Measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open-system behavior of Re–Os in molybdenites. Molybdenite samples from six porphyry copper deposits, one epithermal polymetallic vein deposit, four skarns, and three Fe-oxide Cu–Au deposits were analyzed. The δ97Mo‰ (where δ97Mo = [(97Mo/95Mosample)/(97Mo/95MoICPMS standard)]-1 x 1000 ) for all samples varied from 1.34±0.09‰ to −0.26±0.04‰. This is the largest molybdenum isotopic variation in molybdenite from high-temperature ore deposits recorded to date. δ97Mo‰ of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples. Isotope values for Mo also vary within the individual deposits. In general, molybdenites from porphyry copper deposits have the lightest values averaging 0.07±0.23‰ (1σ). Molybdenites from the other deposit types average 0.49±0.26‰ (1σ). The variations could be related to the fractionation of Mo into different mineral phases during the ore-forming processes. A comparison of the Mo isotope ratios and the Re–Os ages obtained from the same aliquot may possess a geochronological evaluation tool. Samples that yielded robust ages have different Mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages. Another observed relationship between the Re–Os andMo isotope data reveals a weak correspondence between Re concentration and Mo isotope composition. Molybdenites with higher concentrations of Re correspond to lighter Mo isotope values.This material is based upon work supported by the National Science Foundation under grant nos. CHE-0431328 and EAR 05-19144

The •i180 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 300øC. Hydrothermal micas from many of the deposits are characterized by bD 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 bD 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 bD for muscovite, biotite, and hornblende from country rocks, as well as of A•80 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 ba4s 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 a4s-depleted sulfur reservoirs in the phyllites and a4S-enriched reservoirs in the more oxidized lithologies. IntroductionTHE mesothermal quartz lodes of the Juneau gold belt stretch for 200 km along the northern half of the southeast Alaska panhandle (Fig. 1). The deposits are located approximately 125 km inboard of the Gulf of Alaska and about 20 km west of the Coast batholith. Placer gold was first discovered in the area in 1869, and more than 6.7 million oz of lode gold were recovered between 1880 and 1944 (Redman, 1986). Recent exploration indicates a minimum of 3 million oz of additional gold resources (Echo Bay Mines, writ. commun., 1990). The most thorough geologic summary of the Juneau gold belt was compiled by Spencer (1906), who noted many similarities between the deposits of the gold belt and those of the California Mother Lode. Detailed studies of the local geology and ore petrography at the Alaska-Juneau mine, the largest producer along the gold belt, were completed by Wernecke (1932), Wayland (1939), and Twenhofel (1952). Most mineralization in the gold belt is clearly epigenetic, with veining occurring over a wide stratigraphic interval and in a variety of different rock types. There is little evidence for stratiform gold enrichment near the larger orebodies, although there are small volcanogenic sulfide layers in the vicinity of the gold bel...

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