STABLE CHLORINE ISOTOPES IN HYDROTHERMAL PROCESSES

Values Of deltaCl-37 can now be routinely measured with an analytical precision of +/-0.2 parts per thousand, as reported herein on fluid inclusion leachates from the Elmwood-Gordonsville Mississippi Valley-type deposit and formation waters from its host formation; basin brines from the Gulf Coast and Palo Duro Basins; and fluid inclusion leachates and biotites from the Bingham, Silver Bell, and Panguna porphyry copper deposits. Isotopic measurements are not affected by reaction yield in the range 85-100% or by generation Of free 12 during sample preparation. In the Elmwood-Gordonsville deposit, five successive paragenetic stages of ore and gangue (sphalerite, fluorite, calcite 2, barite, and calcite 3) each yielded multiple deltaCl-37 measurements. Ranges of deltaCl-37 vary in alternating fashion between stages with more negative values (sphalerite, calcites 2 and 3) and those with more positive values (fluorite, barite). An interface between two brine systems with deltaCl-37 < - 1.0 and >0.0 parts per thousand appears to have moved back and forth across the zone of mineralization. Brines from the Gulf Coast and Palo Duro basins are apparently bimodal in deltaCl-37 with modes near -0.9 parts per thousand and 0.0 parts per thousand, consistent with the variation in deltaC-37 observed in fluid inclusion leachates from Elmwood-Gordonsville. Biotite in the Bingham deposit has deltaCl-37 values up to 1.7 parts per thousand and chlorine isotopes that are fractionated with respect to salt-rich hydrothermal fluid. Hydrothermal fluids in all three porphyry copper deposits studied have a deltaCl-37 range of -0.1 to 0.4 parts per thousand with the one exception of a -0.5 parts per thousand value at Bingham. Loss of low-deltaCl-37 chloride in vapor may balance chloride with a net deltaCl-37 > 0 in the porphyry deposits.