Economic Geology, v. 113, no. 2, pp. 489–510 Application of Imaging Spectroscopy for Mineral Exploration in Alaska: A Study over Porphyry Cu Deposits in the Eastern Alaska Range Garth E. Graham,1,† Raymond F. Kokaly,2 Karen D. Kelley,1 Todd M. Hoefen,2 Michaela R. Johnson,2 and Bernard E. Hubbard3 1 U.S. Geological Survey, PO Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225 2 U.S. Geological Survey, PO Box 25046, MS 964, Denver Federal Center, Denver, Colorado 80225 3 U.S. Geological Survey, 12201 Sunrise Valley Drive, MS 954, Reston, Virginia 20192 Abstract The U.S. Geological Survey tested the utility of imaging spectroscopy (also referred to as hyperspectral remote sensing) as an aid to regional mineral exploration efforts in remote parts of Alaska. Airborne imaging spectrom- eter data were collected in 2014 over unmined porphyry Cu deposits in the eastern Alaska Range using the HyMap™ sensor. Maps of the distributions of predominant minerals, made by matching reflectance signatures in the remotely sensed data to reference spectra in the shortwave infrared region, do not uniquely discriminate individual rock units. However, they do highlight hydrothermal alteration associated with porphyry deposits and prospects hosted mostly within the Nabesna pluton. In and around porphyry Cu deposits at Orange Hill and Bond Creek, unique spectral signatures are related to variations in chlorite and white mica abundance and their chemi- cal composition. This is best revealed in the longer-wavelength 2,200-nm Al-OH absorption feature positions in pixels spectrally dominated by white mica proximal to porphyry deposits. Similar spectral signatures of chlorite and white mica wavelength positions were also recognized away from the porphyry deposits; follow-up sampling identified these satellite areas to also contain Cu-Mo-Au mineralized rock. Our study confirms that airborne imag- ing spectroscopy has application for regional mineral exploration in exposed mountainous terrain in Alaska. Introduction remain undeveloped. It is likely that additional economic Imaging spectroscopy (also referred to as hyperspectral mineral resources will be discovered, and this drives contin- remote sensing) is a technology that has been utilized with ued exploration activities across the state (e.g., Freeman et al., success for mapping vegetation and mineral abundances over 2015; Athey et al., 2016). However, the vast size, remoteness, many areas of the Earth’s surface (e.g., Clark et al., 2003; and rugged terrain, coupled with a relatively short summer Kokaly et al., 2009, 2013). Spectroscopy can be used to iden- field season, hamper these efforts. New methods like remote tify certain minerals based on their electronic and vibrational imaging spectroscopy (hyperspectral surveying) could help absorptions in the reflected solar range (400–2,500 nm). assess large tracts of land and provide focused target areas for While not effective in identifying many rock-forming miner- ground-based mineral exploration. als such as quartz, feldspars, and pyroxenes, the shortwave The U.S. Geological Survey (USGS) conducted an airborne infrared (SWIR) region (2,000–2,500 nm) is key to identify- imaging spectroscopy study using the HyMap sensor to test this ing carbonates and hydrous minerals (e.g., micas and clays; technology for mineral exploration at high latitudes in Alaska. Clark, 1999; Thompson et al., 1999; Swayze et al., 2014) that The selected study area extends from the Nabesna Glacier to are often products of hydrothermal alteration. Aircraft- or the Canadian border in Wrangell-St. Elias National Park, east- spacecraft-borne platforms are particularly effective for rec- ern Alaska Range (Fig. 1). This area was selected because (1) ognizing the spatial distributions of these minerals associated the region has reconnaissance-level geologic control based on with important mineral deposit types (Crosta et al., 1998; van historical USGS mapping (e.g., Richter, 1973; Richter et al., der Meer, 2006; Hubbard et al., 2007; Bedini, 2012; van Ruit- 1975a), (2) much of the uplands contain bedrock exposure for enbeek et al., 2012; Mars, 2013). Most published studies have effective spectral mapping, and (3) the area includes several focused on areas at midlatitudes, with the technology infre- undisturbed porphyry Cu deposits and prospects. Porphyry quently applied at higher latitudes because data collection deposits are typically characterized by broad alteration zones and analysis are more difficult due to short collection seasons with distinctive mineralogy (e.g., Lowell and Guilbert, 1970; and low sun angle resulting in poor solar illumination. Seedorff et al., 2005; Sillitoe, 2010), including mineral(s) that Alaska is well known for its precious and base metal mines can be remotely sensed using spectroscopy (e.g., Berger et and mineral deposits. Major mines include the Pogo, Fort al., 2003; John et al., 2010; Mars, 2013). These