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

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196 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 There is also the question of the potassic metasomatism that has affected both HM and CM rock samples and which is characteristic of molybdenite deposits in general. The fact that potassic fluids can carry considerable molybdenum is likely a part of the explanation. Additional factors seem to be required to explain the general co-location of the metasomatism and the molybdenite. These factors may relate to the total volume available of enriched fluid or to rapid crystallization owing to the steepness of gradi- ents in intensive or extensive variables within a con- strained structural setting. Crystallization of any extensive portion of a fluid as molybdenite or as other minerals might itself be a factor owing to a consequent increase in volatile content in the remaining fluid. The SEM results that were reported above for the potassic phase occurring on the natural fracture surface permit the additional specu- lation that molybdenum might be transported as a potassic complex (for example, a K-Fe-Mo complex). If this were the case, then deposition of molybdenite would also free potassium; such released potassium might be responsible for the potassic metasomatism in the immediate depositional environment of the molybdenite. Acknowledgments.-Jim Barker (Interior Develop- ment, Fairbanks) provided field guidance in 1991 to rare- earth localities in the Dora Bay area. Dave Taylor provided field assistance in 1992 as a U.S. Geological Sur- vey volunteer. Gary McWilliams captained the research vessel HYAK and also made mineralogic contributions to the investigation. The X-ray diffraction confirmation of scapolite was made by Daniel Webster. Short order chemical analyses were provided by M.W. Doughten, P.H. Briggs, D.L. Fey, J.S. Mee, and D.F. Siems. REFERENCES CITED Barker, J.C., and Mardock, Cheryl, 1990, Rare-earth element- and yttrium-bearing pegmatite dikes near Dora Bay, south- em Prince of Wales Island: U.S. Bureau of Mines Open- File Report 19-90, 41 p. Brew, D.A., Drew, L.J., Schmidt, J.M., Root, D.H., and Huber, D.F., 1991, Undiscovered locatable mineral resources of the Tongass National Forest and adjacent lands, southeastern Alaska: U.S. Geological Survey Open-File Report 91-10, 370 p., 26 maps. Candella, P.A., and Holland, H.D., 1986, A mass transfer model for copper and molybdenum in magmatic hydrothermal systems: the origin of porphyry-type ore deposits: Economic Geology, v. 81, p. 1-18. Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p. Eberlein, G.D., Churkin, Michael, Jr., Carter, Claire, Berg, H.C., and Ovenshine, A.T., 1983, Geology of the Craig quadrangle, Alaska: U.S. Geological Survey Open-File Report 83-91, 30 p., scale 1 :250,000. Gehrels, G.E., and Berg, H.C., 1984, Geologic map of southeast- em Alaska: U.S. Geological Survey Open-File Report 84- 886, 28 p., scale 1:600,000. Herreid, Gordon, Bundtzen, T.K., and Turner, D.L., 1978, Geol- ogy and geochemistry of the Craig A-2 quadrangle and vi- cinity, Prince of Wales Island, southeastern Alaska: Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys, Geologic Report 48, 49 p., 2 maps, scale 1:40,000. Isuk, E.E., 1976, Solubility of molybdenite in the system Na20- K20-Si02-MoS2-H20-C02 with geologic application: Ames, University of Iowa, Ph.D. thesis, 101 p. Smith, J.V., 1983, Some chemical properties of feldspars, in Ribbe, P.H., ed., Feldspar mineralogy: Mineralogical Soci- ety of America, p. 281-296. U.S. Department of Agriculture, Forest Service, 1991, Tongass National Forest Land Management Plan Map, scale 1 :760,320. White, W.H., Bookstrom, A.A., Kamilli, R.J., Ganster, M.W., Smith, R.P., Ranta, D.E., and Steininger, R.C., 1981, Char- acter and origin of Climax-type molybdenum deposits: Eco- nomic Geology, 75th Anniversary Volume, p. 270-316. Reviewers: Bruce R. Doe, Karen D. Kelley, and Holly J. Stein.
