally and vertically into tuff and mudstone. Lack of welding in the tuff, and density-current structures in the mudstone indicate a marine environment. The orebodies were emplaced by the discharge of convectively circulating seawater during a period of volcanic quiescence. Reaction of seawater with the volcanics at elevated temperatures and pressures reduced seawater sulfate to sulfide, transformed the volcanics to keratophyres and quartz keratophyres, and released ore metals into solution. Deposition and banding of the ore minerals were controlled by the temperature of the discharging fluid and the mixing of this fluid with seawater. Conglomerate-textured ore formed as a result of slumping of dense sulfide sediments. Minor tuff eruptions occurred after deposition of the ore