STABLE ISOTOPES OF HYDROTHERMAL CARBONATE MINERALS IN THE BUTTE PORPHYRY-LODE DEPOSITS, MONTANA

We present over 60 new analyses of d18O and d13C of carbonate minerals (calcite, rhodochrosite) from the world-class, porphyry-lode deposits of Butte, Montana. Most of these analyses are of carbonate gangue minerals in the high-grade, polymetallic “Main Stage” veins of Butte. Additional analyses of calcite and rhodochrosite from the active Continental Pit are in progress. Results to date are in broad agreement with the earlier study of Garlick and Epstein (1966, Econ. Geol. 61, 1325-1335). Values of d13C range from -2 to -13 ‰, with the majority of samples falling between -3 and -8 ‰ (VPDB). On average, calcite-C is 1 to 2 ‰ heavier than rhodochrosite-C. The d18O range is similar for both carbonate minerals, ranging from -2 to +10 ‰ (VSMOW). No systematic changes in d13C or d18O were found with depth (up to 4000 feet below surface) or with location within the district. The latter finding is somewhat unexpected, considering the strong zonation in metal content of the Main Stage veins (Central Zone rich in Cu-As, Intermediate Zone rich in Cu-Zn, and Peripheral Zone rich in Ag-Pb-Zn). In hand samples, most of which were taken from the Anaconda Collection (an archived set of samples from throughout Butte which is stored on Montana Tech campus), carbonate minerals are either intergrown with quartz and sulfide minerals, or, more commonly, are late in the vein paragenesis. Assuming a temperature of deposition of 200 to 350C (based on previous literature estimates), the d18O of the ore-forming fluids would have been 5 to 10 ‰ lighter than the carbonate minerals, with a mean in the range of calculated values near zero ‰. The range and mean of the calculated d18O-water is consistent with derivation from meteoric water after variable degrees of high-temperature isotope exchange with the Butte Granite. Furthermore, our estimates of d18O of the ore-forming fluids overlap with those of Sheppard and Taylor (1974, Econ. Geol. 69, 926-946). The lack of any systematic change in d13C or d18O with depth or x-y location in the district is best explained by a long-lived and strongly rock-buffered geothermal system that did not undergo boiling. This suggests a greater depth of formation than is typical of most convecting hydrothermal systems of meteoric origin.