Geologic setting, petrology, and geochemistry of zoned tungsten-bearing skarns at the Strawberry Mine, central Sierra Nevada, California

Abstract

The Strawberry mine, 90 km northeast of Fresno, California, occurs on the margin of a small roof pendant of Early Jurassic metasedimentary rocks and middle Cretaceous metaigneous rocks. Middle Cretaceous granitic intrusions surround and intrude the roof pendant. Adjacent to one granodiorite intrusion, several subvertical marble layers are replaced by scheelite-bearing skarns. The skarns form spears that project as much as 300 m upward and into the marble layers. Between the marble and granodiorite, the skarns comprise a metasomatic column and are subdivided into four zones: (1) wollastonite skarn, adjacent to and replacing marble, and containing wollastonite, grossularite, idocrase, and diopside; (2) garnet skarn, containing andradite-grossularite, diopside-hedenbergite, and scheelite; (3) pyroxene skarn containing hedenbergite-diopside, plagioclase, magnetite, and scheelite; and (4) hornblende skarn, adjacent to granodiorite, and containing hornblende, plagioclase, microcline, magnetite, and scheelite. Important features of the skarn zonation are: sharp contacts between skarn zones; relative constancy of whole-rock, mineral, and modal compositions within each skarn zone; major changes in whole-rock, mineral, and modal compositions across contacts between skarn zones; and systematic replacement between skarn zones, with any given skarn zone replaced by the next lower and adjacent skarn zone toward the granodiorite. Volume remained approximately constant in the replacement of marble by various skarn zones. Marble and skarn exhibit uniformly low porosity. Data on mineral paragenesis and mineral composition indicate that in the replacement of any skarn the most calcic mineral is consumed and the residual calc-silicate minerals in the replacing skarn increase in abundance and are richer in total Fe and MnO. This mineral paragenesis can be related to systematic variations in whole-rock composition. From the marble to the granodiorite, skarns are progressively depleted in CaO and CO 2 , and progressively enriched in SiO 2 , total Fe, MgO, MnO, Al 2 O 3 , Na 2 O, K 2 O, and WO 3 . The skarns probably formed when an aqueous fluid containing a solute fraction relatively depleted in Ca and CO 2 , and relatively enriched in Si, Fe, Mg, Mn, Al, Na, K, and W, was expelled from a granitic magma into country rock. The tendency toward local equilibrium between fluid and marble resulted in formation of skarns with systematic stepwise changes in mineral and rock composition at contacts between skarns. Achievement of local equilibrium occurred mainly by infiltration metasomatism with a possible minor component of diffusion metasomatism.