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Geological Survey of Canada's contributions to understanding the composition of glacial sediments1

W. W. SHILTS Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada K I A OE8

Received March 1 , 1992 Revision accepted September 3, 1992

Drift compositional studies were initiated at the Geological Survey of Canada (GSC) in the mid-1960's in projects that drew heavily on the technology and approaches developed in Fennoscandia over the previous century. As this research progressed and expanded in the 1970's, its Fennoscandian character diminished and, like the geochemical exploration research program that it closely paralleled, drift prospecting began to acquire a distinctly Canadian character, imposed by the geographical and logistical constraints of climate and asymmetric population distribution. GSC research has increasingly focused on understanding and explaining the geochemical expression of the mineralogical composition of glacial sediments that are unaltered or only slightly altered by postdepositional weathering. Because glacial sediments are generated largely by crushing and abrasion, their various mineral and lithological components attain characteristic size modes based on physical properties such as cleavage and hardness. This size sorting by physical properties is expressed geochemically by chemical partitioning in various size ranges. Also, the crushing process disaggregates fresh bedrock and incorporates all of its mineral components, labile as well as stable, into glacial sediment. GSC research has concentrated on the best ways to sample and analyze drift to avoid the compositional size bias and potential weathering alterations of labile minerals, so that geochemical analyses truly represen- tative ofprovenance may be attained. As it has become possible to filter provenance signals from noise generated by weathering, partitioning, sediment facies misidentification, and stratigraphic variations, the principles of glacial dispersal of components of economic and environmental significance have been clarified and dispersal patterns mapped.

Les Ctudes de la composition des mattriaux d'origine glaciaire ou fluvioglaciaire ont commencC a la Commission gCo- logique du Canada (CGC) au milieu des anntes 1960, dans des projets largement inspires de la technologie et des mCthodes d'approche dCveloppCes en Fennoscandie au cours du sikcle dernier. A mesure que ces activitCs de recherches progressaient et croissaient en nombre durant les anntes 1970, l'influence fennoscandienne diminuait et, a l'instard du programme de recherches en prospection geochimique Ctroitement parallkle, la prospection a partir des dCpBts glaciaires a commencC a acqutrir les traits d'une identitt canadienne prescrits par les contraintes logistiques et geographiques du climat et par la rtpar- tition asymetrique de la population. Les recherches h la CGC s'orientaient de plus en plus vers la comprChension et I'explica- tion de l'expression geochimique de la composition mineralogique des sediments glaciaires non alteres, ou que faiblement affect& par une altCration mCtCorique postCrieure leur depBt. Vu que les sMiments glaciaires derivent largement du broyage et de I'abrasion, leurs diverses composantes lithologiques et minCralogiques presentent des modes granulomttriques caractk- risitiques qui dependent des propriCtCs physiques comme le clivage et la durete. Ce tri granulomCtrique dCtermint par les propriCtCs physiques s'exprime gCochimiquement par une repartition chimique et reflkte les diffkrents intervalles granulomC- triques. En outre, le processus de broyage dCsintkgre les roches fraiches du substratum et incorpore dans le ddiment glaciaire I'ensemble de ses minCraux instables et stables. Les recherches a la CGC ont surtout porte sur les meilleures fa~ons d'echantil- lonner et d'analyser les sediments d'origine glaciaire ou fluvioglaciaire afin d'Cviter les erreurs dues i des compositions qui different selon la granulomCtrie et les alterations potentielles des mintraux instables, afin que les analyses gCochimiqus reprB sentent vkritablement la provenance. Comme il est maintenant possible de filtrer les signaux de la provenance des perturba- tions crCCes par I'altCration mCtCorique, la rCpartition chimique selon la granulomCtrie, les erreurs dans I'identification des f a d s de stdiment, et les variations stratigraphiques, on peut mieux comprendre les principes de la dispersion glaciaire des composantes d'importance 6conomique et ecologique et dresser des cartes de dispersion.

[Traduit par la rkdaction] Can. J . Earth Sci. 30, 333-353 (1993)

Introduction Drift prospecting is properly thought of as a Fennoscandian

innovation. Many authors have alluded to the prescient contributions of the Swede Daniel Tilas (1740) to the science of drift prospecting because of his observations of and recognition of glacial transport of erratics of Rapakivi Granite over a century before the theory of continental glaciation was widely accepted elsewhere in Europe and North America. Tilas can probably be considered the progenitor of the systematics of mineral exploration in glaciated terrain, but the scientific base and techniques of using properties of drift directly for mineral prospecting progressed little beyond his level of sophistication until well past the middle of the 20th century (see, for example, Sauramo 1924).

Canadian geoscientists and the Geological Survey of Canada

'Geological Survey of Canada Contribution 21092.

(GSC) have played major roles in developing new techniques for characterizing the composition of glacially transported overburden, techniques that were unimaginable and, in fact, impos- sible, not only in Tilas' era, but as late as 50 or 60 years ago. These advances have come about largely as a result of two major technological developments outside the study of earth sciences: (i) the development of means of rapid transportation and logistical support infrastructures that allow modern glacial processes to be studied and observed in their generally remote locations; and (ii) the development, over the past four decades, of fast and sensitive analytical methods and equipment. This has led to inexpensive, increasingly sensitive, and accurate analyses of the various components that constitute glacial drift.

In Canada, coping with a persistent cover of glacially transported overburden (drift) has been accomplished using three principal strategies: ( i ) Traditional geochemical exploration techniques, largely developed in, and designed for, unglaciated terrains mantled by residual soils on slopes and fluvially trans-

Printed In Canada / Impr~rnC au Canada

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334 CAN. J , EARTH SCI. VOL. 30. 1993

ported sediments in valleys, have been applied, with little modification, to glaciated areas in an attempt to "see through" the cover of various types and thicknesses of glacial sediments. The objective of this approach has been to detect geochemical signals generated directly from bedrock and to differentiate them from the glacially distorted signals produced by the overburden itself. Before about 1970, and to a lesser degree even now, an incomplete understanding of the complexities of drift sedimentology, and therefore of drift geochemistry, has led to the indiscriminate application of exploration techniques developed in nonglacial environments, in an attempt to circumvent the difficulties of drawing a bedrock signal from beneath a seemingly chaotic cover of glacially transported overburden. (ii) Geophysical techniques (aeromagnetic, electromagnetic, airborne radiometric) have been developed, particularly in Canada, to see through the drift cover. Although these techniques have been used with success, they are st31 confounded in many cases by heterogeneity in surficial sediment facies, geometry, or composition. The paper by Teskey et al. in this issue includes some discussion of the GSC's contributions in this field, and it will not be dealt with further here. (iii) Although the GSC has pioneered the application of geochemical and geophysical techniques to exploration in Canada, in the 1960's it began to focus some research on the use of the compositional characteristics of transported materials in drift as a viable means of carrying out mineral exploration. The efforts were concentrated on direct geochemical, mineralogical, and lithological analyses of various sedimentary facies of drift in an attempt to determine which combination(s) of facies and analytical approaches could best lead to the discovery of mineralization. In short, research was focused on the provenance of glacial sediments, which, once determined, can lead to predic- tive inferences about such diverse subjects as location of mineralization, relationships of natural and anthropogenic chemical inputs into the environment, and the dynamic structure and flow lines of the North American ice sheets.

The GSC has been a leader in identifying the sedimentological and diagenetic parameters that are the most important for interpreting the compositional messages derived from the analysis of glacial drift. In this paper, while recognizing the important role that the GSC has played in "conventional" geochemical research, the discussion will be directed mainly toward describing the Survey's role in developing theories to explain drift provenance and applying these theories to practi- cal mineral exploration and environmental problems.

