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Fluid inclusion evidence for a deep origin of porphyry copper-type mineralization at Butte, MT


, : Fluid inclusion evidence for a deep origin of porphyry copper-type mineralization at Butte, MT. Abstracts with Programs - Geological Society of America 31(7): 381

In 1974, Roberts described fluid inclusions associated with early hydrothermal alteration and mineralization at Butte, Montana. The Butte samples did not contain the high-salinity (halite-bearing) and coexisting vapor-rich fluid inclusions characteristic of other porphyry copper deposits that had been studied previously (and since). Rather, the majority of the fluid inclusions in early alteration stages at Butte contained a moderate salinity aqueous phase, one or more opaque daughter minerals, and homogenized to the liquid phase at 350 to 390 degrees C. In other porphyry copper deposits, similar moderate salinity, moderate temperature fluid inclusions were interpreted to represent a mixture of magmatic and meteoric fluids associated with later stages of alteration and mineralization. More recent theoretical and laboratory studies have shown that the type of fluid inclusion found in early alteration and mineralization at Butte is exactly what one would expect for fluid inclusions trapped in a relatively deep (high pressure) magmatic system. The higher pressures attending mineralization at Butte (estimated to be about 1.7 kilobars) are above the two-phase region in the water-salt system. As a result, aqueous fluid immiscibility that is characteristic of other, shallower porphyry systems did not occur at Butte. This interpretation is consistent with results of synchrotron X-ray fluorescence analysis of inclusions from Butte, which indicate that the moderate salinity, moderate temperature fluid inclusions described above are the mineralizing fluids and contain significant amounts of not only copper, but also lead and zinc. Current models for porphyry-epithermal mineral deposits require aqueous phase immiscibility to separate the base metals from precious metals and more volatile elements (Hg, Sb, As, Tl). According to this model, the base metals are precipitated at depth in a porphyry or deep vein system, while the precious metals and volatile elements migrate to shallow levels where they precipitate. Because aqueous phase immiscibility did not occur at Butte, precious metals and volatile elements would have remained in the deeper porphyry system to produce the silver and gold mineralization in the deposit. Roberts, S. (1974) Early hydrothermal alteration and mineralization in the Butte District, Montana. Unpublished Ph. D. Disser., Harvard University, Cambridge, MA, 173 pp.

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