Tungsten-bearing skarns of the Sierra Nevada; I, The Pine Creek Mine, California

Abstract

The Pine Creek mine, located 27 km west of Bishop, California, contains scheelite-bearing skarns found at and near the contacts between a septum of Paleozoic metasedimentary rocks and a mid-Cretaceous quartz monzonite pluton. Underground and surface mapping and drill core logging have suggested that skarn formation and accompanying metal deposition should be characterized as a series of events over a range in temperatures overprinting several different rock types.Metamorphism caused the formation of barren, iron-poor calc-silicate marbles and hornfelses, which were subsequently replaced by iron-rich calc-silicate and ore minerals during metasomatism. High-temperature metasomatism was characterized by the formation of garnet-quartz, garnet-pyroxene, and idocrase-wollastonite-pyroxene zones from marble; by pyroxene-plagioclase + or - garnet skarns from biotite and calc-silicate hornfels; and by pyroxene-plagioclase-quartz rock from quartz monzonite. These high-temperature assemblages were locally over-printed by hydrosilicate alteration characterized by biotiteoplagioclase-magnetite replacement of garnet, amphibole-quartz-chalcopyrite + or - calcite (epidote) replacement of garnet-pyroxene (plagioclase) skarn, and calcite-quartz-epidote-chlorite-fiuorite replacement of wollastonite-idocrase. Contemporaneous alteration of quartz monzonite comprised quartz-biotite-chalco-pyrite veins and locally pervasive biotitization. The latest, lowest temperature alteration con-sisted of vertical pipes of quartz-calcite-zeolite, which cut and altered all other rock types.Patterns of ore deposition were intimately tied to original lithologies and alteration types. Initial scheelite deposition accompanied high-temperature metasomatism of marble but not of hornfels or quartz monzonite. In the early-formed skarn, scheelite is concentrated toward the marble replacement front and depleted away from marble. Molybdenite is concentrated in quartz-rich masses near intrusive contacts and in quartz-garnet veins cutting across metaso-matized hornfels. Scheelite was remobilized and deposited in association with hydrosilicate alteration of skarn and hornfels. Copper was deposited with hydrosilicate alteration, particularly in association with amphibole-quartz calcite replacement of garnet-pyroxene skarn. The sequence of events inferred at Pine Creek appears to be similar to those found in other tungsten skarns from around the world.