Application of Till Mineralogy and Geochemistry to Mineral Exploration

CHAPTER APPLICATION OF TILL MINERALOGY AND GEOCHEMISTRY TO MINERAL 20 EXPLORATION

M.B. McClenaghan and R.C. Paulen Geological Survey of Canada, Ottawa, ON, Canada

20.1 INTRODUCTION Glacial dispersal of debris from a specific bedrock source may be the product of a broad range of transport directions produced by evolving and shifting ice divides and dispersal centers, and may be identified by analyzing a range of particle sizes from the clay-silt till matrix, sand-size mineral grains, pebbles in till, up to boulders and large erratic blocks. Identification of glacial dispersal trains, combined with identification of glacial landforms and ice-scoured bedrock, is a powerful exploration tool in glaciated regions where glacial sediments mask the underlying prospective bedrock. The term ‘drift prospecting’ is defined as the use of the geochemical, mineralogical, and lithological content of glacial sediments to trace metal-rich debris ‘up-ice’ to its bedrock source (DiLabio, 1990a). This chapter will focus solely on the most commonly used type of glacial sediment for drift prospecting—till—because it is a first-order sediment deposited directly by glaciers. Glaciation in the last 2 million years has profoundly affected and shaped much of the northern hemisphere’s landscape (Fig. 20.1) and small isolated areas in the southern hemisphere. For this reason, drift prospecting has become a common exploration method across the glaciated regions of the northern hemisphere and is only rarely used in the southern hemisphere. The intent of this chapter is to succinctly highlight processes and features of glacial dispersal and the application of this knowledge to mineral exploration in glaciated terrain. The first half of the chapter reviews ice-flow reconstructions and glacial dispersal, and explains the specifics of dispersal trains. A relatively newly recognized consideration is the impact that ice streams have had on the glacial landscape, and how they affected glacial dispersal patterns. A basic understanding of subglacial processes, glacial dynamics, glacial process sedimentology (discussed elsewhere in this volume) is requisite to understand glacial dispersal models. Reconstruction of ice flow history in any prospective region is extremely important because ice-flow indicators provide insights into glacial history and erosional vigor that have impacted glacial landscapes. The second half of the chapter summarizes boulder tracing, till geochemistry, and heavy mineral methods that are applied to drift prospecting and is illustrated with several examples. The drift exploration methods used today have evolved over time as analytical methods and our knowledge of

FIGURE 20.1 Extent of the Pleistocene glacial maximum in the Northern Hemisphere indicated by white polygons. Modified from Ehlers, J., Gibbard, P.L., 2007. The extent and chronology of Cenozoic Global Glaciation. Q. Int. 164À165, 6À20. glacial transport have advanced. The chapter builds on the earlier and still relevant reviews of exploration methods in glaciated terrain by Shilts (1976, 1996), Kauranne (1976), Bølviken and Gleeson (1979), DiLabio and Coker (1989), Coker and Shilts (1993),andMcClenaghan et al. (1997, 2000) and summarizes principles reported in greater detail in books by Kujansuu and Saarnisto (1990), Kauranne et al. (1992), McClenaghan et al. (2001),andPaulen and McMartin (2009).

20.2 HISTORICAL PERSPECTIVE Glacial geology applied to the search for mineral deposits can trace its roots to historical observations of boulder transport in Fennoscandia that predates the glacial theory (e.g., Tilas, 1740; Sederholm, 1911; Sauramo, 1924). These early boulder tracing studies had a profound effect on the mineral exploration in Fennoscandia, resulting in the discovery of several mineral deposits. In North America, boulder tracing was employed in the late 1800s and early 1900s to search for sulfide deposits (Burchett, 1944; Lundberg, 1957), iron (Miller, 1901), and gold (Slaght, 1893; Rickaby, 1932). Prest (1911) applied early principles of glacial transport and tracked gold grains in till up-ice to a gold-bearing quartz vein in eastern Canada. By the 1950s, boulder tracing had become a common exploration method in glaciated terrain (e.g., Grip, 1953; Dreimanis, 1958). 20.3 ICE-FLOW RECONSTRUCTION FOR MINERAL EXPLORATION 691

Geochemical analysis of the till matrix became a common tool in the 1970s and remains so today. The tracing of specific heavy minerals in till has been carried out since the early 1900s, but only became a common exploration tool in the 1990s. Boulder tracing, till geochemistry, and till mineralogy have facilitated the exploration for, and discovery of, many important deposits and mines, some of which are listed in Appendix A.

20.3 ICE-FLOW RECONSTRUCTION FOR MINERAL EXPLORATION Exploration in glaciated terrain relies on knowledge of ice flow directions to trace glacial debris back to its bedrock source. Reconstruction of ice-flow patterns uses information about a variety of erosional and depositional landforms and features (Table 20.1; cf., Ryder, 1995; McMartin and Paulen, 2009)

Table 20.1 Summary of Ice Flow Indicators That Are Commonly Used for Reconstruction of Former Ice Sheet Trajectories Agent of Width Landform Classification Formation Relief Length (% of Length) Roches moutonnees´ Bedrock landform Erosion 2À100 m 10À500 m 20À100 Whalebacks Bedrock landform Erosion 1À50 m 5À500 m 10À40 Bedrock steps Bedrock landform Erosion 1À5m 5À200 m 20À100 Rock drumlins Bedrock landform Erosion 5À150 m 50À1000 m 20À50 Drumlin ridges Drift landform Deposition 3À50 m 50À500 m 20À40 Flutings Bedrock & drift Erosion 1À20 m 500À.5000 m ,10 landform Fluted till plain Drift landform Deposition 1À20 m 500À.5000 m ,10 Megascale glacial Bedrock & drift Both 1À10 m Several km ,5 lineations landform Crag and tail Bedrock & drift Both 5À100 m 25À1000 m 10À40 landform Transverse ice-thrust Drift landform Both 1À200 m 100À5000 m 30À75 ridges Hill-hole pair Bedrock & drift Both 10À200 m 100À2500 m 50À100 landform Glacial grooves Bedrock landform Erosion 5À100 cm Several m ,10À40 (outcrop) Striations (outcrop) Bedrock landform Erosion ,1cm 1cmÀ20 m Few mm deep Striations (boulder Drift landform Both ,1cm 1cmÀ2 m Few mm pavement) deep Bullet-shaped boulders Sedimentary Both 20À40 cm 30À200 cm 10À50 structure Pebble fabric Sedimentary Deposition ÀÀ À structure Till fabric Sedimentary Deposition ÀÀ À (micromorphology) structure

Modified from McMartin, I., Paulen, R.C., 2009. Ice flow indicators and the importance of ice flow mapping for drift prospecting. In: Paulen, R.C., McMartin, I. (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada, Short Course Notes 18, pp. 15À34. 692 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

observed in the field and on airborne or satellite imagery and, together, these various forms of ice flow data are used to reconstruct past ice flow trajectories (e.g., Veillette et al., 1999; McMartin and Henderson, 2004; Paulen et al., 2013). The glacial dynamics of former continental ice sheets were complex. Glacial flow was a time-transgressive event, such that the exposed surface of a mineral deposit could undergo intense subglacial erosion and directly contribute to a dispersal train and then be buried by newly formed till deposits and be cut off from further erosion. Yet, the dispersal of mineralized debris can have continued through comminution and entrainment by ongoing ice flow. Thus, the type, provenance, and relative age of striations, streamlined landforms, and till deposits reflect the geographic and glaciological context in the ice sheet. Ice-flow reconstruction in alpine regions is strongly influenced by topography, keeping in mind that build-up of ice masses in alpine terrain could result in switches from topographically controlled glacial flow to flow controlled by glacial dynamics (Stumpf et al., 2000). Glacial flow reconstructions based solely upon landform associations identified by image analysis (e.g., Boulton and Clark, 1990; Shaw et al., 2010) that do not incorporate field observations and previous research, particularly the mapping of striae, may be inaccurate and inadequate. Field observations of erosional indicators and landforms are crucial to reconstructing an accurate model to support mineral exploration (cf., McMartin and Henderson, 2004; Veillette et al., 2017). An example of an ice flow reconstruction is shown in Fig. 20.2 from the Buffalo Head Hills kimberlite field, northern Alberta, Canada, in which data from striated bedrock and boulder pavements, oriented landforms, and pebble fabrics were used (Paulen and McClenaghan, 2015). Ice initially flowed southwest during the main phase of the Laurentide Ice Sheet and then flowed to the southwest and southeast during deglaciation as the ice sheet thinned. The implication of this reconstruction to diamond exploration in this prospective region is that both phases of ice flow must be considered when tracing kimberlitic debris in till back to source. A complex relationship exists between multiple till units (i.e., depositional history) and multiple ice flow phases (i.e., erosional history). Dispersed debris from a mineral deposit may be present in one, some, or all till units in a given region. Hirvas and Nenonen (1990) published various models illustrating the influence of glacial stratigraphy and combinations of flow trajectories on the transport of mineralized debris in till sequences. The provenance of multiple till units, and hence provenance of dispersed debris from mineral deposits, can be elucidated by determining and linking the glacial erosional and depositional records. This linking can be accomplished using a variety of field and compositional evidence. For example, Bird and Coker (1987) used till geochemistry and pebble lithology to decipher gold dispersal in multiple directions within multiple tills down-ice of the Owl Creek gold mine in central Canada. Stea and Brown (1989) used striation orientations, landform trends, pebble lithologies, boulder pavements, and till clast fabrics to decipher and link the glacial flow history to multiple till units in eastern Canada. Rice et al. (2013) utilized striations, distinct heavy minerals, pebble lithologies, till geochemistry, clast fabrics, and till micromorphology to link the erosional history to multiple and thick till units in support of leadÀzinc exploration in the Pine Point Mining District of northern Canada. Dispersal of kimberlite indicator minerals (KIMs) within multiple till units in a glaciated terrain dominated by ice-marginal lobes and surges was documented by Paulen and McClenaghan (2015) in the Buffalo Head Hills kimberlite field (Fig. 20.3) using distinct heavy minerals, till fabrics, and striations. In these, and many more examples, the recognition and sampling of multiple till units is crucial for exploration success in regions of thicker and/or complex glacial sediment cover. 20.3 ICE-FLOW RECONSTRUCTION FOR MINERAL EXPLORATION 693

58° N 117° W

10 km 114°30’ W 114°30’ 56°30’ N

FIGURE 20.2 Reconstruction of the Late Wisconsin Laurentide Ice Sheet flow patterns for the Buffalo Head Hills kimberlite field (kimberlite pipes are indicated as red diamonds), on a topographic digital elevation model. Large arrows represent steady-state flow during the Late Wisconsin glacial maximum, smaller arrows are flow paths from the Peace River and Wabasca River ice lobes during various stages of deglaciation. The dashed polygon indicates a region of ice stagnation characterized by hummocky stagnant ice moraine (MS) and the white star indicates the site shown in Fig. 20.3. Modified from Paulen, R.C., McClenaghan, M.B., 2015. Late Wisconsin ice flow history in the Buffalo Head Hills kimberlite field, north-central Alberta. Can. J. Earth Sci. 52, 51À67.

Research over the past two decades has advanced our understanding of the complexity of continental ice sheets and ice-sheet dynamics. Ice-flow trajectories, and subsequently, glacial dispersal and erosion rates can have varied significantly throughout various regions of an ice sheet. Initial ice-sheet buildup, ice-divide migration, and coalescence of ice masses all may be reflected in glacial dispersal trains (Nurmi, 1975; Ha¨ttestrand and Stroeven, 2002). Klassen and Thompson (1993) 694 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

FIGURE 20.3 Two distinct tills sampled for kimberlite indicator minerals and their relationship to regional ice flow patterns. The lower, sandy orange till, derived from southwestward flowing ice and down-ice of the Buffalo Head Hills kimberlite field, contains .750 KIMs of different species of minerals, as compared to the upper dark-gray silty till that is KIM-poor and derived from southeast surging ice from the Peace River Lowland. Size of pie graphs indicates the relative KIM abundances: olivine (yellow), chromite (dark gray), Mg-ilmenite (light gray), eclogitic garnet (orange), chrome diopside (green), and pyrope (red). Clast fabrics are shown as bidirectional rose diagrams. Person at top of photo for scale. Modified from Paulen, R.C., McClenaghan, M.B., 2015. Late Wisconsin ice flow history in the Buffalo Head Hills kimberlite field, north-central Alberta. Can. J. Earth Sci. 52, 51À67. documented the dispersal of iron formation clasts derived from rocks immediately under the Quebec-Labrador Ice Centre of the Laurentide Ice Sheet. The various dispersal trajectories they documented reflect the complex shifts of the former ice divide (Fig. 20.4). McMartin and Henderson (2004) documented the complex chronology and migration of the Keewatin Ice Divide of the Laurentide Ice Sheet over prospective rocks in northern Canada. Ferbey and Levson (2009) documented glacial dispersal from a CuÀMo mineral deposit within a thick till sequence, in an area underlain by an ice divide of the Cordilleran Ice Sheet, western Canada. 20.3 ICE-FLOW RECONSTRUCTION FOR MINERAL EXPLORATION 695

FIGURE 20.4 An amoeboid-shaped dispersal pattern of iron formation clasts in till dispersed from the Labrador Trough (red polygons). Ice flowed radially towards the northeast, southeast, southwest, and northwest from a migrating ice divide over the region. Modified from Klassen, R.A., Thompson, F.J., 1993. Glacial history, drift composition, and mineral exploration, central Labrador. Geol. Surv. Canada, Bulletin 435, augmented with data from Rice et al. (2017).

