Stream Sediment Indicator Mineral Signatures of the Casino Porphyry Cu-Au-Mo Deposit, Yukon, Canada

A significant part (25%) of Yukon Territory in northwestern Canada is unglaciated, mountainous terrain that lacks fresh outcrop because of the deep bedrock weathering profile and cover of colluvium or loess (Bond and Lipovsky, 2011; Hicken et al., 2017). These geomorphological conditions combined with the cold climate, extreme remoteness (no roads), and large tracts of underexplored terrain (1,000s of km²) present significant challenges to mineral exploration. Historically, only geochemistry of the silt + clay fraction of stream sediments and stream waters has been used for mineral exploration here.

Indicator mineral methods are widely used for till sampling in the glaciated regions of Canada (McClenaghan and Paulen, 2018, and references therein) and have proven to be cost-effective and successful in the discovery of new mineral deposits in this glaciated terrain (e.g., Averill and Zimmerman, 1986; Stea et al., 2009; Neilson et al., 2012; Averill, 2013, 2017; Mc-Clenaghan and Paulen, 2018). However, the application of indicator mineral methods to stream sediment surveys in any terrain, glaciated or otherwise, is new for commodities other than diamonds. In the unglaciated Dawson Range of westcentral Yukon, indicator mineral methods were perceived to be unsuitable, unnecessary, or too costly to include as part of a stream sediment geochemical survey.

Recently, methods for the recovery and identification of porphyry Cu indicator minerals from till (Table 1) have been developed for the glaciated terrain of western Canada (e.g., Chapman, J.B., et al., 2015; Hashmi et al., 2015; Plouffe et al., 2016, 2022; Mao et al., 2017; Pisiak et al., 2017; Plouffe and Ferbey, 2017, 2019) and Alaska (Kelley et al., 2011; Eppinger et al., 2013). The primary purposes of our study are to test the applicability of porphyry Cu indicator mineral methods for stream sediment sampling in the Yukon’s mountainous unglaciated landscape and to compare porphyry Cu indicator suites in glaciated and unglaciated terrains, with the overall goal of improving porphyry Cu exploration methods in this part of the Yukon.

The methods were tested around the Casino porphyry Cu-Au-Mo deposit (Fig. 1), one of Canada’s largest undeveloped porphyry Cu-Au-Mo deposits (Casselman and Brown, 2017). The Casino deposit presents an ideal location to test indicator mineral methods because it has not yet been mined, and the surficial environment has only been minimally disturbed by exploration drilling, is known to contain the potential indicator minerals gold and tourmaline, and has a documented stream silt geochemical signature in local creeks draining the deposit (Archer and Main, 1971).

The earliest exploration in the Casino area was for placer gold deposits in the lower reaches of Canadian Creek in 1911 (Fig. 2; see App. Fig. A1 for map legend). Further upstream in Canadian Creek, on the northwest flank of what is now the Casino deposit, a gold-tungsten placer occurrence was mined for tungsten in 1916 and again in the 1940s (Bostock, 1959; Archer and Main, 1971). Over the years, placer gold mining also took place on Rude Creek (Fig. 2), 12 km southeast of the Casino deposit (Chapman et al., 2014). Other early exploration activity in the area focused on the Ag-Pb-Zn veins at the Bomber occurrence (Yukon Geological Survey, 2018a) on the southern periphery of what is now known as the Casino deposit (Fig. 2).

Prior to the initial diamond drilling that resulted in the discovery of Cu-Au-Mo mineralization, surface indications of the presence of the Casino deposit included the prominent (730 m long) limonite gossan along a small creek that drains into upper Casino Creek (Fig. 2) on the southeast side of the deposit; the presence of the local gold-tungsten placer occurrence; intense hydrothermal alteration; the presence of limonite, jarosite, and weak malachite staining in leached rocks at the surface; the peripheral Ag-Pb-Zn veins; and anomalous Cu concentrations in –80-mesh (<0.177 mm) stream sediment samples in Casino Creek compared to other creeks in the Dawson Range (Archer and Main, 1971). Anomalous Cu and Mo in –80-mesh soil samples collected in 1968 were used to guide initial drilling in 1969 that revealed significant mineralization (Archer and Main, 1971). The current total measured and indicated resources of the deposit are 2.173 billion tonnes grading 0.16% Cu, 0.18 g/t Au, 0.17% Mo, and 1.4 g/t Ag (Casino Mining Corporation, 2021).

The study area is located in west-central Yukon, 300 km north of Whitehorse (Fig. 1) and 150 km south of Dawson at latitude 62°44′N and longitude 138°50′W and is accessed only by fixed-wing aircraft or helicopter. The deposit is in the northern Dawson Range, a series of broad ridges and summits that vary in elevation from approximately 1,000 to 1,800 m above sea level and compose the south part of the Klondike Plateau physiographic region (Bostock, 1970; Mathews, 1986). The local climate is cold and semiarid (Bond and Lipovsky, 2011) with a mean annual temperature of approximately –5.5°C. Permafrost is discontinuous and is most common on N-facing slopes and valley bottoms that are covered by thick, finegrained colluvium and organic veneers (Smith et al., 2004; Bond and Lipovsky, 2011).

The Casino deposit area is underlain by metamorphosed and deformed basement rocks of the Yukon-Tanana terrane, an allochthonous, tectonic terrane that extends more than 2,000 km from Alaska, U.S.A., through Yukon, and south into British Columbia (Mortensen and Friend, 2020). The terrane consists of rocks formed in a mid- to late Paleozoic continental arc system that separated the Yukon-Tanana arc from the western margin of Laurentia (Nelson et al., 2006, 2013; Allan et al., 2013). Within the terrane, the Snowcap assemblage of metamorphosed sedimentary and minor volcanic rocks is unconformably overlain by the Finlayson, Klinkit, and Klondike assemblages that are predominantly arc metavolcanic rocks and associated metasedimentary rocks (Colpron et al., 2006, 2016; Ryan et al., 2013). The Casino deposit is located within the Dawson Range mineral belt, a 175-km-long NW-trending corridor of mineral deposits that include porphyries, epithermal veins, breccias, skarns, structurally hosted veins and breccias, and placer gold deposits (Bennett et al., 2010).

The bedrock geology of the deposit and surrounding area is briefly summarized in this paper from detailed descriptions by Archer and Main (1971), Godwin (1975, 1976), Bower et al. (1995), Ryan et al. (2013), Yukon Geological Survey (2015a, 2018b), Casselman and Brown (2017), and Roth et al. (2020). The Casino deposit is classified as a calc-alkaline porphyry deposit and is centered on the Patton porphyry, a Late Cretaceous (74–72 Ma) stock that intrudes the Mesozoic Dawson Range batholith and Paleozoic Yukon Crystalline Complex schist and gneiss. The intrusion of the small porphyry into these older rocks caused brecciation along its contacts. The porphyry is locally mineralized and is surrounded by a potassic-altered intrusion breccia at its outer contacts. Elsewhere, the porphyry consists of discontinuous dikes (up to 10s of m wide) that cut the porphyry and Dawson Range batholith. The overall composition of the porphyry is rhyodacite, with dacite phenocrysts and a quartz latite matrix.

