Abstract

The Britannia copper-zinc sulfide deposits, previously described as having formed from hydrothermal solutions emplaced into foliated host rocks, are reinterpreted as volcanogenic in origin and to have been deposited from hydrothermal and exhalative solutions related to contemporaneous dacitic volcanism and then deformed during later shearing and faulting. Massive sulfide deposits occur near the upper contact of coarse dacitic tuff. Anhydrite, barite, and chert form related exhalative deposits.Several periods of inhomogeneous strain produced a broad zone of S-tectonites, the Britannia shear zone, which contains all of the known orebodies; metamorphic assemblages are those of the lower greenschist facies of regional metamorphism. Sulfide textures are similar to metamorphic and deformational textures described in the literature. During ore formation and later shearing, the rocks were chemically altered with increases in K 2 O, SiO 2 , and H 2 O and decreases in CaO and total Fe. Following major metamorphism, dacite dikes were intruded into the sheared rocks and were controlled by foliation; sulfides were remobilized into late quartz veins during emplacement of the dacite dikes. A major system of late faults developed subparallel to the foliation.A predeformation reconstruction suggests that the orebodies are segments of two original massive sulfide deposits; this requires a near-vertical displacement along one fault zone followed by subhorizontal offset with a cumulative right-lateral displacement of several thousand feet.

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