An exploration hydrogeochemical study at the giant ...

research-articleResearch article12X10.1144/1467-7873/11-RA-070R. G. EppingerICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska 2012

An exploration hydrogeochemical study at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, USA, using high resolution ICP-MS Robert G. Eppinger, David L. Fey, Stuart A. Giles, Karen D. Kelley & Steven M. Smith U.S. Geological Survey, PO Box 25046, MS 973, Denver, CO, USA *Corresponding author (e-mail: [email protected])

ABSTRACT: A hydrogeochemical study using high resolution ICP-MS was under-

taken at the giant Pebble porphyry Cu-Au-Mo deposit and surrounding mineral occurrences. Surface water and groundwater samples from regional background and the deposit area were collected at 168 sites. Rigorous quality control reveals impressive results at low nanogram per litre (ng/l) levels. Sites with pH values below 5.1 are from ponds in the Pebble West area, where sulphide-bearing rubble crop is thinly covered. Relative to other study area waters, anomalous concentrations of Cu, Cd, K, Ni, Re, the REE, Tl, SO42- and F- are present in water samples from Pebble West. Samples from circum-neutral waters at Pebble East and parts of Pebble West, where cover is much thicker, have anomalous concentrations of Ag, As, In, Mn, Mo, Sb, Th, U, V, and W. Low-level anomalous concentrations for most of these elements were also found in waters surrounding nearby porphyry and skarn mineral occurrences. Many of these elements are present in low ng/l concentration ranges and would not have been detected using traditional quadrupole ICP-MS. Hydrogeochemical exploration paired with high resolution ICP-MS is a powerful new tool in the search for concealed deposits. KEYWORDS: porphyry Cu, exploration, hydrogeochemistry, high resolution ICP-MS, Alaska Exploration for economically important mineral deposits is becoming ever more challenging as it extends increasingly into areas of limited to no outcrop. It is critical, therefore, to develop new and innovative exploration methods that will assist in the search for concealed deposits. Hydrogeochemistry, coupled with extremely high analytical sensitivity, is a technique that helps to meet this challenge, and was applied at the concealed giant Pebble Cu-Au-Mo porphyry deposit of SW Alaska. Pebble contains one of the largest reserves and resources of Cu and Au in the world. The combined West and East zones contain measured and indicated resources of 5 942 million tonnes (Mt) at 0.42% Cu (= 55 billion lb Cu), 0.35 g/t Au (= 66.9 million oz Au), and 250 ppm Mo (= 3.3 billion lb Mo), an inferred resource of 4 835 Mt at 0.24% Cu, 0.26 g/t Au, and 215 ppm Mo (0.30% Cu equivalent), and contains significant concentrations of Ag, Pd, and Re (Northern Dynasty Minerals 2011). Much of the Pebble deposit remains open at depth. Exploration-oriented geochemical and geophysical research at Pebble began in 2007 by the U.S. Geological Survey (USGS), as part of a larger nation-wide project focused on evaluating and developing new methods of exploring for concealed deposits. Geochemical investigations include surface water (ponds, streams, and emerging springs), pond sediment, soil leach, and till indicator mineral studies. Presented here are findings from the surface water hydrogeochemical studies in and around Pebble. This report supercedes previous hydrogeochemical reports by Eppinger et al. (2009, 2010). Findings from other aspects of the USGS studies are described in Anderson et al. (2010), Bedrosian et al. (2009), Kelley et al. (2009, 2010, Geochemistry: Exploration, Environment, Analysis, Vol. 12, 2012, pp. 211–226 DOI: 10.1144/1467-7873/11-RA-070

2011a, 2011b), Minsley et al. (2008), Shah et al. (2009) and Smith et al. (2009). These studies have taken place prior to any surface disturbance of the deposit; exploration activity has been limited to drilling and done using helicopter support to minimize surficial disturbances. The Pebble deposit was chosen for study because it is concealed, undisturbed (except for exploration drill-holes), wellconstrained at depth by drill-hole geology and geochemistry, and is one of the largest porphyry Cu-Au-Mo systems in the world. Further, the discovery of Pebble opens a large area of southwestern Alaska as prospective for porphyry deposits, since they typically occur in clusters within magmatic belts (Sillitoe 1972; Seedorf et al. 2005). The goals of this study are to: (1) determine whether the concealed deposit can be detected with surface samples; (2) better understand the processes of metal migration from the deposit to the surface; and (3) test existing and develop new methods for assessing mineral resources in similar concealed terrains. In 2007, regional background water samples were collected, along with local waters where present, along a 7-km long east– west soil sample transect across the deposit (40 water samples). In 2008, these were augmented with 78 more water samples, mostly from ponds over and around the deposit area. In 2009 and 2010, an additional 50 water samples were collected from background areas and from mineralized occurrences distal to Pebble, resulting in a combined total of 168 samples (Fig. 1). Existing publically-available regional geochemical data covering the Pebble area are sparse at best. The 1º × 3 º quadrangle-based USGS Alaska Mineral Resource Assessment stream-sediment sampling program of the 1970s through early 1467-7873/12/$15.00 © 2012 AAG/Geological Society of London

R. G. Eppinger et al.

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Emerging springs, n = 39 Ponds and streams, n = 129 Bristol Fault Bay PLP exploration zone PLP claim boundary Pebble West outline = 0.3% Cu equiv. Pebble East outline = 0.6% Cu equiv.

Pebble ALASKA

Fig. 1. Location and sample site map for (a) deposit area and (b) and surrounding region. PLP, Pebble Limited Partnership.

1990s did not include the Iliamna quadrangle, in which Pebble is located. The US Department of Energy’s National Uranium Resource Evaluation program of the late 1970s to early 1980s included the collection of pond sediment samples in the Iliamna quadrangle, but no samples were collected within a 6-km radius of the Pebble deposit and the closest down-gradient pond sediment sample was collected c. 11 km distant (Anderson et al. 2009; US Geological Survey 2011a). Review of the USGS National Water Information System database indicates two surface water sites located c. 20 km downstream of the deposit area (US Geological Survey 2011b). There were no existing publically-available geochemical data that could provide clues to the presence of the deposit prior to its discovery. Hydrogeochemistry as an exploration tool has been used for several decades, as summarized by Levinson (1980), Boyle (1988), Giblin (1994, 2001), Mingqi et al. (1995), Taufen (1997), Cameron et al. (1997, 2004), de Caritat & Kirste (2005), Leybourne (2007), and Leybourne & Cameron (2010), and numerous references therein. The use of hydrogeochemistry in the search for porphyry copper deposits is described by Coope (1973), Boyle & Troup (1975), Trost & Trautwein (1975), Miller et al. (1982), Learned et al. (1985), Cook et al. (1999), Cameron & Leybourne (2005), Yang et al. (2006), and Leybourne & Cameron (2006, 2008). However, all of these studies have generally been constrained to the use of traditional atomic absorption spectrometry (used from 1970s to present), inductively coupled plasma atomic emission spectrometry (ICP-AES, mid-1980s to present), and ICP mass spectrometry (ICP-MS, mid 1990s to present) methods. Predicting improved analytical sensitivity in future decades, Hawkes & Webb (1962) stated, ‘The development of hydrochemical methods of prospecting has been hampered by technical difficulties in determining the extremely low concentration of trace elements in natural waters, which are commonly in the order of parts per billion. …as analytical techniques improve the scope of water analysis as a prospecting method will continue to broaden.’ In this study, cations were determined using

high resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS), relatively new analytical instrumentation (commercial analyses available since around 2005) with a large dynamic range (>9 orders of magnitude) and low determination limits (DLs) in tenths to tens of nanograms per litre (ng/l, equivalent to parts per trillion for infinitely dilute water) for most elements. The exceedingly low DLs allow for recognition of element distributions in water that are not possible by traditional analytical methods. Location, physiographic and climatic setting The Pebble deposit is located 380 km SW of Anchorage and c. 27 km NW of the village of Iliamna, in the Bristol Bay region of SW Alaska (Fig. 1). There is no road network and access to the study area is by helicopter. The deposit is situated in a broad glacially-sculpted topographic low at the head of three drainages, Talarik Creek, North Fork of the Koktuli River, and the South Fork of the Koktuli River (Figs 1, 2). The study area is in a zone of discontinuous permafrost. Frozen ground was encountered locally in lowland peat-rich and boggy areas at depths of 20–40 cm. The area falls within the polar domain/subarctic regime ecoregion (section M131C) as defined by Bailey (US Forest Service 2007). Elevations range from c. 300 m above sea level at the deposit itself to 686 m and 841 m at the adjacent Koktuli and Kaskanak Mountains, respectively. The entire area is covered by lichen-rich tundra vegetation, with willow varieties and alder trees that commonly flank drainages. Over the 30-year time period from 1971 to 2000, the Alaska Climate Research Center (ACRC 2009) reports that average daily temperatures at nearby Iliamna ranged from lows of −8.8 to −7.2º C in December through February, to highs of 10.5– 13.3º C in June through August. Average annual precipitation during this same period is 635 mm, with the wettest period in August-September, when monthly rain averages 109 mm. However, climate in the deposit area varies considerably from

ICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska

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Fig. 2. (a) Aerial view to NW across the Pebble deposit with approximate ore body outlines. Scale varies; distance in centre of photo from L to R edge is c. 2 km. Small ponds occupy glacial kettles. Letters and arrows indicate locations and approximate view directions for other photos. (b) View across Pebble West looking east. Iron-stained rubble crop in foreground with naturally acidic ponds behind. Shaded hill in centre distance is till ridge overlying Pebble East. (c) View across Pebble West looking west. Centre of photo shows red-orange ferrioxyhydroxides precipitating in stream and in adjacent rubble crop. Discovery outcrop is the orange-tan slope on left skyline. (d) Looking NW across glacial features over deeply concealed Pebble East. Slope of lateral moraine in foreground. Ponds in photo have circum-neutral pH. All photos by R.G. Eppinger.