Primary copper, gold, and molybdenum mineralization was deposited from hydrothermal fluids in the contact breccias and fractured wall rock and consists of pyrite, chalcopyrite, molybdenite, and minor hübnerite (manganese-tungsten oxide). The primary mineralization is concentrated in the phyllic zone and is surrounded by weakly developed argillic and propylitic alteration zones. Grades decrease away from the contact zone toward the center of the stock and outward into the wall rocks.

Godwin (1975, 1976) suggested that the warm and wet climate of the Paleogene (Zachos et al., 2001; Moran et al., 2006; Vavrek et al., 2012) is the likely time frame for supergene enrichment of the deposit. The deep weathering profile is largely intact because minimal to no glacial erosion of the region has occurred during the last 2 m.y. (Bond and Lipovsky, 2011, 2012a, b). Thus, the deposit has well-formed horizontal zonation consisting of a leached cap, supergene oxide mineralization, supergene sulfide mineralization, and hypogene (primary) mineralization. The leached cap averages 70 m thick, is enriched in gold and depleted in copper, and consists primarily of boxwork textures filled with jarosite, limonite, goethite, and hematite. The deep weathering has obliterated bedrock textures and replaced most minerals with clay minerals. The supergene oxide zone consists of a few isolated lenses within the leached cap and is thought to have formed by more recent fluctuations in the water table. It is rich in copper and contains chalcanthite, malachite, and brochantite along with minor cuprite, azurite, tenorite, neotocite, and trace molybdenite as coatings on fractures and in vugs. The supergene sulfide zone underlies the leached cap, averages 60 m thick, and crops out at surface in places. It has Cu grades commonly double those in the hypogene zone and contains pyrite, chalcopyrite, bornite, and tetrahedrite that may be altered along grain boundaries to chalcocite, digenite, or covellite, as well as molybdenite that is locally altered to ferrimolybdite. Hypogene mineralization underlies the supergene sulfide zone and consists of pyrite, chalcopyrite, molybdenite, sphalerite, bornite, and tetrahedrite. In the hypogene zone, gold occurs as discrete crystals (50–70 µm) in quartz and as inclusions (1–15 µm) in pyrite and chalcopyrite (Huss et al., 2013). On the eastern and northern flanks of the deposit, the supergene oxide zone is absent, the other zones are thinner, and the hypogene zone is closest to the surface (<25 m). Potential indicator minerals of the Casino deposit are listed in Table 2 and were compiled from descriptions of the deposit and minerals present in the Au-W placer in upper Canadian Creek on the northwest flank of the deposit.

Exploration drilling on the adjacent Canadian Creek property, just west of the Casino deposit, confirmed the presence of two porphyry systems: Cu-Au-Mo mineralization (Canadian Creek occurrence, Yukon Geological Survey, 2018c) and porphyry Cu-Au mineralization (Koffee [also known as Zappa] occurrence; Yukon Geological Survey, 2018d) (Fig. 2). The easternmost mineralized rocks, immediately adjacent to the Casino deposit, are now considered to be part of the Casino deposit and included in the deposit’s reported resource estimates (Roth et al., 2020). The northwest part of the Canadian Creek property also hosts structurally controlled gold mineralization that is accompanied by Ag, As, Sb, Mo, Ba, and Bi (Johnston, 2018).

Other mineral occurrences in the area are shown in Figure 2 and include porphyry Cu-Mo-Au mineralization at the Cockfield occurrence (within the land of the Selkirk First Nation) 20 km southeast of the Casino deposit, on Mount Cockfield (Yukon Geological Survey, 2018e). Additional polymetallic vein occurrences are located 10 km to the northeast (Marguerite; Yukon Geological Survey, 2013a), 10 km to the east (Nordex and Idaho; Yukon Geological Survey, 2011a, b), and 12 km to the southeast (Rude Creek; Yukon Geological Survey, 2011c) of Casino. Gold occurrences, thought to be intrusion-related or orogenic in origin, have been reported upstream of samples 1018, 1017, and 1004 (Betty, Buck, and Sonora Gulch; Yukon Geological Survey, 2013b, 2015b, 2018f).

The surficial geology of the Casino area is summarized below from various maps and reports by Duk-Rodkin (2001), Huscroft (2002a, b, c), Duk-Rodkin et al. (2002, 2004), Bond and Sanborn (2006), Jackson et al. (2009), Bond and Lipovsky (2011, 2012a, b), Lipovsky and Bond (2012), and McKillop et al. (2013). The landscape is largely unglaciated. Bedrock outcrop and tors (rocky peaks) are common along the ridges and summits and have disintegrated in situ through mechanical (freeze and thaw) and/or chemical weathering. Surficial material in upland areas flanking ridges and summits consists of colluvium and weathered bedrock intermixed with variable amounts of loess. Material moves downslope by gravity-driven processes—creep, solifluction, landslides, and snow avalanches—and eventually into local creeks. Lower-lying areas are covered with loess.

Isolated alpine glaciers existed on Mount Cockfield during the middle Pleistocene Reid glaciation, extending west into the headwaters and a tributary valley of Victor and Colorado creeks and east into an unnamed tributary that drains into the Selwyn River (Bond and Lipovsky, 2012a). Glacial sediments (end moraines) and cirques are present on the east flank of Mount Cockfield; stream sediments in the creeks draining this east flank are derived, in part, from the glacial sediments. Evidence of past glaciation also exists in the headwaters of Canadian Creek, immediately northwest of Patton Hill, where cirques formed during early Pleistocene (pre-Reid) glaciation (Duk-Rodkin et al., 2002; Bond and Lipovsky, 2012a).

First- and second-order streams (e.g., Casino Creek) occur in narrow V-shaped valleys and contain subangular to subrounded gravel to boulders composed of locally derived bedrock. Higher-order streams occur in broader valleys and are filled with more distally derived colluvium, loess, and rounded gravel. Bond and Lipovsky (2012a) noted that an understanding of the relationship between valley morphology and the variable texture and sources of fluvial sediments is important when sampling for and interpreting stream sediment geochemical surveys. Because loess content in fluvial sediments is variable, Bond and Lipovsky (2012a) recommended that stream samples ideally be collected from high-energy streams in narrow valleys where the loess content is lowest.

