PETROCHEMICAL AFFILIATIONS OF VOLCANOGENIC MASSIVE SULFIDE DEPOSITS OF THE FOOTHILL COPPER-ZINC BELT, SIERRA NEVADA, CALIFORNIA.

摘要

The Foothill Cu-Zn belt, a zone of distinctive volcanic-associated massive sulfide deposits within felsic, Jurassic volcanic sequences, lies within a metamorphic terrane extending the length of the western Sierra Nevada. Five subparallel, northwesterly trending, tectonostratigraphic blocks define the terrane. The eastern, central and western blocks are continuous throughout the foothills while the Smartville and Feather River blocks are restricted to the north. The blocks are separated by regional fault zones. The eastern and Feather River blocks are Paleozoic continental margin, subduction(?), and magmatic arc assemblages unrelated to the Middle to Late Jurassic western, central and Smartville blocks.;The Jurassic blocks preserve an ensimatic magmatic arc dominated by intermediate, island arc tholeiitic pyroclastic sequences interbedded with immature epiclastic sediments. The western block forms the core of the arc containing differentiated felsic, low-K calc-alkalic volcanic centers and associated massive sulfide deposits within the Gopher Ridge and Copper Hill Volcanics. The Smartville block, a differentiated volcano-plutonic complex, has characteristics of both basal portions of ensimatic arcs and marginal basins. Volcanogenic sulfide deposits are associated with local calc-alkalic sequences. The central block exposes a basement terrane comprised of pre-Middle Jurassic ophiolitic and melange-like assemblages beside the Bear Mountains fault zone and a post-Middle Jurassic remnant arc rifted from the western block and preserved within the island arc tholeiitic Logtown Ridge Volcanics. There are no felsic volcanic sequences or volcanogenic sulfide deposits within the block. Whole-rock major oxide and trace element chemistry document the dominance of island arc tholeiitic magma suites in all three blocks and the restriction of calc-alkalic volcanism to the western and Smartville blocks.;Volcanogenic sulfide deposits of the Foothill belt are analogous to Noranda- and Kuroko-type massive sulfides having similarities in ore body mineralogy, morphology, and metal zoning. They are related to calc-alkalic centers of felsic pyroclastic volcanism. Massive lenses of base and precious metal bearing pyritic sulfides are underlain by stringer-zone mineralization within chloritized and sericitized volcanics. Siliceous tuffites and barite lenses overlay sulfides. Wall-rock alteration is documented by whole-rock major oxide chemistry.