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

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160 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Bradley and Kusky (1990). They found four late-fault sets-a conjugate set of strike-slip faults at a large angle to the regional structural fabric, reverse faults parallel to the regional structural grain, and normal faults perpendicular to it. We do not find any conjugate sets of faults. How- ever, the mean strike of the strike-slip faults that we ob- served is perpendicular to the regional (east-west-striking) structural grain as is the mean strike of the strike-slip faults described by Bradley and Kusky (1990). The reverse faults that we measured are parallel to the regional structural fabric, and are antithetic to the synthetic reverse faults along Turnagain Arm. Byrne (1984) recorded both synthetic and antithetic late faults parallel to the structural grain on Kodiak Island (fig. 1). The one normal fault that we measured is perpendicular to regional strike as are the normal faults along Turnagain Arm. Bradley and Kusky (1990) attributed the origin of these late-fault sets to subhorizontal shortening within the accretionary prism and to minor extension along its strike. The north-south-striking strike-slip faults with both dextral and sinistral motion that cut the plutons in the study area are not easily interpreted. Because quartz veins, mineralized zones, and late faults have a similar orientation (compare figs. 12 and 14), veining, mineralization, and faulting are probably related. The north-south strike of quartz veins indicates there was north-south contraction or east-west extension in the study area. The orientations of the late faults are broadly consistent with this interpreta- tion. Felsic dikes within the study area generally have northwest-southeast strikes, but orientations range from north-south, the same as the quartz veins, to east-west. Perhaps the orientation of most of the dikes also indicates east-west or northeast-southwest extension. Alternatively, our data may be biased toward the two or three domains in which the most faults with a known sense of offset were measured, or perhaps there is more than one generation of faulting. IMPLICATIONS FOR ORE DEPOSITS Turbidite-hosted lode-gold deposits are the most common type of epigenetic mineral occurrence in the Chugach-Prince William terrane. The source of the ore- forming fluids has been argued to be from metamorphic dehydration reactions (Goldfarb and others, 1986) or the circulation of meteoric water (Silberman and others, 1981). Regardless of the origin of the fluids, most of these deposits are along late faults, do not show much evidence for hydrofracturing unrelated to faulting, and have common arsenic-bearing sulfides (for example, Mitchell, 1979; Richter, 1970; Pickthorn, 1982, Stuwe, 1986). In contrast, the prospects and occurrences in the study area have base- metal sulfides, no precious metals, little, if any, faulting or shearing across them, and abundant evidence of hydrofracturing. Most of the mineralized zones have a north- south strike, which is similar to the orientation of barren quartz veins and late strike-slip faults. Gold-bearing quartz veins to the east and west of the study area, in the Port Wells and Port Valdez mining districts (fig. I), have some- what different orientations, most commonly striking northwest-southeast. Both types of deposits postdate the plutons in the areas where they are found, and both are found along steeply dipping surfaces. However, the differences in vein mineralogy and structural setting indicate that the polymetallic vein systems have little in common with the gold vein systems. The proximity of mineralized zones to intrusive bodies and the presence of the quartz-fluorite vein suggest a genetic link between magmatism and hydrothermal activity. The abundance of base-metal sulfides and lack of gold- and arsenic-bearing sulfides suggests that chloride complexes were dominant in the ore-forming fluids and that these may have had a magmatic origin. The autobrecciated granite associated with the Granite Cove pluton indicates that at