Application of Imaging Spectroscopy for Mineral Exploration in Alaska: a Study Over Porphyry Cu Deposits in the Eastern Alaska Range

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 spectrometer 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 chemical 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 imaging 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 spectrally predominant mineral and mineral composition variabil- † Corresponding author: e-mail, [email protected] ity maps from the western part of the survey area that contains

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 N C i r k Pl e o PIPv e PIPv n k d W

62°10' N 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. Overlying the volcanic rocks are including the Nutzotin basin in the general study area (Fig. Permian limestone and Permian to Triassic calcareous and car- 1A, B; Berg et al., 1972; Plafker and Berg, 1994; Manuszak bonaceous siltstone and argillite; the carbonaceous argillite has et al., 2007; Goldfarb et al., 2013). The Nutzotin rocks onlap a basal silicic tuff unit and has locally been intruded by dikes, the Wrangellia terrane and extend northward to the Denali sills, and irregular bodies of Triassic gabbro (as much as 50% of fault, but their exposures pinch out to the west (Fig. 1B). the section). The Nikolai Greenstone comprises amygdaloidal The Denali fault displays net dextral displacement of ~350 basalt flows with thin interleaved volcaniclastic beds. Amyg- to 400 km presumed to be mostly of Cenozoic age and sepa- dules are filled with quartz, calcite, chlorite, epidote, pumpel- rates the Mesozoic rocks from Paleozoic and older Yukon lyite, zeolite minerals, and locally Cu. Tanana terrane rocks of ancestral North America (Plafker and Several Cu prospects and deposits have been identified, Berg, 1994). A number of Early Cretaceous plutons intruded mostly within the southwestern portion of the Nabesna pluton deformed Nutzotin flysch and the adjacent leading edge of (Fig. 1C; Mendenhall and Schrader, 1903; Moffit and Knopf, the exposed Wrangellia terrane presumably after the main 1910; Pilgrim, 1930; Van Alstine and Black, 1946; Linn, 1973; contractional event, as evidenced by their lack of deforma- Richter et al., 1975b, Hudson, 2003). The best described are tion. Recent U-Pb age data indicate emplacement between the Early Cretaceous Orange Hill and Bond Creek porphyry ~126.4 ± 1 and 113 ± 0.5 Ma (Graham et al., 2016), similar Cu-Mo(Au) deposits. The deposits and prospects are charac- to and slightly older than previous K-Ar and Ar-Ar cooling terized by distinctive limonite staining from surficial weath- ages (~120–105 Ma [K-Ar, Ar-Ar]; Richter et al., 1975a; Sny- ering of pyritic altered rock (Fig. 2A). Reported alteration der and Hart, 2007). Younger Cenozoic plutonic and volcanic zonation at Orange Hill includes a 400- × 2,000-m zone of rocks are locally abundant (Richter, 1973). The study area lies potassically altered rocks that contain biotite, quartz veinlets, just southwest of the Totschunda fault (a splay off the Denali minor K-feldspar, chlorite, and sericite and an outer 1,000- × fault) whose Pleistocene movement has disrupted the original 3,000-m zone of chlorite and minor sericite (Richter et al., configuration (Fig. 1B, C). 1975b). Much of this altered rock is not exposed, and zonation cannot be observed at the surface. Stockwork quartz Orange Hill-Bond Creek Study Area chalcopyrite-pyrite ± molybdenite veins are exposed up to The Orange Hill-Bond Creek study area, which encom- 1,400 m to the northeast (Fig. 2B, C). Other alteration miner- passes about 300 km2 (~115 mi2) of the original survey area, als include calcite and kaolinite (Van Alstine and Black, 1946). is located approximately 450 km east-northeast of Anchorage Alteration associated with the Bond Creek deposit is evident and 100 km south-southwest of Northway, Alaska (Fig. 1A). in both the Nabesna pluton and adjacent Wrangellia terrane The rugged terrane varies from approximately 900- to more volcanic arc rocks (Fig. 2D). Within the deposit, chalcopyrite than 2,750-m elevation above sea level. The western margin ± molybdenite-bearing quartz-rich and quartz-poor veins are of the study area is the Nabesna Glacier valley floor. Orange present (Fig. 2E). A central zone with abundant chlorite, minor Hill is a conspicuous iron-stained knob that rises several hun- biotite and K-feldspar, and spotty sericite is enveloped in a dred feet above the eastern side of the valley. It is separated 2,000- × 3,000-m zone with minor chlorite, epidote, and anhy- from the higher ridge to the east by California Gulch (Figs. drite (Richter et al., 1975b). Both the Orange Hill and Bond 1C, 2A). To the east and north of the higher ridge are the Creek deposits are cut by late anhydrite veins (now gypsum). west, middle, and east forks of Bond Creek, which flow north Historical non-NI 43-101 compliant estimates at Orange and west. Vegetation, including trees, scrub brush, and tun- Hill vary significantly from 86 Mt at ~0.3% Cu and 0.015% dra, is present in the glacial valley and at Orange Hill, giving Mo (Linn, 1973) to 320 Mt of rock 0.35% and 0.02% Mo way to sparsely vegetated tundra at elevations above approxi- (Richter et al., 1975b). Richter et al. (1975b) estimated Bond mately 1,500 m. Outcrop, rubble crop, and talus predominate Creek to contain 500 Mt of rock with average grade of 0.3% at higher elevations where lichen cover is minimal. Cu and 0.02% Mo at Bond Creek. Both deposits contain

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 492 GRAHAM ET AL.

A BC

FGH

Fig. 2. Select photos from the Orange Hill-Bond Creek study area. A) View looking north at the Fe-stained California Gulch area, Orange Hill deposit. B) Stream cobble of quartz-chalcopyrite-molybdenite stockwork veining from north end of Orange Hill. C) Stockwork veining in diorite/granodiorite in gulch north of Orange Hill. D) View of the Bond Creek ridge. E) Stock- work quartz-chalcopyrite-pyrite veins at creek level, Bond Creek. F) Cirque in Nabesna pluton at head of the east fork of Bond Creek with minor iron staining (loc. 8 discussed in Results section). G) Pyritic fracture surfaces in glacial debris from cirque in F. H) Potassically altered selvage of quartz-sulfide veins from cirque in F.

locally elevated Au concentrations. Other prospects in the field area were collected over two days (July 14 and 21, 2014) area include porphyry prospects (e.g., Nike, Neil), skarn and coincided with the first of three approximately one-week- (Copper King), and polymetallic vein occurrences (Fig. 1C); long field work sessions, completed during the 2014 to 2016 a younger, ca. 22 Ma unnamed Mo occurrence lies northeast summer field seasons. Field spectroscopy was geared toward of Neil (Fig. 1C; Silberman et al., 1977). Limited descriptions collecting calibration site data for atmospheric correction of of these systems are available (Richter et al., 1975b; Hudson, airborne survey data and verifying and validating airborne 2003). Follow-up investigations based on our spectral results results. Geologic samples were collected at and around min- led to recognition of additional mineralized areas hosted in eral occurrences and used for laboratory-based imaging spec- the Nabesna pluton, including at the headwaters of the east troscopy and geochemical characterization to establish the fork of Bond Creek (Fig. 2F). Here, pyrite ± chalcopyrite and distribution of mineralized rock and improve sample density molybdenite in veins and fracture coatings in glacial debris within historical sediment geochemistry datasets. Chemistry were observed, sometimes with K-feldspar alteration halos from targeted sediment and soil sampling in particular pro- (Fig. 2G, H); these areas are discussed in the text. vides important information for interpreting possible relation- ships between imaging spectrometer data and mineralized Methods areas (both previously known and newly recognized) dis- Our study integrated airborne-, field-, and laboratory-based cussed in this paper. The following sections summarize salient spectroscopic data with field- and laboratory-based geologic aspects of methods. More detailed methods descriptions and studies. Airborne imaging spectrometer data in the Nabesna complete spectral and geochemical results can be found in

the Digital Appendix and in USGS data releases (Kokaly et al., data by comparing continuum-removed spectral features in 2017a, 2018; Graham et al., 2017; Hoefen et al., 2018). its reflectance spectrum to continuum-removed absorption features in reference spectra of minerals, vegetation, water, Spectral data collection and processing and other materials. Continuum removal is a technique to Approximately 1,000 line km of imagery were collected using isolate an absorption feature from background spectral varia- a HyMap sensor (Cocks et al., 1998) mounted on a modified tions (Clark and Roush, 1984). The reference spectra used Piper Navajo aircraft. The aircraft was flown at an altitude of in the MICA command file are available to the public in the approximately 5,050 m, resulting in average ground spatial USGS spectral library (Kokaly et al., 2017b). The MICA com- resolution of approximately 6 m. HyMap measured reflected mand file is provided in the Digital Appendix. Output min- sunlight in 126 narrow channels that cover the wavelengths eral classes from the MICA analysis were combined into the of 455 to 2,483 nm. The full set of airborne data, extending grouped classes depicted in mineral predominance maps (see beyond the focus area of this study, is available in Kokaly et Table A1). Representative spectra from the ground-calibrated al. (2017a). HyMap reflectance data for the major classes discussed in this Ground-based spectral collections were completed using study are included in Figure A1 of the Digital Appendix. an Analytical Spectral Devices FieldSpec® 4 standard resolu- The selection of reference spectra in the MICA command tion (ASD FS4) field spectrometer. The ASD FS4 measures file was guided by previously published applications of hyper- 2,151 channels that span the 350- to 2,500-nm reflected solar spectral data for mineral mapping (notably, Clark et al., 2003) range using three detectors. Reflectance data were collected and criteria relevant to this study, including the following: from four calibration sites (with coverage of 86–210 HyMap (1) likelihood a mineral would be present in great enough pixels) in broad alluvial-fluvial gravel bars that were minimally abundance to be detected at the pixel size of the imagery; vegetated and mostly lichen free; these calibration sites were (2) reliability in identifying the spectral features of a mineral used to calibrate flight lines (following procedures in Clark using HyMap, given its wavelength range and spectral resolu- et al., 2002; Kokaly et al., 2013; and Kokaly and Skidmore, tion; and (3) importance of a mineral to lithology and known 2015). Although the rock types in these areas were mixed and mineral resources for the areas covered by the 2014 HyMap varied at the fine-spatial scale, at the HyMap 6-m pixel scale survey. In analyzing the large-area hyperspectral coverage the calibration areas were spectrally homogeneous. of Afghanistan, Kokaly et al. (2013) established a set of min- Corescan’s Hyperspectral Core Imager Mark III™ imaging eral spectra relevant to the first two criteria. To that MICA spectrometer (www.corescan.com.au/services/the-corescan- command file, we added more reference spectra of chlorites system; hereafter referred to as Corescan imaging spectrom- (clinochlore and thuringite), clays (nontronite and hectorite), eter) was used to scan approximately 30 hand specimens from and topaz to address the third criterion. Finally, additional lin- the Orange Hill and Bond Creek porphyry Cu deposits during ear spectral mixtures of various combinations of white mica, the fall of 2015 in order to map the spectrally predominant clays, chlorites, and carbonates were computed and added to minerals and wavelength positions for white mica. Spatial the MICA command file (see Kokaly et al., 2018, for details resolution was approximately 0.5 mm. The Corescan imag- on the adaptation of the Afghanistan MICA command file). ing spectrometer measures 514 channels that span the 450- to Mineral composition mapping: In addition to mineral map- 2,500-nm wavelength range. ping, subtle changes in SWIR absorption feature positions can The wavelength and bandpass characteristics of each spec- provide evidence of compositional variations within mineral trometer used in this study were evaluated using a set of refer- species (Post and Noble, 1993; Swayze, 1997). Examples of ence materials in order to check and cross calibrate data from previous studies include spectral mapping of white mica Al different instruments. HyMap and ASD evaluations agreed composition and/or chlorite of varying compositions to char- with manufacturer-reported values for channel wavelength acterize metamorphic history (e.g., Duke, 1994) and geol- position and bandpass. Based on our analysis of measurements ogy and zoning about mineral deposits (e.g., Herrmann et of the reference materials, the reported channel positions of al., 2001; van Ruitenbeek et al., 2012; Harraden et al., 2013; the Corescan imaging spectrometer were adjusted, notably by Laakso et al., 2015). In our study, we identified pixels with shifting them ~1.4 nm to longer wavelength for channels in that had highest MICA fit to muscovite or illite in our HyMap the 2,200-nm region (for additional details on the wavelength and Corescan imaging spectrometer data and calculated the evaluation and adjustment see Hoefen et al., 2018). 2,200-nm Al-OH absorption feature for each of these pix- Reflectance conversion and predominant mineral classi- els. For our computations, we fit a parabola to three chan- fication: The HyMap data were converted from radiance to nels within the white mica 2,200-nm absorption feature (the reflectance using a multistep calibration process adapted from channel with a minimum in continuum-removed reflectance the procedures in Kokaly et al. (2013). Reflectance images and one channel on either side). The wavelength value of from HyMap and Corescan were processed using the Mate- the axis of symmetry from the fitted quadratic function was rial Identification and Characterization Algorithm (MICA), a used to model the central wavelength position of the feature. module of the USGS PRISM (Processing Routines in IDL The images of continuous values of wavelength position were for Spectroscopic Measurements) software (Kokaly, 2011). converted to classification images with classes in 1-nm incre- PRISM is a freely distributed software (https://speclab.cr.usgs. ments. Representative HyMap spectra for various white mica gov/spectral-lib.html#software) programmed in Interactive wavelength classes are shown in Figure A1 of the Digital Data Language (IDL; Harris Geospatial Solutions, Broom- Appendix. Four billets from three rock samples imaged with field, Colorado). The MICA analysis identifies the spectrally the Corescan imaging spectrometer were scanned to deter- dominant mineral(s) in each pixel of imaging spectrometer mine the wavelength positions of white mica across the billets.

