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U.S. GEOLOGICAL SURVEY
Open-File Report 2007–1126

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Sample Preparation and Analysis

Samples were prepared for analysis at the USGS laboratories in Denver, Colorado. Rock samples were crushed in a jaw crusher, then ground in a vertical pulverizer with ceramic plates to less than 100 mesh (less than 150 microns), and followed by thorough mixing to ensure homogeneity. Additional details of the sample preparation procedures are available in Taylor (1990) and Taylor and Theodorakos (1996). Approximately one-third of the samples were analyzed by the USGS laboratory in Denver, Colorado. The rest of the samples were analyzed by XRAL Laboratories in Don Mills, Ontario, Canada, under contract to the USGS. Rock reference materials and a series of multi-element solution standards were used for calibration and quality-assurance and quality-control (QA/QC) purposes. Detailed descriptions of QA/QC procedures are available in Arbogast (1990) and Taggart (2002).

Analytical Methods

ICP-AES 40-Element (ICP40)

A 0.2-gram portion of each sample was decomposed by using a mixture of hydrochloric, nitric, perchloric, and hydrofluoric acids at low temperature. Digested samples were analyzed for 40 major, minor, and trace elements simultaneously by inductively coupled plasma atomic emission spectrometry (ICP-AES) (Briggs, 1996). Data were considered acceptable if recovery for all 40 elements is plus or minus 15 percent at 5 times the lower reporting limit and the calculated Relative Standard Deviation (RSD) of duplicate samples is no greater than 15 percent. Lower reporting limits are shown in table 1.

ICP-AES 10-Element (ICP10)

All samples were analyzed for 10 loosely bound trace metals (Ag, As, Au, Bi, Cd, Cu, Mo, Pb, Sb, and Zn) by using ICP-AES (table 2). A 1.0-gram sample was partially dissolved with a hydrochloric acid-hydrogen peroxide solution followed by extraction of organic halides with a 10-percent aliquot 336-diisobutylketone (DIBK) solution. Organic halide solutions were then analyzed by using ICP-AES (Motooka, 1996). Lower reporting limits for samples analyzed by the USGS changed between 1993 and 1994 for this method. The higher cutoff for most elements analyzed after 1993 reflect changes in laboratory methodology and instrumentation. In a few cases samples were of insufficient quantity to provide data within the range of the detection limits given (D. Detra, oral commun., 2006). These samples have even higher detection limits and are highlighted in bold red in the data table.

Gold by Fire Assay (FA)

All samples were analyzed for gold (Au) by fire assay. For each sample a 10-gram aliquot was fused with a flux at high temperature to produce a dore bead. Dore beads were then digested with aqua regia, diluted, and analyzed by using directly coupled plasma (DCP) or atomic absorption (AA) spectrometry (O’Leary and Meier, 1996). Samples analyzed by the USGS laboratory had a lower reporting limit of 0.002 ppm for those analyzed by AA and 0.05 ppm for those analyzed by DCP. The lower reporting limit was 0.005 ppm for samples analyzed by XRAL (D. Detra, oral commun., 2006).

Mercury by Cold Vapor Atomic Absorption Spectrometry (CVAA)

All samples were analyzed for mercury (Hg) by digestion of 0.1 gram of sample in a mixture of nitric and hydrochloric acids. Addition of potassium permanganate, sulphuric acid, and potassium persulphate, followed by a sodium-chloride-hydroxylamine solution and dilution completed the sample preparation. Mercury concentrations were determined by using a Perkin-Elmer Flow Injection Mercury System, FIMS-100. The lower reporting limit was 0.006 ppm for samples analyzed by the USGS laboratory and 0.02 ppm for samples analyzed by XRAL (Brown and others, 1997).

Tellurium by Flame Atomic Absorption Spectrometry (AA)

A total of 179 samples collected during the 1993 field season were analyzed for tellurium at the USGS laboratory in Denver. Digestion of 2.0 grams of sample was accomplished by using a mixture of hydrofluoric and sulfuric acids with heat. The residue was treated with hydrocloric acid and hydrogen peroxide. Tellurium was selectively extracted as an organic halide that was subsequently analyzed by flame atomic absorption spectrometry. The lower reporting limit was 0.1 ppm (O’Leary, 1996).

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