HALOGEN ANALYSIS OF QUARTZ HOSTED FLUID INCLUSIONS FROM PORPHYRY CU DEPOSITS: BINGHAM CANYON, BUTTE, AND BAJO DE LA ALUMBRERA

A bulk fluid inclusion halogen study has been carried out on quartz from various alteration zones of three porphyry Cu deposits: Bingham Canyon (Utah), Butte (Montana), and Bajo de la Alumbrera (Argentina). Bromine to chlorine ratios of fluid inclusion leachates have been determined by ion chromatography, and Cl stable isotopic ratios (³⁵Cl/³⁷Cl) of the leachates will be measured by a modified CF-IRMS technique.

Br/Cl ratios for inclusions in potassically altered samples from Bingham Canyon range from 0.32-3.68 X 10^-3M. Br enriched values above seawater (1.54 X 10^-3M) imply involvement of formation water derived from the sedimentary country rock (mixing or overprinting) or the crystallization of Cl-bearing phases (mica, amphibole, apatite) prior to volatile exsolution causing formation of a Br-enriched juvenile magmatic brine. The same effect could also be generated by the direct precipitation of halite from hydrothermal fluids.

Butte samples analyzed from Main Stage mineralization and pre Main Stage sericitic and EDM alteration zones yield values ranging from 0.80-6.77 X 10^-3M. Br/Cl ratios are lowest in the sericitic alteration zone (0.57-1.08 X ^-3M), and higher in Main Stage mineralization (1.39-5.12 X ^-3M). Many of the Butte samples appear to be more Br enriched (or Cl depleted) than typical magmatic fluids, implying a greater involvement of chemically evolved meteoric/formation water or similar processes of hydrous mineral crystallization or halite precipitation.

By contrast, samples from the potassic (overprinted by chlorite-rich assemblages) and the latest phyllic alteration zones from Bajo de la Alumbrera have restricted Br/Cl ratios (1.19-1.25 X ^-3M) implying a direct magmatic origin and mantle affinities.

These preliminary results compliment existing δ¹⁸O and δD isotopic data for these ore deposits and provide insights into the involvement of external meteoric water in each magmatic-hydrothermal system. Unlike other studies of porphyry Cu deposits, we have determined Br/Cl ratios that are in part distinct from mantle-like compositions indicating multiple sources of salinity (e.g., formation water) or modification via processes operating in the source magma chamber (e.g., crystallization). Stable chlorine isotope data from these leachates are in preparation, and will better constrain the origin of salinity in these ore-forming fluids.