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

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84 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 the mountains (Cady and others, 1955). However, its location and Chao (1985). The samples were digested using a se- is poorly defined, and no further data are known for thls min- ries of hydrogen peroxide, hydrofluoric acid, aqua regia, eral occurrence. and hydrobromic acid-bromine solutions. Gold, Te, and T1 were separated and concentrated by extraction into methyl isobutyl ketone and determined by flame AAS. Concen- GEOCHEMICAL SAMPLING trations for Au in the range of 0.002 to 0.050 ppm were TECHNIQUES AND SAMPLE determined by graphite furnace atomic absorption spectro- PREPARATION photometry (GFAAS) on samples that were shown to be Stream-sediment and heavy-mineral-concentrate samples were collected from the active stream channel at 138 sites (fig. 3). Stream sediment was screened to minus- 10 mesh and collected in a stainless steel gold pan. Ap- proximately 2 kg of sediment was taken from the pan and saved as the stream-sediment sample. The pan was re- filled with stream sediment and panned until most of the less dense minerals, organic materials, and clay were re- moved. This panned sample was saved as the heavy-mineral-concentrate sample. In the laboratory, each stream-sediment sample was dried below 50°C, sieved to minus-80 mesh, pulverized, and chemically analyzed. The heavy-mineral-concentrate samples were further separated using bromoform to re- move any remaining lighter minerals, primarily quartz and feldspar, and then separated magnetically into magnetic, paramagnetic, and nonmagnetic fractions. The nonmagnetic fractions were ground and chemically analyzed, and splits were evaluated microscopically for their mineralogical content. Only one heavy-mineral-concentrate sample had insufficient material for geochemical analysis. All of the 138 heavy-mineral-concentrate samples collected were examined microscopically for their mineralogical content. ANALYTICAL TECHNIQUES SEMIQUANTITATIVE EMISSION SPECTROSCOPY The stream-sediment and nonmagnetic-heavy-mineralconcentrate samples were analyzed by a semiquantitative, direct-current arc emission spectrographic (SQS) technique adapted from Grimes and Marranzino (1968). The 35 elements Ag, As, Au, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, Ge, La, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Sb, Sc, Sn, Sr, Ti, Th, V, W, Y, Zn, and Zr were determined in samples analyzed by SQS. In addition, Pd and Pt were determined in the heavy-mineralconcentrate samples by SQS. ATOMIC ABSORPTION SPECTROPHOTOMETRY Concentrations of Au, Te, and T1 in the stream-sediment samples were determined by an atomic absorption spectrophotometry (AAS) technique adapted from Hubert less than 0.050 ppm by the flame AAS technique. The GFAAS technique for Au was adapted from Meier (1980). Mercury was measured in the stream-sediment samples using a modified version of the cold vapor AAS technique (Kennedy and Crock, 1987). The samples were decomposed with nitric acid and sodium dichromate. Mercury was then reduced to elemental Hg with hydroxylamine hydrochloride/sodium chloride and stannous chloride in a continuous flow system that releases the Hg vapor directly into an atomic absorption spectrophotometer. Tungsten was determined by a visible spectrophotometric (VS) method by decomposing the sample with nitric, hydrofluoric, and hydrochloric acids (Welsch, 1983). INDUCTIVELY COUPLED PLASMA-ATOMIC EMISSION SPECTROSCOPY Concentrations of Ag, As, Sb, Bi, Cd, Cu, Mo, Pb, and Zn were determined in the stream-sediment samples by inductively coupled plasma-atomic emission spectroscopy (ICP) using the procedure developed by Motooka (1988). The sediments were decomposed with concentrated hydrochloric acid and hydrogen peroxide in a hot- water bath. The metals were extracted in diisobutyl ketone (DIBK) in the presence of ascorbic acid and potassium io- dide. The DIBK phase was then aspirated directly into the plasma and element concentrations were determined simul- taneously with a multichannel ICP instrument. MINERALOGICAL ANALYSIS The nonmagnetic fraction of the heavy-mineral-concentrate samples contains sulfide minerals, gold, and some nonmagnetic oxides and silicates. Mineral identifications were made with a binocular microscope, recording the amounts of particular minerals in the following way: class 1, less than 1 percent; class 2, 1-5 percent; class 3, 5-20 percent; class 4, 20-50 percent; and class 5, greater than 50 percent. If a particular mineral was not observed, an "N" was recorded. This approach was used to identify the general abundance of minerals in the concentrates that were diagnostic of possible upstream mineral deposits. Cinnabar, gold, scheelite, and barite were the most useful for this study.
