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Lithogeochemistry and images of selected drill core samples from the Mineral Hill alkaline complex, Wyoming

April 2, 2021

This data release contains analytical data and images for a suite of drill core samples from the Mineral Hill alkaline complex (MHAC), northeastern Wyoming. Geochemistry data include major and trace element analytical results for 103 alkaline igneous rock samples. Images include hand sample photographs of halved core and full thin section images captured in transmitted, plane-polarized and cross-polarized light. Samples are from two core holes drilled by Humble Oil Company in 1970, and subsequently acquired and stored by Felix Mutschler at Eastern Washington University. The two skeletonized (incomplete) drill cores were acquired by the U.S. Geological Survey Geology, Minerals, Energy, and Geophysics Spokane office in 2019, through a co-operative agreement with Eastern Washington University. The collection contains full-core pieces from select intervals representative of the different lithologies and textures in the complex. Mineral Hill is a Paleogene-age, ring-shaped, multi-phase alkaline (miaskitic) igneous complex that intrudes Precambrian schists and lower Paleozoic units of the Tinton uplift on the northwest flank of the broader Black Hills uplift (Welch, 1974; Ray, 1979). The outer ring of the complex is composed of foid syenite and compositionally similar rocks, whereas clinopyroxenite (jacupirangite) and other foid-bearing mafic rocks (ijolite and melteigite) form an irregular inner ring. The center of the complex is composed of a central feldspathic diatreme breccia pipe that exhibits pervasive potassic and sulfidic alteration (DeWitt and others, 1986). Alkali feldspar trachyte/latite porphyry is exposed as large masses in a steep-walled, columnar-jointed zone peripheral to foid syenite of the Mineral Hill dome and as high-angle sills within the central area (Ray, 1979). Alkalic lamprophyres throughout the complex are thought to be related to clinopyroxenite and may have formed from a volatile-rich fraction of the pyroxenite magma (Ray 1979). Pseudoleucite porphyry and melteigite/feldspathic ijolite may be variations of foid syenite and foid clinopyroxenite, respectively. Historic mineral production in the area occurred between the 1870s and 1930s: principally alluvial gold, gold and silver at the Treadwell Mine, and gold and copper at the Interocean Mine. Exploration since 2000 has shown that gold and silver at the Treadwell Mine is related to epithermal-style mineralization and adularia-bearing potassic alteration (Eurasian Minerals Inc., 2016). Higher temperature, porphyry-style potassic alteration associated with gold and copper was identified near the Interocean Mine. Investigations of critical mineral potential in the area have focused on Precambrian pegmatites in the Tinton district which produced some cassiterite (Sn) and tantalite-columbite (Ta) ore, and the Spotted Tail and Sand Creek gold placers which produced some cassiterite (Sn) (Smith and Page, 1941; Hausel, 1990). The critical mineral potential of Paleogene intrusive rocks of the Mineral Hill complex has received comparatively little attention. All samples were analyzed by the U.S. Geological Survey contract laboratory, AGAT Laboratories. Major and trace element concentrations were determined by wavelength dispersive X-ray fluorescence, inductively coupled plasma-optical emission spectrometry or inductively coupled plasma-mass spectrometry (ICP-MS). Additional analytical methods included determination of carbonate carbon by combustion and infrared detection (IR); gold, platinum and palladium by lead fusion fire assay and ICP-MS; fluoride by ion-selective electrode; ferrous iron by titration; non-essential and essential water by gravimetric methods; total sulfur by IR; and gold and platinum group elements by nickel sulfide fire-assay followed by instrumental neutron activation analysis. For each method outlined above, an acceptable criteria for the data has been identified based on 1) if recovery of each element is within a designated percentage at five times the lower limit of determination and 2) the calculated relative standard deviation of duplicate samples is no greater than that percentage. Data are reported in a comma-separated values (CSV) file (Mineral_Hill_Drill_Hole_Geochemistry) that lists the samples that were analyzed, drill hole identification number, depth from drill hole collar, latitude/longitude location information, a designation for those samples with accompanying hand sample and thin section images, and brief lithologic descriptions of the samples. All column headings and abbreviations are explained in the accompanying data dictionary (Mineral_Hill_Drill_Hole_Geochemistry_DataDictionary). Hand sample and thin section images are provided in separate folders for each drill hole. References cited: DeWitt, E., Redden, J.A., Wilson, A.B., Buscher, D., Dersch, J.S., 1986, Mineral resource potential and geology of the Black Hills National Forest, South Dakota and Wyoming: U.S. Geological Survey Bulletin 1580, 135 p. Eurasian Minerals Inc., 2016, Eurasian Minerals Options Mineral Hill Property to Coeur Explorations, Inc., Eurasian Minerals Inc. News Release, October 27, 2016, Accessed January 11, 2021 at https://www.emxroyalty.com/site/assets/files/3675/2016-10-27-nr-emx-m77…. Hausel, W.D., 1990, The Treadwell Gold-Silver Mine, Mineral Hill District, Crook County, Wyoming: Geological Survey of Wyoming Mineral Report 90-6, 4 p. Holwell, D.A., Fiorentini, M., McDonald, I., Lu, Y., Giuliani, A., Smith, D.J., Keith, M., and Locmelis, M., 2019, A metasomatized lithospheric mantle control on the metallogenic signature of post-subduction magmatism: Nature Communications, v. 10, no. 3511, 10 p. Ray, J.T., 1979, Petrology of the Cenozoic igneous rocks of the Tinton district, Black Hills, South Dakota - Wyoming: Grand Forks, University of North Dakota, M.S. Thesis, 115 p. Smith, W.C., and Page, L.R., 1941, Tin-bearing pegmatites of the Tinton district, Lawrence County, South Dakota -- a preliminary report: U.S. Geological Survey Bulletin 922-T, p. 595-630. Welch, C.M., 1974, A preliminary report on the geology of the Mineral Hill area, Crook County, Wyoming: Rapid City, South Dakota School of Mines and Technology, M.S. Thesis, 83 p. Werle, J.L., Ikramuddin, M., and Mutschler, F.E., 1984, Allard stock, La Plata Mountains, Colorado - an alkaline rock-hosted porphyry copper -- precious metal deposit: Canadian Journal of Earth Sciences, v. 21, p. 630-641.