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 494 GRAHAM ET AL.

These data were used to select locations for electron probe The mineral predominance map indicates complex lateral microanalysis (EPMA) of white mica chemistry in the respec- variations in spectral signatures in the study area (Fig. 3). In tive thin sections, as described below. general, the Nabesna pluton east of the middle fork of Bond Creek and north of the east fork of Bond Creek is dominated Sediment/soil/rock chemistry and mineralogy by white mica signatures (shown in orange). In the porphyry Sediment data from the Alaska Geochemical Database (Gran- deposit and prospect area to the south of the east fork of itto et al., 2011) were used to establish the regional geochemi- Bond Creek, the spectral patterns are more variable. Expo- cal framework. We collected an additional 96 rock and 66 soil sures at Orange Hill map as white mica, with clinochlore + and sediment samples for geochemical analysis. The fine- white mica more predominant in the southwest portion of the sediment fraction of soils and sediments (–80 mesh) and the hill and abundant montmorillonite (smectite) and gypsum to rock samples were pulverized to –200 mesh (<0.074 mm) and the northwest (Figs. 3, 4). Localized kaolinite-bearing white analyzed at SGS Laboratories for concentrations of 55 major, mica signatures characterize the upper reaches of California rare earth, and trace elements by inductively coupled plasma- Gulch. Zones of white mica, clinochlore + white mica, and atomic emission spectrometry-mass spectrometry (ICP-AES- montmorillonite (smectite) signatures are also indicated in MS) after sodium peroxide fusion. Data were considered the airborne data over variably iron-stained igneous rocks that acceptable if recovery for all 55 elements was ±15% at five crop out in E-W–trending drainages to the northeast. The times the lower reporting limit and the calculated relative mostly plutonic rocks between Orange Hill and the west fork standard deviation (RSD) of duplicate sample analysis was no of Bond Creek have abundant carbonate, chlorite/epidote, greater than 15%. Gold was determined by fire assay. Com- and kaolinite signatures. The altered zone at Bond Creek, as plete results can be found in Graham et al. (2017). mapped by Richter (1973) (loc. 2, Fig. 3; central area, Fig. Select soil samples and four rock samples were also ana- 5), is dominated by white mica signatures in both plutonic lyzed by X-ray diffraction (XRD) to validate the presence of (labeled Kg) and volcanic rocks (labeled PlPv) to the south. spectrally identifiable minerals identified in mineral predomi- Outside of this zone, more abundant clinochlore, chlorite/epi- nance mapping from MICA analysis. The XRD scans were dote, and carbonate classes are present relative to white mica. collected on a PANalytical X’Pert Pro MPD diffractometer Jarosite + white mica and montmorillonite (smectite) map on with Bragg Bertano optics using Cu radiation after following the northeastern flank of the alteration zone. The XRD analy- sample preparation methods of Moore and Reynolds (1997) ses of soils from the Orange Hill and Bond Creek areas yield (App.; Table A2). variable proportions of these same minerals (see Table A2; Graham et al., 2017). White mica chemistry Mapping of the Wrangellia terrane delineates areas of white Electron probe microanalysis (EPMA) was completed on mica, chlorite + white mica, clays, and zones of chlorite/epi- white mica (n = ~159 spots) and lesser chlorite and kaolinite dote generally similar to the porphyry deposit/prospect area from a total of 14 areas on four thin sections from three hand (Fig. 3). The MICA analysis indicates the Nikolai Greenstone specimens from the Orange Hill deposit area. White mica (Trn) signatures are dominated by chlorite/epidote and car- within these billets spanned the approximate range of white bonate (albeit not a pattern restricted to that rock unit). Sev- mica 2,200-nm wavelength positions observed in our airborne eral small areas had spectra that best match serpentine group and Corescan data. The JEOL JXA-8900 Superprobe at the reference standards. XRD analyses of four rock samples col- USGS Central Mineral and Environmental Resources Sci- lected to investigate this spectral signature establish that the ence Center, Denver, Colorado, USA, is outfitted with five rocks actually contain amphibole and chlorite (pale green; Fig. wavelength dispersive spectrometers. The microanalyzer was 3). The initial classification in HyMap data was the result of operated at 15 kV and 20 nA, with a beam diameter of less serpentine and amphibole both having spectral features near than 1 µm. Calibration was checked using well-characterized 2,320 and 2,390 nm (Kokaly et al., 2017b), but only serpen- silicate and oxide standards. The full complement of EPMA tines are included as reference spectra in the MICA analysis. results is provided in Graham et al. (2017). For other units, thin limestone-bearing units within the volcanic arc rocks (unit Pl) map predominantly as calcite. In Results addition, some pixels match the reference spectrum of calcite + dolomite (or a mixture of calcite and epidote or chlo- Airborne spectroscopy rite). Systematic variations in predominant mineral signatures Mineral predominance mapping: Approximately 51% of the suggest exposure of different laterally continuous lithologic Orange Hill and Bond Creek study area produced interpre- units in the volcanic units (PIPV) along Nikonda Creek with table mineral-related spectral signatures (Fig. 3). Vegetated clay, white mica, chlorite + white mica, and local pyrophyllite ground, shown in tan shades, accounts for an additional 21% signatures. The XRD results verify pyrophyllite in soils from of the area. Vegetation occurs mostly below 1,500 m and cov- areas mapped with pyrophyllite signatures (Table A2; Graham ers much of the Orange Hill deposit. Glacial ice and snow et al., 2017). Systematic spectral trends are not obvious to the and wet ground are mapped at higher elevations to the north east. and east in the glacier field (22% of classified pixels). A small Chlorite classes: The predominant mineral classes contain- percentage of the area is unclassified (5%, shown as black pix- ing chlorite, epidote, and clinochlore occur in distinct regions els), mainly due to steep terrane or clouds/cloud shadows that of the study area with little overlap (Fig. 6). Most clinochlore result in poor illumination or ice and meltwater from glaciers pixels are intermixed with abundant chlorite/epidote pixels that cause interference in spectral signatures. (bright green and dark green, respectively) in the Nikolai

142°50'0"W 142°48'0"W 142°46'0"W 142°44'0"W 142°42'0"W 142°40'0"W 142°38'0"W 142°36'0"W 142°34'0"W 142°50'W 142°42’W 142°34’W Kg Survey boundary

Survey boundary

62°16'0" N Kg Tp Tp Glacier Glacier Porphyry deposit Muscovite dominated + prospect area Kg plutonic rocks (”porphyry cluster”) Kg 62°15'0" N 62°15'0"N Kg Tp Glacier Bond Cr East Fork eek Kg

5 62°14'0" N Tp M i d NEN draws 4 d l Figure 5 e Figure 4 F

r k B Glacier 3 o 62°13'0" N n d Kg Kg C Kg er ee California ekk Kg Gulch 1 Nabesna pluton 62°12'0"N 6 Trn 2 Kg PIPv PIPv Wrangellia Pl W e Pl terrane

1'0" N s t PIPv

F Pl Pl o

62°1 PIPv r k Pl

B PIPv o Trn n 142°50'0"W 142°48'0"W 142°46'0"W 142°44'0"W 142°42'0"W 142°40'0"W 142°38'0"W 142°36'0"W 142°34'0"Wd Glacier 142°50'W 142°42’W 142°34’W C Pl Glacier

N N r Spectrally Predominant Mineral(s) ik Survey e N Kg o Pl e n k PIPv Survey boundad boundary White mica Carbonate + mica/clay a ry C 62°16'0" 62°10'0"N r 62°10'0" Kg Tp Glacier e Clinochlore + white mica Calcite+dolomite Tp e PIPv k PIPv Trn Glacier Amphibole + chlorite Porphyry deposit Chlorite + white mica Muscovite dominated N + prospect area Kg Tpplutonic rocks Clinochlore Gypsum (”porphyry cluster”) Pl Tp Kg Pyrophyllite 62°15'0"N Chlorite/epidote PIPv 62°15'0" Kg Pl Trn Survey boundaTp 62°9'0"N ry Kaolinite Jarosite Glacier Bond Cr East Fork eek KgPIPv Kaolinite + white mica GlacierVegetation 0 5 km 5 Montmorillonite (smectite) Ice/snow/water 62°14'0" N Tp M Calcite 142°50'0"Wi d 142°50'0"W 142°48'0"W 142°46'0"W 142°44'0"W 142°42'0"W 142°40'0"W 142°38'0"NEN drawW s 142°36'0"W 142°34'0"W 4 d l 142°50'W 142°42’W 142°34’W Figure 5 e Figure 4 Spectrally F Predominant Mineral(s) Geologic Units o

Survey r

N Kg k Tp Tertiary porphyry (intrudes both Nabesna pluton and Wrangellia) B Glacier Survey bounda boundary3 Whiteo mica Carbonate + mica/clay 62°13'0"N ry n d Kg Nabesna pluton granodiorite, monzonite, and diorite Kg 62°16'0" Kg C Kg Kg Clinochloreer + white mica Calcite+dolomite Trn Nikolai Greenstone Tp Tp Glacier ee ekk Kg Carbonate and calcareous to carbonaceous argillite and siltstone GlacierCalifornia Amphibole + chlorite Pl Porphyry deposit Gulch Chlorite + white mica (combined Pl and TrPa units of Richter (1973). Muscovite dominated N + prospect area 1 Kg Nabesna PIPv Volcanic and volcanicastic rocks of the Wrangellia terrane plutonic rocks Clinochlore Gypsum (”porphyry cluster”) pluton Kg 62°12'0" N 6 Prospects Trn 2 Pyrophyllite

62°15'0"N Chlorite/epidote 62°15'0" Kg Kg 1 Orange Hill 3 Nike 5 Unnamed Mo occurrence Tp PIPv PIPv Kaolinite Jarosite 2 Bond Creek 4 Neil 6 Copper King skarn Glacier Wrangellia nd C Pl W ast Fork Bo reek E '0"N Kg e Kaolinite + whitePl mica Vegetation terrane s t PIPv

F Pl Pl o 62° 11 PIPv Ice/snow/water 5 r Montmorillonite (smectite) k Pl

62°14'0" N B Tp PIPv o Trn M n Calcite d Glacier i d C Pl Glacier d NEN draws 4 N N r l i e e k Figure 5 eFig. 3. Mineral predominance map of the Orange Hill-Bond Creek area. A 3- × 3-pixel majority filter was applied to original Figure 4 o Pl F n Geologic Unitk s PIPv o d r a output to highlight different spectral signatures at this scale. The full set of 77 mineral classes from the original classification k C Tp Tertiary porphyry (intrudes both Nabesna pluton and Wrangellia) 62°10'0" N Glacier r B 62°10'0" e of HyMap data was simplified to the 15 classes shown for clarity. 3 o e

62°13'0"N PIPv n k d Kg Nabesna pluton granodiorite, monzonite, andPIP dioritev Trn Kg Kg C Kg er e Trn Nikolai Greenstone eek Tp California k Pl Kg Pl Carbonate and calcareous to carbonaceousTp argillite and siltstone Gulch (combined Pl and TrPa units of Richter (1973). PIPvPl Volcanic and volcanicastic rocks of the WrPIPangelliav terrane Trn 1 Survey bounda Nabesna 62°9'0" N ry pluton

62°12'0" N 6 Prospects Trn 2 PIPv Glacier Kg 3 0 5 km 1 Orange Hill Nike 5 Unnamed Mo occurrence PIPv Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf PIPv by guest 2 Bond Creek 4 Neil 6 Copper King skarn Wonrangelli 03 Octobera 2021 Pl W 142°50'0"W

'0"N e Pl terrane s t PIPv

F Pl Pl o

62° 11 PIPv r k Pl

B PIPv o Trn n d Glacier

C Pl Glacier

N N r ik e o Pl e n k PIPv d a C 62°10'0" N r 62°10'0" e e PIPv k PIPv Trn

Tp Pl Tp Pl PIPv Trn Survey bounda 62°9'0" N ry PIPv Glacier 0 5 km

142°50'0"W 496 GRAHAM ET AL. 142°52'0"W 142°50'0"W 142°48'0"W

142°52'0"W 142°50'0"W 142°48'0"W N 62°13'0" N 62°13'0" Cu-Mo-Au mineralized

Abandoned Kg River Kg Meander

ault Kt+ Tert.(?) Kt yner F Br dikes ane Nabesna plutonTerr Kt angellia California Gulch Wr Ka 62°12'0" N 62°12'0"N Copper King adits Nikolai greenstone (Richter et al.) Kb Nikonda F

ault Volcanics, Trl and Ka

010.5 2 Kilometers

Spectral Mineral Predominance Geologic unit abbreviations 142°52'0"WhiteW mica Kaolinite + white142°50'0" micaW Amphibole + chlorite 142°48'0"W Kt - biotite-hornblende tonalite Clinochlore + white mica Gypsum Montmorillonite (smectite) Kb - hornblende-biotite diorite Chlorite + white mica Calcite Pyrophyllite Ka - quartz-plagioclase porphyry Trl - limestone hosting Copper King Clinochlore Carbonate + mica/clay Vegetation contact (approximate) fault (approximate) Chlorite/epidote Calcite + dolomite Ice/snow/water Kaolinite

Fig. 4. Expanded view of the mineral predominance map for the Orange Hill deposit (from Fig. 3), without 3 × 3 majority filtering. Superimposed geology mapped by Linn (1973).

Greenstone southeast of Orange Hill, and pixels with chlorite of a glacier that cut volcanic rocks at the southern end of our + white mica (purple) signals coincide with volcanic arc rocks. survey area (loc. 9). The latter zone is at the toe of a glacier Clinochlore + white mica signatures dominate in the vicinity and could be reflecting materials transported from the south, of the porphyry cluster (Fig. 6). Outside of the main cluster, where an undifferentiated mafic intrusion interpreted as a moderate abundances of the clinochlore + white mica pixels probable marginal phase of the Nabesna pluton is exposed are mapped in several areas, including on the north side of (Richter, 1973). and at the headwaters of the east fork of Bond Creek (locs. 7, White mica wavelength position mapping: Figure 7 shows 8; Fig. 6) and in a zone mostly along the edges and terminus the variation in wavelength position for the 2,200-nm Al-OH

142°46'0"W 142°44'0"W142°44'0"W 142°42'0"W142°42'0"W 142°40'0"W

Tp M iddle

62°13'0" N Fork Bond Cr

62°13'0"N Kg 62°13'0" N

Tp 62°13'0"N Tp

ee Kg Clino-Musc+prop k West Fork Bond Cr

Mont.+jarosite

eek Clino-Musc

N TPR a+L N 62°12'0" N

62°12'0" TnR Kg 62°12'0" N Musc. Dominated Nabesna pluton 62°12'0" Wrangellia Richter Terran Alt’d e Zone Clino-Musc Tp PIPvPIPv TPR a+L Clino-Musc+prop

TnR Tp TPR a+L '0"N 1'0"N

PIPv PIPv '0"N

62°1 R TPa+L Tp 1'0"N 62° 11 Tp Tp 62°1 62° 11

010.5K2 ilometers PIPvPIPv

142°44'0"W142°44'0"W 142°42'0"W142°42'0"W 142°40'0"W142°40'0"W White mica Calcite Tp Tertiary porphyry Clinochlore + white mica Carbonate + mica/clay Kg Nabesna pluton Calcareous and carbonaceous Chlorite + white mica Calcite + dolomite TrPa+L siltstone and shale and limestone Clinochlore Amphibole + chlorite PIPv Tetelna volcanics

Chlorite/epidote Gypsum Trn Nikolai greenstone

Kaolinite Pyrophyllite Kaolinite + white mica Vegetation Boundary of altered zone from Richter (1973) Montmorillonite (smectite) Ice/snow/water Jarosite

Fig. 5. Expanded view of the Bond Creek deposit area (from Fig. 3), without 3 × 3 majority filter. Distributions of mineral classes show zonation from a white mica core outward to clinochlore + white mica to clinochlore + white mica with chlorite/ epidote (potential propylitic alteration assemblage; marked “Clino-Musc + prop”) on the western flank of the ridge. Geology from Richter (1973).

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 498 GRAHAM ET AL.

142°50'W 142°40'W

Porphyry deposit + prospect area (”porphyry cluster”)

62°15' N 7 8

ond C East Fork B reek 5

M 4 i d d

l e

F o r 3 k B o 1a n d C e ek 1 6 2 Nabesna pluton

e s Wrangellia t

F terrane o r k

B o n d

N C ik r o e n e d k

62°10' N a C r e e k

0 2 km 0 2 mi 9

MICA output Porphyry propsects Ice/snow Orange Hill Clinochlore + muscovite class 1 4 Neil Spectrally non-chlorite/epidote 2 Bond Creek 5 Unnamed Mo dominant Clinochlore class Chlorite/epidote generic Chlorite+muscovite_intimate class 3 Nike Areas 6-8 discussed in text Vegetation or no classiﬁcation

Fig. 6. Map chlorite-dominated mineral classes. Color scheme changed from Figure 3 to highlight distinct distributions. Numbered locations are discussed in text.

absorption feature measured in white mica-dominated pix- nm (blue and purple), irrespective of the geologic unit ana- els (see Fig. 3). Calculated absorption feature positions are lyzed. However, white micas at longer wavelengths (green, grouped into 12 classes ranging from <2,196 to >2,207 nm yellow, and red) are highly concentrated over a broad area (grading from purple to red, respectively). The majority of in the Nabesna pluton, extending from the Orange Hill area, pixels contain shorter-wavelength positions less than 2,202 through Bond Creek to east of the middle fork of Bond Creek

142°50'W 142°40'W

Porphyry deposit + prospect area (”porphyry cluster”)

62°15' N 7 8

ond C East Fork B reek 10 5

M i 4 d d

l e

F o r 3 k B 1a o n d C e ek 1 6 2 Nabesna pluton

e s Wrangellia terrane t

F o r k

B o n d

N C ik r o e n e d k a 62°10' N C r e e k

White mica 2,200 nm absorption feature wavelength position (nm) ≤ 2,196 2,198 2,200 2,202 2,204 2,206 2,197 2,199 2,201 2,203 2,205 ≥ 2,207 Fig. 7. White mica wavelength position map, for the 2,200-nm Al-OH absorption feature. Warmer colors (green, yellow, orange, and red) show the distribution of longer-wavelength white mica. Note similarity to the distribution of clinochlore + white mica in Figure 6. Longer-wavelength zones include a broad area within the porphyry Cu cluster and several satellite zones. Numbered locations are discussed in the text.

and north to the eastern fork of Bond Creek (area 2; Fig. 7). 1, 1a) yield mixed short to long Al-OH absorption feature This signature incorporates the Nike and Neil porphyry pros- positions. Some intermediate- to long-wavelength signatures pects (locs. 3, 4) and the vicinity of the unnamed porphyry Mo (classes depicted with green, yellow, and red) map in the prospect (loc. 5; Fig. 7). At the Bond Creek deposit, the fea- vicinity of the Copper King skarn (loc. 6). ture extends into the Wrangellia terrane volcanic rocks. Rock Two notable satellite areas with longer-wavelength white exposures at Orange Hill and the northeastern drainages (loc. mica occur north of the east fork of Bond Creek and at the

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 500 GRAHAM ET AL.

headwaters of the east fork (locs. 7, 8; Fig. 7), the same areas gradational at the millimeter scale (e.g., the 15HYPORH002 dominated by clinochlore + white mica pixels as described billet in Fig. 8). Such variation, likely related to overprinting above. A narrow but prominent zone of longer-wavelength alteration/mineralization events and weathering, provides the Al-OH absorption features (loc. 10) is present in a small opportunity to examine white mica wavelength position in drainage southwest of location 7. In the volcanic rocks south relation to chemical changes. of the contact with the Nabesna pluton, localized intermedi- We categorized the chemistry of white micas in the four ate to long wavelengths are observed, but they primarily occur sections into three discrete categories: (1) longest-wavelength in small isolated areas. The most extensive of these areas is positions (≥2,207 nm), (2) shortest-wavelength signatures on the southern edge of our survey on the west fork of Bond (≤2,202 nm), and (3) two categories of mixed short- to long- Creek (loc. 9) and also coincides with clinochlore + white mica wavelength signatures (2,203–2,204 nm). Calculated unit signatures (Fig. 6). These spectrally distinct areas in Wrangel- formulae for white mica with short- and long-wavelength lia terrane rocks (loc. 9 and isolated areas) are not definitively signatures have >6 silicon atoms and insufficient K or Al in associated with mineralized rock. their unit cell to be strictly considered muscovite (Deer et al., 1992). The octahedral site for both end members exceeds 4 Corescan images and mineral chemistry atoms per unit cell, possibly but not necessarily due to some The white mica wavelength positions calculated from Core- component of mixed-layer clays (Gaudette et al., 1964), as scan imaging spectrometer data collected on hand specimens suggested by R1 ordered illite/smectite clays in many of the and billets of altered igneous rocks from Orange Hill span a XRD analyses (see Table A2). range similar to those of the HyMap results. They also dem- Chemical compositional differences of white micas with onstrate spectral complexity at the hand specimen scale. Even short- vs. long-wavelength positions in the thin sections con- within macroscopically uniform rock samples containing pri- firm that the wavelength position is at least in part related marily white mica, spectral variations can be abrupt (e.g., rock to mineral chemistry (representative analyses shown in sample 14HYPORH013A in Fig. 8). In other cases, they can be Table 1). White micas from regions of billets displaying the

Table 1. Representative Electron Microprobe Analyses of White Mica and Chlorite

Sample no. 002 013A(U) 013A(U) 002 002 013A(L) 002 013A(L) Analysis no. 196 17 14 149 172 243 173 221

Class Short wavelength (purple and blue) Long wavelength (red)

Weight percent

Na2O 0.32 0.46 0.46 0.35 0.28 0.27 0.11 0.04 MgO 0.39 1.02 0.81 0.98 2.09 1.69 3.06 2.12 FeO 0.35 2.85 2.97 2.38 2.48 3.37 1.60 1.67 MnO BDL BDL BDL 0.03 0.07 0.03 0.18 0.24 BaO 0.30 0.85 0.21 0.20 0.17 0.18 0.16 BDL V2O3 BDL BDL 0.09 0.18 0.06 0.10 BDL BDL SiO2 46.47 44.76 46.26 47.75 47.94 48.34 49.19 52.89 Al2O3 35.62 34.90 34.72 34.04 32.62 31.87 30.54 28.59 K2O 9.73 9.78 10.36 10.20 10.22 9.99 10.41 8.72 CaO 0.04 BDL BDL BDL 0.04 0.03 BDL 0.23 TiO2 BDL 0.19 0.42 0.34 0.38 0.35 0.12 0.04 Cl 0.01 BDL BDL BDL BDL BDL 0.01 0.02 F 0.36 0.19 BDL 0.38 0.35 0.15 0.68 0.41 Rb2O 0.18 0.15 0.14 0.20 0.16 0.17 0.17 0.18 O –0.16 –0.08 –0.05 –0.17 –0.16 –0.07 –0.30 –0.18 Total 93.62 95.07 96.40 96.88 96.72 96.49 95.94 94.95

Number of cations on the basis of 22O Si 6.28 6.07 6.15 6.32 6.36 6.42 6.58 7.00 Al 1.72 1.93 1.85 1.68 1.64 1.58 1.42 1.00 Total 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 Al 3.94 3.65 3.60 3.63 3.46 3.41 3.40 3.45 Ti 0.00 0.02 0.04 0.03 0.04 0.04 0.01 0.00 Mg 0.08 0.21 0.16 0.19 0.41 0.34 0.61 0.42 Fe 0.04 0.32 0.33 0.26 0.28 0.37 0.18 0.18 Total 4.06 4.20 4.13 4.12 4.19 4.16 4.21 4.06 Ca 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.03 K 1.68 1.69 1.76 1.72 1.73 1.69 1.78 1.47 Na 0.08 0.12 0.12 0.09 0.07 0.07 0.03 0.01 Total 1.77 1.81 1.88 1.81 1.81 1.77 1.81 1.51 Mg + Mn + Fe + Si 6.39 6.60 6.65 6.78 7.05 7.13 7.40 7.62

Complete data in Graham et al. (2017); BDL = below detection limit

C upper billet 14HYPORH013A 14HYPORH013A upper billet A E

lower billet D F

14HYPORH013A upper billet 5 cm

15HYPORH002 I 15HYPORH002 billet G K

H J L

5 cm 15HYPORH002 billet

Spectrally predominant minerals White mica wavelength position (nm) Location of billet/thin section cut white mica ≤2199 2204

clinochlore + white mica 2200 2205 Location of white mica wavelength determination chlorite/epidote 2201 2206 (white mica composition probed at approximately kaolinite + white mica 2202 ≥2207 the same position on corresponding thin montmorillonite 2203 (smectite) section) gypsum carbonate + mica/clay (weak 2300 nm feature) Fig. 8. Photos and mineral maps produced from Corescan imaging spectrometer scans of selected hand samples and associated billets from Orange Hill. A) Rock sample 14HYPORH013A. B) Image of the upper billet cut from A for thin sectioning. C) and D) are the mineral predominance and white mica wavelength position maps, respectively, of hand specimen 14HYPO- RH013A. E) and F) are mineral predominance and white mica wavelength position maps, respectively, of the billet in B. G) and H) are images of the hand specimen 15HYPORH002 and billet, respectively, cut for thin sectioning. I) and J) are the mineral predominance and white mica wavelength position maps, respectively, of hand specimen 15HYPORH002. K) and L) are mineral predominance and white mica wavelength position maps, respectively, of the billet in H). Colors are approximately the same as in Figure 4 for mineral predominance and as in Figure 7 for white mica wavelength classes.

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 502 GRAHAM ET AL.

longest-wavelength Al-OH absorption features have higher and Bond Creek porphyry systems, including historical samples median Si, Mg, and Mn and lower median Al (± Na) than and those collected as part of the current study, contain anoma- those with the shortest-wavelength positions (Table 1; Fig. lous concentrations of Cu, Mo, and/or Au (15.5× median value 9A, B). When abundance (as atoms per formula unit [apfu]) of soils in the conterminous U.S.; Smith et al., 2013). Nearly of total Fe + Mn + Mg + Si is compared to total Al, there is a all samples (n = 25) at Orange Hill and the two drainages to systematic inverse relationship attributed to Tschermak sub- the northeast contain anomalous Cu and Mo; of these, eight stitution (AlVI + AlIV↔[Fe, Mg, Mn]VI + SiIV ) with a greater samples contain ≥60 ppb Au (areas 1, 1a, Fig. 10A, B). Most phengitic component (and lower Al) in micas with longer- samples from both the west and middle forks of Bond Creek wavelength Al-OH features (Fig. 9C). immediately upstream and downstream of the Bond Creek Sediment and soil chemistry: A large number of soil and deposit (loc. 2, Fig. 10A) also contain more than 15 times the stream sediment samples collected proximal to the Orange Hill median value for Cu, and they contain concomitant highly anomalous Mo and/or Au concentrations (Fig. 10B). North and east of the area of known mineral occurrences/ 28 A m=23.70 deposits, samples are also anomalous in Cu, Mo, ± Au. At 26 wt % Al wt % Si location 7 (Fig. 10A, B), soil samples from alluvial fans and m=22.21 one stream sediment sample contain anomalous Cu (3 sam- 24 ples at 269–300 ppm), two contain 15 to 26 ppm Mo, and 22 one contains 30 ppb Au. Three samples collected at the upper m=17.99 20 m=16.61 reaches of the east fork of Bond contain highly anomalous Cu Wt % (235–434 ppm), Au (80–510 ppb), and Mo (8–10 ppm) (loc. 8, 18 Estimated WL Fig. 10A, B). Three samples from drainages northeast of the <2,202 nm headwall to east Bond Creek that flow to the northeast con- 16 2,202 to 2,204 nm tain anomalous Au (50–260 ppb) but are not anomalous in Cu 14 n=81 n=12 n=13 n=99 >2,207 nm or Mo. Finally, sediment from a narrow drainage on the south 12 side of the middle fork of Bond Creek (loc. 10) contains highly 2.5 B m=1.30 anomalous Mo (18 ppm) and Au (150 ppb) and elevated Cu wt % Na wt % Mg (220 ppm). 2 Multielement anomalies are largely absent in the Wrangel- lia terrane rocks and Nabesna pluton outside of the areas dis- m=0.08 1.5 cussed above. Although sporadically distributed samples with highly anomalous concentrations of Cu (>7.5× background) Wt % m=0.60 are common, anomalous Mo and/or Au concentrations are 1 only found immediately adjacent to the Bond Creek deposit (loc. 2), downstream of the Copper King skarn (loc. 6), and .5 m=0.29 locations 11 and 12 (Fig. 10B). Altered and mineralized rock samples confirm local bedrock 0 sources of anomalous metals. Rocks from within and in the 8 C vicinity of the Orange Hill and Bond Creek deposits contain Estimated WL high Cu, Mo, and Au concentrations (median concentrations <2,202 nm of 500 ppm, 18 ppm, and 13 ppb, respectively), consistent with 7.6 2,202 to 2,204 nm >2,207 nm chalcopyrite and molybdenite observed in hand specimens and Increasing phengitic component with previous studies (Van Alstine and Black, 1946; Linn, 1973). 7.2 Mineralized samples were also collected from the drainages northeast of Orange Hill and the headwaters of the east Bond 6.8 Creek drainage. A mineralized grab sample from an alluvial fan below location 7 contained 8,400 ppm Cu, 3,780 ppm Mo, and Mg+Mn+Fe+Si (apfu) Tschermak substitution: (vi) (iv) (vi) (iv) 6.4 Al +Al ↔ (Fe, Mg, Mn) +Si 143 ppb Au. The three highest-grade rock samples collected from glacial debris at the headwaters of east Bond Creek (loc. 8, Fig. 10) contain 800 to 3,900 ppm Cu, <2 to 51 ppm Mo, and 6 4 4.4 4.8 5.2 5.6 6 36 to 200 ppb Au. A grab sample of quartz-sulfide–veined vol- Total Al (apfu) canics from the southern end of our survey yielded a polymetallic signature including 740 ppb Au, 1,120 ppm Cu, 11 ppm Mo, Fig. 9. Chemical variations observed in white mica categorized by wavelength (WL) position into several groups, including shorter wavelength (n = 9,440 ppm Pb, and 21,000 ppm Zn (loc. 9). 81 white mica analyses from two thin sections) and intermediate to longer wavelength (n = 99 analyses from three thin sections). White mica wave- Discussion length positions calculated from Corescan imaging spectrometer data. Box plots showing variations in A) Al and Si, and B) Mg and Na (in wt %). m = Regional mineral predominance maps as a median, n = number of analytical points. C) Inverse relationship between supplement to geologic mapping total Al and Mg + Mn + Fe + Si atoms per formula unit, assuming 22 oxygens per unit cell. Representative electron probe microanalysis data given in Table Mineral predominance mapping as a proxy for geologic map- 1. Complete data in Graham et al. (2017). ping in the Wrangellia terrane has yielded mixed results. On

142°52'W 142°44’W 142°36'W ! ! ! ! ! ! ! ! ! ! !!! ! A ! ! ! ! ! ! ! !!

62°16'N !

!!! This study Historical samples 7 ! 8 ! ! ! ! ! ! ! ! ! ! Copper > 229 ppm (15.5x background) ! ! ! ! ! ! ! EFBC ! ! ! ! ! ! 5 Copper > 111 ppm (7.5x background) 10 !! 62°14'N ! ! Copper < 111 ppm 4 ! ! !! ! ! !!!! ! ! !!! ! !!!!! 3 !! !! !! 1a ! ! ! !!! ! ! 3 km !! 1 ! ! ! ! ! !! ! ! 2

62°12'N ! ! MFBC 6 ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! WFBC ! Nikonda Cree 11 ! ! ! ! ! EFBC East Fork Bond Creek ! ! 62°10'N 12 ! MFBC Middle Fork Bond Creek ! ! ! ! ! k ! WFBC West Fork Bond Creek ! !! ! ! ! Boundary between Nabesna pluton ! ! ! ! Cu 9 ! and Wrangellia ! ! !

142°52'W 142°44’W 142°36'W 2 Area discussed in text ! ! ! ! ! ! ! ! ! !!! ! B ! ! ! ! ! ! ! !!

62°16'N ! Molybdenum > 12.1 ppm (15.5x backgroud) ! !! Molybdenum > 5.8 ppm (7.5x background) 7 ! 8 ! ! Molybdenum < 5.8 ppm and Au ! ! ! <30 ppb ! ! ! ! ! ! ! !! ! EFBC ! Molybdenum >5.8 ppm and ! !! ! ! 5 ! gold >30 ppb (7.5x backgound) ! ! !

62°14'N 10 ! Gold > 30 ppb and molybdenum ! 4 ! <5.8 ppm ! ! ! !!!! ! ! ! !!! 3 ! !!!!! !! !! !! 1a ! ! ! !!! ! ! !! 1 ! ! ! ! !! ! 3 km ! ! ! 2

62°12'N ! 6 ! MFBC ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! WFBC ! Nikonda Cree 11 ! ! ! ! ! ! ! 62°10'N 12 ! ! ! ! ! ! k ! ! ! ! ! ! ! ! ! ! ! Mo and Au 9 ! ! ! !

Fig. 10. Sediment/soil geochemical results showing the distribution of anomalous and highly anomalous A) Cu and B) Mo and elevated gold values. Multielement Cu and Mo and/or gold anomalies are more restricted in distribution than Cu-only anomalies. Area shown is the same as Figure 3. Numbered locations are discussed in the text. Threshold concentrations, above which elements are considered anomalous, were chosen based on multiples of the median abundance values for soil samples collected throughout the entire conterminous United States (Smith et al., 2013).

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 504 GRAHAM ET AL.

the northeast side of Nikonda Creek (Fig. 3), distinct SE- The predominance of jarosite along with montmorillonite trending linear strips of different mineral spectral signatures (smectite) on the steep eastern side of the Bond Creek ridge (from carbonate to white mica to chlorite + white mica) delin- is conspicuous, with jarosite likely an important indicator of eate laterally continuous stratigraphic units with mineralogical weathering of sulfide-bearing rock. variation. However, elsewhere in the mapped area, mineral Large-scale zoning patterns are not obvious within the predominance does not show systematic variation. The lack of Orange Hill deposit owing to extensive vegetative cover (Fig. patterns is likely a consequence of substantial surface mate- 4). However, Linn (1973) identified at least three phases rial movement across mapped boundaries and in part due to of the Nabesna pluton at Orange Hill: a diorite body in the shared spectrally identifiable minerals in multiple geologic southwestern part of the hill in contact with the locally pre- units. For example, whereas calcite is a major component of dominant tonalite on the northern and eastern parts and the Pl unit, which includes limestone, calcareous siltstone, quartz-feldspar porphyry exposed in the headwaters of Cali- and limy shale, it is also an important alteration mineral (along fornia Gulch (Fig. 4). Spectrally, clinochlore + white mica with chlorite and epidote) in the Nikolai Greenstone. Conse- signatures are widespread in the diorite (and some adjacent quently, parts of both units map as calcite (Fig. 3). The wide- tonalite), montmorillonite (smectite) and gypsum occur in the spread andesitic and dacitic volcanic and volcaniclastic units more decomposed tonalite on the northwest side of the ridge, of the Tetelna volcanics are distinct in their lack of carbonate and kaolinite and white mica signatures predominate at and and abundant white mica and locally clay signatures. But in around quartz-feldspar porphyry in the headwaters of Califor- general, there is no consistent pattern to discern discrete sub- nia Gulch. Kaolinite + white mica signatures to the east by the units, and patches of chlorite/epidote that are present in the Copper King skarn coincide with additional quartz-feldspar volcanics are indistinguishable from the Nikolai Greenstone. porphyry (Fig. 4). Therefore, as a first order, the spectral sig- Geologic mapping is not detailed enough to explain the non- natures at Orange Hill empirically reflect alteration products systematic spectral mapping, but the patterns could reflect associated with different rock units. lateral facies changes, structural complexity, mass wasting, The different spectral signatures in tonalite and diorite and overprinting by hydrothermal activity. appear to relate to intensity of alteration as well as initial In the Nabesna pluton, the widespread spectral predomi- composition. Both smectite and chlorite, common alteration nance of white mica with lesser calcite and clay throughout minerals in porphyry Cu deposits, can form by alteration of the northern and eastern parts of the pluton suggests a perva- ferromagnesian minerals, calcic feldspars, and volcanic glasses sive background signature from deuteric alteration or weath- (Ross and Hendricks, 1945). Montmorillonite (smectite) sig- ering (Fig. 3). The abundant chlorite + white mica, chlorite/ natures coincide with broader aprons of friable tonalite on epidote, and clay minerals (sometimes reflecting late dikes) the northwest side of Orange Hill, suggestive of more intense in the vicinity of the porphyry cluster appear similar to those alteration than the clinochlore- and white mica-bearing dio- of the Wrangellia terrane rocks, and without independent rite to the southwest. Whereas mostly near vertical gypsum geologic control, those rocks could be misinterpreted to be veins occur in both tonalite and diorite, the more friable char- part of Wrangellia. Overall, the spectral results do not directly acter of the tonalite results in dispersal of gypsum across the correspond to systematic differences within individual geo- ground surface. The quartz-feldspar porphyry in the head of logic units at the regional scale, precluding geologic mapping California Gulch is compositionally different and lacks mafic based only on mineral predominance. However, the variations minerals. Extensive quartz veining in and around the por- between spectral data and geologic mapping provide impor- phyry indicates widespread hydrothermal fluid circulation. tant indications of secondary (potentially hydrothermal) influ- Alteration and weathering of feldspar and mica within this ences. Utilization of other analytical methods, for example, unit and the rocks it intruded produced secondary minerals supervised classification using end-member spectra for dif- with kaolinite rather than smectite. ferent units, has proven useful for lithologic mapping else- Both clinochlore + white mica and kaolinite + white mica where at northern latitudes (e.g., Harris et al., 2005; Feng et zones are also mapped in the mineralized drainages to the al., 2018). northeast of Orange Hill (Fig. 4) and similarly reflect variably altered and mineralized dioritic to granodioritic rocks. Deposit-scale mapping: Bond Creek and Orange Hill Importantly, the abrupt change in spectral signatures in the At the deposit scale, distributions of spectrally dominant drainages aligns with and thus locally defines the Bryner fault mineral(s) reflect both primary geology and magmatic-hydro- (Fig. 4). The predominance of chlorite/epidote and carbonate thermal overprinting. In the well-exposed Bond Creek deposit signatures in plutonic rocks farther uphill to the east suggests area, a >1.5- × 2-km zone dominated by white mica and distal propylitic alteration associated with the Orange Hill lesser chlorite + white mica signatures closely corresponds to porphyry system. the altered zone mapped by Richter (1973) (dashed zone; Fig. 5). The zone grades outward to the north and south to clino- Chlorite and white mica signatures and their chlore + white mica and then clinochlore + white mica with relationship to mineral occurrences irregular chlorite/epidote and carbonate signatures, consisting Coherent patterns of the three main chlorite-bearing min- of a possible phyllic core flanked by chlorite + white mica to eral predominance classes (clinochlore, clinochlore + white propylitic zones. This pattern is consistent with a fairly deeply mica, and chlorite + white mica) strongly support fundamen- eroded idealized porphyry system (e.g., Lowell and Guilbert, tal differences in chlorite composition or relative proportions 1970; Sillitoe, 2010). Similar mineral assemblage zonation has of chlorite and white mica related to lithology or magmatic- been reported in multispectral studies (e.g., John et al., 2010). hydrothermal processes. The distribution of clinochlore pixels

in Nikolai Greenstone and widespread chlorite + white mica fluids, formation of white micas with near-end-member mus- pixels in the volcanic and volcaniclastic rocks of the Wrangel- covite composition are favored. Below dissociation depths lia terrane suggest lithologic control (Fig. 6). In contrast, the and distal from the main magmatic fluid flux, where fluids are distribution of the clinochlore + white mica class is inferred near neutral pH, phengitic compositions (increased Fe, Mg, to be related to magmatic-hydrothermal processes based on and Si and lower Al) are favored. Spectroscopically, the phen- (1) the close spatial association to the porphyry cluster in gitic compositions have longer-wavelength 2,200-nm Al-OH the Nabesna pluton, (2) the fact that it extends across litho- absorption feature positions. logic boundaries at the Bond Creek deposit, and (3) the near Unlike the above idealized simple system model, many absence of this (or any chlorite-dominant) class elsewhere in porphyry deposits form by multiple overprinting episodes of the Nabesna pluton. The similar but more broadly distributed alteration (e.g., Seedorff et al., 2005; Sillitoe, 2010). Spectral intermediate- to long-wavelength white mica (compare Figs. investigations on drill core from the Cretaceous Pebble por- 6, 7) and the linkage of long-wavelength signatures to phen- phyry Cu-Au-Mo deposit in southwestern Alaska established gitic chemistry in our microprobe analyses (Table 1) strongly that alteration zones within the deposit can be identified by suggest the phengitic white mica is also of magmatic-hydro- classifying the wavelength position of the 2,200-nm feature thermal origin. of the contained micas (Harraden et al., 2013). In agreement The spatial association of the clinochlore + white mica with the model by Halley et al. (2015), short Al-OH absorp- and longer-wavelength white mica classes to multielement tion features of white mica (2,190–2,201 nm) and pyrophyllite Cu-Mo-Au anomalies further supports a causative relation- (2,160–2,170 nm) predominate in the high-grade Cu-Au zone ship with hydrothermal activity (Fig. 11). At the Bond Creek associated with low-pH advanced argillic alteration in the deposit, anomalous concentrations of Cu, Mo, and Au occur eastern part of the deposit. However, longer-wavelength illite- in the upstream portion of the west and middle forks of Bond rich zones (with chemistry and spectral signatures similar to Creek in the immediate vicinity of longer-wavelength white our phengitic white micas [≥2,205–2,210 nm; Table 1]) that mica. Where this zone extends to the east across the middle locally overprint potassic alteration (Harraden et al., 2013, fork of Bond Creek, metal concentrations remain high (e.g., fig. 7A) also contain significant metals. Overprinting illite has loc. 4b, Fig. 11). The mixed spectral signatures at Orange Hill also been recognized at some other porphyry deposits (e.g., and the gullies to the northeast have highly anomalous metal Bingham, Utah; Parry et al., 2002; Red Chris Cu-Au porphyry concentrations. deposit, B.C., Canada; Norris, 2012). Although wavelength Anomalous Cu, Mo, and/or Au coincide with clinochlore + positions are not reported, the illite (± kaolinite) zones can white mica and long-wavelength absorption feature signa- be quite extensive (up to 600–800 m thick and lateral extents tures in several zones outside of the main porphyry Cu clus- of at least 500 × 500 m at Red Chris; Norris, 2012). In each ter (locs. 7–10; Fig. 11). Elevated metal concentrations in case, the presence of illite (and the longer-wavelength Al-OH some rock samples from these locations with up to 8,000 ppm absorption feature position that was identified at Pebble) is Cu, >50 ppm Mo, and >500 ppb Au indicate additional Cu- interpreted to record incursion of lower-temperature, near- Mo-Au mineralized areas that were previously unknown or neutral pH waters either during intermineral (e.g., Pebble; unreported. Gregory et al., 2013) or waning stages of magmatic-hydrother- The absence of significant clinochlore + white mica and mal activity (Red Chris; Norris, 2012). long-wavelength white mica signatures in most of the Wran- The widespread longer-wavelength absorption feature gellia terrane rocks coincides with a general lack of anomalous positions at the Orange Hill and Bond Creek deposits are multielement Cu-Mo ± Au concentrations. Only sporadic Cu consistent with either a deep-level or distal fringe exposure anomalies are present. Most of the occurrences described in of the deposit (below main mineralized shell) in the Halley the Wrangellia terrane are poorly documented and described et al. (2015) model. Alternatively, by analogy with reported as small polymetallic vein occurrences (e.g., locs. 11, 12; Fig. studies at Pebble, Bingham, and Red Chris, and based on 11) and occur in areas without rock exposure (e.g., loc. 12). the mica chemistry and spectral signatures at Bond Creek The abundance of mafic rock (inherently higher Cu concen- and Orange Hill, relatively low temperature fluids may have trations) and weathering of the Nikolai Greenstone (Richter, overprinted earlier alteration minerals. The additional clino- 1973) could account for sporadic high-Cu concentrations in chlore + white mica signature and high Cu, Mo, and/or Au stream sediment samples. in sediment samples draining areas with long-wavelength features support this latter interpretation. Regardless, the Absorption feature shifts in porphyry deposits phengitic compositions of the micas associated with por- Although airborne imaging spectroscopy data have been used phyry formation are on the whole sufficiently composition- for porphyry Cu exploration, there are few detailed inter- ally different to produce the longer-wavelength 2,200-nm pretations of these data in the literature (e.g., Cudahy et al., absorption features that are distinct from those found in plu- 2001; Berger et al., 2003, Coulter et al., 2007); however, there tonic and volcanic arc rocks not affected by the magmatic- are several studies at the hand-specimen scale (e.g., Alva- hydrothermal fluids. As has been observed in this and other Jimenez, 2011; Cohen, 2011; Cohen et al., 2011). Halley et studies, white mica wavelength position maps from imaging al. (2015) described a model of spectral variations related to spectrometers can be used to better understand hydrother- changes in white mica chemistry associated with changing mal systems and potentially serve as a vector to mineraliza- temperature and pH across an idealized porphyry deposit. In tion in some systems (Bierwirth et al., 2002; Laukamp et al, the core of the deposit, where low-pH conditions result from 2011; Yang et al., 2011; van Ruitenbeek et al., 2012; Swayze sulfur dissociation from rising, cooling, oxidized, sulfate-rich et al., 2014; Guo et al., 2017).

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 506 GRAHAM ET AL.

142°50'W 142°42'W 142°34'W ! ! ! ! 62°17'N ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! Porphyry deposit !

+ prospect area ! (”porphyry cluster”) ! !!

62°15'N 7 ! 8 ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! 5 ! ! ! !

! 10 ! 4 ! ! ! ! !!!!! 4b ! ! ! 3 !!! ! !!!!!! ! 62°13'N !! ! !!! ! ! ! 1a ! !!! ! ! !! ! ! ! ! ! ! ! ! ! 1 ! 2 ! 6 ! ! ! ! ! ! Nabesna pluton ! ! ! ! ! ! Wrangellia terrane ! ! ! ! ! ! ! 62°11'N ! ! !! ! ! ! !! ! ! ! 11 ! ! ! ! !

! ! !

! 12 ! ! ! ! ! ! ! ! ! ! !

62°9'N ! ! ! ! ! 9 ! ! 4 km ! ! 2,200 nm absorption feature wavelength position (nm) Cu or Mo or Au >15.5 times median ≤ 2,196 2,199 2,202 2,205 value of conterminous U.S. 2,197 2,200 2,203 2,206 Cu, Mo, and Au all below 2,198 2,201 2,204 ≥ 2,207 15.5 times threshold

Fig. 11. Overlay of mineral chemistry on white mica wavelength map (slightly expanded area from Fig. 7) showing the association of long-wavelength spectral features to highly anomalous metals (Cu, Mo, and/or Au) with the area of known porphyry occurrences and newly identified zones (e.g., locs. 7–9). Sparse elevated metal(s) concentrations occur to the south but are mostly within proximity of the Nabesna pluton-Wrangellia terrane boundary. Note the presence of shortest-wavelength white mica features in both plutonic and volcanic arc rocks distal from the porphyry prospects.

Exploration implications platforms has been used commercially for approximately 20 Satellite remote sensing techniques have been used for several years (Cocks et al., 1998; Coulter et al., 2007). The power of decades to map alteration mineral patterns and identify explo- these high-resolution imaging spectrometers lies in the dense ration targets, commonly at 15- to 90-m pixel size (Goetz and spectral sampling, which permits the identification of subtle Rowan, 1981; van der Meer et al., 2012). High spectral reso- changes in spectral signatures related to mineral chemistry, as lution imaging spectroscopy largely carried out on airborne observed in this study.

There have been limited published airborne imaging spec- remote sensing technologies include airborne imaging and troscopy studies in the Canadian arctic (e.g., Rogge et al., field-deployable imaging spectrometers and multispectral 2014; Laakso et al., 2015, 2016), and to our knowledge, there sensors on multiple satellite platforms that offer high spatial are no published accounts of the application to geologic or resolution (for example WorldView3 with 30-cm pixels), long- mineral deposit studies in Alaska. Difficulties that have likely term time series (30 yr of data from Landsat series satellites), prevented such studies in the past include the requirement of and the possibility for repeat data collection within the short clear weather during collection and the need for exposed bed- Alaska growing season by many satellite sensors (e.g., Land- rock. Furthermore, whereas lithologic determinations can be sat 8, Sentinel-2). Planned satellite missions such as HyspIRI made on rocks with significant lichen cover (~80% by surface (Lee et al., 2015) and EnMAP (Guanter et al., 2016) have the area; Rogge et al., 2014), mineral compositions can be masked potential to provide imaging spectrometer data at 30-m pixel (e.g., Laakso et al., 2015). However, our study demonstrates size. that there are areas in Alaska where this technology could provide direct information for mineral exploration. Despite Conclusions rugged, poorly illuminated terrane, the survey data were suf- This study demonstrates that remotely sensed spectral data ficient to establish distributions of spectrally dominant miner- can be used to identify porphyry Cu-related alteration and als and to identify subtle changes in mineral chemistry that mineralized rock in a remote part of Alaska. Zonation of characterize known porphyry deposits. These signatures can mapped minerals and mineral groups such as white mica, be broad in areas with extensive exposure, as seen in Bond clinochlore + white mica, and chlorite/epidote are consistent Creek, but can also be recognized in more poorly exposed with porphyry models and other studies of porphyry deposits. areas (e.g., Orange Hill). Importantly, focused geochemical Several key conclusions can be drawn from this work: sampling to investigate areas with similar chlorite and white mica spectral signatures resulted in the identification of two 1. The distributions of white mica, chlorite/epidote, carbon- previously undocumented Cu (Au-Mo) occurrences north of ates, and clays utilizing solely the spectrally dominant min- the main porphyry deposit cluster (locs. 7, 8; Fig. 11). eral map derived from imaging spectrometer data only Numerous investigations of other deposit types such as crudely correspond to mapped rock types. However, when volcanogenic massive sulfide (VMS; Jones et al., 2005; van combined with reconnaissance mapping, spectral data can Ruitenbeek et al., 2012; Laakso et al., 2015), orogenic Au help identify areas of potential (magmatic-) hydrothermal (Huntington et al., 2006; Arne et al., 2016), epithermal (Bier- alteration. wirth et al., 2002; Bedini et al., 2009), and iron oxide Cu-Au 2. Two spectrally distinct chlorite-white mica classes (clino- deposits (Laukamp et al., 2011) from around the world indi- chlore + white mica vs. chlorite + white mica) predominate cate large alteration halos associated with mineral chemistry across the study area. The spatial association of the clino- variations. These chemistry variations can be identified by chlore + white mica class with porphyry mineralization imaging spectroscopy in drill core, outcrop, or airborne stud- in rocks from both terranes supports a relationship with ies. The geologic and tectonic settings of mountainous regions hydrothermal processes. in Alaska with significant exposure are permissive of (and in 3. Longer-wavelength white mica 2,200-nm absorption fea- some cases known to host) a number of these other deposit ture positions coincide with the clinochlore + white mica types (e.g., VMS and orogenic- and intrusion-related Au assemblage in the Nabesna pluton and altered volcanic systems). Design of appropriate resolution surveys in these rocks at Bond Creek. The longer-wavelength signatures regions could provide invaluable targeting information in (≥2,206 nm) are a consequence of distinctive chemistry these remote areas. Follow-up sampling and investigations (lower Al and Na and higher Si and Mg) compared to the can then be focused into specific areas, greatly reducing sub- chemistry of white mica with a shorter-wavelength position sequent exploration costs. (≤2,202 nm). Our study focused on analysis of spectral features in areas 4. Focused sampling of stream sediments guided by the map of exposed rock and sediment with clear mineral absorption of longer-wavelength white mica yielded anomalous con- features. Because significant portions of the Orange Hill centrations of Cu, Au, and Mo in areas of previously unrec- deposit are covered by vegetation, follow-on studies should ognized mineralized rock. examine vegetation and soils within mineralized areas in com- 5. An evolving understanding of mineralogical and spectral parison to nonmineralized areas. Variations in soil nutrient variations within and among deposit types is important for and cation content have the potential to lead to variations in interpreting airborne imaging spectrometer data; our study vegetation cover, species composition, and biochemical con- indicates that imaging spectroscopy (hyperspectral imag- centrations (pigments, nitrogen, lignin, cellulose), resulting ing) could prove useful in identifying a number of different in contrasting spectral signatures that could be used to more deposit types in appropriate parts (e.g., exposed mountain- completely define boundaries of mineralized zones. Because ous regions) of Alaska. much of Alaska is vegetated, identification of geobotanical signatures over mineralized rock would be important to utiliz- Acknowledgments ing remotely sensed imaging spectroscopy over more areas. This project was funded through the U.S. Geological Survey Although it may be difficult to establish a robust signature, (USGS) Mineral Resources Program. We want to acknowl- current remote sensing technology offers spectral, spatial, edge the leadership roles played by USGS colleagues Richard and temporal sampling advantages over past applications Goldfarb (retired) and Trude King in the proposal stage of the of remote sensing to this problem. Advantages of current project. William Benzel, Heather Lowers, and David Adams

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 508 GRAHAM ET AL.

(USGS) provided XRD and EPMA analyses of samples and Cohen, J.F., 2011, Mineralogy and geochemistry of hydrothermal altera- helpful discussion of the results. We would like to thank the tion at the Ann-Mason porphyry copper deposit, Nevada: Comparison of large-scale ore exploration techniques to mineral chemistry: M.S. the- U.S. National Park Service for granting us access for helicop- sis, Corvallis, Oregon, Oregon State University, 111 p. (plus appendices, ter-supported fieldwork in Wrangell-St. Elias National Park. p. 112–580). We are grateful to Drs. Anupma Prakash and Marcel Buch- Cohen, J.F., Dilles, J.H., Tosdal, R.M., and Halley, S., 2011, Compositional horn for providing logistical assistance throughout the deploy- variations in hydrothermal white micas and chlorites in a porphyry Cu sys- ment of the airborne hyperspectral sensor during the rainiest tem at Yerington: Geological Society of America Abstracts with Programs, v. 43, no. 4, p. 63–64. summer on record in Fairbanks, Alaska. We also thank the Coulter, D.W., Hauff, P.L, and Kerby, W.L., 2007, Airborne hyperspectral Alaska Science Center, and Andy Allard, Mike McKinnon, and remote sensing: Exploration 07: Fifth Decennial International Conference Fenumiai Ilalio in particular, for their logistical support. The on Mineral Exploration, Toronto, Canada, September 9–12, 2007, Proceed- manuscript was greatly improved based on reviews by John ings, p. 375–386. “Lyle” Mars (USGS), Steve Ludington (USGS emeritus), and Crosta, A.P., Sabine, C., and Taranik, J.V., 1998, Hydrothermal alteration mapping at Bodie, California, using AVIRIS hyperspectral data: Remote Benoit Rivard (University of Alberta, Canada). Any use of Sensing of Environment, v. 65, p. 309–319. trade, firm, or product names is for descriptive purposes only Cudahy, T.J., Wilson, J., Hewson, R., Linton, P., Harris, P., Sears, M., Okada, and does not imply endorsement by the U.S. Government. K., and Hackwell, J.A., 2001, Mapping porphyry-skarn alteration at Yer- ington, Nevada, using airborne hyperspectral VNIR-SWIR-TIR imaging REFERENCES data: International Geoscience and Remote Sensing Symposium 2001 (IGARSS’01), Institute of Electrical and Electronics Engineers (IEEE), Alva-Jimenez, T.R., 2011, Variations in hydrothermal white mica and chlo- University of South Wales, Sydney, Australia, July 9–13, 2001: Proceedings, rite composition in the Highland Valley porphyry Cu-Mo district, British p. 631–633. Columbia, Canada: M.Sc. thesis, Vancouver, University of British Colum- Deer, W.A., Howie, R.A., and Zussman, J., 1992, An Introduction to the rock- bia, 225 p. forming minerals, 2nd ed.: New York, Wiley, 696 p. Arne, D., House, E., Pontual, S., and Huntington, J., 2016, Hyperspectral Duke, E., 1994, Near infrared spectra of white mica, Tschermak substitution, interpretation of selected drill cores from orogenic gold deposits in central and metamorphic reaction progress: Implications for remote sensing: Geol- Victoria, Australia: Australian Journal of Earth Sciences, v. 63, p. 1003– ogy, v. 22, p. 621–624. 1025, doi: 10.1080/08120099.2016.1223171. Eberl, D.D., 2003, User’s guide to RockJock—A program for determining Athey, J.E., Werdon, M.B., Twelker, Evan, and Henning, M.W., 2016, Alas- quantitative mineralogy from powder X-ray diffraction data: U.S. Geologi- ka’s mineral industry 2015: Alaska Division of Geological and Geophysical cal Survey Open-File Report 2003-78, 47 p. Surveys Special Report 71, 45 p., doi: 10.14509/29687. Feng, J., Rogge, D., and Rivard, B., 2018, Comparison of lithological mapping Bedini, E., 2012, Mapping alteration minerals at Malmbjerg molybdenum results from airborne hyperspectral VNIR-SWIR, LWIR and combined deposit, central East Greenland, by Kohonen self-organizing maps and data: International Journal of Applied Earth Observation and Geoinforma- matched filter analysis of HyMap data: International Journal of Remote tion, v. 64, p. 340–353, doi: 10.1016/j.jag.2017.03.003. Sensing, v. 33, p. 939–961, doi: 10.1080/01431161.2010.542202. Freeman, L.K., Athey, J.E., Lasley, P.S., and Van Oss, E.J., 2015, Alaska’s Bedini, E., van der Meer, F., and van Ruitenbeek, F., 2009, Use of HyMap mineral industry 2014: Alaska Division of Geological and Geophysical Sur- imaging spectrometer data to map mineralogy in the Rodalquilar caldera, veys Special Report 70, 60 p., doi: 10.14509/29515. southeast Spain: International Journal of Remote Sensing, v. 30, p. 327– Gaudette, H.E., Eades, J.L., and Grim, R.E., 1964, Symposium on structural 348, doi: 10.1080/01431160802282854. aspects of layer silicates: Nature of Illite Clays and Clay Minerals, v. 13, Berg, H.C., Jones, D.L., and Richter, D.H., 1972, Gravina-Nutzotin p. 33−48. belt—Tectonic significance of an upper Mesozoic sedimentary and volcanic sequence in southern and southeastern Alaska, in Geological Sur- Gillespie, C.D., 1970, Geology of the central Bond Creek area, Nabensa, vey Research 1972: U.S. Geological Survey Professional Paper 800-D, Alaska: M.S. thesis, Corvallis, Oregon, Oregon State University, 67 p., p. D1–D24. illust., 1 map. Berger, B.R., King, T.V.V., Morath, L.C., and Phillips, J.D., 2003, Utility Goetz, A.F., and Rowan, L.C., 1981, Geologic remote sensing: Science, of high-altitude infrared spectral data in mineral exploration: Applica- v. 211, p. 781–791. tion to northern Patagonia mountains, Arizona: Economic Geology, v. 98, Goldfarb, R.J., Anderson, E.D., and Hart, C.R., 2013, Tectonic setting of p. 1003–1018. the Pebble and other copper-gold-molybdenum porphyry deposits within Bierwirth, P., Huston, D., and Blewett, R., 2002, Hyperspectral mapping of the evolving middle Cretaceous continental margin of northwestern North mineral assemblages associated with gold mineralization in the Central Pil- America: Economic Geology, v. 108, p. 405–419. bara, Western Australia: Economic Geology, v. 97, p. 819–826. Graham, G.E., Kelley, K.D., Holm-Denoma, C.S., Ayuso, R.A., Kokaly, R.F., Clark, R.N., 1999, Spectroscopy of rocks and minerals, and principles of spec- Hoefen, T.M., and Selby, D., 2016, Geochronology of Early Cretaceous troscopy, in Rencz, A., ed., Manual of remote sensing, v. 3, Remote sensing porphyry Cu deposits in eastern Alaska: 35th International Geological Con- for the earth sciences, 3rd ed.: New York, Wiley, p 3–58. gress, Capetown, South Africa, August 27-September 4, 2016, www.ameri- Clark, R.N., and Roush, T., 1984, Reflectance spectroscopy: Quantitative cangeosciences.org/sites/default/files/igc/4858.pdf. analysis techniques for remote sensing applications: Journal of Geophysical Graham, G.E., Johnson, M.R., Kelley, K.D., Kokaly, R.F., and Hoefen, T.M., Research, v. 89, p. 6329–6340. 2017, Whole rock, soil, sediment, X-ray diffraction, and electron micro- Clark, R.N., Swayze, G.A., Livo, K.E., Kokaly, R.F., King, T.V.V., Dalton, probe analyses of samples from the Orange Hill-Bond Creek area, Nabesna J.B., Vance, J.S., Rockwell, B.W., Hoefen, T., and McDougal, R.R., 2002, quadrangle, Alaska: U.S. Geological Survey, doi: 10.5066/F78051K1. Surface reflectance calibration of terrestrial imaging spectroscopy data: A Granitto, M., Bailey, E.A., Schmidt, J.M., Shew, N.B., Gamble, B.M., and tutorial using AVIRIS, in R.O. Green, ed., Proceedings of the 10th Jet Pro- Labay, K.A., 2011, Alaska geochemical database (AGDB)—Geochemical pulsion Laboratory (JPL) airborne science workshop: JPL publication 02-1, data for rock, sediment, soil, mineral, and concentrate sample media: U.S. http://speclab.cr.usgs.gov/PAPERS.calibration.tutorial. Geological Survey Data Series 637, http://pubs.usgs.gov/ds/637/. Clark, R.N., Swayze, G.A., Livo, K.E., Kokaly, R.F., Sutley, S.J., Dalton, J.B., Gregory M.J., Lang J.R., Gilbert S., and Hoal K.O., 2013, Geometallurgy of McDougal, R.R., and Gent, C.A., 2003, Imaging spectroscopy: Earth and the Pebble copper-gold-molybdenum deposit, Alaska: Implications for gold planetary remote sensing with the USGS Tetracorder and expert systems: distribution and paragenesis: Economic Geology, v. 108, p. 463–482. Journal of Geophysical Research: Planets, v. 108(E12), p. 5-1–5-43. Guanter, L., Segl, K., Foerster, S., Hollstein, A., Rossner, G., Chlebek, C., Cocks, T., Jenssen, R., Stewart, A., Wilson, I., and Shields, T., 1998, The Storch, T., Heiden, U., Mueller, A., Müller, R., and Sang, B., 2016, Over- HyMap airborne hyperspectral sensor: The system, calibration and perfor- view of the EnMAP imaging spectroscopy mission: International Geo- mance: European Association of Remote Sensing Laboratories (EARSeL) science and Remote Sensing Symposium (IGARSS 2016), Institute of Workshop on Imaging Spectroscopy, Zurich, October 6–8, 1998, Extended Electrical and Electronics Engineers (IEEE), Beijing, China, July 10–15, Abstracts, p. 37–43. 2016, Proceedings, p. 261–263.

Guo, N., Cudahy, T., Juxing, T., and Qingxi, T., 2017, Mapping white mica Laakso, K., Rivard, B., Peter, J.M., White, H.P., Maloley, M., Harris, J., and alteration associated with the Jiama porphyry-skarn Cu deposit, central Rogge, D., 2015, Application of airborne, laboratory and field hyperspectral Tibet using field SWIR spectrometry: Ore Geology Reviews, doi: 10.1016/j. methods to mineral exploration in the Canadian Arctic: Recognition and oregeorev.2017.07.027. characterization of volcanogenic massive sulfide-associated hydrothermal Halley, S., Dilles, J.H., and Tosdal, R.M., 2015, Footprints: Hydrothermal alteration in the Izok Lake deposit area, Nunavut, Canada: Economic Geol- alteration and geochemical dispersion around porphyry copper deposits, ogy, v. 110, p. 925-941. SEG Newsletter, no. 100, p. 1, 12−17. Laakso, K., Rivard, B., and Rogge, D., 2016, Enhanced detection of gossans Harraden, C.L., McNulty, B.A., Gregory, M.J., and Lang, J.R., 2013, Short- using hyperspectral data: Example from the Cape Smith Belt of northern wave infrared spectral analysis of hydrothermal alteration associated with Quebec, Canada: ISPRS Journal of Photogrammetry and Remote Sensing, the Pebble porphyry copper-gold-molybdenum deposit, Iliamna, Alaska: v. 114, p. 137−150. Economic Geology, v. 108, p. 483−494. Laukamp, C., Cudahy, T., Thomas, M., Jones, M., Cleverley, J.S., and Oliver, Harris, J.R., Rogge, D., Hitchcock, R., Ijewliw, O., and Wright, D., 2005, N.H.S., 2011, Hydrothermal mineral alteration patterns in the Mount Isa Mapping lithology in Canada’s Arctic: Application of hyperspectral data inlier revealed by airborne hyperspectral data: Australian Journal of Earth using the minimum noise fraction transformation and matched filtering: Sciences, v. 58, p. 917−936. Canadian Journal of Earth Sciences, v. 42, p. 2173−2193. Lee, C.M., Cable, M.L., Hook, S.J., Green, R.O., Ustin, S.L., Mandl, D.J., Herrmann, W., Blake, M., Doyle., M., Huston, D., Kamprad, J., Merry, N., and Middleton, E.M., 2015, An introduction to the NASA hyperspectral and Pontual, S., 2001, Short wavelength infrared (SWIR) spectral analysis infrared imager (HyspIRI) mission and preparatory activities: Remote of hydrothermal alteration zones associated with base metal sulfide depos- Sensing of Environment, v. 167, p. 6−19. its at the Rosebery and Western Tharsis, Tasmania and Highway-Reward, Linn, G.W., 1973, Geology of Orange Hill, Alaska: M.A. thesis, Berkeley, Uni- Queensland: Economic Geology, v. 96, p.939−955. versity of California, 119 p. Hoefen, T.M., Johnson, M.R., Kokaly, R.F., Graham, G.E., Kelley, K.D., Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-min- and Hubbard, B.E., 2018, Corescan© Hyperspectral Core Imager, Mark eralization zoning in porphyry ore deposits: Economic Geology, v. 65, III system data collected for the characterization of mineral resources p. 373–408. near Nabesna, Alaska, 2014–2016: U.S. Geological Survey, doi: 10.5066/ Manuszak, J.D., Ridgway, K.D., Trop, J.M., and Gehrels, G.E., 2007, Sedimen- F7057DWM. tary record of the tectonic growth of a collisional continental margin: Upper Hubbard, B.E., Rowan, L.C., Dusel-Bacon, C., and Eppinger, R.G., 2007, Jurassic-Lower Cretaceous Nutzotin Mountains sequence, eastern Alaska Geologic mapping and mineral resource assessment of the Healy and Tal- Range, Alaska: Geological Society of America Special Paper 431, p. 345–377. keetna Mountains quadrangles, Alaska, using minimal cloud- and snow- Mars, J.C., 2013, Hydrothermal alteration maps of the central and southern cover ASTER data: U.S. Geological Survey Open-File Report 2007-1046, Basin and Range province of the United States compiled from Advanced 18 p., https://pubs.usgs.gov/of/2007/1046/ofr2007-1046.pdf. Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data Hudson, T.L., 2003, Alaska resource data file—Nabesna quadrangle: U.S. (ver. 1.1, April 8, 2014): U.S. Geological Survey Open-File Report 2013– Geological Survey Open-File Report 03-446, 229 p., https://ardf.wr.usgs. 1139, 6 p., 13 plates, scale 1:1,300,000, doi: 10.3133/ofr20131139. gov/ardf_data/Nabesna.pdf, accessed on May 3, 2017. Mendenhall, W.C., and Schrader, F.C., 1903, Copper deposits of the Mount Huntington, J.F., Quigley, M., Yang, K., Roache, T., Young, C., Roberts, I., Wrangell region, Alaska: U.S. Geological Survey Bulletin 213, p. 141–148. and Mason, P, 2006, A geological overview of HyLogging 18,000 m of core Moffit, F.H., and Knopf, A., 1910, Mineral resources of the Nabesna-White from the Eastern Goldfields of Western Australia,in Dominy, S., ed., Pro- River district, Alaska: U.S. Geological Survey Bulletin 417, 64 p., 2 sheets, ceedings of the 6th International Mining Geology Conference, Darwin, scale 1:250,000. Australia, August 21–23, 2006: Carlton, Victoria, Australia, Australasian Moffit, F.H., and Wayland, R.G., 1943, Geology of the Nutzotin Mountains, Institute of Mining and Metallurgy, p. 45–50. Alaska, with a section on the igneous rocks: U.S. Geological Survey Bulletin John, D.A., Ayuso, R.A., Barton, M.D., Blakely, R.J., Bodnar, R.J., Dilles, 933-B, p. 103–174. J.H., Gray, Floyd, Graybeal, F.T., Mars, J.C., McPhee, D.K., Seal, R.R., Moore, D.M. and Reynolds, R.C., 1997, X-ray Diffraction and the Identifica- Taylor, R.D., and Vikre, P.G., 2010, Porphyry copper deposit model: U.S. tion and Analysis of Clay Minerals. Oxford University Press, 378 p. Geological Survey Scientific Investigations Report 2010–5070–B, 169 p. Norris, J.R., 2012, Evolution and alteration at the Red Chris Cu-Au porphyry Jones, S., Herrmann, W., and Gemmell, J.B., 2005, Short wavelength infra- deposit east zone, northwestern British Columbia, Canada: M.Sc. thesis, red spectral characteristics of the HW horizon: Implications for exploration Vancouver, University of British Columbia, 194 p. in the Myra Falls volcanic-hosted massive sulfide camp, Vancouver Island, Parry, W.T., Jasumback, M., and Wilson, P.N., 2002, Clay mineralogy of phyl- British Columbia, Canada: Economic Geology, v. 100, p. 273−294. lic and intermediate argillic alteration at Bingham, Utah: Economic Geol- Kokaly, R.F., 2011, PRISM: Processing routines in IDL for spectroscopic ogy, v. 97, p. 221−239. measurements (installation manual and user’s guide, version 1.0): U.S. Geo- Pilgrim, E.R., 1930, Upper Nabesna, Chisana, and Snag River area: Alaska logical Survey Open-File Report 2011–1155, 432 p., https://pubs.usgs.gov/ Territorial Department of Mines Miscellaneous Report 78-2, 37 p., 1 sheet, of/2011/1155/. scale 1 inch = 500 feet, doi: 10.14509/832. Kokaly, R.F., and Skidmore, A.K., 2015, Plant phenolics and absorption fea- Plafker, G., and Berg, H.C., 1994, Overview of the geology and tectonic evo- tures in vegetation reflectance spectra near 1.66µ m: International Journal lution of Alaska, in Plafker, George, and Berg, H.C., eds., The geology of of Applied Earth Observation and Geoinformation, v. 43, p. 55−83. Alaska: Geological Society of America, v. G-1, p. 989−1021. Kokaly, R.F., Asner, G.P., Ollinger, S.V., Martin, M.E., and Wessman, C.A., Post, J.L., and Noble, P.N., 1993, The near-infrared combination band fre- 2009, Characterizing canopy biochemistry from imaging spectroscopy and quencies of dioctahedral smectites, micas, and illites: Clays and Clay Miner- its application to ecosystem studies: Remote Sensing of Environment, als, v. 41, p. 639–644. v.113, p. S78−S91. Richter, D.H., 1973, Reconnaissance geologic map of the Nabesna A-4 quad- Kokaly, R.F., King, T.V.V., and Hoefen, T.M., 2013, Surface mineral maps of rangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Afghanistan derived from HyMap imaging spectrometer data, version 2: U.S. Map 789, 1 sheet, scale 1:63,360. Geological Survey Data Series 787, 29 p., https://pubs.usgs.gov/ds/787/. Richter, D.H., Lanphere, M.A., and Matson, N.A., Jr., 1975a, Granitic pluto- Kokaly, R.F., Hoefen, T.M., King, T.V.V., and Johnson, M.R., 2017a, Airborne nism and metamorphism, eastern Alaska Range, Alaska: Geological Society imaging spectroscopy data collected for characterizing mineral resources of America Bulletin 86, p. 819–829. near Nabesna, Alaska, 2014: U.S. Geological Survey Data Release, doi: Richter, D.H., Singer, D.A., and Cox, D.P., 1975b, Mineral resources map 10.5066/F7DN435W. of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Kokaly, R.F., Clark, R.N., Swayze, G.A., Livo, K.E., Hoefen, T.M., Pearson, Field Studies Map MF-655K, 1 sheet, scale 1:250,000. N.C., Wise, R.A., Benzel, W.M., Lowers, H.A., Driscoll, R.L., and Klein, Rogge, D., Rivard, B., Segl, K., Grant, B., and Feng, J., 2014, Mapping of A.J., 2017b, USGS Spectral Library Version 7: U.S. Geological Survey Data NiCu-PGE ore hosting ultramafic rocks using airborne and simulated Series 1035, 61 p., doi: 10.3133/ds1035. EnMAP hyperspectral imagery, Nunavik, Canada: Remote Sensing of Envi- Kokaly, R.F., Johnson, M.R., Graham, G.E., Hoefen, T.M., Kelley, K.D., and ronment, v. 152, p. 302−317. Hubbard, B.E., 2018, Imaging spectrometer reflectance data, mineral pre- Ross, C.S., and Hendricks, S.B., 1945, Minerals of the montmorillonite dominance map, and white mica wavelength position map, Nabesna Quad- group, their origin and relation to soils and clays: U.S. Geological Survey rangle, Alaska: U.S. Geological Survey, doi: 10.5066/F7NV9H6F. Professional Paper 205-B, p. 23−79.

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021 510 GRAHAM ET AL.

Seedorff, E., Dilles, J.H., Proffett, J.M., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A., and Barton, M.D., 2005, Porphyry deposits: Char- acteristics and origin of hypogene features: Economic Geology 100th Anni- versary Volume, p. 251–298. Garth Graham is a research economic geologist Silberman, M.L, Morton, J.L., Cox, D.C., and Richter, D.H., 1977, Potas- with the U.S. Geological Survey. He received his sium-argon ages of disseminated copper and molybdenum mineralization Ph.D. degree in economic geology from the Colo- in the Klein Creek and Nabesna plutons eastern Alaska Range: U.S. Geo- rado School of Mines, Golden, Colorado, in 2011. logical Survey Circular 751-B, p. B54-B56. Most of his research has been focused around base Sillitoe, R.H., 2010, Porphyry copper systems: Economic Geology, v. 105, and precious metals deposits in Alaska. Research p. 3−41. interests include exploration geochemistry, ore Smith, D.B., Cannon, W.F., Woodruff, L.G., Solano, F., Kilburn, J.E., and genesis studies, and imaging spectrometry in mineral exploration. Fey, D.L., 2013, Geochemical and mineralogical data for soils of the conterminous United States: U.S. Geological Survey Data Series 801, 19 p., http:// pubs.usgs.gov/ds/801/. Snyder, D.C., and Hart, W.K., 2007, The White Mountain granitoid suite: Isotopic constraints on source reservoirs for Cretaceous magmatism within the Wrangellia terrane: Geological Society of America Special Paper 431, p. 379–399, doi: 10.1130/2007.2431(15). Swayze, G.A., 1997, The hydrothermal and structural history of the Cuprite mining district, southwestern Nevada; an integrated geological and geophysical approach: Unpublished Ph.D. dissertation, Boulder, Colorado, University of Colorado, 399 p. Swayze, G.A., Clark, R.N., Goetz, A.F.H., Livo, K.E., Breit, G.N., Kruse, F.A., Sutley, S.J., Snee, L.W., Lowers, H.A., Post, J.L., Stoffregen, R.E., and Ashley, R.P., 2014, Mapping advanced argillic alteration at Cuprite, Nevada, using imaging spectroscopy: Economic Geology v. 109, p. 1179–1221. Thompson, A.J.B., Hauff, P.L., and Robitaille, A.J., 1999, Alteration mapping in exploration: Application of short-wave infrared (SWIR) spectroscopy: SEG Newsletter, no. 39, p. 1–13. Van Alstine, R.E., and Black, R.F., 1946, Mineral deposits at Orange Hill, Alaska: U.S. Geological Survey Open-File Report 43-17, 28 p., 1 plate, https://pubs.usgs.gov/of/1943/0017/report.pdf. van der Meer, F., 2006, Indicator kriging applied to absorption band analysis in hyperspectral imagery: A case study from the Rodalquilar epithermal gold mining area, SE Spain: International Journal of Applied Earth Obser- vation and Geoinformation, v. 8, p. 61–72. van der Meer, F.D., Van der Werff, H.M., van Ruitenbeek, F.J., Hecker, C.A., Bakker, W.H., Noomen, M.F., van der Meijde, M., Carranza, E.J.M., de Smeth, J.B., and Woldai, T., 2012, Multi- and hyperspectral geologic remote sensing: A review: International Journal of Applied Earth Observa- tion and Geoinformation, v. 14, p. 112–128. van Ruitenbeek, F.J.A., Cudahy, T.J., van der Meer, F.D., and Hale, M., 2012, Characterization of the hydrothermal systems associated with Archean VMS-mineralization at Panorama, Western Australia, using hyperspectral, geochemical, and geothermometric data: Ore Geology Reviews, v. 45, p. 33–46. Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M, comps., 2015, Geologic map of Alaska: U.S. Geological Survey Scientific Investigations Map 3340, 196 p., 2 sheets, scale 1:1,584,000, doi: 10.3133/sim3340. Yang, K., Huntington, J.F., Gemmell, J.B., and Scott, K.M., 2011, Variations in composition and abundance of white mica in the hydrothermal alteration system at Hellyer, Tasmania, as revealed by infrared reflectance spectroscopy: Journal of Geochemical Exploration, v. 108, p. 143–156.

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/113/2/489/4092994/489-510.pdf by guest on 03 October 2021