least one of the plutons in the study area had a fluid phase that escaped the magma chamber after crystalliza- tion. The escape of fluids from the plutons, which may have also carried base-metal bearing complexes, locally and perhaps regionally hydrofractured the wall rocks to the plutons along a pre-existing joint set. This joint set may have developed if the rocks were within a regional stress field favoring orogen-parallel extension after oroclinal bending. Orogen-parallel extension during late faulting has been proposed by Bradley and Kusky (1990), and is broadly supported by the orientation of the late faults, quartz veins, and dikes in the study area. This scenario, in which fluids escaped from the plutons and hydrofractured the wall rocks along pre-existing fractures, possibly related to orogen-parallel extension, explains the origin of the ore- forming fluids, the lack of offset along most mineralized zones, brecciation of the wall rocks in mineralized zones, void-filling quartz in mineralized zones, and the roughly consistent orientation of most quartz veins. However, it does not explain why such mineralization does not occur elsewhere within the Chugach-Prince William terrane, even though the same rock types and same age and composition plutons are found outside the study area. REFERENCES CITED Bol, A.J., Coe, R.S., GrommC, C.S., and Hillhouse, J.W., 1992, Paleomagnetism of the Resurrection Peninsula, Alaska: Im- plications for the tectonics of southern Alaska and the Kula-Farallon ridge: Journal of Geophysical Research v. 97, p. 17,213-17.232. Bol, A.J., and Gibbons, H., 1992, Tectonic implications of out- of-sequence faults in an accretionary prism, Prince William Sound. Alaska: Tectonics, v. 11, p. 1288-1300. Bol, A.J., and Roeske, S.M., 1993, Strike-slip faulting and block
STRUCTURAL EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE rotation along the Contact fault system, eastern Prince Will- iam Sound, Alaska: Tectonics, v. 12, p. 49-62. Bossart, P., Dietrich, D., Greco, A., Ottiger, R., and Ramsay, J.G., 1988, The tectonic structure of the Hazara-Kashmir syntaxis, southern Himalayas, Pakistan: Tectonics, v. 7, p. 273-297. Bradley, D.C., and Kusky, T.M., 1990, Kinematics of late faults along Turnagain Arm, Mesozoic accretionary complex, south- central Alaska, in Dover, J.H., and Galloway, J.P., editors, Geologic studies in Alaska by the U.S. Geological Survey, 1989: U.S. Geological Survey Bulletin 1946, p. 3-10. Byrne, T., 1984, Structural geology of melange terranes in the Ghost Rocks Formation, in Raymond, L., ed., Melanges: Their origin and significance: Geological Society of America Special Paper 198, p. 21-51. Carey, S.W., 1955, The orocline hypothesis in geotectonics: Pro- ceedings of the Royal Society of Tasmania, v. 89, p. 255-288. Cobbold, P., and Quinquis, H., 1980, Development of sheath folds in shear regimes, Journal of Structural Geology, v. 2., 119-126. Coe, R.S., Globerman, B.R., Plumley, P.W., and Thrupp, G., 1985, Paleomagnetic results from Alaska and their tectonic implications, in Howell, D.G., ed., Tectonostratigraphic terranes of the Circum-Pacific region: Houston, Circum-Pa- cific Council for Energy and Mineral Resources, Earth Sci. Ser., vol. 1, p.85-108. 1989, Rotation of central and southern Alaska in the Early Tertiary: oroclinal bending by megakinking? in Kissel, C., and Laj, C., eds., Paleomagnetic rotations and continental deformation: Boston, Kluwer, NATO-AS1 se- ries, p. 327-339. Condon, W.H., and Cass, J.T., 1958, Map of a part of the Prince William Sound area, Alaska showing linear geologic fea- tures as shown on aerial photographs: U.S. Geological Sur- vey Miscellaneous Investigations Map 1-273. Cowan, D.S., 1982, Deformation of partly dewatered and con- solidated Franciscan sediments near Piedras Blancas Point, California, in Legget, J.K., ed., Trench forearc geology: Geological Society of London Special Publication 10, p. 439-458. Crowe, D.E., Nelson, S.W., Brown, P.E., Shanks, W.C., 111, and Valley, J.W., 1992, Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska: Eco- nomic Geology, v. 87, p. 1722-1746. Dumoulin, J.A., 1987, Sandstone composition