Whole Rock Major and Trace Element Chemistry for Igneous and Metamorphic Rocks from the Yukon-Tanana Upland, Alaska

Methods used for sample preparation of geochemistry analyses by ALS between 2014 and 2020. Prior to crushing, samples for whole-rock analysis were trimmed by USGS staff to remove weathering or heterogeneities, and cut surfaces were lapped to remove any saw metal. Sample material is crushed to >70% passing 2 mm, and a 250 g split is then pulverized in low-chrome steel grinding mills to >85% passing 75 microns, with barren material passing through pulverizers between each sample. Approximately 3-5g aliquots of USGS standard reference materials (AGV2 - andesite, BHVO2 – basalt, DNC1 - dolerite, DTS2b - dunite, GSP2 - granodiorite, QLO1 - quartz latite, W-2 - diabase), plus two internal standards (12AJJ39E, 12AJJ46A) were submitted as unknown samples to monitor accuracy, and replicates of seven of these (AGV2, BHVO2, DNC1, GSP2, and W2, plus the two internal standards) were run in different sessions to monitor laboratory reproducibility. 'Field_Name' in file "geology_Data_Dictionary_Geochem_Data_YTT_Todd_Alaska_2022.csv" provides element (or element oxide), unit (% versus ppm/ppb), and a subscript composite indicating the analytical method. The method subscript begins with "ME_" and is followed by a method code, which refers to analytical packages, each with varying decomposition and analytical techniques, and are described as subsequent Process Steps.

"XRF26" – A calcined or ignited sample (0.9 g) is added to 9.0g of lithium borate Flux (50 % - 50 % Li2B4O7 - LiBO2), mixed well and fused in an auto fluxer between 1050 - 1100°C. A flat molten glass disc is prepared from the resulting melt. This disc is then analyzed by X-ray fluorescence (XRF) spectrometry.

"OA_GRA05x" – The total oxide content is determined from the XRF analyte concentrations and loss on Ignition (L.O.I.) values. A prepared sample (1.0 g) is placed in an oven at 1000°C for one hour, cooled and then weighed. The percent loss on ignition is calculated from the difference in weight.

"ICP06" – A prepared sample (0.200 g) is added to lithium metaborate/lithium tetraborate (LiBO2 /Li2B4O7) flux (0.90 g), mixed well and fused in a furnace at 1000°C. The resulting melt is then cooled and dissolved in 100 mL of 4% nitric acid/2% hydrochloric acid. This solution is then analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and the results are corrected for spectral inter-element interferences. Oxide concentration is calculated from the determined elemental concentration and the result is reported in that format.

"GRA05" - The total oxide content is determined from the ICP analyte concentrations and loss on ignition (L.O.I.) values. A prepared sample (1.0 g) is placed in an oven at 1000°C for one hour, cooled and then weighed. The percent loss on ignition is calculated from the difference in weight.

"MS81" - A prepared sample (0.200 g) is added to lithium metaborate flux (0.90 g), mixed well, and fused in a furnace at 1000°C. The resulting melt is then cooled and dissolved in 100 mL of 4% HNO3 / 2% HCl3 solution. This solution is then analyzed by inductively coupled plasma - mass spectrometry (ICP-MS).

"4ACD81" - Prepared sample (0.25 g) is digested by four (perchloric, nitric, hydrofluoric, and hydrochloric) acids, suitable for the determination of base metals since many sulfides and some metal oxides are only partially decomposed by the borate fusion and some elements such as cadmium and zinc can be volatilized. Not suitable for mercury determination (Aqua Regia digest preferred). This solution is then analyzed by ICP-MS.

"MS42" - Prepared sample (0.25 g) is digested with Aqua Regia (nitric and hydrochloric acids), suitable for the determination of most base metals since many sulfides and some metal oxides are only partially decomposed by the borate fusion and some elements such as cadmium and zinc can be volatilized. Not suitable for nickel and cobalt determination (four acid digest preferred). This solution is then analyzed by ICP-MS.

"MS61" – Prepared sample (0.25g) is digested with HNO₃-HClO₄-HF, and an HCl leach, dissolving nearly all minerals in most geological samples. The resulting salt is dissolved in 100 mL of 4% HNO3 / 2% HCl3 solution. This solution is then analyzed by inductively coupled plasma - mass spectrometry (ICP-MS).

"MS61L" - Same as MS61 (Process Step 9), but with improved lower detection limits.

"ICP21" - Au by fire assay and ICP-AES, 30g nominal sample weight.

"MS25NS" - Nickel sulphide collection fire assay and ICP-MS finish. 30g nominal sample weight.

"XRF05" - A finely ground sample powder (10 g minimum) is mixed with a few drops of liquid binder (Polyvinyl Alcohol) and then transferred into an aluminum cap. The sample is subsequently compressed under approximately 30 ton/in^2 in a pellet press. After pressing, the pellet is dried to remove the solvent and analyzed by WDXRF spectrometry.

"OG62" - NI by HF-HNO3-HClO4 digestion (0.4g sample) with HCl leach, ICP-AES finish; run by default when Ni_ppm_ME_MS41 determination is over limit.