HR: 1400h
AN: V23B-07    [Abstracts]
TI: Water Content of Ore Forming Magma From the Porphyry-Copper-Molybdenum Deposit at Butte, Montana, USA
AU: * Mercer, C N
EM: cmercer@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Reed, M H
EM: mhreed@uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AB: Porphyry copper deposits form where exsolved magmatic volatiles carry metals and sulfur from a large magma volume to a cupola, then ascend and deposit ore. The magma temperature, bulk composition, crystallinity, and volatile contents determine the pressure (and depth) at which the melt liberates ore-forming fluids. Butte fluid inclusion studies indicate that mineralization occurred at pressures of 2 to 3 kbar, which requires a greater depth of formation than most other porphyry deposits. It is possible that the large Butte deposit formed at great depth due to unusually high water content in the parental magma. Estimates of bulk magmatic water contents in porphyry systems range from 1 to 5 wt.% H₂O, but have large uncertainties owing to the rarity of fresh porphyry samples, difficulty of analyzing melt inclusions, and difficulty in comparing natural samples with experimental phase equilibria. We estimate the water content of the bulk magma that fed the Butte porphyry deposit using (1) quantitative textural analysis of quartz porphyry dikes, (2) thermodynamic modeling of phase relations (MELTS), and (3) VolatileCalc, a silicate melt-H₂O-CO₂ solubility model, in combination with existing fluid inclusion data. Counting phenocrysts >0.4 mm, the crystallinity of the Butte quartz porphyry is ~25%, which is smaller than commonly observed in porphyries from other deposits (~50% phenocrysts). MELTS calculations indicate that 25% crystallinity at conditions of deep fluid exsolution, 2 to 3 kbar and 600 to 700°C, requires 6 to 8 wt.% H₂O in the coexisting melt. Similarly, VolatileCalc indicates Butte fluid inclusions which contain 95 mol% H₂O would be in equilibrium with a silicic melt containing 6 to 8 wt.% dissolved H₂O. These H₂O contents represent the amount of water dissolved in the interstitial silicate melt within the partly crystallized dikes, and thus the bulk water content of the parental magma before crystallization is likely to be ~5 to 6.5 wt.% H₂O. These results suggest that the magma responsible for the Butte porphyry deposit was relatively water-rich and thus was able to saturate with volatiles at greater depths than in other porphyry deposits. High water contents may also explain the large size of the Butte porphyry deposit. An alternative possibility is that pressures used in our models are somewhat too high, calling for a re-evaluation of the interpretations of fluid inclusion data.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3625 Petrography, microstructures, and textures
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly