EVOLUTION OF A SHALLOW HYDROTHERMAL SYSTEM IN THE SIERRA NEVADA BATHOLITH: RECORDS FROM A ZONED, LOW δ18O SKARN IN THE MINERAL KING PENDANT
D'ERRICO, Megan, Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA 94305, LACKEY, Jade Star, Geology Department, Pomona College, Claremont, CA 91711, SURPLESS, Benjamin, Geosciences, Trinity University, 1 Trinity Place, San Antonio, TX 78212, LOEWY, Staci L., Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, BARNES, Jaime D., Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712 and VALLEY, John, Department of Geology and Geophysics, Univ of Wisconsin, Madison, WI 53706, mderrico@stanford.edu
Garnets within the Empire Mountain skarn, in the Mineral King roof pendant of the Sierra Nevada, record a 7‰ range in δ18O values across zoned grains, including the lowest known δ18O value of skarn garnet (–4.0‰) in North America (D’Errico et al., 2012, Geology). Field relationships and unusually low δ18O values suggest that heated surface waters contributed throughout all stages of the skarn-forming hydrothermal system, with heat and additional fluids supplied by shallowly (~3.3 km) intruded 109 Ma quartz diorite of Empire Mountain. In situ secondary ion mass spectrometry measurements of δ18O values across single garnet crystals record evidence of punctuated fluid infiltrations, suggesting variable mixing of fluids from meteoric, magmatic and metamorphic sources.
The emplacement of the quartz-diorite magma was inboard of the primary magmatic locus of the Sierran arc at 109 Ma, and intruded into shallow, cold crustal wallrock. Our results indicate the significant thermal temperature contrast and brittle deformation of the pendant rocks were a key control in the skarn forming system. Three-dimensional field relationships reveal ~ 40 x 106 m3 of garnetite remains after erosion, a mass that is larger than typical garnetite skarns in the Sierra and indicative of an exceptionally vigorous hydrothermal system. Brecciation within the skarn rocks and alteration of the Empire Mountain pluton suggests that a fracture-enhanced permeability network facilitated the extensive infiltration of early surface fluids and permitted the incorporation of metamorphic and magmatic fluids in the formation of massive skarn and later alteration of the pluton. Therefore, the Mineral King skarn pendant represents a well-exposed, unrecognized model for efficiently driving shallow level decarbonation in convergent margin arc settings like those of the North American Cordillera.