20.3.1 ICE STREAMS Ice streams are now recognized as prominent features of continental ice sheets that played a vital role in flow dynamics and mass balance, (e.g., Marshall et al., 1996; Stokes and Clarke, 2003; Evans et al., 2008; Stokes et al., 2015). Ice streams are corridors within an ice sheet that flow more rapidly than the surrounding ice; they form the arteries of ice sheets. Palaeo-ice streams discharged large amounts of ice over large distances and were the source of well-defined tracts of far-traveled, or exotic debris (e.g., Dredge, 2000; Stokes and Clark, 2001, Bennett, 2003; Dyke, 2008; Ross et al., 2009). Few studies have focused on dispersal patterns down-ice of mineralized rocks that have been formed or modified by ice streams, in part because former ice stream tracts were not previously recognized or identified in the northern hemisphere, most notably in areas covered by the Laurentide Ice Sheet in Canada. An inventory of Laurentide Ice Sheet ice streams recently published by Margold et al. (2015a, 2015b) should provide the impetus to re-examine many unex- plained or unsourced glacial dispersal trains in northern Canada, with a better understanding of 696 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

Ice

FIGURE 20.5 Ribbon-shaped glacial dispersal train of beryllium (Be) in the ,0.063 mm fraction of till trending northeast from the Strange Lake rare earth element deposit in eastern Canada. The train was formed by a single phase of ice flow towards the northeast. Modified from Batterson, M.J., Taylor, D.M., 2009. Geochemical re-analysis of till samples from the Strange Lake area, Labrador (NTS Map Sheets 14D/5 and 24A/8). Government of Newfoundland and Labrador, Department of Natural Resources, Geological Survey, Open File LAB/1479. glacial context (cf., Stokes et al., 2016). One example of debris dispersed by an ice stream is the plume of metal-rich till down-ice of the Strange Lake rare earth element (REE) deposit in eastern Canada (Batterson and Taylor, 2009). The ice stream produced elongate streamlined landforms enriched in debris from the deposit along a 40 1 km corridor down-ice (northeast) of the deposit (Fig. 20.5).

20.4 GLACIAL DISPERSAL TRAINS Glacial debris eroded from a discrete bedrock source is deposited down-ice in what is commonly referred to as a glacial dispersal train (DiLabio, 1990a). Dispersal trains are three-dimensional (3-D) bodies of till enriched in debris from a specific source relative to the till surrounding the train. They are hundreds to thousands of times larger than their bedrock source, and therefore provide ideal targets for mineral exploration (Fig. 20.6). They commonly have abrupt lateral edges; a (A) Head Tail

Ice flow

Till Background Low Mineralization Bedrock Moderate High

(B)

ice flow 2 1 till Background Low mineralization bedrock Moderate High

Ice flow

Till Background Mineralization Low Bedrock Moderate High

(D)

Ice flow 1 2

Till Background Mineralization Low Bedrock Moderate High

FIGURE 20.6 Conceptual models of glacial dispersal trains showing a buried up-ice component, a head, and a tail, and can be modified or offset by subsequent ice flow: (A) thin till cover whereby mineralized bedrock is dispersed by a single phase of ice flow, creating a dispersal train that is exposed at the surface for its entire length and trends down-ice in one direction; (B) thin till cover whereby mineralized bedrock is dispersed by two phases of ice flow, creating a palimpsest dispersal train exposed at the surface and trending down-ice in two directions; (C) thick till cover whereby mineralized bedrock is dispersed by a single phase of ice flow, creating a dispersal train that is buried proximal to source and exposed at the surface farther down ice, and trends down ice in one direction; (D) thick till cover whereby mineralized bedrock is dispersed by two phases of ice flow, creating a palimpsest dispersal train trending down-ice in two directions. The train is buried proximal to source and exposed at the surface farther down-ice. The original dispersal train created by the older ice flow (1) shown in (A) or (C) is diluted and offset by the second ice flow (2) in (B) and (D). 698 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

contrast over a very short distance between low abundances outside the dispersal train and high abundances within it. Glacial dispersal trains occur at or below the till surface, generally displaced some distance down ice of their source. Abundance of dispersed debris decreases with increasing distance down ice such that a plot of abundance versus distance down-ice commonly approximates a negative exponential curve (Fig. 20.7; Shilts, 1976; Klassen, 1997). Within an ice stream, the concentration gradient of dispersed debris decreases linearly down ice (Fig. 20.8) and not exponen- tially, as the result of rapid ice flow transporting debris far from source with little dilution

FIGURE 20.7 Plots of nickel abundance in till collected at surface in the Thetford Mines area, central Canada. Actual (top) and idealized (bottom) plots show the relationship between the source, head, and tail of a dispersal train. Modified after Shilts, W.W., 1976. Glacial till and mineral exploration. In: Legget, R.F. (Ed.), Glacial Till: An Interdisciplinary Study. Royal Society of Canada, Special Publication, vol. 12, pp. 205À223. 20.4 GLACIAL DISPERSAL TRAINS 699

FIGURE 20.8 Plots of thorium abundance in surface till down-ice of the Strange Lake REE deposit in eastern Canada. The top plot shows data from Batterson and Taylor (2009), with the best fit data curve shown as a dashed line. The lower plot is an idealized dispersal train that shows the relationship between the source, head, and tail of a dispersal train formed by a palaeo-ice stream. Lower plot is modified from Klassen, R.A., 1997. Glacial history and ice flow dynamics applied to drift prospecting and geochemical exploration. In: Gubins, A.G. (Ed.), Proceedings of Exploration 97, pp. 221À232.

(e.g., Klassen, 1997; Dredge, 2000). Dispersal trains have a ‘head’, the highest abundance of dispersed debris at surface, which is commonly, but not universally, overlying or slightly down-ice of the mineralized source. The ‘tail’ of the train is demarcated by the lower abundance of dispersed debris that has been transported and deposited many times farther down-ice than the head, and is commonly the part of the train that is detected first in an exploration program (DiLabio, 1990a). 700 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

Glacial dispersal trains are the net result of glacial erosion of mineralized bedrock sources, transport, and subglacial deposition, all of which can be complicated by multiple cycles of till re-entrainment and deposition, as glacial dynamics change over time. The morphology of dispersal trains is influenced by a number of factors that can be grouped into four broad categories: (1) the bedrock source (composition, hardness, contrast with host lithologies, topographic position, protection from glacial erosion); (2) glacial conditions (basal ice temperature, availability of water which relates to erosion potential of the glacier, ice velocity, ice-flow trajectories); (3) transport and depositional environment (subglacially by lodge- ment, supraglacial transport, meltout processes, etc.); and (4) postdepositional changes (e.g., metal remo- bilization by hydromorphic processes, mineral weathering, downslope movement, etc.). A single phase of ice flow typically produces a ribbon-shaped dispersal train as wide as the bedrock source (Fig. 20.5) that is defined by sharp, straight edges (e.g., McClenaghan et al., 2002a; Batterson and Taylor, 2009). The term ‘palimpsest’ is used to refer to glacial dispersal patterns that are the net result of more than one ice flow event (Stea, 1994; Parent et al., 1996; Stea et al., 2009). Palimpsest dispersal trains have several forms. A change in ice flow direction can: (1) erode the ribbon entirely, (2) rework the ribbon into a fan-shaped train that occurs between the two ice flow vectors (Fig. 20.9) (e.g., Paulen et al., 2013), (3) create a separate new ribbon in a different orientation that results in a bilobate fan (e.g., Charbonneau and David, 1993; Stea et al., 2009) (Fig. 20.10), or (4) rework dispersed glacial debris into stellate or amoeboid-shaped trains through multiple ice flows in very different directions or flow reversals, often associated with ice divides (Klassen and Thompson 1993; Shilts, 1993; Stea and Finck, 2001)(Fig. 20.4). Some limiting factors when exploring for dispersal trains and other patterns include: (1) dispersal may not be evident or detected because mineral-rich debris was glacially transported down-ice beyond the range of sampling area, (2) the bedrock source may not have been subjected to glacial erosion (e.g., buried by glacial or preglacial sediments or covered by younger bedrock units), (3) the mineralized rocks may not contain sufficient volume of indicator minerals to produce a detectable train, and (4) some dispersal trains may have gaps, or be missing the head or some part of the tail due to differential erosion of the mineralized source (e.g., removal of a regolith or gossanous cap), erosion and entrainment of parts of the train by subsequent ice flows, or dilution and masking of a train by till derived from a nonmineralized bedrock source. Dispersal trains form from the transport of debris over distances ranging from tens of meters to tens of kilometers. The scale of dispersal is linked to regional ice flow dynamics through landformÀsediment associations. Transport distances differ spatially with respect to the ice sheet margins and ice divides, and even with landforms. Examples from Finland have shown that glacial transport distances can vary between hummocky and drumlinized terrain (Salonen, 1988). Systematic variations in glacial transport distance can be difficult to define. Shilts (1984) defined four scales of glacial dispersal which have been updated and summarized below: 1. Continental dispersal, which is measured in hundreds to more than a thousand kilometers down- ice from the source and is typically detected using a distinctive bedrock lithology or mineral. An example is the dispersal of the visually distinctive Omarolluk Formation clasts (‘omars’) from the Belcher Islands in Hudson Bay, Canada (Prest, 1990). 2. Regional dispersal, which is measured in tens to hundreds of kilometers down-ice from the source. An example is the dispersal of carbonate detritus by ice streams within the Laurentide Ice Sheet (Hicock, 1988; Dredge, 2000) or at Strange Lake (Fig. 20.5). 20.4 GLACIAL DISPERSAL TRAINS 701

FIGURE 20.9 Fan-shaped glacial dispersal train of (A) tin (Sn) and (B) tungsten (W) in whole till formed by two phases of ice flow to the southeast and to the southwest from the East Kemptville tin deposit in eastern Canada. Modified from Rogers, P.J., Chatterjee, A.K., Aucott, J.W., 1990. Metallogenic domains and their reflection in regional lake sediment surveys from the Meguma Zone, southern Nova Scotia, Canada. J. Geochem. Explor. 39, 153À174. 702 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

65°54’10” N Pyrope garnet + chrome diopside grain counts

0 – 1 111°00’00” W 111°00’00”

2 – 10

11 – 100

101 – 500

501 – 2000

Glacial flow indicators Striae Contwoyto Lake

Streamlined landforms Roche moutonnées 110°38’30” W 110°38’30” 65°45’25” N

FIGURE 20.10 Bilobate glacial dispersal train (outlined in green) of kimberlite indicator minerals in till down-ice of the Contwoyto-1 kimberlite (red star) in northern Canada formed by two phases of ice flow, first to the southwest and subsequently to the north. Modified from Stea, R.R., Johnson, M., Hanchar, D., 2009. The geometry of KIM dispersal fans in Nunavut, Canada. In: Paulen, R.C., McMartin, I. (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada. Short Course Notes 18, pp. 1À13.

3. Regional to local dispersal, which is measured in kilometers to ten kilometers down-ice from the source. Examples include the dispersal of visually distinct minerals from kimberlites in diamond exploration programs, such as those seen in northern Canada (Fig. 20.10). 4. Local dispersal, which is typically measured in meters to hundreds of meters and commonly is at the scale of an exploration property. 20.4 GLACIAL DISPERSAL TRAINS 703

Till thickness, which ranges from less than a meter to tens of meters thick, may also impact the surface footprint of a dispersal train, and may reduce the lower threshold at which dispersal can be detected. A dispersal pattern in till that is extremely thick may have a very diluted signal, a long transport distance, and potentially a complicated glacial history. Several case studies demonstrate the erosion and uptake of mineralized rock from a source, transportation, and deposition in an upward-climbing fashion within increasing distance down-ice (Shilts, 1976; Drake, 1983; Miller, 1984; Klassen, 1997). To better understand how a dispersal train is formed, the transport capacity and first-order patterns of glacial erosion and deposition for a nonsteady-state ice sheet during a glacial cycle must be considered. A measure of glacial erosion intensity is often sought as a proxy for ice-sheet configu- ration (Hildes et al., 2004; McMartin and Henderson, 2004; Kleman et al., 2008). In areas affected by warm-based glaciation, streamlined landform density (Ha¨ttestrand et al., 1999), elongation ratio (Hart and Smith, 1997), inheritance of terrestrial cosmogenic nuclide ratios (Fabel et al., 2002; Gosse et al., 2006; Margreth et al., 2016), and streamlined landform characteristics (Bradwell et al., 2008) have been used to determine erosion intensity. The intensity of glacial erosion is an important consideration for mineral exploration because regions subjected to a higher intensity of erosion can be expected to produce longer dispersal trains where the till was deposited as a thin veneer (,2 m). In regions affected by high-intensity erosion and covered by thick till (.5 m), dispersal trains will be diluted within a short distance down-ice and thus dispersal trains will be shorter. Palimpsest dispersal trains are uncommon in areas that were affected by higher erosion regimes. Regions subjected to a low intensity of erosion, such as those occurring under ice divides, are expected to have older relict landforms preserved and palimpsest dispersal trains. Till deposits, and, in particular, glacial dispersal trains, can also be a proxy for the evolution of erosion intensity. Indicator clasts within till can delimit transport distances (Dreimanis and Vagners, 1971; Dredge, 1988; Kjær et al., 2003; Plouffe et al., 2011; Trommelen et al., 2013). The magnitude of clast, mineral, or elemental dispersal signals depends on the net erosion intensity up-ice at the source, compounded with other factors such as bedrock lithology, duration of glacial transport in one or more directions relative to the source area, inheritance of debris from older deposits, subsequent burial of the source, and dilution of mineralized debris through overprinting and entrainment (Aario and Peuraniemi; 1992; Parent et al., 1995; Stea and Finck 2001; Evans et al., 2006). Adjacent to an ice-divide, subglacial erosion intensity was probably highly variable, owing to additional factors such as initial ice buildup, ice-divide migration, and changes in mass balance with advance and retreat cycles (Kleman et al., 2008). These factors are difficult to assess, however, in the interpretation of the evolution of erosional intensity, especially where till cover is wide- spread, scoured lake basins are rare, and subglacial landform flowsets are fragmented, rare, or heavily overprinted. The formation and distribution of large erosional landforms underneath former ice sheets are primarily governed by the subglacial phase state. Glacial erosion occurred when ice sheets of a given thickness and ice flow rate reached pressure melting conditions (warm-based), typically as ice flow accelerated towards the ice margins. These warm-based zones provided material for deposition for a polythermal ice sheet under steady-state conditions. There is potential for long-distance transport of subglacial debris during ice expansion and deglacial phases, but such material would have to be transported at higher velocities to remain in the migrating entrainment and transport zones of an ice sheet (Kleman et al., 2008), particularly overlying soft sediment beds (Spagnolo et al., 2016). The combination of a relatively short duration of an ice sheet contraction 704 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