Archer and Main (1971) reported that, at the time of discovery, the Casino deposit had an obvious geochemical signature in fine-grained stream sediments (Cu, Mo, Au, and Ag) and waters (Cu) overlying the deposit. Subsequent reconnaissance-scale stream sediment sampling in the region (Geological Survey of Canada, 1987; 19 elements in <0.177-mm fraction using 3:1 HNO₃/HCl) and reanalysis of these stream sediment samples (53 elements in <0.177-mm fraction using 1:3 HNO₃/HCl; Jackaman, 2011; Yukon Geological Survey, 2016; Mackie et al., 2017; Arne et al., 2018) show an obvious multielement geochemical anomaly (Ag, Cu, Mo, Pb, Sb, W) in the local creeks draining the Casino deposit.

Chapman et al. (2014, 2018) compared lode gold signatures in the Casino deposit to large (100 kg) bulk gravel samples from known placer occurrences along Casino, Canadian, and Rude creeks (Fig. 2). They reported that gold crystals in Casino bedrock samples were between 50 and 1,000 µm (long axis of grains) and grains in creek gravel samples measure between 500 and 2,000 µm. Using gold grain trace element chemistry and inclusion compositions, they concluded that the large gold placer occurrence in the middle reaches of Canadian Creek contained a mixture of gold derived from two sources: the Casino porphyry deposit and unknown epithermal mineralization. Barkov et al. (2008) reported the presence of several indicator minerals in a heavy mineral concentrate sample from the same large placer occurrence on Canadian Creek, below the confluence with Potato Gulch (Fig. 2). In addition to Snrich hematite, they recovered ferberite, hübnerite, bismuthinite, daubréeite, tetradymite, and goethite from the sample. The geochemistry of stream and groundwaters sampled in 2017 as part of the Geological Survey of Canada (GSC) study at Casino are reported in Kidder et al. (2022).

A total of 24 fine-grained stream sediment and 22 heavy mineral samples were collected at 22 sites (Fig. 2) around the Casino deposit in September 2017 using GSC protocols described by Friske and Hornbrook (1991) and Day et al. (2013). The small number of sites and their location were constrained by helicopter accessibility in mountainous terrain, time, and cost. Sites were chosen to optimize the study of tourmaline—the main focus of the overall research project—and not other minerals such as gold that may also be present in local placer deposits. Placer deposits were not targeted for sampling, because this study was a test of routine stream sediment sampling sites. Four sites were sampled on Casino Creek draining the south side of the deposit (1007, 1008, 1010, 1014), four sites were sampled on Canadian and Britannia creeks draining the north side of the deposit (1019, 1023, 1025, 1026), and one sample was collected downstream of the Koffee-Canadian Creek occurrences on the west side of the deposit (1012). Two sites were sampled downstream of the Cockfield porphyry Cu occurrence (1002, 1003), and 10 sites on creeks draining other mountains in the local area were sampled to provide a regional context and establish background values.

Field data for, and photographs of, all sample sites are included in McCurdy et al. (2019). At each site, approximately 1 kg of silt and fine sand (geochemistry sample) was collected by hand from various points in the active channel while moving upstream, over a distance of 5 to 15 m. A second bulk 8- to 16-kg sediment sample (heavy mineral sample) was collected from large gravel bars, boulder traps, or tiny pools of sediment in the creeks and wet sieved onsite to remove the >2-mm fragments. Seventeen bedrock drill core samples were collected from mineralized and unmineralized bedrock in the deposit area to identify potential indicator minerals that could be present in stream sediments. Sample numbers reported in this paper are an abbreviation of the sample numbers assigned in the field for ease of labeling maps and data listings in tables (e.g., sample 115J171002 has been abbreviated to 1002).

A total of 17 bedrock and 22 stream sediment heavy mineral samples were processed at Overburden Drilling Management Ltd. (ODM), Ottawa, Canada, to recover heavy mineral concentrates using methods described by McClenaghan et al. (2020). Bedrock samples were broken apart using electric pulse disaggregation methods. Each disaggregated bedrock sample and stream sediment sample was wet sieved, and the resulting <2-mm fraction was passed across a shaking table to prepare a preconcentrate. The preconcentrate was micropanned to recover and count fine-grained gold, sulfide minerals, and other indicator minerals. Gold grain size and shape characteristics were classified using DiLabio’s (1990) scheme (pristine-modified-reshaped) adapted for fluvial transport (S.A. Averill, pers. commun., 2020), and then all panned grains were returned to the preconcentrate. Gold grain counts reported in this paper reflect this stage of the sample processing procedure. Each preconcentrate was then subjected to two heavy liquid separations and a ferromagnetic separation to produce different density concentrates for visual identification of indicator minerals: (1) nonferromagnetic 2.8- to 3.2-specific gravity (SG) concentrate and (2) nonferromagnetic >3.2-SG heavy mineral concentrate. Before examination, the grains were subjected to an oxalic acid wash to facilitate visual mineral identification. The 0.25- to 0.5-, 0.5- to 1.0-, and 1.0- to 2.0-mm fractions of the nonferromagnetic >3.2-SG concentrate and the 0.25- to 0.5-mm fraction of the nonferromagnetic 2.8- to 3.2-SG fraction of each bedrock and stream sediment samples were examined by ODM using a binocular microscope; potential indicator minerals were visually counted, and selected grains were removed for chemical analysis. The mass of all fractions produced and abundances of indicator minerals in the bedrock and stream sediment samples, along with sample processing flow charts, are reported in McClenaghan et al. (2020).

A total of 1,305 fine-grained stream sediment samples collected by the GSC in NTS 115J and 115K (east half) in 1986 were air-dried soon after collection <40°C, disaggregated, and dry sieved to recover the <0.177-mm fraction for geochemical analysis. Gold was determined on a 10-g split using Pb collection fire assay to produce a silver doré bead and detection by instrumental neutron activation analysis (INAA) (Geological Survey of Canada, 1987). In 2011, reanalysis included Cu and Mo determination for archived splits of the 1986 samples using ultra trace aqua regia digestion on a 0.5 aliquot (Jackaman, 2011). A subset of these Au, Cu, and Mo data were plotted as proportional dots using Jenks natural breaks to form base maps on which to plot abundances of gold, chalcopyrite, and molybdenite grains, respectively. The geochemical data provide some context for the interpretation of the new indicator mineral data.

Grain counts for selected indicator minerals are reported both in Table 3 and below as values normalized to 10 kg of <2-mm material to allow comparison between heavy mineral samples of different masses. Individual mineral distribution maps are included in McClenaghan et al. (2020) and are summarized below. Unless otherwise stated, the results are for the 0.25- to 0.5-mm >3.2-SG fraction.

