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

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184 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 samples. The sample is derived from high-grade metamorphic rocks of the Valdez Group, and the reason for the multi-element anomaly is unknown. In the Icy Bay quadrangle, two samples collected near Claybluff Point and a series of samples to the east of Icy Bay and south of Malaspina Glacier have high scores for factor 2. Additionally, 17 of the 30 samples collected in the quadrangle contained greater than 50 pprn Ni, and three of these contained 91-135 pprn Ni. Most of these samples with Ni concentrations above the 90th percentile also had Cu and Cr values above the 90th percentile. These high values for the matic elements could be due to accumulations of mafic volcanic rocks transported down from the higher elevations of Malaspina Glacier to the east of the study area. Samples with high scores for factor 3 contain relatively high concentrations of K, Ba, and A1 and generally low amounts of Ca. This same geochemical association was described by Goldfarb (1984) for NURE stream-sediment samples collected in the Cordova quadrangle, adjacent and west of the Bering Glacier quadrangle. The association was suggested to indicate samples derived from clay-rich zones within marine metasedimentary rocks of the Orca Group. Within the Bering Glacier quadrangle, the Prince William terrane, which is composed of rocks of the Orca Group, lies in a belt across the high elevations of the Bagley Ice Field. No stream- or moraine-sediment samples were collected from this part of the quadrangle during the NURE survey. Samples with the highest scores for factor 3 are generally scattered through the mediumand high-grade metamorphic rocks of the Valdez Group in the northwestern part of the Bering Glacier quadrangle. These samples are probably derived from sedimentary facie~ within the turbidite sequence that have fewer mafic volcanic lithic fragments and plagioclase grains, and (or) greater amounts of potassium feldspar, muscovite, and detrital barite. A cluster of samples with high factor 3 scores from the headwaters of the Duktoth River may also be suggestive of similar facies variations within the Yakutat terrane. An isolated stream-sediment sample with a high score for factor 3, collected from a marshy tributary to the Tsivat River, contained 5,821 pprn Ba. Pickthorn and others (1985) described barium soil anomalies and abundant barite in heavy-mineral concentrate samples from Kayak Island, a few kilometers west of the Icy Bay quadrangle. The source for the barite was shown to be glauconitic horizons in the upper part of the Poul Creek Formation of the Yakutat terrane. A similar source is feasible for the single-site anomaly 75 km northeast in the southem Bering Glacier quadrangle. Factor 4 is dominated by a Sm-Ce-La-Dy-Pb-Eu-Th signature that reflects the distribution of some of the Tertiary felsic igneous rocks that intrude sedimentary units of the Valdez Group. Samples with high scores for factor 4 stretch along the Middle Fork of the Bremner River, the West Fork of the Tana River, and Granite Creek. Those collected from the Middle Fork of the Bremner River are notably enriched in U and Th. Five stream-sediment samples from the watershed contained 23-31 pprn Th and 10-12 pprn U. Goldfarb and others (1989) identified similar thorium and uranium values along the South Fork of the Bremner River within the adjacent Cordova quadrangle to the west. These relative enrichments were attributed to the presence of monazite, apatite, allanite, and other acces- sory minerals that have likely been weathered from felsic igneous rocks. Samples with high scores for factor 5 are enriched in As and depleted in Al and Na. They cluster in the northeast comer of the study area and in the region between the Tana River and Twelvemile Creek. In the former area, samples with highest scores have been derived from rocks of the Alexander and Wrangellia terranes along tributary valleys of the Chitina and Logan Glaciers. Samples that are most depleted in A1 and Na commonly are some of the most Ca-enriched stream-sediment samples from the study area, suggesting detrital contributions from Paleozoic lime- stone and marble. Excluding the earlier described sample from Baldwin Glacier with 186 pprn As, most sediments collected in the northeast comer of the study area with high scores for factor 5 contain 15-33 pprn As. They show background amounts of all base and precious metals. Although the origin of the elevated arsenic values is un- clear, they are not believed to reflect upstream metallic mineral occurrences. A single stream-sediment sample collected below the ridge on the north side of Logan Gla- cier contained high concentrations of Ba, ce, Cs, K, La, Pb, Rb, Sb, Sm, Th, and U. The lead concentration of 39 pprn is the highest value detected from the study area. These data suggest the upstream presence of igneous rocks enriched in lithophile elements. Anomalous samples between the Tana River and Twelvemile Creek target an area likely to contain gold- bearing quartz veins. Such veins are known to be scattered throughout medium-grade metamorphic rocks of the Valdez Group to the west of the Bering Glacier quadrangle (Goldfarb and others, 1986). V.B. Sisson (written commun., 1991) identified a closely spaced pattern of inverted isograds in rocks of the Valdez Group along the Tana River, changing from migmatite to biotite zone of the greenschist facies over only 5 km. Such a metamorphic setting is spatially associated with major gold deposits along the Juneau gold belt in southeast Alaska and at Valdez Creek to the south of the Alaska Range, and in general may be an important feature aiding in the develop- ment of mesothermal gold lodes (Goldfarb and others, 1993). Many of the stream-sediment samples collected immediately east of the Tana River contain 34-39 pprn AS, likely reflecting arsenopyrite commonly found in the aurif- erous veins. A stream-sediment sample collected to the northeast of peak 6425 contained 62 pprn As. One stream-
