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U.S. GEOLOGICAL SURVEY OPEN-FILE REPORT 2004-1371

Whole Rock Geochemical Data For Paleozoic Sedimentary Rocks of the Western Brooks Range, Alaska

John F. Slack,1 Jeanine M. Schmidt,2 and Julie A. Dumoulin2


1 U.S. Geological Survey, National Center, MS 954, Reston, VA 20192
2U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508

INTRODUCTION

This report presents geochemical analyses for 210 unaltered and unmineralized rock samples of Paleozoic age in the western Brooks Range of northern Alaska. These data form the basis for a study by Slack and others (2004a) on the provenance and depositional history of the Paleozoic strata, and on their metallogenic significance relative to the formation of large stratabound Zn-Pb-Ag deposits in the Red Dog mining district. Principal rock types that were analyzed include shale, siltstone, sandstone, chert, bedded siliceous rock, and calcareous radiolarite; selected samples of limestone, phosphate, and argillite, and one siderite concretion, were also analyzed. Note that the phosphate samples are from the central Brooks Range, east of the Red Dog district. The geochemical analyses are presented here in Microsoft Excel and .dbf (part of .zip file) formats in order to facilitate calculations and plotting of data. A related geochemical database on altered and mineralized rocks in the Red Dog Zn-Pb-Ag district is available in Slack and others (2004b).

SAMPLE DESCRIPTIONS

Brief lithologic descriptions of the samples are included in the spreadsheets. All sample types are common rocks except for bedded siliceous rock and calcareous radiolarite. The former lithology is a dark hard rock consisting mostly of fine-grained quartz with minor iron sulfides and hematite, which Slack and others (2004a) interpret as a mixed deposit of biogenic silica with an appreciable eolian component. Calcareous radiolarite is a type of fine-grained limestone composed mainly of radiolarians that have been replaced by calcite (Dumoulin and others, 2004).

SAMPLE PREPARATION

Geochemical analyses were obtained mostly on samples from natural outcrops; some samples are of diamond drill core 3.5 or 4.5 cm in diameter. Samples were cut using water-cooled diamond saws in order to remove oxidized and(or) weathered surfaces. A small number of samples contain veins composed of quartz with or without associated carbonate and sulfides; these veins were also cut out before analysis. All samples were pulverized in an alumina ceramic mortar, which in some cases may have produced very minor contamination by trace amounts of Al, Ba, and(or) rare earth elements (REE).

ANALYTICAL METHODS

Prior to analysis all samples were fused with lithium metaborate/tetraborate to insure nearly complete acid digestion of resistate minerals such as zircon, monazite, rutile, chromite, and barite. Most samples were analyzed by Activation Laboratories (ACT Labs) in Ancaster, Ontario, using methods described on their web site: www.actlabs.com. Major elements, most trace elements, and REE were determined by inductively-coupled plasma mass spectrometry (ICP-MS), using an approach similar to that of Jenner and others (1990). REE in some barite samples were analyzed by high-resolution, magnetic sector ICP-MS using an ion exchange technique in order to eliminate Ba interference on Eu. Volatiles and related components (total C, CO2, Corg, S, SO4) were determined using conventional methods as described in Jackson and others (1987). Fluorine was analyzed by the ion selective electrode technique (Jackson and others, 1987). Data for Sc, Cr, Co, Au, Sb, As, and Se in most samples were obtained by instrumental neutron activation analysis (INAA) (Hoffman, 1992), which provides more precise results than by ICP-MS. Au concentrations in most samples of semimassive and massive sulfide were also obtained by flame atomic absorption (Aruscavage and Crock, 1987) by XRAL Laboratories of Denver, CO, using on splits of the same rock powders that were earlier run for major and trace elements, and REE.

A smaller group of samples was analyzed by the U.S. Geological Survey in laboratories at Denver, Colorado, Reston, Virginia, and Menlo Park, California. Major elements were determined by X-ray fluorescence using the methods of Taggart and others (1987). Concentrations of volatiles and fluorine were determined by the same procedures as outlined above, except that separate determinations of organic carbon were not made. Where data for CO2 are available, organic carbon contents were calculated by percent difference using results for total carbon. Minor elements were analyzed by inductively coupled plasma-atomic emission spectometry (Lichte and others, 1987), except for Rb, Cs, Ba, Zr, Hf, Ta, Th, U, Sc, Cr, Co, Ni, Mo, Zn, Au, Sb, and As and REE (La, Ce, Nd, Sm, Eu, Tb, Yb, Lu) that were determined by INAA (Baedecker and McKown, 1987).

