Date of Award
Master of Science (MS)
The stable isotopic compositions of over 90 hydrothermal carbonate minerals in the Butte porphyry-lode system were analyzed. These samples came from the underground workings as well as the active Continental Pit area. Most material came from polymetallic “Main Stage” veins which post-date the porphyry Cu-Mo mineralization of Butte. Some samples from late calcite-stellerite veins were also included in the study. Rhodochrosite d13C and d18O values range from -8.3 to -2.9‰ (average of -6.7±1.0‰) and -1.8 to 12.8‰ (average of 3.6 ±3.4‰) respectively, while calcite d13C and d18O values range from -9.0 to -2.6‰ (average of -5.4 ± 1.5‰) and -4.4 to 12.3‰ (average of 6.2±2.8‰) respectively. These values are in agreement with the previous work of Garlick and Epstein (1966). Despite the strong ore-mineral zonation of the Main Stage veins of Butte, no systematic changes in stable isotope composition of carbonate minerals are seen across the district, or with depth. Late calcite samples have similar isotopic composition to carbonate minerals from Main Stage mineralization. Some isotopic re-equilibration with a lower temperature fluid may have taken place, with calcite more susceptible to isotopic resetting compared to less soluble and more abundant rhodochrosite. Based on the observed d18O values of rhodochrosite, and an inferred temperature of formation of Main Stage veins of 200 to 350°C, the vein forming fluids would have had d18O values in the range of -12.6 to 4.1, which suggests mixing of magmatic water with an evolved meteoric water. The stable isotopic composition of vein carbonates from Butte have similar d13C but different d18O values compared to the vein carbonate minerals from the nearby Coeur d’Alene district, Idaho. Whereas the two world-class silver-base metal mining districts may have inherited a similar carbon source, the much heavier d18O values of Coeur d’Alene carbonate suggest involvement of metamorphic fluids, as opposed to the porphyry-meteoric fluids that mineralized the veins of Butte.
Stevenson, Ryan, "STABLE ISOTOPES OF HYDROTHERMAL CARBONATE MINERALS IN THE BUTTE PORPHYRY-LODE DEPOSITS, MONTANA" (2015). Graduate Theses & Non-Theses. 44.