that at Iliamna, which is moderated by the 2 600 km2 Iliamna Lake (the largest lake in Alaska and the 8th largest in the USA). Detailed climate data for 2007 collected in the deposit area reveals an August 12th high temperature of 22.8º C and a January 7th low temperature of −33.8º C. In general agreement with the Iliamna 30-year precipitation averages, the highest daily rain event for 2007 was in early September and the cumulative annual precipitation was 879 mm (Hoefler Consulting Group 2008). GEOLOGY AND MINERALIZATION Geology Most of Alaska is an amalgamation of lithotectonic terranes that since the Palaeozoic have moved northerly on the Kula plate and docked to the North America plate (Plafker & Berg 1994). The southern part of the Kahiltna terrane hosts the Pebble deposit (Fig. 3; Bouley et al. 1995). Regionally, the variably metamorphosed southern Kahiltna terrane contains Late Triassic basalt, andesite, and sedimentary rocks that are overlain by Jurassic to Cretaceous volcaniclastic turbidites (Lang

et al. 2007, 2008; Rebagliati & Lang 2008). These rocks are intruded by a diverse post-docking suite of Cretaceous to Tertiary plutons related to the still-active subduction zone located about 500 km to the SE. The plutons occupy a NE-trending structural corridor that is likely related to the crustal-scale Lake Clark translational fault (Lang et al. 2007, 2008; Rebagliati & Lang 2008). At Pebble, 91–89 Ma subalkalic granodiorite bodies (satellites to the Kaskanak Batholith) cut upright Jura-Cretaceous argillite, siltstone, and graywacke (Fig. 3; Lang et al. 2007, 2008; Rebagliati & Lang 2008). Smaller, nearly identical granodiorite stocks and sills that are spatially and genetically related to Cu-Au-Mo mineralization have Re-Os ages (molybdenite) of c. 89.5 Ma (Schrader et al. 2001; Gregory & Lang 2009). The Cretaceous rocks are capped locally along an erosional unconformity by Tertiary volcanic and volcaniclastic sedimentary rocks. Surficial geology at Pebble dominantly reflects Pleistocene glaciation that occurred between 26 000–10 000 years ago. Alpine glaciers swept over and retreated back across the Pebble deposit generally from the NE. Glacial features are predominant in the Pebble area and


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Fig. 3. Geological map of the study area showing pre-Tertiary units projected to the surface, based on drill-core data, and inset map showing lithotectonic terranes. Yellow and red lines are 0.3% and 0.6% Cu equivalent ore body outlines for Pebble West and Pebble East, respectively. Geology from Northern Dynasty Minerals Ltd. Inset regional geology from Wallace et al. 1989.

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ICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska include terminal and lateral moraines, meltwater channels, outwash deposits, numerous kettle depressions now occupied by ponds, and ice-dam glacio-lacustrine deposits (Fig. 2; Hamilton & Klieforth 2010). Unconsolidated glacial deposits at Pebble vary from 0 m locally at Pebble West to 50 m thick over parts of Pebble East and West (Hamilton & Klieforth 2010). Mineralization The Pebble porphyry deposit is the first significant mineralized porphyry system to be discovered in the region and opens the possibility of a major new porphyry province in southwestern Alaska. Pebble has been identified as an example of a major porphyry deposit that occurs in isolation or distal, compared to the more common occurrence of multiple deposits within orogen-parallel belts and provinces (Sillitoe 2010). However, the apparent isolation of Pebble might be the consequence of extensive cover and/or the relative infancy of exploration in the region. Several nearby small concealed porphyry and skarn occurrences are under exploration by the Pebble Limited Partnership (PLP). Pebble was discovered by drilling in 1989 by Cominco American geologists, following their initial investigation of colour anomalies in rubble crop observed from the air two years prior. The deposit’s discovery and geological setting was first described by Bouley et al. (1995). Beginning in 2001, Northern Dynasty Minerals Ltd. (NDM) obtained sole ownership of the deposit and continued exploration, defined limits of the partially exposed Pebble West zone ore body, and in 2006 discovered the richer, deeply-buried, Pebble East zone ore body. In 2007, NDM and Anglo American began a 50:50 partnership and created the PLP. Drilling of the deposit by PLP continued through 2009, although the majority of the drilling was completed by 2008. Through August 2009, a total of 1 085 drillholes covering about 270 000 m of drill-core was used to define the combined ore bodies (Fig. 3) and to determine and monitor the hydrologic setting at Pebble (Northern Dynasty Minerals Ltd. 2011). The Pebble West zone extends from the surface to c. 500 m depth, while the contiguous East zone extends to at least 1650 m depth (Fig. 4). Both zones are open at depth. Mineralized rock in the West zone is centred around several small granodiorite cupolas intruding sedimentary rocks, diorite sills, and breccias while the East zone is generally centred on a large granodiorite pluton (Lang et al. 2007, 2008). The East zone was partially eroded and is unconformably overlain by an eastward thickening, 300 to 600 m wedge of Eocene–Paleocene, postmineralization, volcano-sedimentary rocks (Fig. 4; Lang et al. 2007, 2008). On the eastern side of the East zone, high-grade ore has been truncated and dropped 600 to 900 m by grabenbounding normal faults. Mineralized rock is found mostly in strong potassiumsilicate altered rocks characterized by dominant K-feldspar, variable biotite, and almost no magnetite; and in multigenerational stock work veins containing quartz, carbonate, and sulphides (Lang et al. 2007, 2008). Laterally extensive sericitealtered rocks are peripheral to and overprint margins of the deposit; propylitic and sericite-chlorite-clay assemblages are locally developed. The highest grade ore is found in the East zone and resulted from advanced argillic alteration processes that overprinted previously-altered rocks and were controlled by a syn-hydrothermal brittle-ductile fault zone (Gregory & Lang 2009). Quartz-sericite-pyrite alteration zones do not contain significant Cu or Mo, but consistently exhibit higher Au grades (Kelley et al. 2010). Dominant ore minerals are chalcopyrite, molybdenite, and native gold, the latter mostly within