Multiple standards were analyzed together with the submitted rock samples. Analyses by ACT Labs included data on 8 to 10 compositionally different standards with well-defined elemental concentrations. In addition to these standards, analyses were routinely obtained on duplicate samples and Ohio black shale SDO-1 (http://minerals.cr.usgs.gov/geochem/ohioshale.html). Precision and accuracy for concentrations ≥100× the minimum detection limit (MDL) was generally better than ±5 percent relative, and in many cases such as for major elements was better than ±1 percent relative. For concentrations approximately 10× the MDL, precision and accuracy were about ±10–20 percent relative depending on the method used. Similar procedures were used for the analyses by the U.S. Geological Survey, with comparable results for precision and accuracy.

DATA PRESENTATION

Data for elements, oxides, and other components are presented either in weight percent or parts per million, except for Au and Pt that are in parts per billion. Qualified values (shown by the “<” symbol) represent values less than the specified MDL. In some cases, the MDL for a particular element or component is not uniform, which reflects changing analytical conditions or matrix effects, or use of newer ICP-MS instruments that have higher precision and lower MDLs. For statistical treatment of data and other calculations, it is recommended that qualified values be substituted by one-half the analytical detection limit (Sanford and others, 1993). Note that the abbreviation “n.a.” refers to a lack of analysis for the specified element or component.

Values for the magnitude of the Ce and Eu anomalies are also presented. The magnitude of the Ce anomaly, Ce/Ce*, is calculated as: CeCN/((LaCN)0.667*(NdCN)0.333) where CN represents normalization of Ce, La, and Nd to average chondrites using the data of Nakamura (1974). For the majority of samples for which ICP-MS data are available for all REE, the magnitude of the Eu anomaly, Eu/Eu*, is calculated as: EuCN/(SmCN*GdCN)0.5 with chondrite normalization of Eu, Sm, and Gd. For those samples without ICP-MS analyses for Gd, the Eu/Eu* values were calculated by a curve-fitting Excelmacro program using INAA data for La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu.

REFERENCES CITED

Aruscavage, P.J., and Crock, J.G., 1987, Atomic absorption methods: U.S. Geological Survey Bulletin 1770, p. C1–C6.

Baedecker, P.A., and McKown, D.M., 1987, Instrumental neutron activation analysis of geochemical samples: U.S. Geological Survey Bulletin 1770, p. H1–H14.

Dumoulin, J.A., Harris, A.G., Blome, C.D., and Young, L.E., 2004, Depositional settings, correlation, and age of Carboniferous rocks in the western Brooks Range, Alaska: Economic Geology, v. 99 (in press).

Hoffman, E.L., 1992, Instrumental neutron activation in geoanalysis: Journal of Geochemical Exploration, v. 44, p. 297–319.

Jackson, L.L., Brown, F.W., and Neil, S.T., 1987, Major and minor elements requiring individual determination, classical whole rock analysis, and rapid rock analysis: U.S. Geological Survey Bulletin 1770, p. G1–G23.

Jenner, G.A., Longerich, H.P., Jackson, S.E., and Fryer, B.J., 1990, ICP-MS: A powerful tool for high-precision trace-element analysis in earth sciences: Evidence from analysis of selected U.S.G.S. reference samples: Chemical Geology, v. 83, p. 133–148.

Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, Inductively coupled plasma-atomic emission spectrometry: U.S. Geological Survey Bulletin 1770, p. B1–B10.

Nakamura, N., 1974, Determination of REE, Ba, Fe, Mg, Na, and K in carbonaceous and ordinary chondrites: Geochimica et Cosmochimica Acta, v. 38, p. 757–775.

Sanford, R.F., Pierson, C.T., and Crovelli, R.A., 1993, An objective replacement method for censored geochemical data: Mathematical Geology, v. 25, p. 59–80.

Slack, J.F., Dumoulin, J.A., Schmidt, J.M., Young, L.E., and Rombach, C.S, 2004a, Paleozoic sedimentary rocks in the Red Dog Zn-Pb-Ag district and vicinity, western Brooks Range, Alaska: Provenance, deposition, and metallogenic significance: Economic Geology, v. 99 (in press).

Slack, J.F., Kelley, K.D., and Clark, J.L., 2004b, Whole rock geochemical data for altered and mineralized rocks, Red Dog Zn-Pb-Ag district, western Brooks Range, Alaska: U.S. Geological Survey Open-File Report 2004-1372.

Taggart, J.E., Jr., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., and Stewart, K.C., 1987, Analysis of geologic materials by wavelength-dispersive X-ray fluorescence spectrometry: U.S. Geological Survey Bulletin 1770, p. E1–E19.



U.S. Department of the Interior, U.S. Geological Survey
URL: http://pubs.usgs.gov/of/2004/1371/2004-1371.html
For more information, contact John F. Slack
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