THE TEMPERATURE CONNECTION BETWEEN MAGMATIC AND HYDROTHERMAL REALMS OF THE PORPHYRY-CU-MO DEPOSIT AT BUTTE, MONTANA

Understanding the relationship between magmatic and hydrothermal realms of porphyry-Cu deposits is essential to unraveling their complex evolution. We apply three independent mineral geothermometers to estimate the formation temperatures of magmatic and hydrothermal quartz, rutile, and biotite to establish the thermal relationship between the parental porphyritic magma and hydrothermal veins in the porphyry-Cu-Mo deposit at Butte, Montana.

Magmatic temperatures of porphyry dikes range from 630 to 770ºC while hydrothermal stockwork veins overlap substantially with magmatic temperatures, ranging from <430 to 750ºC. Large temperature ranges spanning 50 to 250ºC are ubiquitous in individual hydrothermal samples. We interpret the highest temperatures from early, deep hydrothermal veins (750ºC) to approximate the magmatic temperature at the time of dike injection and fluid release. These same veins have minimum temperatures (~500ºC) that we interpret to be upper limits for the host rock temperature at the time of vein formation. The large temperature spread within veins between these two thermal limits may record variable cooling of hot magmatic aqueous fluids upon contact with cooler wall rock as hydrofracturing supplied hydrothermal fluids to the crust. Mineral precipitation temperatures within veins are the same as those in adjacent envelopes, indicating approximately contemporaneous formation of veins and envelopes.

Magmatic and hydrothermal samples show no systematic relationship between temperature and depth within the deposit, indicating that the domical shaped metasomatic isograds mapped around the mineralization centers do not portray the temperature distribution at any given time. We observe many anomalous cross-cutting relationships indicating that vein formation temperatures fluctuated up to 130ºC within a single cm³ parcel of rock. We suggest that the temperatures we find indicate discrete cycles of transitory high temperature dike intrusions, hydrofracturing, and vein formation that overprint the cooler host rock thermal gradient. Our new temperatures, combined with previous fluid inclusion data indicate that the porphyry deposit at Butte may have formed at 8-11 km depth, slightly deeper than previously recognized.