Drift prospecting in Canada As a country with over 50% of the formerly glaciated terrain

of the world, and one in which major mineralized areas commonly are covered by glacial drift, it is not surprising that Canada would become a centre for drift prospecting research. As early as 1956, Aleksis Dreimanis of the University of Western Ontario introduced classical Fennoscandian boulder- tracing techniques to Canada in his influential publication on the dispersal of iron-bearing erratics from the orebody that eventually became the Steep Rock Lake iron mine in Ontario (Dreimanis 1956). He followed this research with one of the first drift geochemical surveys, which, although analytically crude by today's standards, introduced the notion that geochemical dispersal patterns in drift could lead directly to the discovery of mineralization (Dreimanis 1960). By the mid- 1960's, H. A. Lee of the Geological Survey of Canada had begun research on the potential for using the mineralogy of glacial sediments to find mineralization in northwestern Ontario:

specifically, gold mineralization and kimberlites with diamond- bearing potential (Lee 1963, 1965, 1968). Shortly after pub- lishing the first results of his drift prospecting research, Lee left the Survey to apply his expertise directly to mineral exploration problems faced by industry.

In the late 19601s, the increasing use of geochemistry in exploration, both by industry and the Geological Survey, began to bring to light many of the problems faced by Finnish explorationists in the 1950's (Kauranne 1959). The principal problem was that "transported overburden," to use an explorationist's common expression, distorted the bedrock signature, in many cases to such an extent that overburden geochemistry bore little relationship to the lithogeochemistry of underlying bedrock. The hope that circulating groundwaters or gases would redistribute exchangeable ions from mineralization in the bedrock to clay minerals or oxides and hydroxides in the drift, thus overriding the transported geochemical signal, proved to be largely futile. R. G. Garrett, faced with such problems in a pilot geochemical study of drift overlying a base-metal orebody at Manitouwadge, Ontario (Garrett 1969), enlisted the aid of D. R. Grant, who discovered that the bulk of the overburden consisted of till dominated by far-travelled carbonate debris from the Hudson Bay Lowland (Grant 1969).

Soon after the Quaternary Research and Geomorphology Division of the GSC (now the Terrain Sciences Division) was formed in 1968, J. G. Fyles, its director, allocated significant funds to projects designed to further drift prospecting research, particularly in areas of perennially frozen terrain of the Cana- dian Shield. Over much of the Shield, significant potential for mineralization exists, but with little permanent population, limited or difficult access, and only a vague body of knowledge about glacial and periglacial history and processes, a uniquely Canadian approach to drift provenance problems was required. It was in this temporal and intellectual setting that the GSC launched the first of its large drift prospecting research projects in 1970. Since that time, significant progress has been made toward the development of a scientific rationale and realistic logistical strategies for studying and applying compositional patterns of surficial sediments to the solution of exploration problems in glaciated terrain.

Seminal GSC projects on drift composition Most of our modern concepts about drift composition in

Canada grew out of a group of projects started in 1965 and largely finished in the mid-1980's. A mapping project in the Quebec Appalachians was started by B. C. McDonald in 1965 (McDonald 1967, 1969) near an area underlain by asbestos- bearing ultramafic bodies of the Quebec ophiolite belt. The ultramafic lithologies are enriched in Ni, Cr, and Co. Glacial erosional and depositional processes acting on the ultramafic outcrops generated large, distinctive geochemical, mineralogical, and lithological dispersal trains that, because of their easy access, have been thoroughly studied and used as models for glacial dispersal (for example, Shilts 1973a, 1975, 1976, 1978a; BClanger 1988; Courtney 1989) (Fig. 1). Tills in the Appalachian region of southern Quebec also are enriched in sand-sized sulphide minerals, which are ubiquitous in the black slates and schists that underlie much of the terrain. Because of the labile nature of the sulphide phases in the near-surface environment, sections through the local tills are ideal for studying the mineralogical and geochemical effects of postglacial weathering (Shilts and Kettles 1990).

In 1970, a major, helicopter-supported project designed to develop methods of prospecting in glaciated, perennially frozen

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H E A D S

FIG. 1. Three-dimensional plot of nickel concentrations along the oph the United States border. (From Shilts 1991.)

terrain was undertaken in the Rankin Inlet - Ennadai green- stone belt of south-central District of Keewatin. Sampling techniques were integrated with various types of traversing, e.g., rotary-wing and fixed-wing aircraft, foot, and (or) inflat- able boat; some of the first attempts at traversing with all- terrain vehicles were also made. The drift sampling projects were further integrated with experimental geochemical exploration projects, utilizing waters, tundra plants, and lake sediments (some collected by divers), carried out under the same logistical umbrella (Klassen 1975; Jones et al. 1976; Edwards et al. 1987). In addition, drift sampling was coordinated with stratigraphic studies of sediment-hosted and volcanogenic mineralization associated with the iron formation that dominates the Rankin - Ennadai belt.

Because of the strong development of a variety of types of patterned ground in this area, the genesis of the patterns and their influence on sediment geochemistry were also investigated (Shilts 1973b, 1977, 19786; Ridler and Shilts 1974; Shilts and Dean 1975) (Fig. 2). From this project, in addition to the raw data and observations of interest to explorationists, the importance of geochemical partitioning by grain size was first recognized (Shilts 197 1). In addition, the contrasting effects of postglacial weathering on till and coarse-grained sorted sediments, such as esker deposits, were described (Shilts 1973b). The Keewatin project was expanded in 1975 to areas near Baker Lake where uranium exploration was being carried out (Klassen and Shilts 1977). The sampling strategies, periglacial process models, and geochemical principles developed during 1 the Rankin Inlet - Ennadai project were applied in this different bedrock setting where uranium mineralization occurs in poorly consolidated sandstones.

Beginning in 1976, regional bedrock and surficial geology mapping projects provided widely spaced drift samples throughout Keewatin and yielded an idea of the shape and causes of the larger dispersal trains (Shilts et al. 1979; Rencz and Shilts 1980; Shilts 1984). At the same time, a number of detailed sampling programs were carried out by R. N. W. DiLabio (DiLabio and Shilts 1977) and R. A. Klassen (Klassen and Shilts 1977).

C v

~iolite belt of southeastern Quebec. Trains trend southeastward toward

FIG. 2. Two- to three-metre diameter mudboils on the surface of a drumlin protruding through a cover of marine sand with frost poly- gons near Kaminak Lake, District of Keewatin. An understanding of sediment-specific patterned ground permitted easy selection of sample sites from airphotos and aircraft during Geologic Survey of Canada Keewatin projects. (GSC 201696-W.)

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336 CAN. J. EARTH SCI. VOL. 30. 1993

9 0

/-'

& 138

104 - 633 @?&

??- 4

136 Tp ,-+

c. ?,I 2 3 Precambrian

ppm Ni in <2pm ppm Cr in <2ym

AKTINEQ GLACIER 0 5 I O K M -

PRECAMBRIAN CRETACEOUS-TERTIARY -BEDROCK BEDROCK w

E, 7 0 -

z I

5 0 - -00 15 10 5 0

DISTANCE UP-ICE FROM SNOUT (km)

FIG. 3. Dispersal of chromium and nickel derived from Precambrian bedrock, as revealed by samples of till from lateral moraines on Bylot Island. (From DiLabio and Shilts 1979.)

In 1977, a project was started on Bylot Island, off the northeast coast of Baffin Island, to study the geochemical and mineralogical composition of sediments transported and deposited by modern glaciers (DiLabio and Shilts 1979) (Fig. 3). This project and subsequent studies on Bylot Island have provided a body of data suitable for comparison with that obtained from ancient glacial deposits and have generated a better understanding of drift compositional data from elsewhere on the Canadian Shield.

In 1980, the GSC undertook a regional drift sampling program on and near the Frontenac Arch of the Grenville structural province of eastern Ontario and contiguous Quebec. Simultane- ously, an extensive regional lake water and sediment survey

was carried out to establish the geochemical linkages between drift composition and lacustrine geochemical environments. Although designed to provide information about the potential impacts of acid rain on drift with variable carbonate content and trace-element geochemistry, the data yielded insights into dispersal patterns and weathering and were used by mineral explorationists (Hornbrook et al. 1986; Shilts 19826; Kettles and Shilts 1989; Kettles et al. 1991).

From 1985 on, research directed specifically toward drift prospecting has continued on a limited scale at the GSC. The principles and models derived from these studies and related research have been widely applied to a series of major drift

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composition projects funded under federal-provincial agree- ments in Quebec, New Brunswick, Nova Scotia, Labrador, District of Keewatin, Ontario, and Manitoba. Routine drift sampling has now become a standard component of almost all surficial geology mapping carried out by the GSC and provincial government agencies.

