Abstract

The JC tin deposit is a stanniferous skarn hosted within Mississippian sediments intruded by the mid-Cretaceous Seagull batholith. The stable isotope compositions of silicate, carbonate, and sulfide minerals have been used to interpret the evolution of fluid sources during sequential stages of skarn formation.The delta 13 C and delta 18 O values of remnant calcite from the earliest contact metamorphic (stage 0) rocks are the result of decarbonation of the protolith marble, coupled with infiltration of variable amounts of meteoric water.Garnet-pyroxene skarn (stage I) was produced by predominantly magmatic fluids during initial skarn-forming metasomatism. Stage II amphibole-rich skarn was formed by magmatic water mixed with up to 50 percent meteoric water. This meteoric water had undergone stable isotope exchange with the JC granite at a low water to rock ratio as it moved upward along the contact of the granite with the surrounding wall rock. The magmatic water component of stage II fluids is inferred to have moved vertically through the solid cap of the granite before mixing with the exchanged meteoric water.Stage III epidote-quartz-calcite skarn formed in the presence of a fluid consisting of magmatic water which had reequilibrated with the granite cap at temperatures between 350 degrees and 500 degrees C and subsequently mixed with up to 45 percent meteoric water. The delta 13 C and delta 13 O values of stage III calcite suggest the presence of an additional fluid component which had equilibrated with the protolith marble at similarly high temperatures. These observations reinforce the hypothesis that originally highly saline stage III fluids were diluted by low-salinity meteoric water at the periphery of the skarn replacement zone and suggest that the surrounding marble horizon may have provided a pathway by which such fluids entered the skarn.During formation of stage IV biotite-quartz-fluorite-cassiterite skarn, the fluid was predominantly magmatic, with no detectable involvement of meteoric water. This stage represented a renewed influx of magmatic fluid following the increased involvement of meteoric water during stages II and III, and may have been related to a discrete episode of aqueous fluid separation from the underlying granitic magma. The return to a predominantly magmatic fluid at stage IV is concomitant with an increase in both fluid temperature and salinity relative to stage III fluids.During the fracturing and formation of breccia zones which accompanied formation of stage VI pyrite-marcasite-carbonate skarn, meteoric water which had initially equilibrated at low temperature (<100 degrees C) with the remaining protolith marble mixed in varying proportions with a second fluid which was in equilibrium with the solid cap of the JC granite.The narrow range of delta 13 S values for all skarn sulfide minerals (-1.0 to +2.5ppm) is consistent with a magmatic source for sulfur throughout skarn formation.Stage III and IV mineral pairs which remained in isotopic equilibrium after their initial crystallization yield oxygen isotope geothermometric temperatures consistent with those determined by fluid inclusion microthermometry.

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