Stream sediments contain between 0 and 44 gold grains (Fig. 3A) in the pan concentrate fraction (Table 3). Abundances are highest in samples from Casino Creek on the south side of the deposit and Canadian and Britannia creeks draining the north side of the deposit (Fig. 4). Gold grains in these sediment samples range in length from 25 to 1,500 µm, with most grains being between 25 and 200 µm and modified to reshaped (DiLabio, 1990) in appearance. No gold grains were recovered from sample 1013 downstream of the Bomber veins on the south flank of the deposit. The two sediment samples downstream of the Cockfield porphyry occurrence contain only one gold grain each. The one sample downstream from the Koffee-Canadian Creek occurrences contains one gold grain. One sample downstream from the Marguerite polymetallic veins northeast of the Casino deposit contains 14 gold grains between 50 and 1,500 µm, most of which are >100 µm in length. These gold grains are accompanied by three cinnabar grains. Two stream sediment samples downstream of the Buck and Betty gold occurrences east-northeast of the Casino deposit contain three and six gold grains, respectively, all of which are coarse (>100 µm).

Chalcopyrite (Fig. 3B) was recovered from half of the stream sediment samples with the largest number of grains in (1) sample 1003 (26 grains) downstream of the Cockfield porphyry occurrence (Table 3), (2) sample 1012 (13 grains) downstream of the Koffee-Canadian Creek porphyry occurrences, (3) sample 1019 (seven grains) downstream of the Casino deposit, and (4) sample 1013 (four grains) downstream of the Bomber veins (Fig. 5).

Pyrite (Fig. 3C) was recovered from all but two stream sediment samples. It is most abundant in (1) sample 1012 (2,113 grains) from the creek draining the Koffee-Canadian porphyry occurrences, (2) sample 1003 (397 grains) downstream of the Cockfield porphyry occurrence, (3) sample 1004 (194 grains) 28 km east of the Casino deposit on Hayes Creek, and (4) sample 1015 (168 grains) on Excelsior Creek 12 km northwest of the Casino deposit. The greatest abundance in stream sediments from creeks draining the Casino Creek deposit are in samples 1019 (115 grains) and 1014 (79 grains) (Table 3). Samples 1017 and 1018 downstream of the two gold occurrences contain only three and 28 pyrite grains, respectively.

Between one and three grains of molybdenite (Fig. 3D) were recovered from four stream sediment samples (Table 3): (1) sample 1014 from Casino Creek, (2) sample 1003, downstream of the Cockfield occurrence, (3) sample 1023, approximately 20 km downstream of the Casino deposit in Britannia Creek, and (4) sample 1012, downstream of the Koffee-Canadian porphyry occurrences (Fig. 6).

Sphalerite grains were identified in three samples by their honey-brown color (Fig. 3E): (1) sample 1013 (11 grains) downstream of the Bomber polymetallic veins, (2) sample 1014 (nine grains) just downstream of the Casino deposit, and (3) sample 1019 (one grain) from Canadian Creek (Table 3). Two samples contained bismuthinite (Fig. 3F) and arsenopyrite grains: sample 1003 downstream of the Cockfield occurrence and sample 1019 downstream from the Casino deposit. Arsenopyrite was also recovered from sample 1018 downstream from the Betty gold occurrence.

Dark-brown to pale-gray tourmaline grains (Fig. 3G, H) were recovered from the 2.8- to 3.2-SG fraction of all but one stream sediment sample (Fig. 7). A few tens of grains were recovered from Casino Creek samples 1008, 1010, and 1014 on the south side of the deposit. Stream samples Canadian and Britannia Creek on the north side of the deposit (1019, 1023, 1025, 1026) contain hundreds to thousands of tourmaline grains (Table 3). Hundreds of tourmaline grains were also recovered from samples (1002, 1003) downstream of the Cockfield porphyry occurrence, sample 1016 on Sunshine creek, and sample 1020 on an unnamed creek feeding into Britannia Creek. The tourmaline distribution map shown (Fig. 7) is an updated version of a map first reported in McClenaghan et al. (2020) and reflects the subsequent recounting (in 2020) of tourmaline grains in sediment samples to take into account the recognition of their variable range of colors, size, and crystal habit (Fig. 3G, H).

Scheelite is a white to beige mineral that fluoresces blue under short-wave ultraviolet light—a diagnostic feature used to help identify the mineral in heavy mineral concentrates. It was recovered from all but two stream sediment samples (Table 3). The highest values for the study area (23–40 grains) are in creeks draining the Casino deposit, in the Cockfield occurrence, and in samples 1016 from Sunshine Creek.

Jarosite is a dull yellowish to light-brown mineral (Fig. 8A) and was recovered from the 2.8- to 3.2-SG fraction of stream sediments. Samples 1025 and 1026 from Britannia Creek and lower Canadian Creek, downstream from the Casino deposit, contained the most jarosite (1,200–1,300 grains; Table 3). Samples that contained hundreds of jarosite grains include those from Casino Creek and downstream of the Cockfield and Koffee-Canadian Creek porphyry occurrences. No jarosite was recovered from samples downstream from the Marguerite polymetallic veins or the two gold occurrences.

Pyrolusite was visually identified in stream sediment heavy mineral concentrates by its dull black, amorphous appearance (Fig. 8B), and its identity was confirmed by scanning electron microscopy (SEM). Approximately 2,500 grains were recovered from Meloy Creek (sample 1013) downstream of the Bomber veins. In addition to pyrolusite, approximately 86 grains of plumbojarosite (Fig. 8C) were recovered from the same sample (Table 3). One plumbojarosite grain was also recovered from sample 1002, downstream of the Cockfield porphyry occurrence. Beudantite, a secondary Pb-As sulfate mineral, was recovered from sample 1010 from Casino Creek (Table 3). Goethite (Fig. 8D) was recovered from all but two stream sediment samples.

All but two samples contain white barite grains, with most samples containing <1,000 grains (Table 3). Stream sediment samples from Casino and Meloy Creek on the south side of the Casino deposit contain hundreds of grains. Samples from Canadian and Britannia creeks on the north side of the deposit contain tens to hundreds of grains. Notable abundances were recovered from samples that are not downstream from porphyry mineralization: sample 1016 from Sunshine Creek (7,194 grains), 1004 from Hayes Creek (9,302 grains), and 1022 from an unnamed creek 18 km northwest of the Casino deposit (31,746 grains).

Potential porphyry indicator minerals green epidote, apatite, and andradite were recovered from stream sediments (Table 3), but their distribution patterns do not reflect the presence of the porphyry mineralization.