FAVORABLE AREAS FOR METALLIC MINERAL RESOURCES IN AND NEAR THE HORN MOUNTAINS 85 RESULTS Anomalous concentrations of elements and minerals used in this study were determined by identifying breaks in the frequency distribution of each data set, usually between the 90th and 98th percentiles. Anomalous concentrations were also based on background concentrations of various lithologies in the Iditarod quadrangle (McGimsey and others, 1988), where rock types are similar to those exposed in the Horn Mountains area. Table 1 summarizes data used in this report and selected anomalous concentrations. Some elements and minerals were not included in table 1 because they were not considered pathfinders for metallic mineral resources in this study area, or because no samples contained anomalous values for that element or mineral. Cinnabar is common in the heavy-mineral-concentrate samples; 56 percent of the concentrates contain microscopically visible cinnabar. Heavy-mineral-concentrate samples containing greater than 1 percent cinnabar are considered diagnostic of possible upstream mineralized areas, and all of these samples cluster in specific areas considered favorable for epithermal Hg-rich deposits. Most samples with less than 1 percent cinnabar are found as single-site anomalies that are difficult to evaluate; however, a few of these samples help delineate areas favorable for Hg-rich deposits when samples from these areas also contain element anomalies typical of this deposit type. INTERPRETATION AREAS FAVORABLE FOR POLYMETALLIC VEIN DEPOSITS Two areas surrounding the Horn Mountains are favorable for polymetallic vein deposits (fig. 3). Delineation of these areas is based on bedrock geology and geochemi- cally or mineralogically anomalous stream-sediment or heavy-mineral-concentrate samples. AREA 1 Area 1 is located in the southern Horn Mountains (fig. 3), where mafic to felsic volcanic rocks and sedimentary rocks of the Kuskokwim Group are cut by intermediate to felsic intrusions of the Horn Mountains stock (fig. 2). Stream-sediment samples collected in this area contain as much as 0.70 pprn Au, 0.68 pprn Ag, 120 pprn Cu, 54 pprn Pb, 370 pprn As, 35 pprn Sb, 0.81 pprn Hg, 16 pprn W, 1.9 pprn Cd, 150 pprn B, and 0.2 pprn Te. Heavy-mineral concentrates collected in area 1 contain as much as 30 pprn Au, 15 pprn Ag, 2,000 pprn Pb, 300 pprn Sb, 1,000 pprn Bi, 7,000 pprn Ba, and 700 pprn W, and several contain microscopically visible gold, scheelite, and barite. This area is consistent with an upstream polymetallic vein occurrence found near the headwaters of Whitewing Val- ley (Bundtzen and others, 1993); additional similar undis- covered veins are possible in this area. AREA 2 A geochemical anomaly on the northern side of the Horn Mountains includes several samples collected from tributaries of Getmuna and Jungjuk Creeks (fig. 3). In area 2, intermediate to felsic intrusions of the Horn Moun- tains stock cut mafic to felsic volcanic rocks (fig. 2). Stream-sediment samples collected in the area contain as much as 0.026 pprn Au, 0.95 pprn Ag, 290 pprn As, 57 pprn Sb, 4.0 pprn Hg, 5 pprn W, 57 pprn Pb, 6.4 pprn Bi, 1.5 pprn Cd, 200 pprn B, and 0.6 pprn Te; heavy-mineralconcentrate samples contain as much as 30 pprn Au, 5 pprn Ag, 1,500 pprn Bi, 500 pprn Pb, 200 pprn Sb, 1,000 pprn W, 1,000 pprn B, and 1 percent scheelite. Two polymetallic vein occurrences are known in area 2 (Bundtzen and others, 1993) along Getmuna and Jungjuk Creeks (fig. 3). With the exception of Pb, base-metal anomalies are lacking in samples collected from area 2. However, as previously mentioned, base-metal anomalies in samples collected around volcanoplutonic complexes are sometimes inconsistent with the base-metal-sulfide-bearing nature of known polymetallic vein deposits. Although base-metal anomalies are not predominant, area 2 is considered favorable for polymetallic vein deposits on the ba- sis of other geochemical anomalies and bedrock geology consistent with areas where polymetallic veins have been recognized. AREAS FAVORABLE FOR EPITHERMAL MERCURY-RICH VEIN DEPOSITS Data from the Horn Mountains area suggest that eight areas are favorable for epithermal mercury-rich vein deposits. Lode deposits of this type are not known in these areas, but geochemical anomalies in samples from these areas and rocks that host known epithermal Hg-rich vein deposits in southwest Alaska are similar to that in these eight areas. AREA 3 Four sample sites delineate area 3 on the western side of the Horn Mountains (fig. 3). Bedrock geology in this area consists of mafic to felsic volcanic rocks and minor sedimentary rocks of the Kuskokwim Group that are adja- cent to the Horn Mountains pluton (fig. 2). Anomalous stream-sediment samples contain as much as 1.7 pprn Hg, 120 pprn As, and 7 pprn W; a heavy-mineral-concentrate sample contains 500 pprn Au, 20 pprn Ag, microscopically
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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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Page 51 and 52: Table 1. Conodont faunules and lith
Page 55 and 56: DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
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Page 97 and 98: GOLD AND CINNABAR IN HEAVY-MINERAL
Page 107 and 108: EARLY CENOZOIC DEPOSITIONAL SYSTEMS
Page 111 and 112: EARLY CENOZOIC DEPOSITIONAL S YSTEM
Page 125 and 126: RESERVOIR FRAMEWORK ARCHITECTURE, C
Page 137: 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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Table 3. Summary of geochemical sig
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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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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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