phase and inward migration of erosion zones would inhibit long-distance transport during deglaciation, except for ice-stream corridors. Over the past 25 years, the tracing of physically robust and distinct minerals, known as ‘indicator minerals’ (see detailed explanation below), has provided new insights into glacial dispersal patterns and transport distances where the traditional methods of boulder erratics and till geochemistry have failed. The dispersal train from the Izok Lake volcanogenic massive sulfide (VMS) deposit in northern Canada is one of example of this. Here, indicator minerals were detected at least 40 km down-ice from the deposit but till geochemistry was only able to detect elements derived from the deposit up to a maximum of 6 km down-ice and boulder tracing detected clasts dispersed up to 1 km down-ice (McClenaghan et al., 2015a).

20.4.1 DISPERSAL MODELS Various models are used to estimate the length of down-ice dispersal distances and evaluate how they covary with glacial entrainment and deposition processes. Quantitative numerical models that predict glacial dispersal have been summarized by Stea and Finck (2001). These models include the exponential decay model (e.g., Shilts, 1976; Bouchard and Salonen, 1989; Parent et al., 1996; Klassen, 1999) which is commonly used to describe glacial dispersal in basal tills (Shilts, 1973; Levson, 2001), and the linear decay model (Dyke and Morris, 1988; Klassen, 1997; Batterson and Liverman, 2000; Primmer et al., 2015) which is more commonly used to describe englacial dispersal in basal melt-out tills (Klassen, 2001), fast flowing ice (Dredge, 2000), and in regions of higher topographic relief (Paulen, 2001; Ozyer, 2011). Exponential decay models describe both enrichment of till by anomalous material and dilution of till by background material using negative exponential functions for enrichment and dilution. Most models illustrate the functional forms of these exponential equations including how the exponential model half-distances, also known as ‘renewal distances’, are associated with different decay constants (Bouchard and Salonen, 1990; Finck and Stea, 1995). These exponential decay model half-distances are inversely related to the ‘entrainability’ of the bedrock beneath a glacier and the basal ice velocity (Dyke, 1984; Clark, 1987; Aario and Peuraniemi, 1992). These decay constants and their associated half-distances can vary significantly with substrate lithology and hardness (Bouchard and Salonen, 1989; Puranen, 1990; Charbonneau and David, 1995). Linear decay models describe enrichment and dilution in till across a mineralized source using linear functions for enrichment and dilution (Klassen, 1997). Like the exponential decay models, the linear decay model half-distances are also inversely related to bedrock entrainability. Both of these models assume that no loss of entrained material, such as loss from ascension (shearing) into the englacial environment, took place down-ice from the source. However, if material already entrained within the glacier was lost, these increasing rates of entrainment of mineralized material would not be required with distance down-ice to achieve exponential or linear enrichment. Unfortunately, for the loss of entrained material to cause sufficient enrichment such that constant entrainment from bedrock was possible, the material lost would have to have been selectively composed of background material, a condition that became increasingly unlikely as the composition of the entrained material became more anomalous with increasing distance down-ice from a background-mineralized rock contact (because anomalous material was being added to the englacial load). This statement does contradict the linear and exponential decay models that appear to 20.4 GLACIAL DISPERSAL TRAINS 705

empirically fit geochemical, mineralogical, and lithological concentration data down-ice from known sources. However, Stanley (2009) stated that both models have mathematical consequences that are inconsistent with glacial processes and suggested that the exponential and linear decay models fail to adequately or reliably describe glacial dispersal trains. Hooke et al. (2013) argue that a thermal regime of basal meltwater refreezing, followed by shearing and folding of basal ice layers broaden and thicken a dispersal plume down-ice, which is later deposited. However all of these models fail to account for dispersal trains with linear enrichment and dilution patterns, especially those deposited by fast-flowing ice (Dredge, 2000). A dispersal model that appears to be a ‘best-fit’ with observed glacial dispersal trains is the aggradational-constant entrainment decay model (Stanley, 2009). This model treats units of englacial debris in ice and subglacial material (deformable bed) at the base of the glacier as ‘cells’. Each cell contains a specific geochemical, mineralogical, or lithological composition and is consistent with overall flow mechanisms of ice. These cells form the basal part of the glacier, including the deforming bed, as a series of layers that are stacked vertically on top of each other. Each layer is composed of cells that are adjacent to each other, and material is entrained into these cells from bedrock by regelation, glacial creep, internal thrusting, or abrasion, or from subjacent cells by internal shearing processes, and then transported upwards into overlying layers by internal shearing. The amount of material entering a cell from below is equal to the amount of material transported upward to the overlying cell (Fig. 20.11). Thus, as the glacier moves, the resultant dispersal train is transported down-ice and upward through the glacier. This migration and mixing of metal-rich and

Ice flow 10 00000000236810111110 9 00000001358101111119 8 000000025810121211108 7 000000148111313121086 6 000000371114141311864 5 0 0 0 0 0 1 6 12 16 16 14 11 8 6 4 3 4 0 0 0 1 1 2 12 18 18 16 12 8 6 4 2 1 Till layers Till 3 0 0 1 3 6 9 21 22 18 13 8 5 3 2 1 0 2 0 2 7 1319263021137 4 2 1 1 0 0 1 0 173244536127125 2 1 0 0 0 0 0 0 100 100 100 100 100 0000000000 BedrockMineralization Bedrock

FIGURE 20.11 Concentration (as a percentage) of metal-rich debris in till to illustrate the aggradational-constant entrainment decay model down-ice from mineralized bedrock. Mineralized bedrock is initially eroded and smeared down-ice as till layers are accreted, the size (length and width) of the dispersal train increases down-ice, and the concentrations of metal-rich material decrease by dilution down-ice. Modified from Stanley, C.R., 2009. Geochemical, mineralogical, and lithological dispersal models in glacial till: physical process constraints and application in mineral exploration. In: Paulen, R.C., McMartin, I. (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada. Short Course Notes 18, pp. 35À48. 706 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

metal-poor material causes the metal-rich debris to be diluted and dispersed upwards in the till profile, as it is transported down-ice and upwards through the glacial sediment sequence. As a result, material eroded from mineralization is effectively dispersed by down-ice movement such that, when subsequently deposited beneath the glacier, it produces a dispersal train within the till layers. This aggradational-constant entrainment decay model is geometrically consistent with the descriptive 3-D model described by Miller (1984), and 3-D dispersal trains that have been described (e.g., Nurmi, 1976; Averill, 2013; Oviatt et al., 2015; Averill, 2017). Application of these models, in combination with reconstruction of ice flow history in a given prospective region, will help mineral exploration geologists better interpret dispersal trains at surface, tighten their up-ice search vector, and provide insight into transportation distances in regions of thicker till (Fig. 20.6).

20.5 MINERAL RESIDENCE SITES IN TILL Till is the end-product of glacial erosion, transport, comminution, and deposition and consists of two particle size modes: (1) rock fragments or clasts (.1mm),and(2)matrix(,2 mm) that is dominated by mineral fragments (Dreimanis and Vagners, 1971). The minerals in the till matrix are crushed and abraded to their terminal grain size mode. The characteristic terminal grain size mode for each mineral is determined largely by the original grain size, hardness, cleavage, and the cementation or metamorphic history of the mineral in its bedrock source (Shilts, 1993). In till, quartz, magnetite, feldspars, pyroxenes, and other minerals commonly dominate the sand-size fraction, carbonates dominate the silt-size fraction, and phyllosilicates and hematite dominate the clay-sized fraction. Thus, the bulk chemical compositions of different-size fractions of till reflect the dominant minerals in each size fraction. In experimental studies involving the genesis of the fine fraction of tills, Ma¨kinen (1995) observed that as the degree of sediment grinding increased the contents of some metals (cobalt, copper, iron, manganese, nickel, and zinc) decreased in comparison to the overall composition of the coarser fraction. Elements that are common in coarser sand-size fractions include chromium typically in chromite, tungsten with scheelite, and tin with cassiterite (Shilts, 1993). Base metals, gold, and platinum group element contents tend to be higher in the fine (,0.06 mm) till fraction (Nevalainen, 1989; Salminen et al., 1989; DiLabio, 1995). Exploration programs should select sample preparation and analytical protocols based upon the target mineral deposit, the elements con- tained within or associated with it, and the likely terminal mode of ore minerals. Doing so will achieve the strongest signal:noise ratio that will better outline the head and tail of a dispersal train or other pattern.

20.6 EXPLORATION METHODS 20.6.1 BOULDER SAMPLING Mapping the distribution of mineralized boulders, cobbles, or pebbles, collectively referred to as ‘boulder tracing’, is one of the oldest exploration methods in glaciated terrain (e.g., Milthers, 1909; Dreimanis, 1958; Hyvarinen et al., 1973; Saltikoff, 1984; Sarala and Peuraniemi, 2007). Since the late 1800s, boulder tracing has provided the most obvious and direct indicators of the presence of 20.6 EXPLORATION METHODS 707

48°51’N 91°33’W

5 km

Ore outcrop > 2% ore pebbles in till < 2% ore pebbles in till Limits of main train Extreme limits of train Striations 92°01’N 48°32’N

FIGURE 20.12 Distribution of iron formation clasts in till down-ice of the Steep Rock Lake iron deposit in central Canada. Modified from Nichol, I., Bjo¨rklund, A., 1973. Glacial geology as a key to geochemical exploration in areas of glacial overburden with particular reference to Canada. J. Geochem. Explor. 2, 133À170, after Dreimanis, A., 1958. Tracing ore boulders as a prospecting method in Canada. Can. Inst. Min. Metallur. Trans. 61, 49À56. mineralization or a specific bedrock lithology (e.g., kimberlite, carbonatite, etc.). Boulder tracing has aided in the exploration for, and discovery of, a broad range of deposits including diamond hosted in kimberlite (Stewart et al., 1988; Carlson et al., 1999), rare metals (Ford et al., 1988; Batterson and Taylor, 2009), uranium (Steele, 1988; Kirchner and Tan, 1994; Hirvas and Ma¨kinen, 1989), VMSs (Morrison, 2004; Day et al., 1987), gold (Dreimanis, 1958; Plouffe et al., 2011), sediment- hosted leadÀzinc (Morrissey and Romer, 1973; Nawrocki and Romer, 1979), intrusion-hosted tin (Rogers et al., 1990), and iron ore (Fig. 20.12; Dreimanis, 1958). Modern boulder tracing methods involve determining the areal distribution of boulders at the surface, the rate of decrease of boulder abundance, the distance down-ice, and the geochemical, mineralogical, and isotopic composition of the boulders. Bouchard and Salonen (1990) provide an overview of the occurrence and dispersal of boulders and their utility to mineral exploration.

20.6.2 SOIL SAMPLING Postglacial weathering of till can significantly change its chemical and mineralogical composition in the zone of oxidation above the water table or above the permafrost table (Shilts, 1975; Shilts and Kettles, 1990; McMartin and McClenaghan, 2001) by destroying the labile minerals such as 708 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

sulfides. Thus, the geochemical signatures in soils formed on till are the combined result of clastic glacial dispersal and subsequent aqueous and gaseous dispersal. Therefore, metal contents may be depleted or enriched in soil in comparison to unweathered till. The 3-D geometry of glacial dispersal trains defined by soil geochemistry may be more difficult to interpret and trace up-ice to the source if hydromorphic processes have modified the soil composition (e.g., Kaszycki et al., 1996; Paulen, 2001; Lett and Jackaman, 2002; Hall et al., 2003). B-horizon soil is the highly oxidized and chemically weathered till that forms at the surface and typically extends down to a depth of 0.5À1.0 m. It is commonly enriched in amorphous Al- and/or Fe-oxyhydroxides, organic matter, and clay (Spirito et al., 2011). C-horizon till underlies the B-horizon and is the parent material (glacial sediment) that the B-horizon soil has developed on and that has been minimally affected by soil forming processes. C-horizon till is the target sampling medium because it provides an unimpeded signal of clastic glacial dispersal that is least affected by weathering. Some of the earliest exploration programs in Canada sampled the traditional ‘B horizon soil’ (e.g., Byers, 1956; Ermengen, 1957; Govett, 1973; Miller, 1979) adapted from use in areas of residual overburden and designed to avoid/ignore the effects of glaciation (Shilts, 1984). In glaciated terrain, soil may be developed on a variety of sediment types in addition to till, including gla- ciofluvial, glaciolacustrine, eolian, colluvium, or weathered bedrock. Each of these soil types can have very different transport and deposition mechanisms that can complicate the interpretation of soil geochemical data. Dreimanis (1960) recognized early on that the optimal approach to mineral exploration in glaciated terrain would be to sample till—material eroded, transported, and deposited directly by glaciers—and that metal-rich debris within till is a product of mechanical dispersal. Since the early 1980s, most exploration programs, case studies, and regional geochemical surveys have sampled till and not soil.

20.6.3 TILL SAMPLING Till geochemistry and indicator mineral methods are both widely used tools for mineral exploration in glaciated terrain, and both methods can be applied to the same till sample. Reviews of drift prospecting methods published over the last 15 years include recommendations for till sampling survey design and collection procedures and protocols (Levson, 2001; McMartin and McClenaghan, 2001; McMartin and Campbell, 2009; Paulen, 2009a; McClenaghan et al., 2013a). The size of area to be sampled, the commodity and size of deposit sought, and the expected size of a dispersed anomaly (see discussion above about scale of dispersal) dictate the sampling density (Table 20.2) and pattern, such as grid, random, or focused along roads. Till samples are collected at a reconnaissance to regional scale within a geological province; regional scale within a mineral district or at a regional to local scale to detect a mineralized zone or deposit (Table 20.2). Other factors that will influence the design of a till sampling survey include budget, time frame, accessibility of the area of interest, distribution and thickness of surficial materials, landforms, ice flow history, permafrost distribution, land tenure, and the mineralogical and analytical methods to be used to detect glacial dispersal (i.e., heavy minerals versus matrix geochemistry) (Coker and DiLabio, 1989; Hirvas and Nenonen, 1990; Kauranne et al., 1992; Plouffe, 1995). In areas affected by continental ice sheets, topography is generally subdued and therefore has less influence on the design of a sampling grid as compared to areas affected by alpine glaciation. A till sampling grid 20.6 EXPLORATION METHODS 709

Table 20.2 Varying Scales of Till Sampling Surveys Conducted for Mineral Exploration Sample Sample Sample Type Minimum Target Survey Scale Density Spacing and Size Analysis Comments Geologic Reconnaissance 1 sample/ 10À25 km Geochemistry Matrix Low sample density, province/ 100À500 km2 (3 kg) and geochemistry, often random sample domain heavy mineral indicator pattern (101 kg) minerals Mineralized Regional 1 sample/ 4À10 km Geochemistry Matrix Low to moderate district, 10À100 km2 (3 kg) and geochemistry, sample density, often kimberlite heavy mineral indicator sampled in offset lines field (101 kg) minerals perpendicular to regional ice flow Cluster of Regional to 1 sample/ 1À2 km Geochemistry Matrix Moderate sample deposits local 1À4km2 (3 kg) and/or geochemistry, density in either offset heavy mineral indicator grid or random sample (101 kg) minerals pattern (nearest neighbor) Individual Local to 100À1000 25À250 m Geochemistry Matrix High sample density, deposits property samples/ (3 kg) and/or geochemistry, infilling previous 1km2 heavy mineral indicator surveys, tight grid or (101 kg) minerals lines perpendicular to dominant direction of transport

Modified from McMartin, I., McClenaghan, M.B., 2001. Till geochemistry and sampling techniques in glaciated shield terrain: a review. In: McClenaghan, M.B., Bobrowsky, P.T., Hall, G.E.M., Cook, S.J. (Eds.), Drift Exploration in Glaciated Terrain. Association of Exploration Geochemistry-Geological Society of London Special Publication 185, pp. 19À43. with similar sample spacing in every direction (Fig. 20.13) is used where ice flow patterns or the character of the buried bedrock sources are not well known (McMartin and McClenaghan, 2001; McMartin and Campbell, 2009). In some areas, sample distribution will be controlled by access along roads or bodies of water (Figs. 20.12 and 20.14). In areas where ice flow patterns are known, samples may be collected along lines perpendicular or parallel to ice flow, with spacing between lines equal to or much greater than spacing between samples (Figs. 20.9 and 20.15). In mountainous terrain, ice flow can be controlled by topography, and sampling lines should therefore be oriented with respect to ice flow directions. Given the steep slopes present in these areas, till may be covered by postglacial sediments (e.g., colluvium) which must be differentiated from till and not sampled.

20.6.3.1 Sample mass A small till sample, 1À3 kg, is sufficient for bulk geochemical analysis of the till matrix and for archiving. A larger till sample, 10À20 kg if sandy till or 20À40 kg if clay-rich till, is required for satisfactory recovery of indicator minerals (e.g., McClenaghan et al., 2013a; Plouffe et al., 2013). Both large and small till samples can be collected from the same hole or overburden drill hole interval and placed in separate bags/pails to facilitate shipping to different sample preparation/ analytical facilities. If the .2 mm fraction of till is to be used for provenance studies, pebbles can be collected from the same hole at the time of field sampling or set aside during sample preparation at the processing laboratory. (A)

(B)

Palaeoproterozoic Neoarchean

FIGURE 20.13 Gold content in till samples collected on a regularly spaced grid in the Meliadine area of northern Canada: (A) gold content of the ,0.063 mm fraction determined by INAA; (B) number of gold grains in the ,2mm fraction normalized to a 10 kg sample mass of ,2 mm material. Ice flow arrows in top right corner indicate the trend of the three phases of ice flow that affected the area; red is the youngest ice flow. Modified from McMartin, I., 2009. Till composition along the Meliadine Trend near Rankin Inlet, Nunavut: applications to gold exploration in permafrost terrain. In: Paulen, R.C., McMartin, I. (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada, Short Course Notes 18, pp. 153À166. FIGURE 20.14 Zinc content in surface till samples collected on an irregular sample grid controlled by road and river access in low relief terrain in western Canada. Data are plotted on a digital elevation modal generated from the Shuttle Radar Topography Misson. Modified from Plouffe, A., Paulen, R.C., Smith, I.R., 2006. Indicator mineral content and geochemistry of glacial sediments from northwest Alberta (NTS 84L, M): new opportunities for mineral exploration. Geological Survey of Canada, Open File 5121; Alberta Energy and Utilities Board, Special Report 77 and Paulen, R.C., Paradis, S., Plouffe, A., Smith, I.R., 2011. Pb and S isotopic composition of indicator minerals in glacial sediments from NW Alberta, Canada: implications for ZnÀPb base metal exploration. Geochem. Explor. Environ. Anal. 11, 309À320. 712 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

63°05’N No. grains pyrope

91°27’W 0 ice flow N 1 2–10 11–50 51–100 101–500 501–1000

Kimberlite dyke Dyke trace

Inset 1

91°12’W

0120.5 Km 63°00’N

FIGURE 20.15 Pyrope content in till sampled along lines perpendicular to ice flow direction (southeast) with sample spacing along lines shorter than the spacing between lines on the West Churchill property in northern Canada. Modified from Strand, P., Banas, A., Baumgartner, M., Burgess, J., 2009. Tracing kimberlite indicaomineral dispersa trains: an example from the Churchill Diamond Project, Kivalliq region, Nunavut. In: Paulen, R.C., McMartin, I. (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada, Canada, pp. 167À175. Sampling density was increased during subsequent field seasons to follow-up on anomalous samples from previous seasons, which led to the discovery of two diamondiferous kimberlite dykes (green stars).

20.6.3.2 Till facies The facies of till sampled is another important consideration for assessing glacial dispersal. Basal (or subglacial traction) till is the optimal sample medium because it was deposited at the base of an actively flowing glacier with little or no reworking by water and is generally locally derived. It may occur at any depth between the ground surface and the top of bedrock (Levson, 2001). In local- and property-scale surveys, basal till sampling close to the bedrock surface is most effective because the composition of the till most closely resembles that of the underlying bedrock.

20.6.3.3 Sample depth In areas of simple stratigraphy and a single, thin (,2 m) till unit, glacial dispersal trains reach the surface at, or very close to, source (Fig. 20.6A and B) (e.g., Hirvas and Nenonen, 1990), and thus surface till sampling can be used to detect these trains. Surface samples should be collected at .0.5 m depth to obtain a till sample from the C-horizon. 20.6 EXPLORATION METHODS 713

In areas of complex stratigraphy with multiple till sheets, glacial dispersal trains may occur in one or more till layers (e.g., Fig. 20.6C and D). Trains may also occur deep within a single thick (.5 m) till unit. These deeper trains may not be accessible by surface till sampling. In such set- tings, profile sampling methods must be employed to collect vertically adjacent till samples below the surface, to characterize the till stratigraphy, and to determine lateral and vertical variations in the bulk geochemical composition of till (e.g., Hirvas and Nenonen, 1990; Sarala and Peuraniemi, 2007). When using deep sampling methods, the lateral sample spacing needs to be adjusted (discussed below) depending on the mobility and efficiency of the sampling equipment. In permafrost areas, a till sample may be collected from a shallow (,1 m) hole dug in a mudboil, a feature that develops in the active layer during the maximum summer thaw period (Shilts, 1978; McMartin and Campbell, 2009) when hydrostatic pressure pushes fresh till up to the surface producing a vegetation-free patch (Fig. 20.16A). Mudboils are easily recognized as round to oval patches that are barren of vegetation and surrounded by ridges of vegetation and small rock fragments (Fig. 20.16B). Till in mudboils is commonly well homogenized and relatively unoxidized, thus representative samples may be collected at a shallow depth (B0.3 m) from the center of the mudboil (Shilts, 1978). Where mudboils are poorly developed, old and inactive, or scarce, such as in some areas of discontinuous permafrost or coarse-grained bouldery till, samples can be collected directly below the thin soil profile on the mudboil surface. Most till sampled in glaciated terrains is a product of warm-based continental ice sheet erosion, transportation, and deposition. However, in areas affected by cold-based glacial conditions (e.g., parts of Canada’s north, or at high elevations in mountainous areas), till relict sediment or regolith may still be collected from rare mudboils (Fig. 20.16C), cryogenic sediments pushed into the center of boulder rings (sorted circles), or in between boulders in felsenmeer terrain (e.g., Tremblay and Paulen, 2012).

20.6.4 SAMPLING METHODS Surface till samples can be collected from holes dug using a hand-held shovel or mechanical exca- vator, or from vertical till sections exposed along rivers, lakes, or manmade road cuts or excava- tions. As indicated above, in thick drift areas with potentially complex stratigraphy, glacial dispersal trains may be intersected at any depth and in more than one till unit. Canadian examples of deep till sampling surveys include those conducted in prospective areas such as the Abitibi Greenstone Belt (e.g., Bird and Coker, 1987; Sauerbrei et al., 1987; McClenaghan, 1994), the Rainy River area (Averill, 2013), the central Interior Plateau of British Columbia (Ferbey and Levson, 2009; Averill, 2017), and the Athabasca Basin (Geddes, 1982; Wilson, 1985). Auger, percussion, pneumatic, and rotary drills recover till at depth, as well as bedrock samples (e.g., Paulen, 2009b; Sarala, 2015). Diamond drills can generally recover cores of compact, fine- grained till, but not of loose, sandy till. Reverse circulation drills recover a slurry of till cuttings up to 1 cm in diameter (McMartin and McClenaghan, 2001), however, most clay-sized material and approximately 30% of the silt-sized material in till is lost by this drilling method. Cross- contamination of vertically adjacent samples through the drilling fluid can occur with this drilling method. The rotasonic drilling method recovers an 8À9 cm diameter continuous core of till, through boulders and into bedrock, allowing detailed till sampling, identification of till stratigraphy, and no loss of fine-grained material (McMartin and McClenaghan, 2001; Sarala, 2015). For all drilling methods, till samples may be contaminated by the small fragments released during the (A) Sedges, Mosses, Stones Lichens Surface

Till

Permafrost

(B)

(C)

FIGURE 20.16 (A) Mudboil schematic cross-section emphasizing the mixing and push of fresh till to the surface due to hydrostatic pressure during each summer thaw period; (B) photograph of an active mudboil in till in permafrost terrain, till samples can be collected from the freshest material in the center of the mudboil; (C) photograph of a mudboil of relict sediment and/or regolith (outlined by dashed line) in a region of cold-based glaciation. (A) Modified from McMartin, I., Campbell, J.E., 2009. Near surface till sampling protocols in shield terrain, with examples from western and northern Canada. In: R.C. Paulen and I. McMartin (Eds.), Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada. Geological Association of Canada, Short Course Notes 18, pp. 75À95. 20.6 EXPLORATION METHODS 715

operation of drill equipment, including brass from fittings, diamonds from drill bits, tungsten- carbide fragments from drill bits (Coker and Shilts, 1993), and by drilling grease (contains molybdenum, lead, zinc) (Averill, 1990). Deformation of till structures and stratigraphy can result from the drilling process (Smith and Rainbird, 1987).

