B#246;hlke et al..pdfVIP

EconomicGeology Vol. 81, 1986, pp. 296-322 Rb-Sr,K-Ar, andStableIsotopeEvidencefor the AgesandSourcesof Fluid Componentsof Gold-BearingQuartzVeinsin the Northern Sierra NevadaFoothillsMetamorphicBelt, California J. K. BOHLKE Departmentof Geologyand Geophysics,Universityof California,Berkeley,California94720and U.S. GeologicalSurvey, Mail Stop941,345 MiddlefieldRoad,MenloPark,California94025 AND R. W. KISTLER U.S. GeologicalSurvey,Mail Stop937, 345 MiddlefieldRoad,MenloPark,California94025 Abstract Gold-bearingquartzveinsoccurin and near majorfault zonesin deformedoceanicand island-arcrockswest of the mainoutcropof the SierraNevadacompositebatholith.Veins typicallyoccupyminorreversefaultsthatcrosscutblueschistto amphibolite-grademetamorphic rockswhosemetamorphicagesrangefromearlyPaleozoictoJurassic.Veinmicasandcarbonate- quartz-micaassemblagesthatformedby hydrothermalmetasomatismof ultramaficwall rocks in the Alleghany,GrassValley, Washington,and Mother Lode districtsyield concordant K-ArandRb-Srages.The datedveinsaresignificantlyyoungerthanprogrademetamorphism, penetrativedeformation,andaccretionoftheir hostrocksto the continentalmargin.New and previouslypublishedmineralizationagesfrom 13 localitiesin the Sierrafoothillsrangefrom about140to 110 m.y.ago,with meanandmedianbetween120 and115 m.y.The agerelations suggestthat mineralizingfluidswere setin motionby deepmagmaticactivityrelatedto the resumptionof east-dippingsubductionalongthe westernmarginof North Americafollowing the LateJurassicNevadancollisionevent.