OXYGEN, HYDROGEN, AND SULFUR ISOTOPE STUDIES IN THE JUNEAU GOLD BELT, SOUTHEASTERN ALASKA - CONSTRAINTS ON THE ORIGIN OF HYDROTHERMAL FLUIDS

The delta-18O values of gold-bearing quartz from the Juneau gold belt range from 15.2 to 20.8 per mil, indicating that ore fluid values ranged from 7.2 to 12.8 per mil at an estimated temperature of 300-degrees-C. Hydrothermal micas from many of the deposits are characterized by delta-D values of -75 to -53 per mil, and ore fluids were calculated to have values of -35 to -15 per mil. In contrast, extracted fluid inclusion waters have a broad range of delta-D values, from -48 in relatively undeformed quartz to about -110 per mil for some of the more deformed quartz veins. This range of more than 60 per mil reflects various mixtures from inclusions containing isotopically heavy, primary ore fluids and those containing low-temperature, isotopically light meteoric waters that were trapped in late fractures during uplift of the veins to shallow crustal levels. These results indicate a deep crustal source for the ore fluids, most likely of metamorphic origin. Values of delta-D for muscovite, biotite, and hornblende from country rocks, as well as of DELTA-18O for quartz-feldspar and quartz-plagioclase, provide little evidence of deep circulation of meteoric water along the length of the gold belt. The provinciality of delta-34S data suggests a regional metamorphic fluid of approximately -6 per mil that acquired much of its sulfur from lithologies near sites of ore deposition. Sulfur ratios of sulfide minerals from auriferous quartz range from -17.8 to -5.6 per mil in black phyllite hosts, from -6.0 to -1.3 per mil in less reduced metasedimentary rocks, and from -3.8 to +1.2 per mil in relatively oxidized igneous rocks. The regional fluid is interpreted to have driven desulfidation reactions in relatively S-34-depleted sulfur reservoirs in the phyllites and S-34-enriched reservoirs in the more oxidized lithologies.