20.6.5 TILL GEOCHEMISTRY The term ‘indicator element’ is used here to refer to an element that is an economically valuable component of the mineralization being sought and which may be used to detect a mineral deposit (Boyle, 1974; Rose et al., 1979). The term ‘pathfinder element’ is used here to refer to elements associated with the mineral deposit, but that are not an element of main economic interest (Boyle, 1974; Rose et al., 1979). Key indicator and pathfinder elements for various deposit types are summarized in Table 20.3 from several sources (Boyle, 1974; Rose et al., 1979; Levinson, 1980; Lehtonen et al., 2015). 20.6.5.1 Size fractions The ,0.063 mm (À250 mesh) silt 1 clay sized fraction of till is the most common size fraction geochemically analyzed for mineral exploration (e.g., Lestinen et al., 1991; McClenaghan, 1994; Tarvainen, 1995; McClenaghan et al., 2011; McClenaghan and Peter, 2016) because: (1) many minerals in the mineral deposit are readily comminuted to this size range over short distances and thus are enriched in this size fraction (Nevalainen, 1989; Coker and DiLabio, 1989; Shilts, 1995, 1996); (2) this fraction contains phyllosilicates that are common alteration or gangue minerals and can be (primary) enriched in these elements of interest; (3) phyllosilicates will scavenge cations released during till weathering (Shilts, 1993, 1996); and (4) it can be prepared for analysis rapidly and inexpensively (Lett, 1995; Levson, 2001). The ,0.002 mm (clay-sized) fraction of till may also be geochemically analyzed (e.g., McMartin et al., 1996; Plouffe et al., 2016) because of its greater capacity to retain elements released during weathering, and to avoid potential textural influences on matrix geochemistry (Shilts, 1975, 1996). Disadvantages of using the ,0.002 mm fraction include (1) the large volume of material (up to 1 kg) needed to produce a sufficient mass (1À2 g) of clay-sized material for geochemical analysis (Klassen, 2003), (2) the strong influence of hydromorphic processes on the geochemistry of this size fraction that may distort or completely mask the signature of clastic glacial dispersal, and (3) the higher cost of the centrifuging that is required to recover this size fraction. Geochemically analyzing the ,0.177 mm (À80 mesh) sand 1 silt 1 clay sized fraction of till is generally not recommended because it includes abundant sand-sized quartz and feldspar grains which will dilute and mask the geochemical signature of mineralization or specific rock lithologies. 20.6.5.2 Analytical methods

Aqua regia (3:1 HCl:HNO3) is a partial digestion that is often used to analyze till because it targets the part of the total concentration that most likely reflects mineralization or provides geochemical contrast, i.e., sulfides, native gold, platinum, palladium, arsenides, selenides, tellurides, carbonates, most sulfates, phosphates, some oxides (e.g., uranium) and their oxyhydroxides (e.g., iron, manganese), some silicates, and organically bound elements (Chao, 1984; Hall, 1999). Cations in some silicate minerals will be leached by aqua regia, especially in micas and clay minerals (Koljonen Table 20.3 Listing of Selected Mineral Deposit Types and Corresponding Common Indicator Minerals That Can Be Recovered From, and Pathfinder 1 Indicator Elements That Can Be Analyzed for, in Till Indicator/ Ore Pathfinder Published Reviews and Deposit Type Elements Elements Common Indicator Minerals Examples Kimberlite- C Ba, Cr, K, Cr-pyrope, Cr-diopside, McClenaghan et al. hosted LREE, Mg, Nb, eclogitic garnet, Mg-ilmenite, (2002a), McClenaghan diamonds Ni, P, Rb, Sr, chromite, diamond and Kjarsgaard (2007), Ta, Ti Nowicki et al. (2007) Volcanogenic Cu, Pb, Ag, As, Au, Ba, Chalcopyrite, sphalerite, galena, Averill (2001), massive Zn, Ag, Bi, Cd, Cu, Hg, pyrrhotite, gold, pyrite, gahnite, McClenaghan and Peter sulfide Au In, Pb, S, Sb, Tl, staurolite, cassiterite, (2016), McClenaghan Zn spessartine, sillimanite, et al. (2015a,b) andalusite, beudantite, jarosite, barite, tourmaline, hogcomite, nigerite Sediment- Ag, Cu, Ag, Cu, Pb, S, Chalcopyrite, sphalerite, galena, Tarplee and Meer (2010), hosted lead- Pb, Zn Zn pyrite, barite, spessartine, Paulen et al. (2011), zinc smithsonite, anglesite, cerussite Oviatt et al. (2015) Gold Au, Ag Ag, As, Au, B, Gold, scheelite, tourmaline, Averill (2001), Sarala Ba, Bi, Cu, Co, rutile, sulfides, tellurides, PGM, et al. (2009), Fe, Hg, Mn, Sb, barite, cinnabar McClenaghan and Se, Te, U, W Cabri (2011) Magmatic Ni, Cu, As, Au, Cr, Cu, Pentlandite, chalcopyrite, pyrite, McClenaghan and Cabri NiÀCuÀPGE PGE Mg, Ni, PGE, S millerite, platinum group (2011), Averill (2009, minerals, chromite, Cr-diopside, 2011), McClenaghan et al. enstatite, olivine, Cr-andradite (2011) Rare metals REE, Li, Be, Ce, Cl, F, Pyrochlore, columbite, Batterson and Taylor Nb, Ta, Li, Nb, U, P, Ta-minerals, allanite, zircono- (2009), Lehtonen et al. Zr REE, Ta, Th, Y, silicates, apatite, monazite, (2015), Simandl et al. Zr fluorite, rhabdophane, (2015); Mackay et al. arfvedsonite (2015) Porphyry Cu, Mo, Au, Ag, Cu, Chalocopyrite, chalcocite, Averill (2011), Kelley CuÀAuÀMo Au, Ag Mo, S pyrite, molybdenite, gold, et al. (2011), Hashmi et al. silver, epidote, tourmaline, (2015), Plouffe et al. apatite, andradite, barite, (2016) monazite, rutile, titanite, zircon, jarosite, malachite Porphyry Sn, W, As, Ag, Be, Bi, Cassiterite, scheelite, Snow and Coker SnÀW Mo Cd, Cu, F, In, wolframite, molybdenite, (1987a,b), McClenaghan Mo, Pb, S, Te, chalcopyrite, Bi sulfides, et al. (2017a,b) W, Zn sulfides, fluorite, topaz, tourmaline Uranium U As, Ba, Cu, F, Uraninite (apitchblende), Geddes (1982), Boyle La, Ni, P, Pb, thorianite, tourmaline, sulfides, (1982), Campbell (2009); Th, Ti, U, Y, Zn, monazite, allanite, zircon, Robinson et al. (2016) Zr baddelyite, niccolite, UÀTh anatase, UÀTh rutile, brannerite, magnetite

PGM, platinum group minerals; PGE, platinum group elements; REE, rare earth elements. aPitchblende—brown or black pitchy massive form of uraninite. Data compiled from Boyle (1974), Rose et al. (1979), Levinson (1980), and Lehtonen et al. (2015). 20.6 EXPLORATION METHODS 717

et al., 1992). Minerals minimally attacked by aqua regia include barite, chromite, gahnite, garnet, cassiterite, ilmenite, rutile, sphene, monazite, and zircon (Dolezalˇ et al., 1968). An aqua regia digestion is commonly used for till geochemical surveys conducted by government agencies (e.g., Koljonen et al., 1992; Lett, 1995; Sarala et al., 1998; Bajc and Hall, 2000; Parkhill and Doiron, 2003; Barnett, 2007; McClenaghan et al., 2011) and by industry (e.g., Snow and Coker, 1987a,b; Brereton et al., 1988). Another partial digestion, (8:1 HNO3: HCl) that is weaker than conventional aqua regia is commonly used to capture uranium bound in uranium oxides and loosely bound in weathered material (Campbell, 2009). A total digestion of till is carried out to determine major, minor, and trace element contents for identifying signatures related to specific bedrock lithologies, major rock-forming minerals, or resistate minerals that are not digested by aqua regia (e.g., Taipaleetal.,1986; Koljonen and Malisa, 1991; Koljonen et al., 1992; Lahtinen et al., 1993; Tarvainen, 1995; Tarvainen et al., 1996; Klassen, 2003; Plouffe et al., 2006). For example, total digestion methods are useful for determining REE contents in till when exploring for kimberlites (e.g., McClenaghan and Kjarsgaard, 2007), carbonatites, or peralkaline granites (e.g., Lehtonen et al., 2015). Abundances of important pathfinder elements such as tungsten, tin, barium, selenium, chromium, and zirconium are best determined by ‘total’ digestion methods rather than those employing aqua regia (e.g., Chao, 1984; Chao and Sanzolone, 1992). Total/near total digestion methods include: (1) lithium meta/tetra borate fusion-nitric acid digestion which is optimal for determining the content of REE and other high field-strength elements (e.g., Cremer and Schlocker, 1976; Chao and Sanzolone, 1992); (2) four-acid digestion which is optimal for determining total copper, nickel, cobalt, lead, zinc, molybdenum, and silver, but is not a total digestion for REE, some oxides of aluminum, iron, hafnium, manganese, tin, tantalum, tungsten, and zirconium, for chromium from chrome-bearing minerals, or for barium from barium-bearing minerals (Koljonen et al., 1992; Noras, 1992). Also, volatilization during fuming may result in some loss of silicon, arsenic, antimony, and gold (Chao and Sanzolone, 1992); (3) lead fire assay on a minimum of 30 g of material which is optimal for determining total gold, platinum, and palladium content in areas with potential to host gold- or platinum group elements (PGE)-bearing mineralization (Hoffman et al., 1999; Hall and Oates, 2003). Instrumental neutron activation analysis (INAA) is a nondestruc- tive technique that can be used when the sample cannot be destroyed. Till heavy mineral concentrates (HMCs) (described below) may be analyzed for gold and other elements by INAA in order to preserve the concentrates for subsequent visual examination and determination of the abundance and character of indicator minerals present. Lead isotopic determinations of bulk till matrix can be useful for determining if copperÀleadÀzinc-rich till is sourced from VMS mineralization (Hussein et al., 2003 and references therein). The development of inductively coupled plasma-mass spectrometry (ICP-MS) in the late 1980s and through the 1990s was one of the key advances for applied till geochemistry as it allowed for the determination of a broad suite of trace elements including potential indicator and pathfinder elements for a wide spectrum of mineral deposit types. For example, till geochemical data may now be used to determine if glacially eroded VMS mineralization is silver-rich or gold-rich, or if it is enriched in sal- able (gold, silver, indium) or deleterious trace elements (selenium) (McClenaghan and Peter, 2016). Portable X-ray fluorescence spectrometry (pXRF) can be used to determine metal contents of till while still in the field in order to actively guide till sampling and follow-up of geochemical anomalies in the same field season. It may also be used in the laboratory to prioritize till samples for conventional lab-based geochemical or mineralogical analysis. Several recent studies report the advantages and disadvantages of applying pXRF analyses to moist versus dry till, sieved versus 718 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

unsieved till, and making determinations through plastic sample bags versus Prolene film-covered cups (Hall and McClenaghan, 2013; Kjarsgaard et al., 2014; Hall et al., 2016; Sarala, 2016). Quality assurance (QA) and quality control (QC) measures must be included at all phases of a till analytical program, including the collection of field duplicates, insertion of blanks and ‘blind’ laboratory duplicates, and insertion of certified reference standards. Recommendations for QA/QC measures for geochemical analysis of till include those suggested by Thomson (1983), Gustavsson (1992), and McClenaghan et al. (2013a).

20.6.6 INDICATOR MINERAL METHODS Similar to indicator elements described above, indicator minerals are mineral species that indicate the presence of a specific mineral deposit, alteration, or rock lithology and can be recovered from surficial sediments (Averill, 2001). Ideal indicator minerals recovered from glacial sediments are those that are found in few if any rocks other than in the mineral deposit, alteration, or lithology being sought. Their physical and chemical characteristics allow them to be readily recovered from large till samples in sufficient abundance for ready identification, viable grain counts, and analysis. These characteristics include: visual distinctiveness, moderate to high density (.2.9gcm23), silt to sand-size, ability to survive glacial erosion and transport, and ability to survive pre- and/or postglacial weathering (Brundin and Bergstrom, 1977; Averill, 2001). Peuraniemi (1990), Averill (2001),andLehtonen et al. (2015) provide extensive lists of common heavy minerals in till that have been combined to produce Table 20.3, a list of the most common indicator minerals of the major mineral deposit types. Indicator mineral methods differ from the traditional till geochemical methods described above in that the indicator grains reflect mechanical dispersal and individual grains can be visually examined, counted, and analyzed. The use of indicator mineral methods provides: (1) mineralogical evidence of the presence of a (unmetamorphosed or metamorphosed) mineralized target, hydrothermal alteration zone, or specific rock lithology; (2) information about the proximity to source; (3) low detection limits down to only a few grains in a single sample that is equivalent to ppb-level geochemical analyses; and (4) the ability to visually identify and remove, if necessary, grains from anthropogenic sources (McClenaghan, 2005). During the 1970s and 1980s, it was common for till HMCs to be pulverized and then geochemically analyzed (e.g., Garrett, 1971; Peuraniemi, 1985; Bird and Coker, 1987; Peuraniemi, 1990) because there was limited capability to systematically visually examine and identify indicator minerals. For example, tin content in till was used as a proxy for the presence of cassiterite (Peuraniemi, 1987; Rogers et al., 1990), and tungsten content for scheelite (e.g., Peuraniemi, 1985; Johansson et al., 1986; Salminen and Haartikainen, 1986). Methods are now commercially available to identify suites of indicator minerals in a till HMC for a broad range of deposit types, as listed in Table 20.3. Indicator minerals can be readily identified and counted in the silt to sand-sized heavy (.3.2 g cm23) and mid-density (2.9À3.2 g cm23) fractions of till (e.g., Averill, 2001; McClenaghan, 2011), thus at present, the HMC is less commonly analyzed geochemically. Indicator minerals are sparse in till, however, the presence of one sand-sized grain of a particular indicator mineral in 10 kg of till may be significant. Till samples are processed to reduce the mass of material to be examined by removing coarse fragments (typically .2 mm), clay-sized material, and low-density minerals. Most indicator minerals have a moderate to high density, therefore processing techniques involve density separation in combination with sizing. The most common density separation methods used for processing till samples include the shaking table, dense media separation, 20.6 EXPLORATION METHODS 719

