Collaboration: Temporal Evolution of Hydrothermal Fluid Compositions and Construction of a 3 Dimensional Geologic Model for the Giant Butte Magmatic-Hydrothermal Ore Deposit

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EAR-0001230 & 0001272Dilles, J./Reed, M.The Butte district, Montana, contains North America's largest copper reserves and is among the world's largest producers of copper and silver. Butte is a superb example of a giant porphyry Cu-Mo magmatic-hydrothermal ore deposit where data and samples from more than 100 years of underground mining plus hundreds of drill holes, including 11 vertical holes of 1.5 to 2.3 km depth, provide exceptional access to the system. Butte has served as the basis for much of modern understanding of ore formation processes, and our goal is to build improved hypotheses, that include the links of hydrothermal ores to magmas, via application of new and state-of-art analytical techniques. In the completed phase of research, we used isotopic dating techniques to establish that porphyry magmas that produced mineralization were emplaced about 76 to 68 million years ago, but that hydrothermal ore fluids and rocks cooled to less than 300 degrees C between 64 and 61 million years ago. Application of additional isotopic dating (using U-Pb, Ar-Ar and Re-Os) will enable us to understand this age difference. Our earlier work with isotopes of hydrogen and oxygen shows that magmatic brines produced high-temperature copper-molybdenum ores (biotite alteration) as well as later iron-sulfides (sericitic alteration). Those findings will be enlarged by application of state-of-the-art techniques (laser-ICP-MS and cathodoluminescence) on micrometer-scale samples of ore fluids trapped in quartz to establish temperature, pressure, and chemical composition of various ore fluids. We plan to synthesize the geology, geochemistry, and hydrothermal alteration mineralogy, and ore mineralogy into a series of maps and cross-sections that illustrate the three-dimensional spatial and temporal evolution of a giant magmatic-hydrothermal system.