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chalcopyrite. High-grade gold-bearing bornite-covellite-digenite± enargite deposits were discovered at Pebble East in 2006 (Lang et al. 2007, 2008; Gregory & Lang 2009). Molybdenite is paragenetically younger. Supergene chalcocite and covellite are found in the upper parts of the West zone, but supergene minerals are not present in the East zone. Other mineral occurrences in proximity to Pebble and under active exploration by PLP include the 38 Porphyry, 37 Skarn, 25 Gold, 52, 65, and 308 zones, and the Sill prospect (Fig. 1). Published information on these occurrences is sparse to nil. Hawley (2004) speculates that the 38 Porphyry and 37 Skarn, and an adjacent unnamed porphyry occurrence (=IL003 in his report) probably have similar ages as Pebble. The unnamed porphyry occurrence is located c. 1-2 km immediately north of 52 zone (Fig. 1). Also present in the vicinity of Pebble are at least two Tertiary epithermal precious metal occurrences, the Sharp Mountain and the Sill occurrences (Schrader 2001; Fig. 1). There are also scant reports of minor historic placer gold occurrences downstream of Pebble along the Koktuli River (Hawley 2004). METHODS A total of 168 surface water samples were collected from ponds (98), emerging springs/seeps (39), and streams (31), from numerous sites in a 22-km2 area around Pebble, and from a greater 760-km2 surrounding region (Fig. 1). Samples from rare drill-hole seeps (8) in the deposit area were also collected. Thorough descriptions of collection, analysis, and quality control (QC) procedures, assessments of data quality, and databases containing analytical results for all sample media collected are found in Anderson et al. (2011), which supercedes interim geochemical data releases by Fey et al. (2008, 2009) and contains all USGS geochemical, geophysical, and mineralogical data collected in the project. Sample collection Water samples were collected in mid-summer, generally precipitation-free periods. Clean procedures were used throughout, following the protocols outlined in Ficklin & Mosier (1999). On-site measurements included pH, specific conductance, alkalinity, acidity, dissolved oxygen, turbidity, water temperature, and a qualitative discharge estimate. For stream samples, widthintegrated (collected across the source) and depth-integrated (collected below riffles) representative water samples were collected in 1-litre polypropylene bottles. Non-integrated pond and spring samples were collected in 1-litre polypropylene bottles. Bottles were rinsed with source water before collection. The samples were filtered on-site at 0.45 µm using disposable filters. Sub-samples for cation analysis were placed in acidrinsed polypropylene bottles and acidified on site with ultrapure HNO3. Filtered, unacidified sub-samples for anion analysis were refrigerated until analysed. Filtered samples for Fe2+ and dissolved organic carbon (DOC) analyses were acidified on site with ultrapure HCl. In 2007, samples for Hg analysis were collected in acid-rinsed glass bottles with Teflon lids and preserved with ultra-pure HCl; however, no Hg was detected at the detection limit of 0.02 micrograms per litre (µg/l, equivalent to parts per billion for infinitely dilute water), so samples for Hg analysis were not collected thereafter. Sample analysis Analyses for 63 cations by HR-ICP-MS were done on filtered/acidified samples by Activation Laboratories Ltd., using a Finnigan Mat ELEMENT 2 instrument. Anions by ion chromatography, Fe2+ and DOC by spectrophotometry, and


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Hg by cold vapour-atomic fluorescence (CVAF) spectrometry, were all determined in USGS laboratories. The sector-field HR-ICP-MS instrumentation differs from standard quadrupole ICP-MS in that ion focusing is achieved through separate electric and magnetic field sectors, as compared to a quadrupole mass filter which uses alternating AC and CD voltages on four parallel rods to affect the flight path of ions. Moens & Jakubowski (1998) and MacFarlane et al. (2005) provide overviews of theory behind and operational details for the HR-ICP-MS instrument. For specific parameters used in the present study, the reader is referred to Activation Laboratories Ltd. (1336 Sandhill Drive, Ancaster, Ontario L9G 4V5). Instrument details for analyses performed at USGS laboratories are found in Taggart (2002). Mercury was also determined as part of the HR-ICP-MS suite of elements, but these data were not used, since only the CVAF samples were properly preserved to keep Hg in solution for analysis. Quality control Quality assurance/quality control concerns were addressed through the use of field blanks, site duplicates, analytical duplicates, and standards, which together comprised c. 15% of the samples analysed. De-ionized water field blanks were collected on 11 different days to evaluate process contamination. Site duplicates were taken at 10 sites to evaluate repeatability and site variation. Instrumental precision was constrained by analysis of laboratory duplicate solutions, and analytical error is typically less than 5%. Finally, standard reference material (SRM) water standards were analysed with sample batches to assess instrumental accuracy. All aspects of the QC assessment yielded impressive results (Anderson et al. 2011). There was no contamination by sample processing. In blanks, very low concentrations of 30%