jigging, centrifugal concentration, spiral concentration, and panning (e.g., Gent et al., 2011; McClenaghan, 2011). Approximately 90% of gold and platinum group mineral (PGMs) grains in source rocks and till are silt-sized (,0.063 mm) (Shelp and Nichol, 1987; Sibbick and Fletcher, 1993; Averill, 2001; McClenaghan and Cabri, 2011), thus preconcentration techniques to recover these fine-grained minerals from till must allow for the recovery of silt-sized heavy minerals. After preconcentration using one of the methods described above, the preconcentrate is further refined using a heavy liquid of a specific density to produce a sharp separation between heavy (sink) and light (float) minerals at a known density. Commonly used liquids include methylene iodide with a density of 3.3 g cm23, tetrabromoethane or the low-toxicity heavy liquid lithium heteropolytungstate, both with a density of 2.9. The density of the heavy liquids can be increased or decreased to capture specific minerals. The most common density separation for till samples is performed at 3.2 g cm23 so as to include the key indicator minerals Cr-diopside (3.25À3.55 g cm23), and olivine (3.27À3.37 g cm23)(McClenaghan and Kjarsgaard, 2007; McClenaghan, 2011). Some indicator minerals require a slightly lower density separation at 2.8 or 3.0 g cm23,suchasfluorite (3.01À3.25 g cm23), apatite (3.1À3.2 g cm23), and tourmaline (3.1À3.2 g cm23). The ferromagnetic and paramagnetic fractions are then separated from the nonmagnetic fraction to further reduce the volume of material to be examined and to isolate fractions for the counting of specific mineral species. Once the HMC is prepared, the 0.25À2.0 mm nonferromagnetic fraction is examined optically to determine the indicator mineral abundance and size. In addition to mineral abundance, mineral chemistry is an important modern exploration tool that provides insights into the nature and/or grade of the bedrock source. For example, the chemistry of KIMs in till can be diagnostic of the diamond grade of a kimberlite source (McClenaghan and Kjarsgaard, 2007; Nowicki et al., 2007). Mineral grains are characterized by measuring the abundances of major oxides, and minor and trace elements by scanning electron microscopeÀenergy-dispersive X-ray spectroscopy (SEM-EDS), electron microprobe analysis, or laser ablation-inductively coupled plasmaÀmass spectrometry (Layton Matthews et al., 2014;Lehtonen et al., 2015). Microchemical characterization techniques so well developed for placer gold (e.g., Chapman et al., 2009; Chapman and Mortensen, 2016) can now be applied to gold and PGM grains in till, to provide insights as to their bedrock source. Magnetite is a major accessory mineral in many types of mineral deposits as well as igneous, metamorphic, and sedimentary rocks, and thus is common in till. It is an ideal indicator mineral in till because it is physically robust (hardness 5 5.5À6.0), resistive to chemical alteration during glacial transport, and easily recovered from till because of its high density (5.1À5.2 g cm23) and ferromagnetic character. The chemical composition of grains in till can be used to discriminate between a magmatic or hydrothermal origin of the source rocks (e.g., Nadoll et al., 2012; Boutroy et al., 2014; Dare et al., 2014; Makvandi et al., 2016). Much progress has been made in the application of ICP-MS for isotopic analysis of single mineral grains in till. Radiogenic isotopes can provide additional information about potential mineralization types (e.g., VMS versus other base metal deposits) that may help refine exploration targets (e.g., Paulen et al., 2011; Oviatt et al., 2015) or determine the age of preglacial gossan formations (e.g., Kelley et al., 2011). This new lower-cost method may be faster and easier for the exploration- ist to use than traditional isotopic methods using the bulk till matrix. The rapid SEM scanning techniques, quantitative evaluation of minerals by SEM and mineral liberation analysis, can be used to automatically identify and characterize very small (,0.25 mm) indicator mineral grains in till HMC (Wilton and Winter, 2012; Layton-Matthews et al., 2014; Lehtonen et al., 2015). Morphology and surface textures of indicator minerals in till may provide indications of distance of 720 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

glacial transport (e.g., DiLabio, 1990b; Huhta, 1993; McClenaghan and Kjarsgaard, 2007). These features can be observed using optical microscopy and using SEM in backscatter mode. QC must be monitored at each stage in heavy mineral processing, indicator mineral identification, and subsequent mineral chemical analyses. Guidelines for QC procedures, as well as for the reporting of indicator mineral results for till, are reviewed in McClenaghan (2011) and Plouffe et al. (2013).

20.6.7 DIAMONDS Kimberlite, the primary host rock for diamond deposits in glaciated terrain, is a mineralogically and chemically distinct point source. A kimberlite’s variable size (50À200 m in diameter), shape (ellipti- cal pipe or narrow, elongated dyke), competence, and degree of postglacial weathering influence the size and areal extent of dispersed kimberlitic debris (McClenaghan and Kjarsgaard, 2001, 2007). In most cases, kimberlite is softer than its surrounding host rock and may be capped by soft pregla- cially weathered kimberlite. As a result of both factors, dispersal trains from kimberlite pipes are typically kilometers in length. Trains can be ribbon-shaped (Fig. 20.17), fan-shaped (e.g., Lehtonen et al., 2005; McClenaghan et al., 2012), or bilobate (Fig. 20.10), depending on ice flow patterns.

65º19' N 111º26' W <------Ice flow

Ranch Lake kimberlite

Cr-pyrope/20 kg till 0 1 – 10 11 – 50 50 – 150 151 – 300 Geological Survey of Canada till sample 301 – 445 2 km Canamera Geological Ltd. till sample

111º59' W 111º59' 65º11' N

FIGURE 20.17 Ribbon-shaped glacial dispersal train trending westward from the Ranch Lake kimberlite, Lac de Gras as defined by Cr-pyrope content in the 0.25À0.5 mm HMC fraction of till. The train was formed by a single phase of ice flow towards the west. Modified from McClenaghan, M.B., Ward, B.C., Kjarsgaard, I.M., Kjarsgaard, B.A., Kerr, D.E., Dredge, L.A., 2002a. Indicator mineral and till geochemical dispersal patterns associated with the Ranch Lake kimberlite, Lac de Gras region, NW Territories, Canada. Geochem. Explor. Environ. Anal. 2, 299À320. 20.6 EXPLORATION METHODS 721

In contrast, dispersal trains from kimberlite dykes that trend perpendicular to ice flow are typically wide and short (,1 km) (e.g., Kirkley et al. 2003; Strand et al., 2009). Glacial dispersal from kimberlite is typically traced using the abundance of the visually and chemically distinct indicator minerals Cr-pyrope, eclogitic garnet, Mg-ilmenite, chromite, Cr-diopside, forsteritic olivine, and diamond (Fig. 20.18 and Table 20.3) in the 0.25À2.0 mm nonferromagnetic fraction of till. Kimberlite boulders are also used to identify dispersal. Mineral chemistry is used to differentiate KIMs in till from visually similar minerals from other bedrock sources and to indicate the potential presence of diamond in the kimberlite source (e.g., McClenaghan and Kjarsgaard, 2007; Nowicki et al., 2007, and references in both). Surface textures on mineral grains can provide indications that their source is kimberlitic, or of glacial transport distance (Mosig, 1980; Afanase’ev et al., 1984). For example, resorbed octahedral crystal faces on chromite grains (Fig. 20.18F) are typical of a kimberlitic genesis versus magmatic or hydrothermal origin (McClenaghan and Kjarsgaard, 2001, 2007). The presence of soft kelyphite reaction rims on Cr-pyrope (Fig. 20.18B) indicates that its source is kimberlitic and that glacial transport has been relatively short (Garvie and Robinson, 1984; McCandless, 1990). Kimberlites have distinctive major and trace element compositions compared to their host rocks, thus till geochemistry may be useful for detecting the presence of kimberlitic debris down-ice (McClenaghan and Kjarsgaard, 2007). Its usefulness will depend on the geochemical contrast between the kimberlite and the surrounding bedrock such that elements in the kimberlite with concentrations that are 10 times greater than the host rock(s) they are emplaced into will be the most useful pathfinder elements (e.g., McClenaghan et al., 2002b,2004; McClenaghan and Kjarsgaard, 2007). Elements such as magnesium, nickel, chromium, niobium, tantalum, titanium, barium, potassium, rubidium, strontium, phosphorous, and light REE (Table 20.3) should be the most viable kimberlite pathfinder elements. Element abundances in till should be determined using a total digestion method such as lithium meta/tetra borate fusion (discussed above) in order to fully digest the oxide and silicate minerals and to determine the total concentrations of elements.

20.6.8 VOLCANOGENIC MASSIVE SULFIDE DEPOSITS VMS deposits are important sources of copper, lead, zinc, and silver. Glacial dispersal from VMS deposits has typically been traced using mineralized boulders and till geochemistry (McClenaghan and Peter, 2013, 2016, and examples therein). Indicator and pathfinder elements for VMS deposits that have proven effective for till geochemistry are listed in Table 20.3, and include the metals of economic interest in these deposits: copper, lead, zinc, and silver. Over the past 15 years, a suite of indicator minerals has been developed for VMS deposits (Table 20.3) that includes minerals present within mineralization and associated hydrothermal alteration zones (Averill, 2001; McClenaghan et al., 2015b). Detection of glacial dispersal of sulfide minerals down-ice is complicated by their chemical instability in the surface weathering environment. Thus, oxide and silicate indicator minerals can be more useful for this deposit type (Table 20.3). Gahnite is a zincian spinel that occurs around and within metamorphosed VMS deposits (Spry and Scott, 1986; McClenaghan et al., 2015a). It is a useful indicator mineral of VMS deposits because it is physically robust (hardness 5 8), chemically stable in oxidizing environments, and it can be easily recovered from till because of its high density (4À4.6 g cm23) and distinctive bluish green color (Fig. 20.19)(Lalonde et al., 1994; Paulen et al., 2013). An example of gahnite glacial dispersal down-ice from the Izok Lake VMS deposit in northern Canada is shown in Fig. 20.20. 722 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

FIGURE 20.18 Color photographs of KIMs: (A) purple to pink Cr-pyrope; (B) Cr-pyrope with dark green-gray kelyphite rims (k); (C) Cr-diopside; (D) eclogitic garnet; (E) Mg-ilmenite; (F) chromite showing resorbed crystal faces; (G) forsteritic olivine; and, (H) diamond. Mineral photography by Michael J. Bainbridge. Eclogitic garnet grains provided by Mineral Services. FIGURE 20.19 Color photographs of VMS indicator mineral gahnite (Zn-spinel) recovered from till down-ice of the Izok Lake VMS deposit in northern Canada: (A) gahnite with adhering minerals, an indication of proximity to source mineralization; and, (B) gahnite without adhering minerals and subrounded shape, an indication of longer glacial transport distance. Mineral photography by Michael J. Bainbridge.

N 65°40’N No. gahnite grains/10 kg (0.25–0.5 mm)

113°00’W Ice flow Range No. of samples 4 51–1739 (9) ? Iznogoudh Lake 21–50 (19) ? 6–20 (22) ? 2–5 (32) 1 (18) 0 (3)

Phase 3 dispersal (NW flow) Izok ? Lake Izok Deposit ?

Phase 1 112°43’W dispersal 1 km (SW flow) 65°36’N

FIGURE 20.20 Fan-shaped glacial dispersal train of gahnite abundance in the 0.25À0.5 mm HMC fraction of till normalized to a 10-kg sample mass down-ice of the Izok Lake volcanogenic massive sulfide deposit in arctic Canada. The fan- shaped train was formed by two phases of ice flow towards the southwest (blue polygon) and subsequent reworking by a younger ice flow towards the northwest (yellow polygon). Green polygon indicates the part of the train modified by both phases of ice flow. Modified from McClenaghan, M.B., Paulen, R.C., Layton-Matthews, D., Hicken, A.K., Averill, S.A., 2015a. Glacial dispersal of gahnite from the Izok Lake ZnÀCuÀPbÀAg VMS deposit, northern Canada. Geochem. Explor. Environ. Anal. 15, 333À349. 724 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

The morphology of gahnite shape may provide clues to relative distance of glacial transport. At source, grains in till are commonly interlocked with quartz and mica (Fig. 20.19A). As transport distance down-ice increases, glacial crushing of grains liberates the interlocking minerals to leave only gahnite (Fig. 20.19B) (McClenaghan et al., 2014a). The trace element chemistry of indicator minerals may be useful for establishing that the bedrock source of the mineral is VMS mineralization (e.g., gahnite: Heimann et al., 2005; O’Brien et al., 2015; magnetite: Makvandi et al., 2016). Secondary minerals such as beudantite, jarosite, and goethite form during the oxidation of primary sulfide mineralization. Where preglacial gossans have formed on VMS mineralization, these gossan minerals can survive glacial erosion and transportation and be useful indicator minerals in till down-ice (e.g., McClenaghan et al., 2015b).

