ORIGIN AND EVOLUTION OF HYDROTHERMAL FLUIDS THAT FORMED THE PORPHYRY CU-MO DEPOSIT IN BUTTE, MONTANA: A MULTI-FACETED FLUID INCLUSION STUDY

To understand the origin and evolution of hydrothermal fluids that formed the enormous Cu-Mo-Pb-Zn-Ag deposits in Butte, Montana, we analyzed H and noble gas isotopes, active gases, and aqueous solutes using a variety of bulk fluid inclusion analytical techniques. Analyses were performed on samples from the deep zone of potassic alteration; from quartz-sericite-pyrite (QSP) alteration that post-dates potassic alteration; and from late poly-metallic Main Stage veins.

He isotopic ratios in fluid inclusions in quartz from deep, potassic veins are between 0.01 and 0.05 R/R_A, similar to He composition of crustal fluids. Inclusions in later QSP and Main Stage quartz have He compositions of 0.17 to 0.21 R/R_A, suggesting mixing between crustal and deep magmatic sources. In contrast to quartz, fluid inclusions in sulfide minerals in these veins have He isotopic ratios of 2.6 to 2.9 R/R_A, indicating abundant excess ³He that likely originated in the mantle or a deep magma source. We conclude that inclusions in quartz exchanged He with the crust after trapping, but that sulfide minerals maintained the original isotopic ratios. Post-entrapment He exchange was most complete in deep parts of the deposit that stayed hot longest, while He exchange was less complete in shallower veins that cooled more quickly.

H isotopes in fluid inclusions in these samples show a trend from highest δD in the deep potassic core (-94>δD>-113), to intermediate values in QSP mineralization (-116>δD>-123) and lowest values in Main Stage mineralization (-154>δD>-168). This trend suggests that fluids trapped in the deepest veins are magmatic in origin with minor overprinting by later fluids, while later, shallower, cooler veins have increasing contributions from meteoric water.

Aqueous solute analyses show CO₃^2-> Cl^- in all vein types where fluid unmixing did not occur. Cl/Br (1000-4000) and Na/Cl (0.6 to 2.4) ratios plot between the fields of magmatic and metamorphic fluids. Active gas analyses show that in most samples H₂O>CO₂>H₂S>SO₂>HCl. N, Ar, and CH₄ were also commonly detected.

These results along with previous microthermometry and LA-ICP-MS analyses suggest a magmatic source of volatiles and Cu. This magmatic fluid was involved in all mineralization events at Butte, but it was progressively diluted by meteoric waters in shallow parts of the deposit.