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Cations, anions, and field parameters

Fig. 6. Box-and-whisker plots showing summary statistics for selected ions and field parameters in spring, stream, and pond water samples. All analyses by HR-ICP-MS except pH, Cond, Alk, DOC, SO42-, Cl-, and F-. Lower determination limits for traditional ICP-MS are indicated by dotted line for comparison. ∑REE, sum of rare-earth elements; for other abbreviations, see Fig. 5.

Similarly, Tl and Re (Figs 12 and 13, respectively) concentrations are highest in the low-pH ponds and springs at Pebble West, but at exceedingly low concentration ranges (ones to tens of ng/l). Nearly the entire upper quartile of Tl and Re concentrations (>3 and >4.1 ng/l, respectively) are found in samples within or flanking the west and south sides of Pebble West (Figs 12a, 13a). These elements would not have been detected using standard ICP-MS (Fig. 6). Trace elements: Pebble West and East A broad, 4-km wide area encompassing both Pebble West and the deeper Pebble East areas contains numerous spring and pond samples that are circum-neutral in pH (Fig. 7a) and have attendant anomalous cation concentrations in elements that are commonly mobile at higher pH. However, the absolute concentrations are commonly at sub-µg/l to single-digit µg/l concentration ranges, revealing patterns that would not be seen with standard ICP-MS. Nearly the entire upper quartile of

samples containing anomalous Mo lie within this area at concentrations ranging from 0.369–3.13 µg/l (Fig. 14a). Other cations that behave similarly include V (upper quartile 236– 3420 ng/l; Fig. 15), As, and Mn, at concentration ranges generally amenable to standard ICP-MS, although very few samples would have shown detectable As and V by ICP-MS (Fig. 6). Possible low-level dispersion of V is evident for about 12 km along Upper Talarik Creek downstream of the deposit area, but similar dispersion is not evident below Frying Pan Lake along the South Fork Koktuli River (Fig. 15b). Exceedingly low concentrations of Ag (from 2.1–13.1 ng/l; Fig. 16), and W (from 5–273 ng/l; Fig. 17) reveal patterns that target both Pebble West and East. While the range for Ag concentrations is close to the lower determination limit of 2.0 ng/l by HR-ICP-MS, suggesting that caution should be used in evaluating Figure 16, HR-ICP-MS is a particularly rigorous technique for Ag (Paul Lamothe, personal comm., 2010). Similarly, distributions of In, Sb, Th, and U show patterns targeting both Pebble West and East, all at ng/l concentration


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Pebble West outline = 0.3% Cu equivalent Pebble East outline = 0.6% Cu equivalent Fault PLP mineral occurrence PLP claim boundary

(a)

Fig. 7. pH in water samples from the (a) deposit area and (b) and surrounding region. PLP, Pebble Limited Partnership.

* ( ( (( ( ( ( ( ( ( * * (

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ANIONS

Fig. 8. Piper diagram showing major ion composition (Piper 1944). Values for ions are in %meq/l. Sample site duplicates are shown as pairs with a unique coloured symbol that generally plot on top of one-another, indicating minimal site variation for major ions.

ICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska 10000

Pond Spring/Seep Stream n = 167

1000

Fe3+

LDL

100

10 LDL 1

1

10

100

Fe2+

1000

10000

Fig. 9. Fe3+ vs Fe2+ in pond, spring, and stream waters. LDL, lower determination limit.

ranges. The anomalous distributions over Pebble West and Pebble East for these six elements would not have been revealed using standard ICP-MS (Fig. 6). Possible low-level dispersion is evident along Upper Talarik Creek for Ag (Fig. 16b), Th, and U, and along both Upper Talarik Creek and the South Fork Koktuli River for Sb. Anomalous concentrations of all of these elements are commonly found in samples from springs and ponds overlying or adjacent to major NEand NW-trending faults in the Pebble East area (Figs 16a, 17a). Curiously, Zn, which is commonly mobile under higher pH conditions, does not group with these elements; the highest Zn concentrations are from samples in the low pH ponds and springs at Pebble West. Other anomalous sites West and SW of the Pebble deposit area are several concealed, undisturbed porphyry and skarn occurrences (Fig. 1). Currently under exploration by PLP, only minimal helicopter-supported drilling has been done on the 37, 52, 38, 308, 65, and 25 zones, and the Sill occurrence. Surface waters from areas surrounding these occurrences are generally circum-neutral in pH (Fig. 7). Several elements target these occurrences, all exhibiting lowlevel anomalous concentrations, particularly in the vicinity of the 52, 37, 308, and 38 zones to the southwest of Pebble. These elements include Cu (Fig. 10b), the REE (Fig. 11b), Tl (Fig. 12b), Re (Fig. 13b), V (Fig. 15b), Ag (Fig. 16b), W (Fig. 17b), as well as As, Cd, Sb, Th, and SO42-. Molybdenum in particular stands out in targeting the area, with a spring immediately NE of the 52 zone having the highest Mo concentration in the entire study, at 3.13 µg/l (Fig. 14b). An adjacent stream and two samples near the Tertiary Sill epithermal occurrence also have high Mo concentrations. The low-level multi-element anomalies in surface waters surrounding the occurrences would not have been discerned using traditional ICP-MS. Drill-hole seeps Samples collected from eight drill-hole seeps in the Pebble deposit area reveal geochemical information about the potentially deeper groundwater system (Anderson et al. 2011).