factors, along Knox, and Kensington Au deposits, the Greens Creek Ag- with reconnaissance ground sampling completed as part of this Au-Pb-Zn volcanogenic massive sulfide deposit, and the Red project, form a framework against which our imaging spectros- Dog Zn-Pb-Ag deposit. World-class deposits, including the copy survey results can be compared and validated. Donlin Creek Au and the giant Pebble porphyry Cu-Au-Mo, In this paper, we present mapping results, including spec- trally predominant mineral and mineral composition variabil- † Corresponding author: e-mail, [email protected] ity maps from the western part of the survey area that contains © 2018 Gold Open Access: this paper is published under the terms of the CC-BY license. ISSN 0361-0128; doi: 10.5382/econgeo.2018.4559; 22 p. Submitted: May 16, 2017 Supplementary files available online. 489 Accepted: November 7, 2017 Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 490 GRAHAM ET AL. Alask Yukon Ti ell fault Chistochina Continental ntina fault A B arew Chisna i-F a backstop al e Den Nutzotin Baulto Totschunda fault Horsfeld Alaska alkeetna fault Yukon Alaska Rang T Wrangellia Wr Northway angell M Orange Hill - Bond Creek area Canada ountains DezadeashD Study area Kahiltna t Accretionary Anchorage Anchorage Complex Alexander Yukon Neacola BC t Undifferentiated Klukwan ar Jurassic-Cretaceous Pebble Peninsular rder Ranges faul flysch basins Lake Clark faul Bo C Glacier Tp Glacier Glacier Porphyry deposit Kg 62°15'N Kg + prospect area Glacier East Fork Kg Bond Creek 5 Kg Middle NE drainages 4 Bond C F Glacier r or 3 eek Kg k Glacier California Kg Gulch Glacier 1 6 Trn 2 Kg Nabesna pluton Pl W e Trn Pl s Wrangellia terrane t F Pl o r k B Trn o n Glacier d Glacier y Pl C N N i r k Pl e o PIPv e PIPv n k d W 62°10' a est C r e Fo e PIPv k rk Bond Cree Pl Nabesna Glacier valle Glacier Nabesna Pl PIPv Survey boundary PIPv Glacier k 4km 142°50'W 142°42’W 142°34’W Geology Deposits/Prospects Covered (glacier, till, Nikolai Greenstone (Trn) 1 Orange Hill Cu-Au-Mo 4 Neil prospect vegetation) Tertiary porphyry (Tp) Metasediments and limestone (Pl) 2 Bond Creek Cu-Au-Mo 5 Unnamed Mo prospect Strongly altered rock at Volcanic and volcaniclastic rocks 3 Nike prospect 6 Copper King skarn Bond Creek (from Richter, 1973) Wrangellia terrane (PIPv) Nabesna pluton (Kg) Fig. 1. Geologic framework of the Orange Hill-Bond Creek study area. A) Location of study area with box outlining area shown in B. B) Generalized terrane map showing flysch basins and relationship to Wrangellia, modified from Goldfarb et al. (2013). C) General geology of the study area. Geology modified from Richter (1973) and Wilson et al. (2015). Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 IMAGING SPECTROSCOPY OF PORPHYRY Cu DEPOSITS, EASTERN ALASKA RANGE 491 the largest exposed porphyry Cu-Au-Mo deposits—Orange The geology of the study area features Early Cretaceous Hill and Bond Creek. We examine whether geologic units rocks of the Nabesna pluton to the north that intrude Wrangel- can be distinguished based on mineral predominance map- lia terrane to the south (Fig. 1C). A number of geologic inves- ping and whether the porphyry deposits can be identified. We tigations were completed in this area (e.g., Mendenhall and establish that different chlorite and/or white mica spectral Schrader, 1903; Moffit and Knopf, 1910; Pilgrim, 1930; Moffit signatures are directly related to mineral chemistry and that and Wayland, 1943; Gillespie, 1970; Linn, 1973; Richter, 1973). these differences are useful for identifying known as well as The most comprehensive mapping was completed by Richter previously unknown Cu-Au-Mo–mineralized areas. Finally, (1973), from which the following geologic descriptions of the we discuss utilization of the technology in other portions of plutonic and arc rocks are derived. The Nabesna pluton consists the state of Alaska and in other northern latitude areas. predominantly of biotite-hornblende granodiorite with lesser quartz monzonite, biotite quartz diorite, and hornblende dio- Regional Geology rite. There are sparse exposures of medium- to coarse-grained The geology of the southern margin of Alaska records con- trondhjemite and light-gray quartz-plagioclase porphyry. To tinental growth. The Wrangellia superterrane (composed of the south of the roughly E striking contact, the Wrangellia ter- the Peninsular, Alexander, and Wrangellia island arc terranes; rane rocks include volcanic and volcaniclastic rocks, limestone, Fig. 1B) was accreted to the paleocontinental margin in the siltstone, and the Nikolai Greenstone. The Pennsylvanian and/ Jurassic to Cretaceous (Plafker and Berg, 1994). Sediments or Permian volcanic rocks include fragmental volcanic rocks, deposited into the intervening closing ocean basin are now tuffs, and andesite flows intruded by silicic quartz-eye effusive preserved in a discontinuously exposed belt of flysch basins, or shallow intrusive bodies.