Drift dispersal and compositional models from Geological Survey of Canada research

The unconsolidated sediments that lie on either side of a terminal moraine marking the maximum extent of a Pleisto- cene glacier have radically different compositional characteristics. Outside a glacial boundary, overburden is formed largely by chemical weathering and decomposition of bedrock, and by fluvial and gravity transfer of altered materials downslope and through drainage systems. Compositional signals of soils and sediment in these areas (except in regions of aeolian activity) can be related confidently to nearby bedrock or, in the case of fluvial sediments, to specific drainage basins from which the sediments were derived. In an unglaciated terrain, geochemical characteristics of overburden reflect most strongly the adsorption or incorporation of chemical components on or in secondary mineral phases such as Fe -Mn oxides and hydroxides and clays. Except in the case of resistate minerals, such as cassiterite, chromite, and gold, geochemical analyses rarely reflect primary minerals derived directly from unmineralized bedrock.

The mineralogy of surficial materials in a glaciated terrain is very different from that of unconsolidated soils and sediments in an unglaciated terrain. Glacial overburden consists of many different sediment facies, deposited in glacial as well as in preglacial, interglacial, and postglacial environments. Drift is extremely variable in thickness, ranging from less than one metre to hundreds of metres over short distances. But most significantly, from the viewpoint of the mineral explorationist or environmental geochemist, glacial deposits, and sediments derived from them, are composed largely of unweathered, crushed bedrock detritus, the product of clast-on-clast and (or) clast-on-bedrock impacts of stones suspended in, or dragged along by, ice.

The compositional implications of the fundamental difference in origin between overburden in glaciated and unglaciated terrains are profound. For example, geochemists sampling soils in glaciated terrain must be cognizant of the fact that, except for the uppermost parts of the thin postglacial solum, the chemistry reflects the mineralogy of the sample, not the amount of adsorbed or absorbed ions that were released from primary mineral phases by weathering. Fragments of both labile and stable primary ore minerals, glacially extricated from their bedrock sources, may be found, unaltered, in the overburden. From an environmental point of view, many easily altered minerals, with chemical components that could be noxious (i.e., arsenopyrite (As), sphalerite (Cd)) if released into the surface or subsurface hydrologic environment, are present in the overburden and can be destabilized by anthropogenic activities or natural changes in weathering regime. .

In contrast to unglaciated regions, where the unconsolidated cover is redistributed within discrete drainage basins by mass wasting and fluvial processes, glaciated regions were covered by ice sheets that flowed over complex topography, carrying components from drainage basin to drainage basin. Thus, the composition of a sample of glacial sediment can reflect the integration of a multitude of bedrock sources from the centre

of glacial outflow to the site where the sample was collected. Though glacial components subsequently may be redistributed through drainage basins by streams and mass wasting processes, the compositional characteristics of a sample of glacial sediment, or of nonglacial sediment derived from it, are not necessarily related to those of the drainage basin in which the sample was collected. In areas of mountain glaciation, where the high relief tends to confine glaciers to specific drainage basins, the compositional signal of drift more closely cor- responds to that which would be expected for unglaciated mountains. Nevertheless, glacial sediments from this setting share many characteristics of drift with areas of continental glaciation, since they are composed largely of a heterogeneous mixture of unweathered, crushed bedrock detritus (Evenson and Clinch 1987).

Derivative concept of drift sampling At the GSC in the mid-1970's, an informal nomenclature

was developed to express these complex ideas (Shilts 1976; Coker and DiLabio 1989). Unconsolidated sediments of the glaciated landscape were referred to as "derivatives" of bedrock, not only to reflect the physical nature of the processes that reduced bedrock to a stony mud, but to emphasize the variety of glacial and nonglacial processes by which its components, once crushed, were transported and deposited to form the modern landscape typical of glaciated regions.

Till, formed of crushed bedrock debris and deposited directly beneath, or from, ice by lodgment and various melting-out processes, is the $rst derivative of bedrock. Its components may have undergone one or more episodes of glacial transport, but owe their present location mainly to the simple, linear movement of the glacial ice that transported them. Thus, the composition of till usually reflects ice-flow history.

Glaciofluvial sediment, which forms eskers, kames, and proglacial outwash, can be regarded as a second derivative of bedrock. The sand and gravel that make up glaciofluvial deposits are primarily derived from debris carried by the glacier. This debris would have been deposited as till had it not been eroded from the glacier bed or from the ice by subglacial or proglacial meltwaters, which flush the finest components (rock flour) through and out of the glaciofluvial system. Second-derivative sediment has undergone one or more additional phases of transport following its movement in ice from the original bedrock source. Thus, compositional data derived from glaciofluvial sediments must be interpreted in light of their potential for reflecting two or more transportational cycles, and, therefore, varying directions and distances of dispersal.

Glaciolacustrine and glaciomarine sediments generally com- prise the fine-grained debris that is washed from the glacier's load by glaciofluvial processes and flushed through the glaciofluvial system. Subsequent to glacial and glaciofluvial transport, silt and clay can remain in suspension for some time because of their fine grain size, and they can be deposited far from their source owing to dispersal by currents in lakes or the sea. Thus, glaciolacustrine and glaciomarine sediments are the third derivative and have a complex transportational history, starting at a bedrock source and involving glacial, fluvial, and marine or lacustrine transportation. Consequently, the geochemistry and mineralogy of these sediments tends to be homo- geneous over large areas and cannot be related readily to the bedrock sources originally tapped by glaciers. Fine-grained sediments deposited by density underflows (Gustavson 1975) issuing directly from conduit openings at the front of a glacier submerged in proglacial waters, if recognized, may represent

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a much more local provenance than distal fine-grained sediments. These sediments can accumulate rapidly in depressions r immediately in front of the glacier and may not undergo much more dispersal than the glaciofluvial sediments or tills from which they are derived. Because of the graded, laminated 60 I nature of underflow deposits and their common association with similarly laminated sediments deposited from suspension, considerable sedimentological expertise is required to collect 2 useful samples from them. They generally are not regarded as 40 a useful sample medium. c

N Fluvial, lacustrine, or aeolian processes may redistribute

glacial sediments across the landscape, producing fourth or higher derivatives. These materials have had a varied glacial and nonglacial transportation history and may have been resedi- mented many times since glaciation. Unlike the lower-order derivatives, which were deposited rapidly under climatic and sedimentological conditions unfavourable for penecontempora- neous chemical alteration, postglacially redistributed, higher- order sediments may be subjected to varying degrees of chemical alteration before or during transport. The consequence of this weathering is to flush ions from the system in solution or to redistribute ions from labile mineral phases into stable secondary phases, such as Fe-Mn oxides and hydroxides and clay minerals.

Traditionally, environmental geochemistry or geochemical prospecting research at the GSC has diverged at the conceptual boundary between the lower derivative sample media and those higher than third order. Glacial sedimentologists have tended to sample glacial or glaciofluvial sediments that have undergone relatively limited transport and little weathering. Explora- tion geochemists, on the other hand, have concentrated on higher order sediments, with their potential for focusing or intensifying the effects of local geochemical environments by concentrating ions in secondary mineral phases (i.e., Fe oxides-hydroxides) with high ion exchange capacity by means of various low- temperature geochemical reactions. The former approach, though producing compositional data that are somewhat easier to interpret, requires a fairly high level of sedimentological expertise, as well as relatively deep exposures, which often are not readily available. Drift sampling, therefore, can require expensive or laborious techniques (drilling, digging) to obtain material that is relatively unaltered. Historically, at the GSC, the two sampling philosophies have complemented each other, one or the other being appropriate in each of the varied glacial - geochemical landscapes that make up the Canadian landscape.

Geological Survey of Canada S contributions to understanding and using drift composition

Numerous summaries of the development of boulder tracing and other forms of drift prospecting in other countries have been published (i.e., Sauramo 1924; Grip 1953; Wennervirta 1968; Tanskanen 1980; DiLabio and Coker 1989; Kujansuu and Saarnisto 1990), and symposia have been held at regular intervals to discuss the latest methods of exploration in glaciated terrain (Prospecting in Areas of Glaciated Terrain series of Imperial College of Mining and Metallurgy). The following discussion will highlight the author's perception of the main contributions that GSC scientists have made to the general field of drift compositional studies. These contributions, made

. ' . . . . - - .. . -. . .., ..

L r. 'm

-.@$< c . "SIEVED" (-250 mesh) e-..

FIG. 4a. Plot of zinc concentrations in sieved <64 pm separates from till against wt. % clay ( < 2 pm) in < 64 pm fraction, Carr Lake area, District of Keewatin. See Fig. 11 for location. (From Shilts 1991 .)

Partitioning Several problems related to the interpretation of geochemi-

cal analyses of sieved fine fractions from till collected from areas of patterned ground became obvious during the Keewatin drift prospecting project in the Rankin Inlet - Ennadai green- stone belt (1970- 1975). In the Keewatin study, clay-sized ( < 2 pm) particles constituted 2 -20 % by weight of the < 2 mm portion of till. This considerable variation was observed throughout the area sampled, as well as within single excavations made in mud boils. When samples were sieved to <63 pm, the sample-to-sample variations were enhanced. By plotting trace-element concentrations of the <63 pm fractions of till against weight percent material < 2 pm (Figs. 4a, 4b), it was evident that background metal levels varied almost directly as a function of the texture of the samples (Shilts 1971, 1975). This observation spurred a 20 year study that has yielded significant insight into the effects of chemical partitioning in glacial sediments.

Partitioning effects in drift were first described in a classic series of papers by Dreimanis and Vagners (1971, 1972) on the behaviour of carbonate minerals subjected to glacial transport. Briefly, they concluded that carbonate clasts entrained by a glacier were quickly reduced in transit, by clast-to-clast or clast-to-bedrock contact, to a bimodal size distribution of larger, multimineralic carbonate clasts and smaller, silt-sized, mono- mineralic carbonate mineral grains. The silt-sized grains were considered to be the optimum or terminal grain size mode for carbonate minerals. The fact that carbonate minerals were reduced primarily to silt size and no farther was related to their physical properties, for example, cleavage and hardness. Theo- retically, all mineral species subjected to glacial abrasion should have a characteristic terminal mode, and it should be within this mode that their chemical composition is most strongly expressed. Therefore, the chemistry of various fractions of unweathered glacial sediments should directly reflect the miner-

principally over the past 30 years, have been integrated with alogy of the modes that dominate each size fraction. externally established models and techniques to form the cur- Early in the Keewatin project, it was observed that, in rent GSC research program of drift composition studies. general, the clay-size mode of many glacial and derived sedi-

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Wt.% <2 pm in <64 pm Zn (ppm) in <64 pm

FIG. 4b. Maps of zinc concentrations and textural variation, Carr Lake area, near Kaminak Lake, showing how false anomalies can be generated by partitioning processes. See Fig. 11 for location. (Modified from Shilts 1971.)

ments is particularly enriched in many cation species and that, consequently, analyses of the silt-clay fractions of samples of similar provenance but with differing clay contents yielded significantly different geochemical analyses. Clay-sized particles, which are dominated by various phyllosilicate phases, are generally enriched in metallic elements, which often can be related to sulphide or other types of mineralization (Fig. 5). The metallic elements seem to be largely structural and not adsorbed, because selective leaching experiments fail to dis- place them (Shilts 1984).

As a result of these observations, samples from the GSC's drift prospecting program have been subjected routinely to clay ( < 2 km) separation before geochemical analysis since about 1971. Although this procedure avoids the false provenance signals generated by textural variations, it is not necessary in all cases, nor is it generally desirable in evaluating coarse- grained, chemically immobile minerals, such as gold, chromium, and tin.

Table 1 gives some indication of the widely varying chemistry of different size fractions of till. For further discussions of the effects of partitioning, the reader is referred to DiLabio (1982), Nikkarinen et al. (1984), Shilts (1984, 1991), and Shilts and Wyatt 1989.

Weathering of glacial sediments Many minerals in drift are labile under surface weathering

conditions. Postglacial weathering takes place in the zone of

oxidation above the groundwater or permafrost table and can alter drift geochemistry to considerable depths (Shilts 1975, 1976, 1984; Rencz and Shilts 1980; Peuraniemi 1984; Shilts and Kettles 1990). Furthermore, the effects of weathering on the chemistry of relatively impermeable silt- and clay-rich tills are quite different from those on some of its more permeable silt- and clay-poor derivatives, such as esker or other ice- contact gravels (Shilts 19736; Shilts and Wyatt 1989). In an oxidizing environment, labile minerals, such as sulphides and carbonates, are generally destroyed above the water and permafrost tables; their chemical constituents are carried away in solution or precipitated or scavenged locally by clay-sized phyllosilicates and by secondary oxides -hydroxides, depending on the element and the local geochemical environment. At poorly drained sites where the water table is at or is close to the surface, or where the surface is underlain by an organic mat, a reducing environment inhibits the destruction of primary labile minerals (Peuraniemi 1984).

Destruction of labile components also takes place in porous and permeable glacial sediments, particularly in well-sorted, glaciofluvial sands and gravels. Weathering of primary silicate minerals that are unstable in the near-surface environment produces secondary, mixed-layer clays, hydroxides and oxides, which can be physically translocated downward through the deposit (Shilts 1973b; Shilts and Wyatt 1989). This is especially important in eskers and other coarse-grained deposits, which have little or no primary fine fraction. Many such deposits stand

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340 CAN. J . EARTH SCI. VOL. 30, 1993

FIG. 5. Partitioning of As, Cu, and U by grain size in till samples from several geological settings in Canada, United States, and Europe. (From Shilts 1984.)

TABLE 1. Partitioning in till over ultramafic bedrock, northern Ungava, Quebec

Pd Pt Ni Co Cr Cu Au (PPb) (PPb) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ) (PPb)

Raglan 3" Bulk sample (<6.0 mm) 1100 330 2966 91 1305 4460 120 <63 pm (silt + clay) 980 210 3466 84 669 5 400 160 2.0-6.0 mm 2100 500 2579 91 2360 4230 620 0.25-2.0 mm 1400 490 3249 117 1760 4430 230 63 -250 pm 1100 220 2306 79 904 3 380 130 45-63 pm 900 180 2164 62 913 3300 160 2-45 pm 720 300 2418 72 629 3620 210 <2 pm 2300 110 6355 118 882 > 10000 60

Raglan 5b Bulk sample ( < 6.0 mm) 130 60 679 53 845 477 20 <63 pm (silt + clay) 100 96 592 41 407 433 44 2.0-6.0 mm 130 85 727 60 1440 411 4 0.25 -2.0 mm 140 75 674 64 1010 436 6 63 -250 pm 110 50 446 40 538 346 8 45-63 pm 68 100 406 34 469 291 34 2-45 pm 78 70 490 37 445 344 26 < 2 pm 390 70 1601 104 1045 1405 10

NOTES: Samples collected and donated by Michel Bouchard, UniversitC de Montreal. "Sample of highly altered till collected from a mud boil 8 m down-ice from platinum group elements -

sulphide mineralization. hSample of apparently unaltered till collected from a till plain 170 m down-ice and downslope from gossan

near Raglan 3.

as ridges or hummocks, and the bulk of the sediments lie above In till, the fine fractions were produced largely by the physical the ground-water table. Owing to the enhanced scavenging crushing of well-crystallized primary minerals with much lower ability (exchange capacity) of the secondary mineral phases, exchange capacities than the clay-sized phyllosilicate oxide - the fine fractions of glaciofluvial and other sandy or gravelly hydroxide debris formed by weathering of sands or gravels. deposits have elevated background concentrations of trace ele- Weathering of both near-surface till and glaciofluvial sediments relative to the same size fractions of nearby till (Fig. 6). ments restricts the use of heavy minerals in prospecting for

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SHILTS

FIG. 6. Concentration of manganese in < 2 pm fraction of till and esker sediments, District of Keewatin. Enhancement of Mn in esker is a result of weathering effects. See Fig. 11 for location. (From Shilts and Wyatt 1989.)

most economic minerals, with the exception of resistate ore minerals or their indicators (e.g., cassiterite, gold, chromite, pyrope). Weathering also causes a contrast between the mineralogy and chemistry of fine fractions of till compared with sorted sediments derived from till. For this reason, accurate facies characterization of samples is absolutely essential to interpreting geochemical data in glaciated terrain. Plotting analyses of fine fractions of oxidized samples of till with those of gravelly or sandy sorted sediments can result in nonprovenance- related geochemical enrichment (Fig. 6). In the case illustrated in Fig. 6, esker samples have anomalously high concentrations of cations compared with adjacent till (see also Shilts 1973b).

Appalachian tills Because till and associated sediments in the Appalachian

region of Quebec are rich in sulphide minerals, deep till sections in this region have been studied to determine the effects of postdepositional weathering on mineralogy and chemistry (Shilts 1975, 1984; Shilts and Kettles 1990).

Shilts and Kettles (1990) studied weathering processes in several natural stream banks near Thetford Mines, Quebec. At all sites, hard, olive-grey till with subequal amounts of sand,

silt, and clay in its matrix weathers to a brown to tan colour to a depth of about 2 - 3 m below the ground surface. The till is cobble rich and contains few boulders; where unoxidized, it has an easily seen component of sand- to granule-sized pyrite cubes and fragments. The pyrite is derived from underlying and surrounding quartz -albite - sericite schists. A northeast- striking belt of chlorite-epidote schists with known base-metal potential (Harron 1976), less than 15 krn up-ice (northwest) from the sections studied, is a possible source of other sulphides, such as sphalerite, chalcopyrite, and galena. These and other sulphide minerals from this belt probably contribute Zn, Cu, Pb, and other cations to the large concentrations of Fe in the pyrite-dominated heavy mineral separates from local (unoxidized) tills.

The effects of weathering on labile minerals are reflected by variations in trace-metal concentrations in sand-sized, heavy mineral (specific gravity > 3.3) separates from till samples collected vertically through typical stream-cut sections. For all elements studied, except chromium, there is a sharp decrease in metal concentrations in heavy minerals at and above the oxidized zone (Fig. 7). In some sections there is a corresponding increase in the concentrations of some cations in the clay-sized

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342 CAN. I. EARTH SCI. VOL. 30. 1993

-& concentration in heavy minerals 0

(D

uo concentration m c 2 p m 0 z

heavy minerals

clay fraction

FIG. 7. Profiles showing the effects of weathering on the chemistry of clay and heavy mineral separates from till near Thetford Mines, Quebec. The substantial decrease in Fe is the result of oxidation of pyrite which dominates the heavy mineral fraction of unweathered tills in this region. (From Shilts and Kettles 1990.)

fractions of oxidized samples, which suggests that clay-sized phyllosilicates and (or) secondary oxides and hydroxides have scavenged some of the metal released by weathering of the heavy mineral fraction (Shilts and Kettles 1990). Because sul- phur and iron also decrease markedly in the zone of oxidation, it is probable that pyrite and other sulphide phases were the major hosts for metal that has been redistributed in the zone of oxidation (Shilts and Kettles 1990).

This study and similar GSC studies (e.g., Podolak and Shilts 1978) demonstrate that weathering effects can be important well below the shallow (< 1 m thick) postglacial solum. The effects of greatest importance in mineral exploration and environmental geochemistry are the destruction of primary labile mineral phases and the redistribution of their cations into secondary mineral phases or into groundwater. Thus, it is of paramount importance to understand vertical geochemical variation when evaluating geochemical patterns obtained from heavy minerals separated from the silt and coarser fractions of near-surface samples of till. The postglacial weathering problem is, of course, not nearly so important in exploration for metals that occur in resistate minerals such as chromium, tin, and gold. Because of selective weathering, even concentrations of stable minerals may be augmented in oxidized samples if large concentrations of labile minerals (e.g., pyrite) that were present in the original unaltered sediments have been removed by weathering processes.

Dispersal patterns Assuming that geochemical and other analyses have been

carried out so as to minimize nonprovenance-related variations, such as those caused by (i) misidentification of sediment facies or stratigraphic position, (ii) partitioning, and (iii) postdeposi-

maps will show, ideally, patterns related to glacial dispersal. Glacial dispersal can take several forms, depending on where

the source outcrops are located with respect to former centres of outflow and with respect to certain dynamic features within former ice sheets (Boulton 1984; Hicock et al. 1989; Shilts and Smith 1989; Bouchard and Salonen 1990).

As a glacier transports an indicator2 component away from a particular source, the concentration of the component is attenuated, both by the addition of debris eroded from the dispersal area and by the deposition of the component along the way. Generally, the decline in the concentration of an indicator component with distance can be plotted as a negative exponential curve; high concentrations near the source decline rapidly to levels slightly above background, which are then maintained for distances several times greater than the width of the source outcrop. The zone of rapid decrease has been termed the "head" of dispersal and the extended zone of lower frequencies, the "tail" of dispersal (Shilts 1976). The area of dispersal is called the "dispersal train," and the curve itself is the "dispersal curve" (Shilts 1976) (Fig. 8).

The shape and dimensions of the dispersal curve and dispersal train are influenced by a number of factors:

(1) The lithology, structure, and topography of the source area influence how much of a component will be eroded and available for transport. If the source area is a topographically positive feature and is composed of "soft" rock (e.g., lime- stone, serpentinized peridotite) or rock that is highly fractured, it is likely to be a source of abundant debris through repeated glaciations. Hard, massive outcrops of rocks, such as rhyolite and basalt, provide comparatively less debris, even if they stand as positive features. If the source outcrops are in narrow

tional weathering, the real variations of geochemistry, miner- 2An indicator is a glacially transported rock, mineral, or chemical alogy, or clast lithology may be contoured to provide maps of component that is derived from, and can be traced to, a specific compositional variations related to sediment provenance. These source area.

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'0° 1 Background I Head of Dispersal I - Tail of Dispersal -

1100 1 1200 Direction of Glacial Flow 1300 1 A

W\ - ----. N~ckel N~ckel ~n In Clay S~ l t and Fract~on Clay Fract~on (<2pm) (<64pm) * 1 1- *: -- Nlckel ~n Coarse Stlt Fract~on (44-64pm)

Ultrabasic Outcro ( -2200 ppm Ni)

DISTANCE (km)

FIG. 8. Dispersal curves showing the variation in nickel concentrations in fine and coarse fractions of till down-ice from an ophiolitic complex at Thetford Mines, Quebec. Nickel is particularly enriched in the clay fraction because of the dominance of Ni-rich serpentine, which was reduced preferentially to clay sizes. (Modified from Rencz and Shilts 1980.)

depressions, parallel to the general direction of glacial flow, the increased velocity of ice flowing through the-constriction may generate much more debris than flow on adjacent flatter terrain. In such cases dispersal trains may be particularly well developed (Shilts and Smith 1989, p. 51).

(2) The topography of the dispersal area has an important effect on the shape and continuity of both the dispersal curve and dispersal train. In the simplest case, where the dispersal area has relatively little relief, the shapes of the curve and the train are controlled principally by (i) the rate of dilution by debris eroded from the dispersal area and mixed with the indicator component and (ii) the rate of deposition of the indicator component in the dispersal area. In a topographically diverse dispersal area, ridges, escarpments, valleys, and other features may block or divert debris carried in the ice, destroying, dis- placing, or truncating dispersal curves or trains. Blocking and

- -

diversion are particularly common in parts of the ~anadian and Fennoscandian Shield, the Appalachians, and the Cana- dian Cordillera.

(3) Since till is deposited in a variety of ways, it is important to recognize the circumstances under which debris is transported in, and released from, glacial ice. In general, glaciers carry two types of sediment load, a concentrated basal load and lower concentrations of debris scattered throughout, or on, the rest of the ice mass. Basal debris forms relatively dense, compact till, which is lodged beneath a glacier or is melted out of the sole of the glacier during the waning stages of glaciation. Debris carried higher in the ice (englacial debris) or on the ice surface (su~raelacial debris) is melted out with or without

~L u

accompanying deformation, or it slumps off the ice surface by various mass-wasting processes during retreat of an ice sheet or a valley glacier.

These two groups of till facies, basal and supraglacial, are important to recognize in dispersal studies, because they can differ radically in composition (Shilts 1973~). In general, supraglacial deposits tend to be dominated by the lithologies of the topographically higher (often local) or more distant elements of the dispersal area, whereas basal deposits tend to be dominated

by lithologies of topographically lower elements. However, where glaciers were advancing up river valleys cut deep into highlands, they may have carried their basal load far up the valleys, in which case the resulting basal deposits may contain significant amounts of debris of distant origin (Holmes 1952; Shilts 1973a, 1976; Aber 1980).

(4) In the vicinity of ice divides in Keewatin and Quebec, regional dispersal trends do not necessarily agree with other indicators of ice movement, especially striae (Shilts 1984; Shilts and Smith 1989). Similarly, in some regions where directions of ice flow are known to have shifted considerably, individual or multiple tills can show substantial vertical variations in composition, especially in geologically complex areas (Shilts 1976, 1978a) (Fig. 9). The reasons for vertical and spatial variations during a single glacial event are presently poorly known, but probably are related to the changing dynamics of the base of the glacier in a given region with time. For example, ero- sion may be enhanced at one or more stages of the glacier's occupation of a particular area. The length of time that a glacier flowed in a constant direction may also affect the distance that debris is transported, a consideration that is particularly important in the vicinity of ice divides, many of which came into existence or moved to their last Dosition late in a glaciation. It is known that basal ice flow velocities increase exponentially away from centres of outflow, with the result that much of the debris in transit near the centre takes a great deal of time to move any appreciable distance (Boulton 1984, p. 219).

(5) The magnitude of dispersal and the proportion of "far- travelled" to "local" components in till and its derivatives are concepts about which it is difficult to generalize, notwithstand- ing attempts by Clark (1987), Bouchard and Salonen (1990), and others to do so. The complexity of factors governing dispersal in any particular region precludes the formulation of universal rules. Many studies have drawn conflicting conclu- sions about dispersal, the conflicts being largely due to the local nature of the studies with the attendant influence of local factors on till composition.

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344 CAN. 1. EARTH SCI. VOL. 30, 1993

BOREHOCE 29 BOREHOLE 3 0 ft rn

abundant fresh pyrle, heterolihic-local shale, volcaniclastii

and ultrarnafic clasts

right brown colour, rn- weathered pyrite and ultramafic

contorted larnhatlons h debris flow, 20 COW--upward sequence

olive-green to brown cob, mimr weathered clasts

N i As As (heavy ' 6 3 p m ) ((2 p m ) minerals)

% kaolin in clay

ite

eppm asample site

FIG. 9. Vertical variations of trace elements and clay minerals in complex glacial and nonglacial deposits, Rivibre Gilbert, Quebec. v.g. , column reports numbers of visible gold grains recovered. (From Shilts and Smith 1988.)

Because of the negative exponential character of the ideal dispersal curve, the common observations that most material is dispersed a short distance from its source and that the bulk of any till sample consists mostly of "local" debris can be considered to be generally true. However, the presence of abundant local pebbles and cobbles should not be taken to mean that the finer fractions of the till are likewise of' local origin, nor should the opposite be assumed. For example, some glaciers on Bylot Island carry almost 100% coarse Precambrian debris at their snouts, which are at least 10 krn down-ice from the nearest Precambrian outcrop. The local bedrock comprises unconsolidated or poorly consolidated Cretaceous -Tertiary sediments, which are so easily disaggregated that they rarely form clasts coarser than granule size. The sand and finer sizes of the glacier load are made up predominantly of detritus derived from these Cretaceous -Tertiary sediments rather than the resis-

tant Precambrian rocks (DiLabio and Shilts 1979). This example underscores the caveat about universal rules for quantifying dispersal patterns based on studies of a limited range of clast or particle sizes.

(6) The concentration of a component decreases gradually in the down-ice direction until it merges with and becomes indistin- guishable from natural or analytical variations in background. Generally, the more distinctive a component is with respect to the chemistry or lithology of rocks in the dispersal area, the farther it can be traced. Chromium, for instance, is depleted in most crustal rocks; therefore, chromium anomalies generally can be traced for long distances from ophiolitic or other ultramafic complexes. Likewise, red volcaniclastic pebbles derived from outcrops of the Late Proterozoic Dubawnt Group west of Hudson Bay have been traced for long distances, even where present in very small amounts, because they have been

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Proterozoic Dubawnt ' ... redbed outcrops

/ Approximate Dubawnt : / Group erratic isopleth .,

/ -weight % of 2-6mm size :, granules from tlll or till-like '., sediment . . Sample point

,I' Boundary between Paleozoic - - (Mesozoic) and Precambrian bedrock

FIG. 10. Continental-scale dispersal of distinctive red volcanogenic erratics of the Dubawnt Group. (Modified from Shilts 19826.)

dispersed across lithologically dissimilar terrain underlain by Archean gneissic bedrock and light-coloured Paleozoic lime- stones (Shilts 1982~).

Scales of dispersal To recognize the sometimes subtle geochemical expressions

of dispersal tails, it is important to realize that glacial dispersal can be mapped at a variety of scales. In glaciated terrain, the composition of a sample is a composite of many overlapping dispersal trains emanating from multiple sources up-ice from where the sample was collected. For convenience of discussion, four geologically meaningful scales of dispersal have been defined (Shilts 1984): (1) continental, (2) regional, (3) local, and (4) small scale. The significance of these scales for property- level prospecting or regional environmental studies would not have been evident from most of the detailed work carried out by private-sector or university-based geologists because of the necessarily limited scale typical of their research projects. Because of the large scale of GSC projects and the national scope of its syntheses, larger scales of dispersal have been recognized and factored into applied studies.

(1) Dispersal on a continental scale is measured in hundreds

to more than a thousand kilometres (Shilts 1982b, 1984; Prest and Nielsen 1987; Stewart and Broster 1990) (Fig. 10). Very far-travelled coarse clasts and very small amounts of fine- grained debris of potential economic interest (gold, for example), if detected and not properly related to a distant source, may be thought to come from local sources, creating severe exploration problems. The occurrence of diamonds in drift of the American midwest (Stewart et al. 1988) is perhaps the best example of this. Although very rare, these diamonds are easily identifiable and may come from kimberlite dykes in the Hudson Bay Lowland, from which an immense train of Paleozoic debris has been dispersed southward and westward during repeated glaciations.

(2) Dispersal on a regional scale is measured in tens to hundreds of kilometres. Such dispersal has been mapped in the District of Keewatin for till overlying Archean and younger Precambrian bedrock (Fig. 1 1). The reasons for the consistently elevated metal levels in some parts of this region are not clearly understood, but some anomalies near Rankin Inlet have recently been the site of intense exploration for base metals. At this low sample density some areas of metal enrichment may have been caused by the areal homogenization, with distance, of several

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346 CAN. J. EARTH SCI. VOL. 30, 1993

FIG. 11. Regional-scale dispersal of zinc in < 2 pm fraction of till from District of Keewatin. Polygonal outlines are areas of local-scale sampling and represent approximately 6000 sample sites. Numbers refer to approximate locations of figures in this paper. (Modified from Shilts 1984.)

relatively small bodies of geochemically distinctive rocks. For instance, elevated nickel, cobalt, and chromium concentrations could be the result of glacial dispersal from clusters of komatiitic or other ultramafic bodies that crop out a few hundred kilometres north of Baker Lake. In other cases, trace-element levels in till are known to be suppressed because far-travelled, metal-poor debris was mixed with more metal-rich debris derived from local rocks (Shilts and Wyatt 1989). The large dispersal train of clay-

sized hematite and kaolin from the easily eroded Dubawnt Group (Donaldson 1965) has depressed levels of metal from local rocks over a large area, making the higher levels of metal in till outside the train appear anomalous by contrast. Also, within the area of the Dubawnt dispersal train, the geochemical expression of mineralized outcrops is muted because of the diluting effects of Dubawnt detritus.

(3) Local glacial dispersal is reflected in trains less than a

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few kilometres long derived from restricted sources that may be economically significant. It can be detected by reconnaissance sampling at the scale of one sample per 1-4 km2 (Fig. 12). Over 10 000 km2 of terrain in southern Keewatin has been sampled at this scale, and similar reconnaissance sampling has been conducted in Fennoscandia as part of the Nordkalott project (Kautsky 1986). Geochemical anomalies detected at the local scale are much more easily related to mineralization than are those of the larger scales of dispersal. At this scale of sampling, the tails of dispersal trains from potential orebodies are likely to be detected, but sample and analytical control (taking into account sediment facies, partitioning, and weathering effects) must be precise enough to allow them to be differentiated from the background and from the tails of trains of regional or continental scale.

(4) Small-scale dispersal is manifested by trains hundreds of metres long and can usually be related to specific outcrops. It is usually encountered in the last stages of mineral exploration. Boulder train tracing, which is routinely carried out in Finland (Hirvas 1989), is designed to map this scale of dispersal. While there are many published case histories of small-scale dispersal (e.g., DiLabio 1981), a small nickel dispersal train extending down-ice from outcrops of presently uneconomic, nickel- copper mineralization in District of Keewatin has been chosen for discussion because it also illustrates the importance of sediment facies and weathering conditions and of choosing proper sample processing and analytical techniques (Fig. 13). This train of nickel is clearly defined by analysis of the < 2 pm fraction of till, but is very poorly expressed by analyses of sand-sized heavy minerals (specific gravity > 3.3) and of the < 64 pm fraction, the former because weathering has destroyed the labile, Ni-bearing sulphides, and the latter because the metal-poor, quartz -feldspar-rich silt fraction dilutes the sample in an unpredictable way. Note also in Fig. 13 that Ni concentrations in the <64 pm fraction of samples from the Copper- needle Esker, which bisects the sampling area, are 4-7 times higher than those in similar fractions of nearby till, as predicted in the discussion of weathering.

Some important exceptions to classical dispersal patterns The classic "head-and-tail" form of dispersal is a useful

model for interpreting the composition of a glaciated landscape, but recent research has shown that there are other important types of dispersal trains, particularly in regions formerly or presently covered by continental ice sheets. It has long been known that shifts in the direction of ice flow during a glaciation can form fan-shaped dispersal patterns (Flint 1971), although within a fan there may be one or more distinctive, ribbon-shaped dispersal trains. The edges of a fan represent the absolute limits of dispersal of a component and reflect the maximum range of ice-flow directions across an area during one or more glaciations (Flint 1971; Salonen 1987).

Amoeboid patterns near ice divides One variation of the fan pattern is the amoeboid patterq3

represented by dispersal trains near or on the major, late-glacial ice divide in Labrador - Nouveau Quebec (Klassen and Thompson 1989). Similar examples have been described by Blais (1989) (Fig. 14), Stea et al. (1989), and Lowell et al . (1990) in the vicinity of ice divides in Quebec, Nova Scotia and Maine, respectively. Because these ice divides migrated in a complex way around and across source outcrops or suddenly

'Author's terminology.

ZlNC IN TILL (<Z pm)

,, zim-Coppr M l " . r ~ n

7 GladslFbw DlndKn

FIG. 12. Local-scale dispersal of zinc, Henninga - Turquetil Lake area, District of Keewatin. Note secondary train of zinc-rich till trail- ing away from mineralized zones. See Fig. 11 for location. (Modified from Shilts 1984.)

developed in areas of former unidirectional ice flow as a result of drawdown of parts of the glacier margin into rising marine waters, debris was dispersed first one way, then another. The ultimate result of this constant shifting of ice-flow centres is irregular or amoeboid patterns of dispersal (Fig. 14).

Distorted patterns in zones of shifting flow Even far from centres of ice flow, where lobes of ice from

different centres coalesced, the zone of coalescence probably shifted with time, depending on the relative health of compet- ing ice masses (Veillette 1989). Irregular or amoeboid dispersal patterns can result from such shifting where the "suture" marking the zone of coalescence swept back and forth across source outcrops. This occurred, for example, in northern Manitoba, where Keewatin and Labradorean ice merged (Kaszycki et al. 1988). The zone of confluence shifted repeatedly, according to the climatic ascendency of one or the other ice sheet. This produced, in the case of the Wheatcroft Lake arsenic dispersal train, westward striae formed by Labradorean ice in an area where southward dispersal was effected by Keewatin ice. Both types of dispersal evidence are preserved in the same place, probably because the later Labradorean flow event was short lived and did not distort significantly the earlier formed, southward dispersal train.

Trains formed by late glacial streaming in continental ice sheets

A third major type of dispersal train, formed by rapidly flowing streams of ice within the decaying Laurentide Ice Sheet, has been recently recognized (Hicock 1988; Aylsworth and Shilts 1989; Dyke and Dredge 1989, pp. 198 - 199; Thorleifson and Kristjansson, in press). The areal pattern of this type of dispersal train is similar to that of the classical dispersal ribbon (Shilts 1976), and it is commonly expressed geomorphologi- cally by trains of drumlins and abundant eskers. The compositional profile, however, is radically different from that of the ideal dispersal curve, and the width of the train commonly bears little relationship to the width of the source outcrops.

In profile, the composition of a train formed by "streaming" is flat, maintaining constant high concentrations of the dispersed component from the source outcrops to the end of the train where the concentrations drop abruptly (Fig. 15). There is little incorporation of local debris into the train in the area

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LEGEND r--. '-,,> Copperneedle Esker

200 Esker sample, Ni (ppm)

Til l sample

Glacial transport direction

. .

fraction;contour interval lOppm

Nickel (ppm) in < 6 4 ~ m fraction; contour Interval lOppm

9 5 9 0 lop0 2 0 p 0 metres I " " . Nickel (ppm) in clay (<2um) fraction of t i l l

FIG. 13. Detailed or "property-scale'' dispersal of nickel in different fractions of till and esker sediments, Southern Lake, District of Keewatin. Note enhancement of dispersal pattern achieved by using analyses of the clay fraction. Sulphides are removed by weathering from heavy mineral fractions. See Fig. I 1 for location. (Modified from Shilts 1984.)

of dispersal. The end of this type of train is marked by a sharp drop of the source component to background levels and probably marks the position of the front of the retreating glacier at the time the ice stream was active. The width of the dispersal train reflects the width of the ice stream and not that of the source outcrops. Compositions also change abruptly at the sides of the train. For instance, on Boothia Peninsula in northern District of Keewatin, a sharply bounded train of carbonate detritus, several tens of kilometres wide, emerges from a small area within a large Paleozoic carbonate basin (Dyke and Dredge 1989). Like- wise, several narrow trains of carbonate debris extend south- westward as fingers across the Canadian Shield from the large

Paleozoic platform that underlies Hudson Bay and the Hudson Bay Lowland. The till in these trains, even 100 km from the source outcrops, is almost totally composed of clasts eroded from Paleozoic and Proterozoic outcrops within and adjacent to Hudson Bay (Hicock 1988).

The Ontario ice-stream dispersal trains described by Hicock (1988) and Thorleifson and Kristjansson (in press) have important implications for the application of drift compositional studies to mineral exploration, as first noted by Garrett (1969) and Grant (1969). The fact that exotic ice-stream drift is relatively undiluted by local bedrock makes it a mask that conceals the compositional signal of underlying bedrock. In many places,

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the exotic till overlies till composed of more local components, presumably with dispersal characteristics reflecting the normal head-and-tail type of dispersal train. The local till is assumed to have been deposited earlier in the glaciation when the retreat phase was marked by ice-sheet instability resulting in ice streaming. Local till forms surface deposits adjacent to former ice streams, but may be encountered only in boreholes in areas covered by exotic, ice-stream drift.

