Mineralogical and Chemical Data for Heavy-Mineral Concentrate Samples Collected in the Taylor Mountains Quadrangle, Alaska, 2004-2008

Sample Collection: A total of 819 pan concentrated stream-sediment samples were collected during a geological survey of the Taylor Mountains quadrangle, Alaska. About 10 percent of the samples are site duplicates collected for quality control. Samples are composites of active alluvium collected primarily from first- or second-order streams as shown on USGS 1:63,360-scale maps. Each composite sample was created by collecting sediment from several places at the sample site, generally along a 30-ft. stretch of the channel and combining the increments to form a single sample for the site. When possible, moderately coarse gravel material was selected from areas where heavy minerals tend to drop out of the actively flowing stream water, such as at point bars or inside curves in the stream course. Sediment was wet-sieved through a 10-mesh (2-mm) screen until a 14-inch stainless steel or plastic pan was full. The material was then carefully panned until about 100 grams of the heaviest material was left in the pan. The panned material was swirled in the pan until a heavy mineral trail developed. This trail was examined with a 10X hand-lens for minerals of interest, such as gold or cinnabar. The panned material was saved in 4-mil plastic zip-loc bags for processing at the USGS laboratory in Denver, Co.

Laboratory preparation of the heavy mineral concentrate sample: In the laboratory, the panned samples were air dried and then sieved through a 30-mesh (0.60-mm) sieve. The minus-30 mesh fraction was further separated using a heavy liquid, bromoform, to remove the remaining lighter minerals having a specific gravity of 2.85 or less. A Frantz Isodynamic Separator was used to split the concentrate samples into two mineral phases; a relatively magnetic phase containing magnetite, ilmenite, garnet, amphibole, pyroxene, epidote, and other iron-rich silicates, and, a relatively nonmagnetic phase containing most ore-related and low-iron oxide and silicate minerals. Refer to https://www.usgs.gov/energy-and-minerals/mineral-resources-program/science/analytical-chemistry/ for links to laboratory processes.

Mineralogical Analysis of the non-magnetic fraction of the heavy mineral concentrate sample: The non-magnetic fractions of the heavy mineral concentrate samples were scanned visually under a ring-light illuminated binocular microscope using as much as 56X magnification. Shortwave ultraviolet illumination was also used to identify scheelite and powellite. In most cases, the mineral grains could be identified from their physical properties, but occasionally X-ray diffraction or chemical analysis was used to confirm some mineral identifications. Minerals of primary interest include gold, cinnabar, monazite, barite, cassiterite, scheelite, and iron sulfides such as pyrite, arsenopyrite, and chalcopyrite.

Geochemical Analysis of the non-magnetic fraction of the heavy mineral concentrate sample: A subset of 93 of the nonmagnetic heavy mineral concentrate samples were analyzed for 55 major, rare-earth, and trace elements by inductively coupled plasma-atomic emission spectrometry-mass spectrometry (ICP-AES-MS) using a modification of Meier and Slowik (2002) (https://www.usgs.gov/media/files/60-elements-icp-oes-ms-na2o-fusion-method; U.S. Geological Survey, 2010). A 0.10 g aliquot of the non-magnetic fraction of heavy mineral concentrate sample was decomposed using a sodium peroxide sinter heated to 450°C. The resultant cake was leached with water and acidified with nitric acid. After an addition of tartaric acid, aliquots of the digested samples were aspirated into the ICP-AES-MS. Data were considered acceptable if recovery for all 55 elements was plus or minus 15 percent at five times the lower reporting limit and the calculated relative standard deviation (RSD) of duplicate sample analysis was no greater than 15 percent.