Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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238 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Table 1. Analytical data for vein minerals from mines in the Juneau gold belt [qz-a is -60 +I00 mesh; qz-b is -100 +I40 mesh. Minerals: qz, quartz; carb, carbonate; ank, ankerite; alb, albite; phlog, phlogopite; mic, white mica; chl, chlorite; dolo, dolomite; calc, calcite. Kensington mine "mic-a" specimens were treated with 1N HCI for 10 minutes to remove intergrown carbonate. For the A-J mine, hydrogen (GD) and carbon (G13c) fractionation values are given in the last column along with oxygen values. nd, not determined] Sample Rb(PPm) Sr(PPm) RbISr 87~h - -- ,86s, I -- 87~,,86~, --I -- ~180 - - A-J mine, lat S8°18'26" N., long 134"20'47" W. 86RN16 phlog 427.5 9.98 42.8 125.15 0.80914*7 +13.1 6D=-96 01 6Aqz-a 1.55 3.28 .473 1.368 .7090429 +18.0 01 6Aqz-b 0 1 6Acarb .I38 .245 3.62 983 .038 .0002 .I10 .001 .70829*9 .70822*1 +17.7 +15.9 613c=-10.6 Treadwell mine, lat 58"16'0OW N., long 134"22'40" W. 87RN276B ank 0.2 4844 0.0001 0.0003 0.7036222 nd 87RN283A alb 1 .O 44 1 .002 .006 .70352+.2 nd 87RN280A qz .02 1.13 .O 15 .I419 .70443*9 nd 87RN276B mic 349.4 220.5 1.585 4.584 ,7072222 nd 87RN281A chl 21.4 114.3 .I87 .542 .70345*2 nd Kensington mine, lat 58°S2'11" N., long 13S006'10" W. 90RN101A mic 62.3 802.4 0.078 0.225 0.7034722 nd 90RN101A mic-a 117.5 3400 ,035 .1 .70344+2 nd 90RN101B qz .045 2.6 .017 .05 .70384+2 nd 90RN 10 1 C dolo .866 566 .002 .004 .70336+2 nd 90RN102A mic 8 8 226 .389 1.126 .70404+2 nd 90RN102A mic-a 110.5 26.5 4.17 12.07 .71143+2 nd 90RN 102B calc 1.07 803 .OO 1 ,004 .70326r2 nd negative, -6.2 to -17.8 per mil) and heaviest (most positive, -3.8 to 1.2 per mil) S34~ reported by Goldfarb and others (1991a) are from the Alaska-Juneau, and from the Treadwell and Kensington mines, respectively. Even though much of the veining at Alaska-Juneau is hosted by metagabbro sills, this area is largely underlain by black phyllite. The extremely negative sulfur isotope values observed suggest that the sulfur in the vein sulfide minerals was derived from the local, reduced sedimentary rock source (Goldfarb and others, 1991a). The radiogenic strontium isotope composition (0.7081) of the Alaska- Juneau vein minerals is also compatible with having a source in local sedimentary wallrocks. In contrast, the heavier sulfur isotope values from ore veins at the Treadwell and Kensington deposits, hosted in monzoniticmonzodioritic plutonic rocks, are compatible with sulfur derivation from sulfide minerals with 634~ close to 0 per mil from earlier sulfide-bearing veins in the plutons (Goldfarb and others, 1991a). This sulfur isotopic composition is typical of many magmatic sulfides, and it correlates positively with the unradiogenic strontium (0.7033-0.7036) that also probably had its source in the local granitoid rock hosts at these mines. The strontium isotope data indicate that the quartz and carbonate from the Alaska-Juneau vein precipitated in isotopic equilibrium at the same time. If the carbonate were pure calcite, the about 2 per mil oxygen isotope fractionation between quartz and carbonate (table 1) would indicate their temperature of formation to be about 400°C (Friedman and O'Neil, 1977, fig. 24). Although this is not the case and the carbonate is ferroan dolomite, this temperature is probably not too far out of line given that geothermometry based on arsenopyrite (k pyrite-pyrrhotite or pyrite-sphalerite) indicates temperatures of vein formation in the neighborhood of 300400°C in the Juneau gold belt (R.J. Newberry, unpublished data). Using a conserva- tive estimate of 350°C for vein formation at the Alaska- Juneau mine, an oxygen-isotope fractionation between quartz and water of about +6 per mil is indicated. There- fore, the vein-forming fluid had a 6180 of +11.9 per mil, well within the range of oxygen isotope compositions of metamorphic water and also the compositions of other Ju- neau gold belt ore fluids reported by Goldfarb and others (1991a). Using the measured SD of -96 from the phlogopite (table I), the calculated 6D for the ore fluid at 350°C is -66 per mil from water-theoretical phlogopite fractionation (Suzuoki and Epstein, 1976). This calculated hydrogen isotopic composition is like that measured from fluid inclusions in quartz rather than that calculated for Juneau gold belt ore fluids by Goldfarb and others (1991a). This suggests that our Alaska-Juneau vein had a minor post- depositional interaction with meteoric water. The carbonate
RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMATICS OF VEIN MINERALS, JUNEAU GOLD BELT 6I3c of -10.6 per mil (table 1) is similar to carbon isotopic compositions of C02 measured in hot springs at Steamboat Springs, Colorado, and at The Geysers and Lassen Peak, California. Figure 2. Strontium evolution diagram showing isochrons defined by minerals from Alaska-Juneau, Treadwell, and Kensington deposits in Juneau gold belt. phlog, phlogopite. Figure 3. Expanded view of Treadwell and Alaska-Juneau data. Note that quartz and chlorite from Treadwell locality are not in isotopic equilibrium with ankerite, albite, and white mica from there. Mineral abbreviations as in table 1. CONCLUSIONS 239 The Alaska-Juneau, Treadwell, and Kensington deposits in the Juneau gold belt are Eocene in age. The strontium in the vein minerals of the Alaska-Juneau deposit in the Wrangellia terrane was apparently scavenged from sedimentary host rocks, whereas vein minerals of the Treadwell and Kensington deposits have strontium derived from their plutonic rock hosts. The Kensington deposits are hosted by the 106-Ma Jualin pluton (G.R. Tilton, writ- ten comm. to D.A. Brew, 1985) in the Wrangellia terrane close to Gravina overlap assemblage rocks. We note here that Goldfarb and others (1991a) reported an erroneous 113-Ma age for the Jualin pluton. Its strontium isotopic characteristics are similar to those of the Treadwell deposits that are localized within a zone of monzonite sills within a sequence of slate in the Gravina overlap assemblage rocks. The mineralizing fluids responsible for the gold belt veins have oxygen isotope characteristics of evolved metamorphic waters. REFERENCES CITED Bohlke, J.K., and Kistler, R.W., 1986, Rb-Sr, K-Ar, and stable isotope evidence for the ages and sources of fluid compo- nents of gold-bearing quartz veins in the northern Sierra Nevada foothills metamorphic belt, California: Economic Geology, v. 81, p. 296-322. Borthwick, J., and Harmon, R.S., 1982, A note regarding CIF, as an alternative to BrF5 for oxygen isotope analysis: Geochimica et Cosmochimica Acta, v. 46, p. 1665-1668. Brew, D.A., and Ford, A.B., 1978, Megalineament in southeastern Alaska marks southwest edge of Coast batholithic com- plex: Canadian Journal of Earth Sciences, v. 15, p. 1763-1772. Brew, D.A., Karl, S.M., Barnes, D.F., Jachens, R.C., Ford, A.B., and Horner, R., 199 1, A northern Cordilleran ocean-conti- nent transect: Sitka Sound to Atlin Lake, British Columbia: Canadian Journal of Earth Sciences, v. 28, p. 840-853. Friedman, Irving, and O'Neil, J.R., 1977, Compilation of stable isotope fractionation factors of geochemical interest: U.S. Geological Survey Professional Paper 440-KK, 12 p. Godfrey, J.D., 1962, The deuterium content of hydrous minerals from the east-central Sierra Nevada and Yosemite National Park: Geochimica et Cosmochimica Acta, v. 26, p. 1215-1245. Goldfarb, R.J., Leach, D.L., Pickthorn, W.J., and Paterson, C.J., 1988, Origin of lode-gold deposits of the Juneau gold belt, southeastern Alaska: Geology, v. 16, p. 440443. Goldfarb, R.J., Newbeny, R.J., Pickthorn, W.J., and Gent, C.A., 1991a, Oxygen, hydrogen, and sulfur isotope studies in the Ju- neau gold belt, southeastern Alaska: Constraints on the origin of hydrothermal fluids: Economic Geology, v. 86, p. 66-80. Goldfarb, R.J., Snee, L.W., Miller, L.D., and Newberry, R.J. 1991b, Rapid dewatering of the crust deduced from ages of mesothermal gold deposits: Nature, v. 354, p. 296-298.
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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GOLD AND CINNABAR IN HEAVY-MINERAL
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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Table 1. Isotopic ages of intrusive
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Table 1. Isotopic ages of intrusive
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GEOCHEMICAL EVALUATION OF STREAM-SE
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