Hydrogen and Oxygen Isotope Evidence for the Origins of Water in the Boulder Batholith and the Butte Ore Deposits, Montana

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Biotite, hornblende, and whole-rock delta D values for 23 plutonic rocks from the composite Boulder batholith (age 78 to 68 m.y.) range from --63 to --155 per mil (SMOW). Biotites and hornblende with delta D = --63 to --90 (9 samples) are from rocks which have normal igneous whole-rock delta 18 O values (7.0 to 8.3) and quartz-K-feldspar fractionations. D/H ratios of these unaltered rocks probably approximate primary magmatic values. Samples with delta D = --130 to --155 and/or low whole-rock delta 18 O values, (i.e. --4 18 O values; these rocks probably interacted with very small quantities of heated meteoric waters. Meteoric water-rock interaction was a wide-spread phenomenon during cooling and late-stage crystallization of the Boulder batholith but only locally occurred on a large scale. Correlation between the 18 O/ 16 O and 206 Pb/ 204 Pb ratios indicate that some of the minor 18 O/ 16 O and D/H variations are probably primary features of the magmas.At Butte, the isotopic data for hydrous alteration minerals (biotite, sericite, pyrophyllite, clays) and the calculated delta 18 O water values at the selected temperatures are:Stage delta D Mineral delta 18 O Mineral delta 18 O Water T degrees CPre-Main Stage -160 to -170 +3 to +5 +6 to +9 600Main Stage. Central -115 to --180 --9 to +12 -12 to +8 300Intermediate --130 to -155 -4 to +7 -7 to +4 300Peripheral -130 to -145 -2 to +4 - 5 to +1 300Vein quartz also spans much of this 20 per mil delta 18 O range, which is the largest ever reported from a single ore deposit. Excluding two sericites, the hydrogen isotope composition of Main Stage alteration minerals at Butte is very low and extremely uniform (delta D = -140 + or - 15). Calculated D/H ratios of the hydrothermal fluids at approximately 300 degrees C are --110 + or - 15. The low D/H ratios of pre-Main Stage (EDM) biotites indicate equilibration with similar deuterium-depleted solutions of meteoric hydrothermal origin, in contrast with the isotopic evidence for a magmatic hydrothermal origin for alteration biotite in porphyry copper deposits. It is therefore possible that the hydroxyls of the very fine grained EDM biotites were exchanged during the Main Stage hydrothermal activity. The most 18 O-rich Main Stage fluids are associated with the pervasive sericitization in the Berkeley pit area, and the most 18 O-depleted solutions formed the advanced argillic assemblages and some vein minerals in major through-going fractures. Meteoric waters were dominant in all of these Main Stage hydrothermal fluids throughout the Central, Intermediate, and Peripheral Zones. The large variations in delta 18 O water for the Main Stage mineralization indicate that fluids of radically different isotopic composition were present at different times and/or places during the evolution of the alteration assemblages; this is probably a result of variable degrees of interaction between the wall rocks and the hydrothermal fluids or to mixing of meteoric waters with different histories.