Studies of porphyry Cu indicator minerals in glacial sediments have reported two groups of indicator minerals: (1) minerals that can be directly linked to porphyry mineralization based on their spatial distribution and abundance in surficial sediments and (2) minerals for which their mineral chemistry must be used to establish the link to porphyry Cu mineralization (Plouffe and Ferbey, 2017). Group 1 indicator minerals that were recovered from samples downstream from porphyry Cu mineralization in this study (Casino, Koffee-Canadian Creek, Cockfield) and are known to occur in the Casino deposit include chalcopyrite, pyrite, gold, tourmaline, molybdenite, sphalerite, jarosite, and goethite. Stream sediments in the study area also contain group 2 minerals that also occur in the Casino deposit: epidote, scheelite, zircon, and magnetite. Mineral chemistry studies of tourmaline, scheelite, and magnetite grains from bedrock and selected stream sediment samples around the Casino deposit are ongoing in the hopes that they will provide insights into the bedrock source(s) of these mineral grains.

Gold is present in the supergene and hypogene mineralization at Casino. Not unexpectedly, the largest gold grain counts are those from creeks that are downstream of the Casino deposit (i.e., Casino Creek and Canadian Creek; Fig. 4). In addition to the large abundance of grains in these streams, the gold grain dimensions and morphologies reflect their source. The largest dimension of approximately 80% of the gold grains from creeks immediately draining the Casino deposit is 25 to 200 µm (App. Table A1), similar to that reported for bedrock samples. Several 25- to 100-µm gold grains were recovered from mineralized, tourmaline-bearing Patton porphyry bedrock samples examined in this study (McClenaghan et al., 2020); this is similar to the gold grain size range (50–70 µm) reported by Huss et al. (2013) for the hypogene zone. Chapman et al. (2014) reported a broader size range (5–1,000 µm) in Casino bedrock samples. Additional supporting information includes the gold grain shapes in samples proximal to the Casino deposit (1007, 1008, 1019) that are a combination of modified and reshaped, reflecting some but not extensive fluvial transport (App. Table A1). Although they were not collected at the same sites, comparison of gold grain abundance patterns to that for gold concentrations in the 1986 regional stream sediment samples (Fig. 4) show that the greatest gold values for both methods are in the creeks draining the Casino deposit. The presence of gold grains in local creeks draining the Casino deposit and other porphyry occurrences indicates that it has survived both in situ chemical weathering of the deposit and subsequent mechanical erosion by periglacial downslope processes and fluvial transport. There are five samples in the study area that do not report gold grains, reflecting the lack of Au-bearing mineral occurrences within their drainage basins.

Chalcopyrite, molybdenite, and pyrite are indicators of hypogene mineralization. Sphalerite formed in paragenetically late veins in the deposit and is present in the distal Bomber Pb-Zn veins (Godwin, 1975; Casselman and Brown, 2017). Bismuthinite has been reported in the hypogene zone of the Casino deposit (Huss et al., 2013). This mineral assemblage, when found together in stream sediments, is diagnostic of porphyry mineralization. Comparison of chalcopyrite grain abundance to Cu concentrations in the 2011 regional stream sediment samples (Fig. 5) shows that the greatest abundances for both methods are in the creeks draining the Casino deposit, the Koffee-Canadian Creek occurrences (sample 1012), and the unnamed creek draining the southeast side of the Cockfield occurrence (sample 1003).

Molybdenite is not a physically robust mineral, with a Mohs hardness of 1 to 1.5 and a perfect (001) basal cleavage, which contribute to its ease of destruction during glacial transport and its absence as a porphyry Cu indicator mineral in glaciated terrain (Table 1). Unglaciated areas, such as surrounding the Casino deposit, provide an opportunity for molybdenite to survive and be recovered in indicator mineral surveys. Few stream sediment samples in this study contain molybdenite; however, those that do are coincident with the stream sediment samples that contain elevated (>1.5 ppm) concentrations of Mo (Fig. 6). These samples are directly downstream of known porphyry Cu-Mo-Au occurrences including from the Casino deposit, the Koffee-Canadian Creek occurrences, and the Cockfield occurrence.

The presence of sulfides in stream sediments is somewhat unexpected because the terrain, except for east of Mount Cockfield, is largely unglaciated and the long-term weathering of bedrock was expected to have destroyed sulfide minerals in rocks at or near surface. Archer and Main (1971) noted that pyrite was visible in some small fragments of leached feldspar porphyry on the surface of the Casino deposit when they were broken open, indicating that there is potential for sulfides to be present at or near surface. The presence of sulfide minerals in stream sediments may be the result of some combination of creeks directly eroding less oxidized mineralized parent material, ongoing cryoturbation and downslope gravity movement contributing less oxidized material to local creeks, and/or fluvial erosion of older gravel deposits. Former placer mining operations on Canadian Creek may have contributed some sulfide minerals to Canadian and Britannia creeks.

Tourmaline is present throughout the Casino deposit as radiating crystals in disseminations, in veins, and in breccias (Beckett-Brown et al., 2019, 2021, in press a, b); thus, the recovery of tourmaline from local creeks draining the Casino deposit is not surprising. It is present in the potassic, phyllic, propylitic, and argillic alteration zones and in the weathered leached cap, supergene oxide, and supergene sulfide zones as well as the hypogene zone. Its presence in the leached cap and stream sediments around the Casino deposit indicates its robustness in the secondary weathering environment and its physical durability during fluvial transport. Sediments in creeks draining the north side of the deposit contain significantly more tourmaline (100s to 1000s of grains) than creeks draining the south side (10s). Tourmaline abundance is high even in a few samples from creeks that do not drain areas of known porphyry mineralization (samples 1016, 1022), and these grains may be metamorphic in origin. Tourmaline has been identified in the Snowcap assemblage, which crops out in both the drainage basins of 1016 and 1022 (Morneau, 2017). The radiating crystal shape of some of the tourmaline grains (Fig. 3H) in samples that are proximal to Casino is a strong indicator of the presence of porphyry mineralization (Beckett-Brown et al., in press a, b) in contrast to grains forming as individual crystals, more commonly derived from metamorphic rocks (Fig. 3G).

The presence of scheelite has not been reported for the Casino deposit or its peripheral veins; however, its high concentrations in the creeks draining the Casino deposit (relative to other sample sites) and its presence in the upper Canadian Creek Au-W placer suggest it may be a useful indicator mineral for porphyry mineralization. Examination of the trace element geochemistry of scheelite grains using the discrimination scheme of Poulin et al. (2017, 2018) is ongoing and may confirm the utility of scheelite as a porphyry indicator mineral.