Summary of intellectual and societal contributions of GSC drift prospecting research

It is difficult to differentiate the unique contributions of the GSC to drift prospecting research from those of other individuals and groups. As research on drift composition and glacial sedimentation accelerated during the 1970's and 19807s, it became increasingly difficult to discern where ideas originated, particularly those passed on by word of mouth before their actual publication. It is clear, nevertheless, that the GSC, largely through of its commitment of funds and personnel, provided intellectual leadership in drift prospecting and many other areas of glacial sedimentological and environmental research.

From the preceding discussion, I believe that it is fair to summarize the major intellectual contributions of the GSC to drift compositional research as follows: (i) The concept of head-and-tail of the classic dispersal train was introduced as a result of GSC work as early as 1976 and is now so commonly used that its source is seldom acknowledged any more. Like- wise, recent GSC research (Dyke and Dredge 1989; Thorleifson and Kristjansson, in press) has played a major role in clarify- ing the difference between dispersal trains formed by rapidly moving ice streams within the decaying ice sheet and those formed by normal glacial dispersal processes. (ii) The significance of various scales of dispersal (continental to small scale) naturally evolved from the regional nature of GSC drift compositional projects, an approach usually not feasible for individual researchers or companies. (iii) The effects of weathering on labile minerals to considerable depths in glacial sediments, although addressed by other researchers, were first clearly defined and integrated into sampling and analytical methodologies by GSC scientists. (iv) The concept that chemical partitioning is related to the mineralogy of various size fractions, which in turn is related to physical partitioniqg of minerals into various size classes, was introduced by the GSC as early as 1971 (Shilts 1971). The concept, however, owes much to the research of Dreimanis and Vagners (1971, 1972), who pioneered the idea of terminal modes based on the physi-

I cal properties of minerals and on their behaviour under the 1 stresses of glacial abrasion.

Technically, the GSC has pioneered the use of various types 1 of overburden drilling and logging techniques for geochemical and stratigraphic studies, particularly the use of reverse circu- lation (Skinner 1972) and rotosonic drills (Shilts and Smith 1988; Smith 1992). The methodology of physical separation of

I clay fractions from till (Shilts 1975) and the routine use of methylene iodide for heavy mineral separations also arose from research in GSC laboratories. GSC scientists were also among the first to use the scanning electron microscope with energy dispersive backscattering to identify heavy mineral species and to study their glacially produced or modified morphol- ogy (Shilts 1 9 8 2 ~ ; DiLabio 1990).

As recently as 1969, geochemist R. Garrett observed, after encountering ice-stream tills during an early geochemical study by the GSC that " . . . thorough knowledge of the Pleistocene history of any area is very necessary before correct sampling

Nickel Concentrations (ppm) in < 63 pm Fraction of Till

) Lntramailc bedrock outcrop

I 0 r46'

71'

FIG. 14. Amoeboid pattern of glacial dispersal of Ni from ultrarnafic outcrops, Rivikre des Plante (100 km south of Quebec), near the Quebec Ice Divide. Flow, indicated by arrows, was first southeastward, and then north-northwestward. (From Blais 1989.)

techniques and interpretation criteria can be devised" (Garrett 1969, p. 48). In that statement is embodied the main contribution of the GSC to solving compositional problems in Canada's glaciated landscape: the integration of the principles of glacial geology with the techniques and data provided by low- temperature geochemical research. Although originally directed toward solving problems in mineral exploration, drift geochemical research has increasingly found environmental applications. The principles of glacial dispersal, even if they are not always understood, are now widely recognized as important by scientists studying such wide-ranging environmental subjects as acid rain, forest decline, geochemical effects of reser- voir flooding, and geomedicine. If not for the GSC's early commitment to regional drift compositional studies and its subsequent publication and publicization of both the results and the possible applications of the research, this fundamental body of knowledge, despite Garrett's admonition of more than two decades ago, probably would not be factored into modern applied research.

Finally, although drift prospecting could have been applied only by utilizing glacial sedimentological data as they became available, in reality the compositional principles discovered in the course of carrying out drift prospecting research actually contributed to and constrained sedimentological research. Fun- damental questions about the history and dynamics of various ice sheets have been answered using large-scale compositional data generated by GSC drift sampling projects (e.g., Shilts et al. 1979; Shilts 1980; Dyke and Dredge 1989). Thus, we can conclude that drift prospecting, which could have been pursued at a purely technical level with little opportunity for innovation or contribution to glacial geology, has been carried out in such a way that it actually has nurtured, enhanced, and advanced this discipline. This happy circumstance would not

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Metasedirnentary and metavolcanic rocks 80

K i l o m e t r e s

FIG. 15. Flat dispersal curve of Paleozoic carbonate debris from an area of a southwest-trending ice stream south of Hudson Bay. (From Thorleifson and Kristjansson, in press.)

likely have occurred outside the unique research environment standing of postdepositional alteration of the chemistry of drift provided by the GSC. and its influence on the mobilization and concentration of ele-

ments like mercury will come increasingly from drift composi-

The future The future of drift prospecting research at the GSC is to be

found, ironically, in its past. As a result of the investments of money and intellect in this activity in the past, its implications for the science of glacial geology have outstripped the general knowledge base. Compositional principles conceived 10-20 years ago are just now having their impacts on glacial geology and glaciology, in the sense that they constrain modern physical models of ice-sheet dynamics and sedimentation. One of the first, previously unconstrained glaciological hypotheses to be rejected was the concept of a single-domed Laurentide Ice Sheet (Shilts et al. 1979; Dyke et al. 1982). This concept foun- dered on the mass of compositional data generated by early GSC drift prospecting projects west of Hudson Bay and was replaced by various versions of the now widely accepted multiple-dome model, which was originally proposed by GSC geologists in the 19th century, largely on the basis of their observations of drift composition! Many modern glacial con- troversies, such as the role of massive meltwater flows in creating glacial landforms, probably will be constrained and resolved by the hard facts of objective compositional data derived from glacial sediments.

In terms of the future applications of drift prospecting, the glacial distortion of the compositional signals generated by bedrock will continue to be mapped at various scales and factored into mineral exploration and environmental geochemical studies. Without such mapping on a national scale, the understanding of the relative contributions of nature and humans to environmental geochemical anomalies will continue to elude us. For instance, the presently renewed concern about high mercury levels in the food chain cannot be resolved without some understanding of the redistribution of mercury-bearing components of bedrock by glacial processes. Furthermore, an under-

tion research. Finally, the use of drift prospecting in mineral exploration

will wax and wane in concert with the fate of the mineral industry. As orebodies become harder to locate, largely because those in areas with little drift already have been found and exploited, drift-covered bedrock terrane with high mineralization potential increasingly will be investigated, and drift prospecting methods will be integrated with geophysical and drilling techniques to obviate the masking effects of drift.

Drift prospecting has thrived and evolved at the GSC over the past 25 years. Because it has been approached as an adjunct of glacial sedimentation research, its potential applications have far exceeded those that would have resulted had it been merely an eztension of the well-established field of exploration geochemistry. It is hoped, and expected, that the research initiated in the "novelty" days of drift prospecting will continue to evolve and contribute to a better understanding of glacial geology, glaciology, techniques of mineral exploration, and the myriad of environmental problems that confront us now and in the future.

Acknowledgments I am grateful to my colleagues, both in and outside the GSC,

for their ideas and discussions, which have contributed to the necessarily brief distillation of ideas presented here. In particular, I thank my colleagues in the former Sedimentology and Mineral Tracing section, J. Aylsworth, S. Courtney, R. DiLabio, C. Kaszycki, I. Kettles, R. A. Klassen, M. Lamothe, M. Rappol, H. Thorleifson, and P. Wyatt; W. Coker and E. Hornbrook of the Geochemistry Subdivi- sion and R. Ridler, presently of Ridler and Associates, Inc. E. Evenson, S. Hicock, R. A. Klassen and J. Wheeler read and critically commented on the manuscript, as did J. Clague.

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The author is grateful for their helpful suggestions, most of which he has incorporated, but takes full responsibility for any oversights or shortcomings in the text.

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