219

These seeps are not included in Figure 6 or the discussion above. All the seeps are circum-neutral in pH (median pH 6.5) and have the highest alkalinity concentrations (median 77 mg/l, maximum 150 mg/l) found in the study. The highest specific conductance values (median 248 µS/cm, maximum 392 µS/cm) are from drill-hole seeps and probably reflect the deeper groundwater source, with associated higher dissolved major constituents. The most reduced waters found in the study are from iron-rich drill-hole seeps that are likely derived from anoxic deeper groundwater sources. Dissolved organic carbon concentrations are lowest in drill-hole seeps. Relative to the entire water dataset, elements found in anomalous concentrations in drill-hole seeps include Ag, Ba, Ca, F-, Fe, Fe2+, Fe3+, Mg, Mo, Na, Sr, U, V, and W. However, except for iron species and major elements, the absolute element concentrations in drill-hole seeps are generally in the low µg/l to very low ng/l range. Elevated Cu and REE are not found in drill-hole seeps, likely due to the circum-neutral pH of the waters. DISCUSSION Natural processes Anomalous element concentrations in surface waters in the Pebble deposit area form three broad groups that may be related to three different natural processes: (1) low pH ponds and groundwater passing through underlying faults at Pebble West; (2) circum-neutral groundwater passing from depth along graben-bounding faults at Pebble East; and (3) displaced hydrogeochemical anomalies in glacial kettles in till on west edge of and immediately south of Pebble West. These processes are further described below. (1) At Pebble West, the four low-pH ponds (3.6–4.8) with associated high metal and sulphate contents found in five samples, reflect an undisturbed naturally acidic and metalliferous local ecosystem, common in mineralized areas where sulphideand/or related ferrihydroxide-bearing minerals in bedrock interact with water and oxygen (Eppinger et al. 2000, 2007; Eppinger & Fuge 2009). Mineralized bedrock is under shallow cover in this area (Figs 2b, 2c). In detail, the low-pH ponds straddle a NW-trending fault (identified in drill-core) that likely provides a conduit for groundwater passage through and interaction with the underlying shallow mineralized bedrock. At two of the low-pH ponds (sites 08PB255 and 08PB256; Anderson et al. 2011), the pond substrates contain thick accumulations of orange-red sands that under microscopic inspection reveal rounded grains of iron oxide with concretion-like concentric textures. We hypothesize that these iron oxide grains reflect Fe2+-rich groundwater upwelling through the fault system, mixing with near-surface waters, and precipitating at the pond bottom as Fe hydroxides and hydroxysulphates, releasing H+ in the process (Nordstrom & Alpers 1999; Smith 1999). Sunlight photoreduction, bacterial processes, and wind-generated wave action in the shallow ponds might all contribute to the accumulation of the oolith-like concretionary iron oxide sands in the ponds. Anomalous concentrations of Cu, Cd, F-, K, Ni, Re, REE, SO42-, Tl, and Zn found in these low-pH ponds and adjacent springs are likely derived from the upwelling waters interacting with shallow mineralized bedrock (Figs 10-13). In agreement, analyses of iron-rich pond sediment core samples from these two ponds reveal that the highest trace element concentrations are found in the upper few cm of core (Anderson et al. 2011), likely due to adsorption onto the precipitating iron oxide grains. Similar iron-rich orange, sandy sediment is present in springs NE of the 52 zone and


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Fig. 10. Cu in filtered/acidified water samples, determined by HR-ICP-MS, for (a) deposit area and (b) and surrounding region. PLP, Pebble Limited Partnership; ND, not detected at low limit.

