Abstract

In the West Shasta copper-zinc district, Kuroko-type massive sulfide deposits are present in Early Devonian volcanic rocks consisting of the Copley Greenstone and the overlying Balaklala Rhyolite. The Copley Greenstone is composed of pillow lava (mainly basalts with subordinate andesites) and pyroclastic deposits. High Mg andesites occur near the top of the 1,800-m-thick volcanic succession. The Balaklala Rhyolite, 1,000 m thick, consists of highly siliceous rhyolite flows, conglomerates, and tuffs. Both units were deposited in a submarine environment. The sulfide deposits are localized in the rhyolitic flows of the upper part of the middle unit of the Balaklala Rhyolite.Petrological and geochemical data show that these lavas belong to a tholeiitic suite formed in an immature island arc. At the close of Copley eruptions, the island arc underwent rifting and tectonic extension. This is suggested by the presence of boninites, a feeder dike system for the Balaklala Rhyolite, and bimodal volcanism. The extensional environment is similar to that classically observed in the domain of the Kuroko massive sulfide deposits. This rifting led subsequently to the development of a back-arc basin represented by the Trinity basalts associated with an ophiolitic sequence.

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