20.6.9 SEDIMENT-HOSTED LEADÀZINC DEPOSITS Sediment-hosted leadÀzinc deposits include sedimentary exhalative (SEDEX) and Mississippi Valley-type (MVT) deposits, as well as Irish-type deposits that can be considered to be transitional between the first two (Wilkinson, 2014). SEDEX and MVT deposits are major sources of global zinc resources (Goodfellow and Lydon, 2007; Wilkinson, 2014). Glacial dispersal from SEDEX deposits has been traced using mineralized boulders and till geochemistry (e.g., Morrissey and Romer, 1973; Nawrocki and Romer, 1979; Tarplee and Meer, 2010). These studies have identified indicator and pathfinder elements, including the main metals of economic interest (zinc, lead, silver). Indicator minerals for this deposit type include sulfide minerals and may also include spessartine, cassiterite, and barite (Table 20.3). MVT and Irish-type deposits are carbonate-hosted leadÀzinc deposits (Paradis et al., 2007; Wilkinson, 2014), thus, till derived from these deposits has a carbonate-rich matrix which buffers the till as it weathers and allows the less-stable sulfide minerals to survive. As a result, sphalerite and galena (Fig. 20.21) can be detected down-ice from these deposits using both till geochemistry (Fig. 20.14) and indicator mineral methods. Secondary minerals such as smithsonite formed from the oxidation of sphalerite, and anglesite as well as cerussite formed from the oxidation of galena may also be useful indicator minerals in till (Table 20.3)(Oviatt et al., 2013). Lead isotopic compositions of individual galena and sulfur isotopic compositions of discrete sphalerite grains in till may

FIGURE 20.21 Color photographs of (A) sphalerite and (B) galena grains recovered from till down-ice of the O-28 lead-zinc deposit in the Pine Point Mining District of northern Canada. Mineral photography by Michael J. Bainbridge. 20.6 EXPLORATION METHODS 725

be useful for characterizing the bedrock source (e.g., Paulen et al., 2011; Oviatt et al., 2015). A few case studies and regional geochemical surveys have been carried out around, or in search of, carbonate-hosted leadÀzinc deposits (e.g., Allan, 1974; Hannon and Scott, 1975; Nawrocki and Romer, 1979; Paulen et al., 2011; Oviatt et al., 2013). Glacial dispersal from these deposits is characterized mainly by the ore-forming elements lead and zinc (Table 20.3).

20.6.10 LODE GOLD DEPOSITS Glacial dispersal from lode gold deposits was initially traced using mineralized boulders (e.g., Dreimanis, 1958). Today, gold grains recovered from till are the best indicator of the presence of gold mineralization (e.g., McMartin, 2009; Averill, 2017). Sulfides, arsenides, tellurides, PGM, and other oxide and silicate minerals listed in Table 20.3 are also useful indicators (McClenaghan and Cabri, 2011 and references therein). In rare cases, secondary minerals formed during pre- or postglacial oxidation may be useful indicator minerals (e.g., pyromorphite, malachite, goethite) (Averill and Zimmerman, 1986; Peuraniemi, 1990). Gold grain methods for till include documenting gold grain abundance, size, shape, and composition (e.g., Chapman et al., 1990; Huhta, 1993; McClenaghan, 2001; Sarala et al., 2009). Because gold is malleable, the degree of rounding and bending of the gold grains recovered from till provides useful information about glacial transport distance (Averill, 2001). DiLabio (1990b) presented a graphically descriptive classification scheme that is widely used today in Canada for describing gold grain condition (shape, crystal face, surface texture) that relates to glacial transport distance (pristine, modified, or reshaped; Fig. 20.22).

(A) (B) (C)

10 µm 10 µm 10 µm

FIGURE 20.22 Secondary electron images of gold grains showing examples of the three ‘conditions’ used to classify gold grains recovered from till: (A) pristine gold grain with equant molds suggestive of former quartzÀfeldsparÀcarbonateÀsulfide gangue; (B) modified gold grain with vestiges of equant gangue molds and edges that are slightly curled; and, (C) reshaped gold grain showing pitted surfaces and well curled edges. Modified from McClenaghan, M.B., 2001. Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada. In: McClenaghan, M.B., Bobrowsky, P.T., Hall, G.E.M., Cook, S.J. (Eds.), Drift Exploration in Glaciated Terrain. Geological Society of London, Special Publication 185, pp. 201À223. 726 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

Noteworthy examples of the successful application of indicator mineral methods to the discovery of gold deposits in Canada include the Casa Berardi deposits (Sauerbrei et al., 1987), the Rainy River deposit (Averill, 2013), and most recently, the Blackwater Lake deposit (Averill, 2017). Electrum, native copper, chalcocite, bornite, molybdenite, galena, and pyromorphite grains in till were used to identify a glacial dispersal train which led to the discovery of the Partridge Zone at Waddy Lake in central Canada (Averill and Zimmerman, 1986). Fig. 20.13B is an example of gold grain distribution in a regional till survey conducted over a prospective greenstone belt in northern Canada, showing a significant gold anomaly in till formed by south and southeast ice flow (McMartin 2009). Consistent gold grain recovery, counting, and shape classification methods have been used for more than 30 years in Canada. Use of these consistent methods allows for the comparison of industry- and government-led surveys results within and between projects over time, and the establishment of background thresholds for gold grain content in some regions such as the Abitibi Greenstone Belt (e.g., Averill, 1988). Till geochemistry is also an important gold exploration tool (e.g., Bird and Coker, 1987; Sarala and Peuraniemi, 2007; Sarala et al., 2009). Fig. 20.13A is an example of gold contents in the ,0.063 mm fraction of regional till samples around gold occurrences along the Meliadine Trend in northern Canada. Indicator and pathfinder elements for gold deposits are listed in Table 20.3, and include gold and silver. The optimal analytical methods for determining the total gold content of a till sample include fire assay-ICP-MS or INAA on a 30 g aliquot to reduce the nugget effect. Several studies have demonstrated only poor to moderate correlations between gold concentration in the ,0.063 mm fraction and number of gold grains in the heavy mineral fraction (e.g., MacEachern and Stea, 1985; McClenaghan, 1992, 1994). These studies indicate that both heavy mineral and geochemical methods should be used in concert for gold exploration programs.

20.6.11 MAGMATIC NICKELÀCOPPER DEPOSITS Magmatic nickelÀcopper deposits are significant sources of nickel, copper, and PGE. Recent Canadian examples of till geochemistry studies around magmatic nickelÀcopper deposits include those conducted in the Thompson Nickel Belt, Sudbury (Bajc and Hall, 2000; McClenaghan et al., 2011, 2014b), and Lac des Isles regions (Barnett, 2007) in central Canada. Table 20.3 lists indicator and pathfinder elements summarized from these studies. A suite of indicator minerals for magmatic nickelÀcopper deposits has been developed that includes nickel, copper, and PGE-bearing sulfides, arsenides, antimonides, and gold, together with one or more of Cr- and Mg-bearing oxides and silicates (Table 20.3)(Averill, 2009, 2011; McClenaghan and Cabri, 2011; McClenaghan et al., 2013b). Of these minerals, chromite, Cr- diopside, forsterite, and enstatite have distinguishing color, size, surface textures, and mineral inclu- sions that allow them to be visually distinguished from those found in kimberlite (Averill, 2009). Sperrylite is a common PGM that has been recovered from till down-ice from several magmatic nickelÀcopper deposits (McClenaghan and Cabri, 2011). It is a particularly useful indicator mineral because it is relatively stable in the surface weathering environment, is physically robust (hardness 5 6À7), visually distinct (tin white), and a direct indicator of the presence of PGE. The composition of some indicator minerals such as chromite, olivine, and magnetite, can be useful for identifying the presence of nearby nickelÀcopperÀPGE mineralization (e.g., McClenaghan et al., 2013b; Dare et al., 2014; Sappin et al., 2014). One of the better examples of glacial dispersal from 20.6 EXPLORATION METHODS 727

49°13’N 0 89°32’W

N 0 Lac des Iles 0 0 North Lac des 0 28 10 4 Iles Intrusion 0 2200 34 5858 10 633 3232 0 8 5757 1414 4400 50 8 0 0 1 0 76 45 4 1 30 1 0 7 0 235 0 2 0 20 4 Mine block 00 0 3434 1 intrusionIntrusion 0 222 3 9 0 Ice flow 0 0 1 0 7 0 2222 0 3737 53535 07 3535 6 0 2424 20 1 3838 0 3636 0 330 0 1 0 2 0

Zone of Pd mineralization Study area 36 Sample site and number of chromite grains normalized to 1 km 10 kg sample mass

30 Chromite abundance contour line 89°40’W 49°09’N

FIGURE 20.23 Pink ribbon-shaped glacial dispersal train showing the number of chromite grains in the 0.25À0.5 HMC mm fraction of till, normalized to 10-kg sample mass, at the Lac des Isles palladium deposits in central Canada. The train was formed by a single phase of ice flow towards the southwest. Modified from Barnett, P.J., Averill, S., 2010. Heavy mineral dispersal trains in till in the area of the Lac des Iles PGE deposit, northwestern Ontario, Canada. Geochem. Explor. Environ. Anal. 10, 391À399.

a PGE deposit is shown for the Lac des Iles area of central Canada in Fig. 20.23 (Barnett and Averill, 2010). A ribbon-shaped glacial dispersal train defined by Cr-andradite and chromite content in till extends at least 5 km down-ice (southwest) from the palladium mineralization (Averill, 2009; Barnett and Averill, 2010).

20.6.12 RARE METAL DEPOSITS Rare metal deposits are significant sources of REE and other high field strength elements such as tantalum, niobium, and zirconium, and are hosted in peralkaline intrusions, carbonatites, pegmatites, and peraluminous granites (Simandl et al., 2012, 2015). Indicator and pathfinder elements for these deposits include various combinations of the elements listed in Table 20.3. One geochemical example of glacial dispersal from an REE deposit is shown in Fig. 20.5 for the Strange Lake REE 728 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

deposit in eastern Canada. A ribbon-shaped glacial dispersal train defined by Be content in till extends at least 30 km down-ice (northeast) from the intrusion (Batterson and Taylor, 2009). Other useful examples of glacial dispersal from REE-rich sources include Ford et al. (1988), and Sarapa¨a¨ and Sarala (2013). Airborne or ground gamma-ray spectrometry may be used in combination with till geochemistry to detect glacial dispersal trains of uranium, potassium, or thorium-rich debris from these unusual host rocks (e.g., Ford et al., 1988). Indicator minerals of rare metal deposits contain high concentrations of the REE, niobium, tantalum, yttrium, and zirconium and are listed in Table 20.3. They include carbonates, phosphates, Na-pyroxenes, sodium and potassium amphiboles, titanium-bearing minerals, and zirconosilicates (e.g., Birkett et al., 1992, 1996; Makin et al., 2014).

20.6.13 PORPHYRY DEPOSITS Porphyry deposits are important sources of copper and molybdenum, as well as gold, silver, and tin (Sinclair, 2007). Till geochemical and mineralogical case studies and surveys carried out around porphyry deposits in glaciated terrain are described here in two sections: (1) the well-known copperÀgoldÀmolybdenum porphyry deposits of the North American Cordillera and (2) tinÀtungsten deposits. 20.6.13.1 CopperÀgoldÀmolybdenum porphyry deposits Copper porphyry deposits are significant sources of copper, molybdenum, gold, and silver. Till geochemistry is an important exploration tool for these deposits as demonstrated in several studies and regional till surveys (e.g., Nurmi and Isohanni, 1984; Sibbick and Kerr, 1995; Sibbick et al., 1997; Levson, 2001). More recent Canadian case studies include examples from the Highland Valley, Gibraltar, and Mount Polley 1 Huckleberry porphyry deposits, and the Woodjam prospect (Ferbey and Levson, 2009; Hashmi et al., 2015; Plouffe et al., 2016). From these studies a list of the most common indicator and pathfinder elements has been summarized in Table 20.3. Indicator minerals of porphyry deposits include the minerals chalcopyrite, chalcocite, molybdenite, and gold, as well as high-density oxide and silicate minerals such as andradite garnet, epidote, rutile, titanite, and zircon (Table 20.3) (e.g., Averill, 2011; Kelley et al., 2011; Hashmi et al., 2015; Plouffe et al., 2016). In contrast to other deposit types described here, the indicator mineral suite for porphyry deposits also includes mid-density (2.9À3.2 g cm23) minerals such as tourmaline, apatite, and fluorite (Averill, 2011), as well as jarosite, that formed prior to glaciation from the oxidation and supergene enrichment of iron sulfides (Kelley et al., 2011). The composition of some indicator minerals such as epidote and magnetite in till can provide further indications of the presence of porphyry mineralization nearby (e.g., Cooke et al., 2014; Nadoll et al., 2015; Canil et al., 2016). 20.6.13.2 TinÀtungstenÀmolybdenum porphyry deposits Intrusion-hosted tinÀtungstenÀmolybdenum deposits are an important source of strategic metals. The use of till geochemistry to explore for these deposits in glaciated terrain is well documented from case studies and exploration programs carried out in the 1970À1990s (e.g., Szabo et al., 1975; Snow and Coker, 1987a,b). One notable example is the use of till geochemistry combined with boulder tracing to outline a tin-rich dispersal fan (Fig. 20.5) and discover what would become the 20.6 EXPLORATION METHODS 729

East Kemptvlille tin deposit in eastern Canada (Rogers et al., 1990). Recent studies by McClenaghan et al. (2014c, 2017a,b) demonstrate the multielement geochemical signatures of two deposits (Sisson, Mount Pleasant) in eastern Canada. In general, indicator and pathfinder elements include tin, tungsten, molybdenum, and in some cases, indium (Table 20.3). In contrast to geochemical methods, the use of indicator mineral methods to explore for tungsten deposits in general is a more recent development. Older reports of the recovery of tin- or tungsten- bearing minerals from a till sample are mostly from follow-up investigations to determine the mineralogical composition of a single metal-rich till sample (e.g., Brundin and Bergstrom, 1977; Lindmark, 1977; Toverud, 1984; Johansson et al., 1986). Indicator minerals include the main ore minerals, scheelite (Fig. 20.24A and B), wolframite (Fig. 20.24B), cassiterite (Fig. 20.24C), and molybdenite, as well as accessory sulfides, Bi-bearing minerals, fluorite, and topaz (Table 20.3)(Averill, 2001). The visual identification of the scheelite in till HMC is greatly aided by the mineral’s bluish-white fluorescence under short-wave ultraviolet light (Fig. 20.24B). Recent studies by McClenaghan et al. (2017a,b) have documented the indicator mineral signatures of tin and tungsten-bearing porphyry deposits through case studies around the Sisson and the Mount Pleasant deposits.

