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Application of Till Mineralogy and Geochemistry to Mineral Exploration

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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 bed- rock. 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 sedi- ment 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 dis- persal 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 explo- ration methods used today have evolved over time as analytical methods and our knowledge of Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00022-1 © 2018 Elsevier Ltd. All rights reserved. 689 690 CHAPTER 20 APPLICATION OF TILL TO MINERAL EXPLORATION 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 explo- ration 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 observa- tions 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 sul- fide 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 miner- alogy 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 min- eralized 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 analy- sis (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, ori- ented landforms, and pebble fabrics were used (Paulen and McClenaghan, 2015). Ice initially flo- wed 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
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