of the Valdez and Orca Groups, Prince William Sound, Alaska: U.S. Geologi- cal Survey Bulletin 1774, 37 p. Gibbons, H., 1988, Microstructures and metamorphism in an accretionary prism in Prince William Sound, Alaska: Santa Cruz, University of California Santa C m, M.S. thesis, 191 p. Goldfarb, R.J., Leach, D.L., Miller, M.L., and Pickthorn, W.J., 1986, Geology, metamorphic setting, and genetic constrains of epigenetic lode-gold mineralization within the Creta- ceous Valdez Group, south-central Alaska, in Keppie, J.D. Boyle, R.W., and Haynes, S.J., eds., Turbidite-hosted gold deposits: Geological Association of Canada Special Paper 32, p. 87-105. Grantz, Arthur, 1966, Strike-slip faults in Alaska: U.S. Geologi- cal Survey Open-File Report 267, 82 p. 161 Hudson, Travis, Plafker, George, and Peterman, Z.E., 1979, Pa- leogene anatexis along the Gulf of Alaska margin: Geology, v. 7, p. 573-577. Jansons, U., Hoekzema, R.B., Kurtak, J.M, and Fechner, S.A., 1984, Mineral occurrences in the Chugach National Forest, southcentral Alaska: U.S. Bureau of Mines Open File Re- port MLA 5-84. Karig, D.E., 1986, The framework of deformation in the Nankai Trough, in Karig,D.E., Kagamia, H., and others, Initial Re- ports of the Deep Sea Drilling Project, volume. 87: Wash- ington, U.S. Government Printing Office, p. 927-940. MacKevett, E.M., Jr., and Holloway, C.D., 1977a, Metalliferous and selected non-metalliferous mineral deposits in the east- em part of southern Alaska: U.S. Geological Survey Open- File Report 77-169A, 99 p., 1 sheet, scale 1:1,000,000. 1977b, Metalliferous mineral deposits in the western part of southern Alaska: U.S. Geological Survey Open-File Re- port 77-169-F, 38 p., 1 sheet, scale 1 : 1,000,000. Mitchell, P.A., 1979, Geology of the Hope-Sunrise (gold) mining district, north-central Kenai Peninsula, Alaska: Stanford, Cali- fomia, Stanford University, M.S. thesis, 243 p. Moffit, F.H., and Fellows, R.E., 1950, Copper deposits of the Prince William Sound district, Alaska: U.S. Geological Sur- vey Bulletin 963-B, 80 p. Moore, J.C., Diebold, J., Fisher, M.A., Sample, J., Brocher, J., Talwani, M., Ewing, J., von Huene, R., Rowe, C., Stone, D., Stevens, C., and Sawyer, D., 1991, EDGE deep seismic reflection transect of the eastern Aleution arc-trench layered lower crust reveals underplating and continental growth: Geology, v. 19, p. 420-424. Mutti, E., and Ricci-Lucchi, F., 1978, Turbidites of the northern Apennines-introduction to facies analysis: International Geology Review, v. 20, p. 125-166. Nelson, S.W., Miller, M.L., Barnes, D.F., Dumoulin, J.A., Goldfarb, R.J., Koski, R.A., Mull, C.G., Pickthorn, W.J., Jansons, U., Hoekzema, R.B., Kurtak, J.M., and Fechner, S.A., 1984, Mineral resource potential of the Chugach Na- tional Forest, Alaska: U.S. Geological Survey Miscella- neous Field Studies Map MF-1645-A. Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest, Alaska: U.S. Geologi- cal Survey Map MF-1645-B. Nilson, T.H., and Zuffa, G.G., 1982, The Chugach terrane, a Cretaceous trench-fill deposit southern Alaska, in Trench- forearc geology-Sedimentation and tectonics on modem and ancient active plate margins: Geological Society of London Special Publication 10, p. 213-227. Pickthorn, W.J., 1982, Stable isotope and fluid inclusion study of the Port Valdez district, southern Alaska: Los Angeles, University of California, M.S. thesis, 66 p. Plafker, George, Jones, D.L., and Pessagno, E.A., Jr., 1977, A Cretaceous accretionary flysch and melange terrane along the Gulf of Alaska margin, in Blean, K.M., ed., The United States Geological Survey in Alaska-Accomplishments during 1976: U.S. Geological Survey Circular 751-B, p. B41-B43. Plafker, George, Keller, Gerta, Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Summary of data on the age of the Orca Group, Alaska, in Bartsch-Winkler, S. ed., The United States Geological Survey in Alaska: Accomplish-
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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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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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