GEOCHEMICAL EVALUATION OF STREAM-SEDIMENT DATA sediment sample collected along the east side of the Tana River contained 0.24 pprn Au. An adjacent sample contained 201 pprn Zn, a highly anomalous value for the NURE survey and likely indicative of sphalerite associated with the precious metals. A few additional isolated anomalies were identified through examination of single-element distribution maps. A stream-sediment sample containing 259 pprn Zn and 36 pprn As may be associated with the earlier described hy- drothermal mineralization near Slender Lake. Four samples with anomalous zinc values extend along the southwestern edge of the Bering Glacier quadrangle in the Yakutat terrane. One sample collected from Canyon Creek contained 498 pprn Zn, the highest concentration from the NURE survey. These data suggest an eastward extension of sphalerite-bearing strata of the Poul Creek and Yakataga Formations delineated by Goldfarb and others (1985). Two samples with 42 pprn As were collected along the Kiktukh River in the southwestern comer of the Bering Glacier quadrangle. Five kilometers to the east a single stream-sediment sample from the Tashalich River contained 1.99 ppm Au. Since these three samples reflect glaciofluvial outwash from Bering Glacier, the anomalies could be derived from locations throughout half the study area, and these data are thus difficult to relate back to their source. One sample from the Bremner River on the western edge of the study area contained 0.28 pprn Au, indicative of the placer gold accumulations that are recognized within rocks of the Valdez Group a few kilometers downriver. CONCLUSIONS Factor analysis of stream-sediment data from the NURE survey was used to aid in the delineation of geochemically anomalous samples within the Bering Glacier and Icy Bay quadrangles. Much of the variation in the geochemical data reflects differences in lithology. Samples enriched in the lithophile elements are largely 185 been identified through interpretation of the NURE data. Samples with anomalous values for Fe, Cu, Co, and Ni (likely indicating the presence of pyrite and chalcopyrite), as well as for gold, silver, and arsenic, are common throughout the Chitina Valley batholith. These anomalies suggest the presence of metal-bearing quartz veins associated with copper porphyry and molybdenum porphyry systems. To the east of this area, a cluster of samples with anomalous arsenic between the Tana River and Twelvemile Creek are suggested to reflect the upstream presence of mesothermal gold-bearing quartz veins within metasedirnentary rocks of the Valdez Group. Despite the fact that two new areas with favorability for the presence of metallic mineral resources have been delineated, the NURE stream-sediment data are generally inadequate for a comprehensive mineral resource assess- ment of the Bering Glacier and Icy Bay quadrangles. Stream-sediment samples were collected over less than half of the land area on the Bering Glacier quadrangle. Although much of the higher elevations of the quadrangle is glaciated and was ignored during the NURE survey, stream- or moraine-sediment sampling is still possible in these areas. In fact, not a single sediment sample was collected from within the Prince William terrane. Many of the collected stream-sediment samples were recovered on the more accessible large outwash plains and along major rivers, and thus their usefulness for identifying upstream sources is limited. Also, the lack of collection of heavy- mineral-concentrate samples during the NURE drainage survey further hinders its usefulness. Collection of such a sample medium, concentrating a relatively large stream volume, is critical when trying to detect a few rnetalIic grains along a stretch of stream channel. REFERENCES CITED derived from upstream outcrops of Tertiary granitic rocks within the Valdez Group, whereas many of the samples Brabb, E.E., and Miller, D.J., 1962, Reconnaissance traverse across the eastern Chugach Mountains, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-34 1, I sheet, scale 1:96,000. with high concentrations chalcO~hile are Eyles, N., 1990, Glacially derived, shallow-marine gold placers derived from volcanic rocks of the Wrangellia terrane. of the Cape Yakataga district, Gulf of Alaska: Sedimentary Differences in content of K, Ba, and A1 apparently reflect Geology, v. 68, p. 171-185. facies variations in marine sedimentary rocks. Samples Gardner, M.C., Bergman, S.C., Cushing, G.W., MacKevett, derived from Paleozoic carbonate rocks in the Wrangellia E.M., Plafker, G., Campbell, R.B., Dodds, C.J., and Alexander terranes are relatively enriched in As and McClelland, W.C., and Mueller, P.A., 1988, Pennsylvanian Ca and depleted in A1 and Na. Elements enriched in refractory minerals, such as many of the rare earths, chromium, and titanium, occur in anomalous amounts in samples collected along the coastal plain. These samples delineate the area of well-studied gold and heavy-mineral beach placers of the Yakataga mining district. Two new areas, geochemically favorable pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska: Geology, v. 16, p. 967-971. Goldfarb, R.J., 1984, A preliminary geochemical interpretation of the Chugach Wilderness, southern Alaska, in Coonrad, W.L., and Elliott, R.L., eds., The United States Geological Survey in Alaska-Accomplishments during 1981: U.S. Geological Survey Circular 868, p. 89-92. Goldfarb, R.J., Leach, D.L., Miller, M.L., and Pickthorn, W.J., for the presence of metallic mineral resources, have also 1986, Geology, metamorphic setting and genetic constraints
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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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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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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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