(b) B

REE, ng/L ND (0.2)

97.5th Pebble West outline = 0.3% Cu equivalent Pebble East outline = 0.6% Cu equivalent Fault PLP mineral occurrence PLP claim boundary

Fig. 11. Sum of rare earth elements in filtered/acidified water samples, determined by HR-ICP-MS, for (a) deposit area and (b) and surrounding region. ∑REE, sum of rare-earth elements; PLP, Pebble Limited Partnership; ND, not detected at lower determination limit.

north of Kaskanak Mtn (Fig. 1), and likely reflect the same processes. Also present on the shorelines of the acidic ponds at Pebble West are accumulations of black ‘moss’ mats attached to cobbles and small boulders. Similar-looking black ‘moss’ was observed in naturally acidic waters at Red Mountain, Alaska (Eppinger et al. 2007), where further study revealed that

the moss was actually the unusual acid- and metal-tolerant liverwort, Gymnocolea inflata (Gough et al. 2006). (2) In the Pebble East area, where the deposit is under thick cover, waters are circum-neutral in pH and attendant elements exhibiting anomalous concentrations include Ag, As, In, Mn, Mo, Sb, Th, U, and V, all having various degrees

ICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska 155°16'W

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221

Fig. 12. Tl in filtered/acidified water samples, determined by HR-ICP-MS, for (a) deposit area and (b) and surrounding region. PLP, Pebble Limited Partnership; ND, not detected at lower determination limit. 155°16'W

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Fig. 13. Re in filtered/acidified water samples, determined by HR-ICP-MS, for (a) deposit area and (b) and surrounding region. PLP, Pebble Limited Partnership; ND, not detected at lower determination limit.

of mobility under circum-neutral pH (Smith & Huyck 1999). Many of these same elements were found in anomalous concentrations in rare circum-neutral drill-hole seeps that likely tap the deeper groundwater system and the unoxidized ore body at depth. Several of the ponds and springs overlie or are adjacent to NE-trending graben-bounding and other

NW-trending significant faults identified in drill-core (Figs 14-17). We hypothesize that the faults serve as conduits for the upward migration of water at depth and in contact with the ore body. While speculative, migration of the deep groundwater to the surface and mixing with shallow waters could be facilitated by common low-level seismic activity


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related to the active subduction zone located c. 500 km to the SE (US Geological Survey 2011c). Similar processes have been proposed for the migration of metals in Chile (Cameron et al. 2002; Kelley et al. 2006). (3) Immediately south of and along the western edge of Pebble West are several ponds with: pH between 5.0 and 6.0

(Fig.7); low-level anomalous concentrations (up to 90th percentile) of Tl, Re, the REE; and even higher concentrations (above 95th percentile) of Mo, V, Ag, and W (Figs 11-17); as well as anomalous In and Th. Springs were not observed in the area. The ponds fill glacial kettles in till deposited by Pleistocene Brooks Lake glaciation that swept generally southwestwardly

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across the deposit area from the NE and north (Hamilton & Klieforth 2010). Porphyry copper indicator minerals (PCIMs®) identified in several till samples from this same area include hundreds of grains of pristine and slightly modified gold, and up to 1500 grains of jarosite (Kelley et al. 2011a, b). Gold and jarosite grain counts diminish down-ice from around Frying

Pan Lake, where one of several terminal moraines is present (Hamilton & Klieforth 2010). The pristine gold and high grain counts of gold and jarosite in till, proximal to but not directly over the Pebble deposit, represent an offset PCIM anomaly immediately down-ice of the deposit. The water chemistry in the kettles may reflect dissolution of ore-related and secondary

224


minerals in the enclosing till, rather than hydromorphic processes as described in (1) and (2) above. Natural versus anthropogenic sources The Pebble deposit and surroundings have been drilled extensively (n = 1 064) as exploration has progressed in the last decade since its discovery. Factors that argue that the hydrogeochemical anomalies over the deposit are natural and not related to drilling include: (1) the presence of a common suite of anomalous elements in natural seeps over the deposit and in natural seeps at the edge of the deposit away from any drill-holes; (2) the presence of the same suite of anomalous elements in soils (commonly having low pH) over the deposit that are not proximal to drill-holes (Smith et al. 2009); (3) the presence of a similar suite of elements at similar magnitudes in natural seeps from nearby porphyry and skarn prospects, such as the 37, 38, 308, 52, and 25 zones, where drilling has been minimal and distant from sampled seeps; (4) in the Pebble area, the presence of some ponds with the anomalous suite of elements that are higher in elevation than nearby drill-holes; (5) observed seeps from capped drill-holes were sparse (n = 8) and flow from them was minor (generally in the 10ths to less than 10 litres/min range); (6) the vast majority of drill-hole sites that were observed were capped and dry. Nevertheless, a tracer or isotopic study would be necessary to conclusively rule out minor drill-hole contribution to the metal loads in the surface waters. Unconventional elements Some of the elements present in anomalous concentrations in water samples from the Pebble area are not traditional pathfinder elements used in the search for porphyry deposits. These include U, In, Re, and V. Low-level anomalous U concentrations (14.5 to 125 ng/l) are present in ponds, springs, and streams in the deposit area, particularly over Pebble East, dispersing downstream along Upper Talarik Creek. Geochemical analysis of numerous rock types in drill-core over the deposit area (Northern Dynasty Minerals, unpublished data, 2008) indicate that the highest U concentrations are in the Tertiary volcanic and volcaniclastic rocks that unconformably overlie the deposit at depth over Pebble East (Fig. 4). Therefore, the low-level anomalous U concentrations in water are likely derived from the Tertiary units rather than the underlying porphyry ore body. In agreement, none of the water samples from surrounding mineral occurrences contain anomalous U concentrations. Low-level anomalous concentrations of In (0.31–3.1 ng/l), Re (15.6–158 ng/l), and V (357–3420 ng/l) are present in water samples that cluster around the Pebble deposit. These elements also identify one or more of the outlying mineral occurrences in the Pebble region (Figs 13, 15). These elements may serve as additional pathfinders to the more traditional Cu, Mo, REE, Sb, SO42-, Tl, W, precious metal, and other elements, for porphyry Cu-Au-Mo deposits in the region. Rhenium, a trace element in molybdenite, is likely common in waters in contact with molybdenite-bearing mineral deposits, but at concentration levels that render it undetectable by traditional methods. Similarly, In, a trace element in chalcopyrite, is likely common in waters in contact with porphyry and other chalcopyrite-bearing mineral deposits, but at ultra-low concentration levels. Vanadium is present in unusually high concentrations (>300 ppm) in ores