Jarosite, plumbojarosite, beudantite, and goethite were recovered from several stream sediment samples. They are products of preglacial chemical weathering and are present in the leached cap of the Casino deposit (Archer and Main, 1971; Huss et al., 2013); thus, their presence in stream sediments downstream of the Casino deposit is not surprising. Jarosite, specifically, is noteworthy in that it is commonly a product of supergene weathering of primary sulfides (mainly pyrite) under highly acidic conditions (generally pH values <3). Thus, the jarosite present could be derived from the leached cap of porphyry systems (i.e., Casino) or potentially other sulfidebearing gossans. The highest concentrations of jarosite were recovered in samples 1025 and 1026, which are downstream of Casino deposit. Additionally, there are significant concentrations of jarosite in Casino Creek to the south of the Casino deposit. The presence of pyrolusite in sample 1013 was not unexpected, because black manganese coatings were observed on stream cobbles and pebbles at this site. Drill core samples from this part of the deposit are characterized by high Mn concentrations relative to the rest of the deposit and supergene weathering of this part of the deposit likely formed pyrolusite. A pipe discharging water from the Bomber vein adit into the headwaters of Meloy Creek may also have contributed to the formation of pyrolusite coatings on other mineral grains in the stream bed at this location. Averill (2017) demonstrated the utility of pyrolusite as an indicator of glacial dispersal from the supergene cap of the Blackwater Lake Au-Ag deposit, British Columbia.

The presence of barite was noted in the polymetallic veins associated with the Casino deposit on its south flank by Archer and Main (1971). Stream sediment samples on the south side of the deposit contain more barite than those on the north side, and this may be a reflection of the barite in the polymetallic veins. The extremely high barite abundances in samples 1004, 1016, and 1022, not downstream from porphyry mineralization, may reflect the presence of bedded sedimentarytype barite, which occasionally can be associated with Pb and Zn deposits, but the lack of any sphalerite and galena in these streams rules that out.

The indicator mineral distributions described above are summarized in Table 4 for individual samples. Unfortunately, there are not yet other indicator mineral studies in this unglaciated terrain to compare to our results. The other challenge for interpreting the results of this study is the presence of other mineral occurrences so close to the Casino deposit. Sample 1009 best represents background in the area as it drains a region that does not contain any known mineralization. Additionally, the sample does not contain any notable indicator minerals in the stream sediments (e.g., gold, chalcopyrite, and pyrite). Due to the absence of other indicator mineral studies in the region, we cannot be confident on the determination of background based on a single sample. As a result of these limitations, thresholds between background and anomalous indicator mineral abundances could only be crudely estimated by comparing stream sediment samples in this study with one another and with other studies from glaciated terrain listed in Table 1. These estimated threshold values are listed in Table 4. This method of background estimation is not optimal and will be improved as additional heavy mineral sampling is conducted in the unglaciated parts of the Dawson Range.

Table 4 also lists trace element geochemical signatures in the <0.177-mm fraction of each sample. McCurdy et al. (2019) used a subset of the GSC reconnaissance stream sediment data (Geological Survey of Canada, 1987; Jackaman, 2011) to calculate thresholds between background and anomalous trace element concentrations across the region. These threshold values along with the 2017 data listing for selected pathfinder elements are included in Appendix Table A2. The elements that have anomalous concentrations in the 2017 stream sediment samples are those listed in Table 4.

Stream sediments from Casino (samples 1014, 1010, 1008, 1007) and Canadian creeks (1019, 1026) draining the Casino deposit display similar indicator mineral signatures that include various combinations of gold, chalcopyrite, pyrite, molybdenite, sphalerite, jarosite, tourmaline, and goethite (Table 4). The Casino deposit has the strongest indicator mineral anomaly above the thresholds, reflecting the large size of this deposit in comparison to the other porphyry occurrences in the area. The multielement geochemical signature of the deposit is also significant in comparison to the other porphyry occurrences in that it includes the most elements and some of the highest values for Ag, Au, Bi, Cd, Cu, Mo, Pb, Sb, Te, W, and Zn. Stream sediment samples from Britannia Creek (1023, 1025) farther downstream from the Casino deposit also display a strong indicator mineral signature that includes gold, chalcopyrite, pyrite, molybdenite, jarosite, and tourmaline. In contrast, the geochemical response in these two samples is very weak and limited to Sb and W. The combined signature of the Koffee-Canadian Creek porphyry Cu-Au occurrences is reflected in sample 1012. The sample contains gold, chalcopyrite, molybdenite, jarosite, and the largest number of pyrite grains (2113) of any sample in this study. However, sample 1012 displays a weak geochemical signature consisting only of Sb, Te, and W. Chalcopyrite, pyrite, molybdenite, bismuthinite, arsenopyrite, jarosite, and tourmaline in stream sediments from two creeks (1002, 1003) draining the area around the Cockfield occurrence are accompanied by various combinations of elevated Ag, As, Bi, Cd, Cu, Mo, Pb, Sb, Te, and W concentrations.

Sample 1013 downstream of the Bomber Pb-Zn-Ag veins, on the south flank of the Casino deposit, has a different indicator mineral signature from the Casino deposit, dominated by plumbojarosite and pyrolusite and the highest sphalerite abundance of any sample in this study (11 grains). Other indicator minerals at this site include chalcopyrite and pyrite. This mineral assemblage is typical of distal-style mineralization that is commonly observed surrounding porphyry systems and is somewhat reflected in the strong multielement (Ag, Bi, Cd, Pb, Sb, and Zn) geochemical signature of the sample and the highest Pb value (568 ppm) of the 2017 samples.

Sample 1004 contains notable concentrations of andradite and barite, in addition to gold, chalcopyrite, pyrite, and tourmaline. The geochemical signature of this sample does not reflect the presence of the gold or chalcopyrite but does contain anomalous concentrations of As, Pb, Sb, and W. Together these compose an interesting anomaly that may reflect the presence of the Sonora Gulch occurrence 10 km upstream. The occurrence is reported to have potential to host structurally or lithologically controlled Au-Ag mineralization as well as porphyry mineralization (Yukon Geological Survey, 2018f). Sample 1020, downstream of the Maguerite polymetallic vein occurrence, displays an indicator mineral signature that includes gold, pyrite, goethite, jarosite, and cinnabar, which is most distinctive along with the gold. Remarkably, this sample does not have a corresponding geochemical signature. Sample 1016 from Sunshine Creek also contains a notable abundance of andradite and barite, in addition to containing chalcopyrite, pyrite, jarosite, and tourmaline. In contrast to the interesting indicator mineral assemblage at this site, the sample has an unremarkable geochemical signature with only slightly elevated Sb. No mineral occurrences are known upstream of the site; thus, the source(s) of these indicators remains to be discovered. The gold, chalcopyrite, pyrite, bismuthinite, and arsenopyrite recovered from sample 1018 may be derived from the Betty orogenic Au showing located 2 km upstream. This indicator mineral signal is much more obvious than the weak Cd-Sb signature in the sample and the Au signature in the regional stream sediment data in the local creeks around the site (Fig. 4). Samples 1015 is noteworthy because it contains chalcopyrite and pyrite and is one of only three samples found to contain andradite. These minerals are accompanied by a weak geochemical signature consisting of Bi and Sb. Sample 1022 contains chalcopyrite, pyrite, barite, and tourmaline and has no geochemical signature. No mineralization is known upstream of either sample sites.