FIGURE 20.24 Color photographs of: (A) scheelite grains recovered from till overlying the Sisson tungstenÀmolybdenite deposit in eastern Canada, under visible light; (B) scheelite grains from (A) under short-wave ultraviolet light; (C) wolframite, and (D) cassiterite grains recovered from till-down ice of the Mount Pleasant tinÀtungsten deposit in eastern Canada. Mineral photography by Michael J. Bainbridge. 730 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

20.6.14 URANIUM DEPOSITS The highest-grade uranium deposits in the world occur at, or in close proximity to, the unconformity between Proterozoic quartz-rich sandstones in basins and underlying Aphebian and Archean basement rocks, including those in the Athabasca Basin in central Canada (Jefferson et al., 2007; Potter and Wright, 2015). These are termed unconformity uranium deposits. Other uranium deposit types include pegmatites and other granitic deposits, uraniferous iron oxide copperÀgold deposits, ancient fluviatile placers, and uraninite veins and lodes (Boyle, 1982; Tauchid and Underhill, 1997). The tracing of radioactive boulders is a relatively simple and widely used exploration method for uranium deposits in glaciated terrain (e.g., Lundberg, 1973; Gustafsson and Minell, 1977; Hirvas and Ma¨kinen, 1989). The radioactive boulder trains of the Athabasca Basin are among the most numerous and well documented in the world (e.g., Kirwan, 1978; Ramaeker et al., 1982; Wilson, 1985; Kirchner and Tan, 1994). The geochemistry of glacial erratics (composite sampling) in the Athabasca Basin has also proven to be useful (Earle, 2001). Till geochemistry is also an important uranium exploration tool (e.g., Geddes, 1982; Simpson and Sopuck, 1983; Wilson, 1985; Campbell, 2009) and employs indicator and pathfinder elements that are summarized in Table 20.3. Uranium is highly mobile in the oxidizing surface environment, thus the interpretation of uranium geochemical anomalies in till must take into account the potential for both clastic dispersal and hydromorphic dispersion (e.g., Bjo¨rklund, 1976; Boyle, 1982; Peuraniemi and Aario, 1991). Because the ,0.002 mm fraction of till has a greater cation exchange capacity than the ,0.063 mm fraction (Shilts, 1975), it has greater potential to scavenge uranium from hydromorphic solutions, and thus it is a commonly analyzed size fraction. Airborne or ground gamma-ray spectrometry, in combination with surficial geology and geochemistry, can help delin- eate glacial dispersal from radioactive sources and prioritize areas of interest for uranium exploration (Fortin et al., 2015). Boyle (1982) suggested a list of potential indicator minerals that is summarized in Table 20.3 and vary with uranium deposit type. Geddes (1982) reported one of the only descriptions of the use of indicator minerals to outline a uranium-rich dispersal train using niccolite and pitchblende in till. This till sampling led to the discovery of a uranium deposit in the eastern Athabasca Basin.

20.7 CONCLUSIONS In glaciated terrain, glaciers have eroded metal-rich debris from mineralization and transported and deposited the debris down-ice in dispersal trains, fans, and other patterns, all of which form larger exploration targets than their bedrock sources. Till geochemical and indicator mineral methods, combined with bedrock mapping and geophysical methods, are used to detect and trace the dispersed debris up-ice to mineralization. Glacial dispersal may be the result of one or more phases of ice flow and vary in distance down-ice from a few tens of meters to in excess of 100 km. Knowledge of the complexity of continental ice sheets and ice-sheet dynamics during their growth, evolution, and deglaciation, provides additional clues to elucidating the nature of dispersal in complex glacial stratigraphy and/or glacial history such as corridors of fast-flowing ice. The dispersal pattern often detected at the surface is the net result of all cycles of erosion, transportation, and deposition by one or more glacial events. REFERENCES 731

Boulder tracing and till geochemistry have been exploration tools in glaciated terrain for more than 60 years. The development and advancements of ICP-MS analytical instruments since the 1980s now allow for the determination of an extensive suite of indicator and pathfinder elements for a broad range of commodity and deposit types. Portable XRF is increasingly being used in the field to characterize till geochemistry and guide sampling. Since the early 1990s, the use of indicator mineral methods in glaciated terrain has expanded rapidly such that they are now routinely used to explore for a broad range of commodities and deposit types. Indicator minerals include the more traditional gold and sulfide minerals, as well as more physically robust and chemically stable oxide and silicate minerals. In some cases secondary sulfate and phosphate minerals, formed during preglacial weathering of mineralization, survive glacial erosion and transport and can be useful indicator minerals. Microanalytical techniques have evolved such that indicator mineral species may now be rapidly identified in the very fine size fraction in addition to the traditional .0.25 mm size range of HMCs. Isotopic compositions may now be determined for single mineral grains and compared to the known characteristics of various mineral deposit types. Utilizing indicator mineral methods in tandem with till geochemical methods allows for the simultaneous exploration of a broad range of commodities. Although not discussed in this chapter, till compositional data generated during an exploration program can also provide useful data for environmental assessments and planning.

ACKNOWLEDGEMENTS This chapter reflects the cumulative experiences of the authors in drift prospecting and till sampling surveys, but also draws on the published work of fellow researchers at the Geological Survey of Canada (GSC) and the Geological Survey of Finland. The GSC’s Geomapping for Energy and Minerals (GEM) and Targeted Geoscience Initiative (TGI) programs at Natural Resources Canada have supported the recent research in drift prospecting and glacial studies reported here. The authors thank the following individuals and companies for contributing information and/or figures: Pertti Sarala (Geological Survey of Finland); Jan Peter, Isabelle McMartin, Janet Campbell (GSC); and Stu Averill (Overburden Drilling Management Limited). Tom Nowicki (Mineral Services) provided the eclogitic garnets that were photographed in Fig. 20.18. Pertti Sarala is also thanked for his contributions to Appendix A. Michael Bainbridge produced the color photographs of the mineral grains. Natural Resources Canada Earth Sciences Library, Ottawa staff are thanked for their help in retrieving some of the older reference material used in this review. We thank Tracy Barry and Paul Champagne (GSC) for their production of some of the digital figures included in this chapter. Alain Plouffe, Jan Peter, and Wendy Spirito (GSC) are thanked for their thorough and thoughtful reviews of the manuscript. Natural Resources Canada Earth Sciences Sector (NRCan ESS) Contribution No. 20160420.

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APPENDIX A: MINERAL DEPOSITS DISCOVERED BY TILL SAMPLING AND BOULDER TRACING A significant number of mineral deposits (e.g., precious metals, base metals, uranium, iron, diamonds) have been discovered in glaciated terrain using various drift prospecting techniques. DiLabio (1989) compiled an extensive list of discoveries that were made in Canada and Fennascandia prior to the diamond rush in the early 1990s and the associated expansion in the use of indicator mineral methods. This appendix is an updated list of selected examples of the discovery of major mineral deposits and mines in which boulder tracing, till geochemistry, and indicator mineral methods have contributed. The deposits are organized by commodity. Note that the mapping of mineralized boulders, cobbles, or pebbles is collectively referred to here as ‘boulder tracing.’

Deposit Locale Commodity Method Reference Taivalja¨rvi Eastern Finland Ag Boulder tracing Papunen (1986) Storbodsund Sweden Ag Boulder tracing Grip (1953) Equity Ore British AgÀCu Soil (till) Ney et al. Columbia, geochemistry (1972) Canada Chappell British AgÀAu Soil (till) Barr (1978) Columbia, geochemistry Canada Hirsikangas Western Au Boulder tracing Kontoniemi and Finland Mursu (2006) Osikonma¨ki Southeast Au Boulder tracing Kontoniemi Finland (1998) Kittila¨ Central Au Till geochemistry Pankka et al. Lapland, (2006) Finland (Continued) 750 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION

Continued

Deposit Locale Commodity Method Reference Pahtavaara Northern Au Till geochemistry Korkiakoski Finland et al. (1989) McGill Mines Ontario, Au Boulder tracing Rickaby (1932) Canada Duport Ontario, Au Boulder tracing Thomson Canada (1936) Lamaque Mines Quebec, Au Boulder tracing Wilson (1948) Canada Fisher-Quebec Quebec, Au Boulder tracing Tremblay Canada (1950) Malarctic Gold Quebec, Au Boulder tracing Halet (1948) Fields Canada Rainy River Ontario, Au Indicator minerals Averill (2013) Canada Quesnel River British Au Soil (till) Fox et al. Columbia, geochemistry (1987) Canada Casi Beradi Area Quebec, Au Indicator minerals Sauerbrei et al. Canada (till) (1987) Hoyle Township Ontario, Au Indicator minerals Harron et al. Canada (till) (1987) Waddy Lake EP Saskatchewan, Au Indicator minerals Averill and Zone Canada (till) Zimmerman (1986) Kemi Mine Northern Cr Boulder tracing Maier et al. Finland (2015) Otanma¨ki Central Finland FeÀTiÀV Boulder tracing Sarapa¨a¨ et al. (2013) Steep Rock Ontario, Fe Boulder tracing Dreimanis Canada (1956) Josephine Ontario, Fe Boulder tracing Moore and Canada Armstrong (1948) Burin Peninsula Newfoundland, Fluorite Boulder tracing Van Alstine, Canada (1948) Hitura Mine Central Finland Ni Boulder tracing Isohanni et al. (1985) Lynn Lake Manitoba, NiÀCu Boulder tracing Brown (1947) Canada East Kemptville Nova Scotia, Sn Till geochemistry Rogers et al. Canada and boulder tracing (1990) Rabbit Lake Saskatchewan, U Boulder tracing Tan (1977) Canada Key Lake Saskatchewan, U Boulder tracing Tan (1977) Canada APPENDIX A 751

Continued

Deposit Locale Commodity Method Reference Cluff Lake Saskatchewan, U Boulder tracing Wilson (1985) (D deposit) Canada Midwest Lake Saskatchewan, U Boulder trains Kirwan (1978) Canada Paukkajanaar a Eastern Finland U Boulder tracing Wennervirta Mine (1967) Sisson New WÀMo Till geochemistry Snow and Brunswick, Coker (1987a,b) Canada Nigadoo Mine New ZnÀPbÀCu Boulder tracing MacKenzie Brunswick, (1958) Canada Photo Lake Manitoba, ZnÀCuÀAu Till geochemistry Kaszycki et al. Canada (1996) Mogador Quebec, ZnÀPb Boulder tracing Dreimanis (Vendome) Canada (1958) Izok Lake Nunavut, ZnÀPbÀCu Boulder tracing Money and Canada Heslop (1976) Buchans Newfoundland, ZnÀPbÀCuÀAg Boulder tracing Neary (1981) Canada Fyre Lake Yukon, Canada ZnÀPbÀCuÀAgÀAu Boulder tracing Peter et al. (2007) Newfoundland Zinc Newfoundland, Zn Soil (till Brummer et al. Mines Canada geochemistry) (1987) New Calumet New CuÀPbÀZn Boulder tracing Alcock (1941) Brunswick, Canada Sullivan Mine British PbÀZn Boulder tracing Burchett (1944) Columbia, Canada Tynagh Ireland CuÀPbÀZn Boulder tracing Donovan and James (1967) Chidliak kimberlite Nunavut, Diamonds Indicator minerals Pell et al. province Canada (2013) Buffalo Head Hills Alberta, Diamonds Indicator minerals Eccles (2011) kimberlite province Canada (eskers and till) Lac de Gras Northwest Diamonds Indicator minerals Fipke et al. kimberlite province Territories, (eskers and till) (1995) Canada Otish Mountains Quebec, Diamonds Indicator minerals Birkett et al. kimberlite field Canada (till) (2004) Kirkland Lake Ontario, Diamonds Indicator minerals Brummer et al. kimberlite field Canada (eskers and till) (1992a,b)