GEOCHEMICAL CHARACTER OF UPPER PALEOZOIC AND TRIASSIC GREENSTONE AND RELATED METAVOLCANIC ROCKS OF THE WRANGELLIA TERRANE IN NORTHERN SOUTHEASTERN ALASKA ABSTRACT Greenstone and other mafic metavolcanic rocks of late Paleozoic and early Mesozoic age are widespread in northern southeastern Alaska. Major- and trace-element similarities between Upper Triassic greenstone of the Wrangellia terrane (Wrangellia) of the Chilkat Peninsula and greenstone of the Upper Triassic upper part of the Gastineau Volcanics near Juneau indicate that Wrangellia extends southward as a long narrow belt along the west margin of the Coast Mountains. AmphiboIite near Juneau has major- and trace-element characteristics similar to those of Upper Triassic metabasalt, which suggests that parts of Wrangellia were involved in amphibolite-facies- grade metamorphism of the western metamorphic belt of the metamorphic and plutonic complex of the Coast Moun- tains. Greenschist and greenstone of northern Admiralty Is- land (Barlow Cove) are of unknown age but probably correlative with the Triassic Hyd Group. They have many major- and trace-element signatures of metabasalt of Wrangellia, which suggests that Wrangellia is an integral part of the Alexander terrane of insular areas of southeast- em Alaska. The recognition of Wrangellian mafic volcanism in these areas greatly increases the known extent of this basaltic province, which is among the world's largest of this type. All of the greenstones of the Gastineau Volcanics near Juneau are tholeiitic in composition, but the Permian(?) and Triassic(?) lower part of the formation has trace-element signatures different from those of the Upper Triassic higher part. The lower part of the Gastineau shows an en- richment in large-ion-lithophile elements and has other characteristics of ocean-island basalt, which contrast with the enriched-MORB or back- or inter-arc-basin basalt character of the Upper Triassic metabasalts. Subduction- zone components (Ta and Nb depletions, in particular) are not found in any of the volcanic rocks of Wrangellia in the Juneau area. The metabasalts record different settings of volcanism within a long-lived oceanic volcanic system, the late Paleozoic and Triassic(?) activity of which was related By Arthur B. Ford and David A. Brew to ocean-island volcanism and the later activity to Late Triassic rifting or superplume events. Accretion of the volcanic terrane, which either originated or became combined with, the Alexander terrane, to North America, may have been associated with Late Cretaceous volcanism of the Douglas Island Volcanics and with Barrovian regional metamorphism at the end of the Cretaceous. INTRODUCTION Linear belts of mafic metavolcanic rocks of late Pa- leozoic, Triassic, and Triassic(?) age, among others, have long been recognized in northern southeastern Alaska (Knopf, 1911; Martin, 1926; Buddington and Chapin, 1929). This report focuses on one of these belts, termed the Wrangellia terrane (Jones and others, 1977; 1987), of late Paleozoic and Triassic age. This belt was recently identified on the Chilkat Peninsula (Plafker and others, 1989) (fig. 1). For brevity, the terrane is herein also termed Wrangellia, following general usage (Jones and others, 1986; MacIntyre, 1986; Barker and others, 1989; Samson and others, 1990). Faunal evidence and paleomagnetic data support a non-North American origin of Wrangellia (Jones and others, 1986). Metavolcanic rocks of Triassic and Triassic(?) age ex- tend from the Chilkat Peninsula south to Juneau (Brew and Ford, 1985) and beyond to other parts of mainland southeastern Alaska. Metavolcanic rocks of similar age and type occur on Admiralty Island and elsewhere, as discussed below. Triassic mafic volcanic rocks also occur widely in British Columbia (Souther, 1971 ; Mortimer, 1987; Henderson and others, 1992). The Triassic and older metavolcanic rocks of southeastern Alaska are variously assigned to different tectonostratigraphic terranes (Brew and Ford, 1984, 1993; Monger and Berg, 1987; Plafker and others, 1989; Silberling and others, 1992; Gehrels and others, 1992). Originally defined as Triassic only (Jones and others, 1977), a definition still followed by some (Lassiter and DePaolo, 1992), Wrangellia is now generally
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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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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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