Stream sediment samples from first-order creeks draining the Casino deposit display strong porphyry Cu mineralization signatures in indicator minerals in the heavy mineral fraction (0.25–0.5 mm, fine to medium sand) and strong to weak geochemical signatures in the <0.177-mm (silt + clay) fraction (Table 4). The two samples (1025, 1023) farthest downstream (14 and 18 km, respectively) from the deposit in the second-order Canadian Creek display only an indicator mineral signature; a geochemical signal indicating porphyry Cu mineralization is not detectable. The indicator mineral signature in sample 1012, downstream of the Koffee-Canadian Creek, is very obviously a porphyry Cu signature but is accompanied by a weak geochemical signal. Samples such as these beg the question “Which would an exploration company prefer to base an exploration follow up decision upon—the presence of gold, chalcopyrite and molybdenite grains in a sample or its weakly anomalous values of Sb, Te, and W?”

For some samples, however, indicator minerals provide a stronger, more obvious signal than the fine fraction geochemistry. Samples 1008, 1023, and 1025 have unremarkable stream sediment chemistry that would not normally generate further interest, yet the presence of gold grains, chalcopyrite, scheelite, and jarosite in these three samples is worthy of further investigation. This lack of geochemical signature further demonstrates the value of including indicator minerals methods for mineral exploration.

The glaciated terrain studies listed in Table 1 have documented various combinations of porphyry Cu indicator minerals including gold, chalcopyrite, pyrite, tourmaline, apatite, magnetite, and green epidote, as well as secondary minerals such as jarosite and the skarn-related minerals andradite garnet and titanite in till samples down-ice of known deposits. Some of these minerals were also recovered from stream sediments in this study, with the greatest abundances in creeks that directly drain porphyry Cu mineralization. The exceptions are andradite garnet, apatite, and green epidote, which were recovered from the stream sediments samples in this study but for which their spatial distribution was not indicative of presence of porphyry mineralization. Andradite garnet was not expected to be indicative of mineralization at Casino, as there is no skarn mineralization associated with the deposit.

Additional minerals identified in this study as useful porphyry indicator minerals include molybdenite as well as the secondary minerals plumbojarosite, pyrolusite, and beudantite that likely reflect distal Pb-Zn mineralization that commonly envelopes porphyry systems. The absence of molybdenite as an indicator mineral in the glaciated terrain studies is likely related to its softness and thus its inability to survive the extreme crushing and grinding by glacial processes. The absence of plumbojarosite, pyrolusite, and beudantite in in the glaciated terrain studies is likely related to the extreme erosive power of glaciers and their ability to remove soft preglacial regolith, in which these minerals would be common.

Two other studies of porphyry Cu indicator minerals in sediment samples in weathered terrain are listed in Table 1. The first, by Averill (2011), reported data for alluvium samples collected around the Quebrada Blanca porphyry Cu deposit in Chile (Table 1). Similar to Casino, jarosite is a key indicator mineral in sediments around the deposit. The spatial distribution of andradite garnet is diagnostic only at Quebrada Blanca as this mineral reflects skarn mineralization, which is not present at Casino. Alunite and turquoise are indicator minerals unique to Quebrada Blanca. The second is a recent study by Kelley et al., 2021, 2022) of indicator minerals in stream sediments around the Taurus and Bluff porphyry systems, 150 km to the east of Casino in Alaska. They examined the <0.25-mm heavy mineral fraction using automated mineralogy and reported the recovery of chalcopyrite, bornite, and svanbergite (aluminum phosphate sulphate) and of apatite with a distinct chemistry downstream of porphyry mineralization. The presence of chalcopyrite in stream sediment is the only similarity to the results of our study.

Most of the Yukon has been covered by reconnaissancescale fine-grained stream sediment geochemical surveys; however, gaps in coverage exist because some areas have not been sampled and some areas have been sampled at the low sample density typical of GSC reconnaissance-scale surveys (one site per 10–14 km²) (Ballantyne, 1991; Friske and Hornbrook, 1991). One limitation of stream sediment geochemistry is that fine-grained stream sediment samples that are collected at the drainage outlets of large catchment areas can be affected by significant sediment dilution and large lithological variations across the catchments. This can result in subdued to absent geochemical signatures of mineralization (Fletcher, 1997; Herberlein, 2013; Jackaman, 2021). Indicator mineral methods can help solve these issues. They can be used to fill in gaps in existing or new stream sediment surveys as they are suitable for collection in third- and fourth-order streams as well as first and second (Jackaman, 2021). Collection of indicator mineral samples at the outlets of each drainage basin, as demonstrated for a recent stream sediment survey in south-central British Columbia (Jackaman, 2021), can address the dilution issues that fine stream sediment geochemistry samples suffer. Other advantages of using indicator mineral methods are that they (1) provide physical evidence of nearby mineralization (gold grains, chalcopyrite, molybdenite, sphalerite) or the weathering/alteration of primary mineralization (jarosite, goethite, plumbojarosite, beudantite), (2) can be examined with a binocular or scanning electron microscope and chemically analyzed using an electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to determine trace element contents that will provide detailed information about the nature of the mineralizing system, (3) may be present at sites that do not have a coincident anomalous geochemical signature in stream silt samples, and (4) may be present at sites that do have geochemical anomalies and provide specific insights into the mineralogy of a source (i.e., Au-bearing porphyry-style mineralization).

In the 1960s, the GSC collected heavy mineral concentrates as part of a stream sediment and water geochemical survey of the glaciated region surrounding the Keno Hill silver district, 200 km northeast of the Casino deposit (Gleeson and Boyle, 1976). They noted the presence of indicator minerals (e.g., gold, tourmaline, sulfides) in selected samples but did not systematically determine the mineral types and abundances in each sample. Their study was one of the first to demonstrate the utility of recovering indicator minerals from sediment samples for mineral exploration in the Yukon or elsewhere in Canada and was an inspiration for the current study.

Indicator mineral information can be especially important for reconnaissance-scale exploration, where the presence of a few indicator grains in broadly spaced stream sediment samples may indicate that a region is worthy of more detailed sampling. Government and exploration surveys would benefit most from the addition of indicator mineral sampling to stream sediment surveys, a recent example being the regional-scale heavy mineral stream sediment survey in the Boundary district of south-central British Columbia, Canada (Jackaman, 2021).

