Abstract

The Turner Albright Zn-Cu-Ag-Au-Co deposit represents an important ophiolite-hosted (Cyprus-type) massive sulfide occurrence in the West Coast accreted terranes of the United States. Recent exploration efforts have located an estimated 3,300,000 tons of mineralization with an average grade of 3.33 percent Zn, 1.46 percent Cu, 0.44 oz/ton Ag, 0.11 oz/ton Au, and 0.06 percent Co.The deposit occurs as a series of pods in the basal basaltic pillow lava sequence within the Josephine ophiolite complex. The ophiolite represents a 157-m.y.-old accreted terrane which comprises part of the Western Jurassic lithotectonic belt of the Klamath Mountain province in southwestern Oregon and northern California.The mineralized horizons, known as the Main Upper and Lower zones and the Upper High-Grade pods, consist (in order of abundance) of pyrite, sphalerite, chalcopyrite, marcasite, and native gold, with minor to trace amounts of tetrahedrite, galena, arsenopyrite, and pyrrhotite. The mineralized areas occur as discrete pods containing massive (50-100%) and semimassive (20-50%) sulfides, which commonly occur stratigraphically above zones of nonmassive (5-20%) or veined sulfides. The massive sulfides exhibit fine- to medium-grained granular to porous and massive textures locally interrupted by colloform banding and sulfide breccias. Semimassive sulfides typically exhibit brecciated textures with minor colloform banding. Nonmassive sulfides occur as veins and disseminations in pillow basalts below the other sulfides. Thin mudstones typically overlie the massive sulfides, whereas brecciated chert and basaltic volcaniclastic units occur in association with the semimassive sulfides. Alteration of the basalts consists of minor to moderate chloritization, accompanied by limited silicification and pyritization in the mineralized areas.The Turner Albright deposit was formed by metal-bearing fluids generated by the circulation of seawater through a pillow basalt and sheeted dike complex within a back-arc rifting environment. These fluids vented as sea-floor hot springs, resulting in the precipitation of semi-massive and massive sulfides. Stringer zones of nonmassive sulfides are interpreted as conduits for exhalative fluids.

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