from Pebble (Kelley et al. 2010); and black rutile in drill-core samples from Pebble contains high V concentrations (avg. 6.3 wt %, Kelley et al. 2010, 2011b). The anomalous V concentrations in water samples appear to reflect the unusually high V signature at Pebble. The non-traditional pathfinder elements In, Re, and V may be useful for assessing for similar porphyry deposits in the region, but would likely not be detected in water samples without the use of HR-ICP-MS. CONCLUSIONS The use of HR-ICP-MS opens new possibilities for using hydrogeochemistry in exploration. In the Pebble deposit area, several anomalous elements (As, Cu, K, Mn, Mo, Ni, REE, V, and Zn) with concentrations in the µg/l and mg/l ranges could have been detected using standard quadrupole ICP-MS. However, because concentrations are near the lower end of the analytical working range for several of these elements (As, Cu, Mo, Ni, and V; Fig. 6), results for these elements would have been nondetects and blank map patterns for many of the samples. Anomalous Ag, In, Re, Sb, Th, Tl, U, and W from this area, with concentrations in the ng/l range, generally would not have been detected by standard ICP-MS and the patterns for these elements over Pebble would not have been revealed. Results from the HR-ICP-MS technique enlarge the target around the area of exposed and shallowly-concealed ore and extends over concealed mineralized rock on the flanks. Most of the patterns for low-level anomalous element concentrations found in surface waters near outlying mineral occurrences (37, 52, 308, 38, 25, 65 and Sill zones; Figs 10-17) would not have been revealed without the use of HR-ICP-MS. In contrast to traditional ICP-MS, low-level anomalous concentrations for most of the cations determined by HR-ICP-MS are robust and well within the working range for the method (Fig. 5). The above features clearly demonstrate the advantages of HR-ICP-MS in hydrogeochemical exploration. Similar advantages would be gained in environmental hydrogeochemical studies. However, a strong QC program is essential for successful use of HR-ICP-MS. Other conclusions include: (1) The low-level HR-ICP-MS data exhibit good QC at ng/l levels and consistent, interpretable map patterns; (2) Cations having anomalous concentrations around Pebble West include Cu, Cd, K, Ni, Re, the REE, and Tl; anomalous anions include F- and SO42-. These ions are commonly mobile under low pH; natural low-pH ponds and springs are present in parts of Pebble West where the ore body is under shallow cover; (3) Cations having anomalous concentrations at Pebble East and parts of Pebble West (areas where cover is much thicker) include Ag, As, In, Mn, Mo, Sb, Th, U, V, and W. Waters from these areas are circum-neutral, likely the controlling factor for the mobility of these elements; (4) The conduits for water movement from depth to the surface are believed to be large graben-bounding NE- and smaller NW-trending faults in the deposit area. While speculative, regional low-level seismicity related to the active subduction zone to the SE could facilitate the water movement and mixing; (5) Low-level anomalous concentrations found in glacial kettles located down-ice and to the immediate west and southwest of the deposit may reflect displaced geochemical anomalies that are related to dissolution of PCIMs present in the surrounding till that were scraped from the deposit; (6) Low-level anomalous concentrations for Ag, As, Cd, Cu, Mo, Re, the REE, Sb, Th, Tl, V, W, and SO42- are present

ICP-MS at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska in surface waters surrounding the 37, 52, 38, 308, 65, 25 and Sill zones, all concealed porphyry, skarn, and epithermal mineral occurrences that are distal to Pebble; (7) Hydrogeochemical exploration paired with HR-ICP-MS delineates the concealed Pebble deposit and surrounding concealed mineral occurrences and together make a powerful new tool in the search for concealed deposits. We thank the Pebble Limited Partnership, particularly Jim Lang, Mark Rebagliati, Keith Roberts, Lena Brommeland, Robin Smith, Gernot Wober, and Sean Magee, for logistical and scientific support for this work. Eric Hoffman and Activation Laboratories Ltd. are thanked for contributing the HR-ICP-MS analyses. Reviews by Richard Wanty and Patrice de Caritat improved the manuscript. The use of trade, product, or firm names in this article is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Received 20 April 2011; revised typescript accepted 12 August 2011.