Based on gold grain alloy compositions and mineral inclusion assemblages, Chapman et al. (2014, 2018) concluded that gold in the Canadian Creek placer occurrence downstream of Potato Gulch (Fig. 2) was a mixture of grains derived from the Casino deposit and from shallow epithermal mineralization. The chemistry and inclusion compositions of gold grains in GSC samples are under investigation and will be compared to the results of Chapman et al. (2014, 2018) in future work to determine if their method of using placer gold grains is suitable for gold grains in routine stream sediment samples in the Dawson Range and in stream sediments in general.

Mineral chemistry characterization of tourmaline, scheelite, jarosite, and magnetite from bedrock and stream sediments from the Casino area is ongoing, and results are anticipated to provide insights into the bedrock sources and distance of fluvial transport for these minerals. Tourmaline that displays a distinct radiating crystal shape has survived intense chemical weathering and subsequent fluvial transport to accumulate in creeks around the Casino deposit. Characterization of the trace element chemistry of these specific mineral grains in the Casino area to determine if they have a specific porphyry signature that could be diagnostic when recovered from stream sediments has been partially completed (Beckett-Brown et al., in press a, b), and sampling at the site and laser ablation analysis will be carried out in 2022. Till samples from the Woodjam porphyry Cu deposit are also being examined to determine if this same diagnostic crystal form survives glacial transport and will be useful in glaciated terrains.

Automated SEM-based methods (e.g., mineral liberation analysis, quantitative evaluation of materials by SEM, TESCAN integrated mineral analyzer [TIMA-X]) are useful for detecting indicator minerals in the fine (<0.25 mm) fraction of heavy mineral concentrates of sediment samples that cannot be seen because they are too small to be observed using a binocular microscope or occur as inclusions within other minerals (Lehtonen et al., 2015; Lougheed et al., 2020, 2021; Layton-Matthews and McClenaghan, 2022; Kelley et al., 2022). Automated SEM-based methods are not yet routinely applied to heavy mineral samples in industry or government surveys. Case studies are needed to demonstrate the mineral signatures around various mineral deposit types using this smaller size fraction and large numbers (10,000s to 100,000s of grains). Ongoing GSC research will include mineral liberation analysis of the <0.25-mm heavy and mid-density mineral fractions of selected Casino bedrock and stream sediments to determine the indicator minerals present, inclusion mineralogy, mineral associations, and how these small minerals are distributed in creeks as compared to the 0.25- to 0.5-mm minerals.

In 2022, additional indicator mineral sampling was conducted around the Casino deposit, in local creeks, and at the upper Canadian Creek placer to document the threedimensional distribution and chemistry of tourmaline in the deposit and determine if the W- and Sn-bearing minerals in the upper Canadian Creek placer are related to the porphyry mineralization.

Future work will also include comparing the results of this indicator mineral study to others ongoing in the Dawson Range by the Yukon Geological Survey and at the Taurus deposit in Alaska by the U.S. Geological Survey. Regional-scale stream sediment indicator mineral sampling targeting drainage basin outlets by the GSC in the southern part of the Dawson Range Mineral Belt is planned for the next few years to assess the effectiveness of the indicator mineral method to assess mineral potential of this highly prospective, remote, and underexplored region.

This study is the first to include detailed sampling and determination of indicator minerals around a major porphyry Cu deposit in Canada’s unglaciated terrain. It has demonstrated the applicability of porphyry Cu indicator mineral methods in the mountains of south-central Yukon—an area that is challenging for mineral exploration because of its deeply weathered bedrock, cover of loess and colluvium, remote location, and cold climate. In order to maximize the return on the high cost of conducting stream sediment surveys in this remote mountainous terrain, indicator minerals methods should be included as part of stream sediment surveys. Indicator mineral methods can detect the presence of both fresh and deeply weathered mineralization, overcoming another challenge of the Dawson Range.

The Casino deposit has a strong indicator mineral signature, significantly more pronounced than any other mineral occurrence in the study area. The deposit and/or peripheral mineralization has an obvious indicator mineral signature in stream sediments that consists of combinations of gold, chalcopyrite, pyrite, molybdenite, sphalerite, jarosite, goethite, and tourmaline. The distinct radiating crystal shape of the tourmaline grains may be a strong indicator of the presence of porphyry mineralization. In addition to the primary mineralization at Casino, distal Pb-Zu-Ag vein mineralization related to the deposit has its own distinct indicator mineral signature of sphalerite, pyrolusite, plumbojarosite, pyrite, and chalcopyrite, further increasing the indicator mineral footprint of the Casino mineralized system. These minerals have survived the intense chemical weathering that formed a leached cap and supergenes zones over the Casino deposit and/or fluvial transport. It is not any one mineral alone that is diagnostic of porphyry mineralization; instead, it is the combination of ore minerals, secondary minerals, and tourmaline in stream sediments that is most indicative of the presence of porphyry mineralization.

The Casino indicator mineral signature is detectable at least 18 km downstream of the deposit. Similar indicator mineral patterns were detected in creeks downstream from two local porphyry occurrences (i.e., Koffee-Canadian Creek, Cockfield). The indicator mineral data reported here offer a guide to what might be expected downstream of porphyry Cu-Au deposits in the unglaciated terrain of the Yukon and perhaps elsewhere globally in unglaciated terrains. The porphyry Cu indicator mineral assemblage for Casino is similar to that established for glaciated terrain and further emphasizes the utility of indicator mineral methods for porphyry Cu exploration. The addition of indicator mineral methods to a stream sediment survey will help in the interpretation and prioritization of exploration targets and follow up. Indicator minerals provide information that fine-fraction stream sediment geochemistry cannot, including sand- to silt-sized fragments that are visual confirmation of the presence of mineralization, alteration, or their weathering products. Trace element analysis, isotopic age determinations, and stable isotopic analysis of individual mineral grains can provide valuable insights into the nature of the bedrock source upstream.

This paper is a contribution to Geological Survey of Canada’s Targeted Geoscience Initiative program. We gratefully acknowledge Western Copper and Gold Corporation and the Casino Mining Corporation, and in particular M. Mioska and H. Brown, for providing access and logistical support and sharing knowledge of the deposit. J. Bond, Yukon Geological Survey, shared geologic information and advice concerning the Casino area. Michael J. Bainbridge is thanked for his impressive photographs of such small mineral grains. This paper benefited from thoughtful reviews by J. Bourdeau, R. Chapman, and K. Kelley. This is Natural Resources Canada contribution 20210222.

Beth McClenaghan is a graduate of the University of Waterloo and Queen’s University. She is a research scientist at the Geological Survey of Canada, where she has worked for the past 30 years, and is head of the Geochemistry Section. Her research focuses on methods development for the application of till geochemistry and indicator mineral methods to mineral exploration in glaciated terrain, with particular emphasis on precious, base, and critical metals and diamonds. She is an adjunct professor at Queen’s University, a fellow of the Association of Applied Geochemists, and editor of their quarterly newsletter, EXPLORE.