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
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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 representative 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 gCologique 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 rtpartition asymetrique de la population. Les recherches h la CGC s'orientaient de plus en plus vers la comprChension et I'explication 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 caractkrisitiques 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 granulomCtriques. En outre, le processus de broyage dCsintkgre les roches fraiches du substratum et incorpore dans le ddiment glaciaire
s
I'ensemble de ses minCraux instables et stables. Les recherches a la CGC ont surtout porte sur les meilleures f a ~ o n d'echantillonner 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 perturbations 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.
Printed In Canada / Impr~rnCau Canada
(GSC) have played major roles in developing new techniques
for characterizing the composition of glacially transported overburden, techniques that were unimaginable and, in fact, impossible, 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-
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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 predictive 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 practical 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 bouldertracing 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 mid1960'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 diamondbearing potential (Lee 1963, 1965, 1968). Shortly after publishing 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 lithogeochemistryof 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 Canadian 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 asbestosbearing 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
SHlLTS
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HEADS
1
v
C
FIG. 1. Three-dimensional plot of nickel concentrations along the oph~iolitebelt of southeastern Quebec. Trains trend southeastward toward
the United States border. (From Shilts 1991.)
terrain was undertaken in the Rankin Inlet - Ennadai greenstone 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) inflatable boat; some of the first attempts at traversing with allterrain 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
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).
FIG. 2. Two- to three-metre diameter mudboils on the surface of
a drumlin protruding through a cover of marine sand with frost polygons 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.)
-
CAN. J. EARTH SCI. VOL. 30. 1993
336
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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. Simultaneously, 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
SHILTS
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composition projects funded under federal-provincial agreements 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 Pleistocene 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 activi.
ties 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
337
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 corresponds 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 comprise 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 homogeneous 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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338
CAN. J. EARTH SCI. VOL. 30, 1993
a much more local provenance than distal fine-grained sediments. These sediments can accumulate rapidly in depressions
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
nature of underflow deposits and their common association
with similarly laminated sediments deposited from suspension,
considerable sedimentological expertise is required to collect
useful samples from them. They generally are not regarded as
a useful sample medium.
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 resedimented many times since glaciation. Unlike the lower-order
derivatives, which were deposited rapidly under climatic and
sedimentological conditions unfavourable for penecontemporaneous chemical alteration, postglacially redistributed, higherorder 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. Exploration 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 lowtemperature 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
principally over the past 30 years, have been integrated with
externally established models and techniques to form the current GSC research program of drift composition studies.
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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 geochemical 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 greenstone 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 < 6 3 pm,
the sample-to-sample variations were enhanced. By plotting
trace-element concentrations of the < 6 3 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, monomineralic 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. Theoretically, 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 mineralogy of the modes that dominate each size fraction.
Early in the Keewatin project, it was observed that, in
general, the clay-size mode of many glacial and derived sedi-
339
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SHILTS
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 displace 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 coarsegrained, 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 icecontact 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.)
TABLE1. Partitioning in till over ultramafic bedrock, northern Ungava, Quebec
Raglan 3"
Bulk sample (<6.0 mm)
<63 pm (silt + clay)
2.0-6.0 mm
0.25-2.0 mm
63 -250 pm
45-63 pm
2-45 pm
< 2 pm
Raglan 5b
Bulk sample ( < 6.0 mm)
<63 pm (silt + clay)
2.0-6.0 mm
0.25 -2.0 mm
63 -250 pm
45-63 pm
2-45 pm
< 2 pm
Pd
(PPb)
Pt
(PPb)
Ni
Co
Cr
(PP~) (PP~) (PP~)
Cu
(PP~)
Au
(PPb)
1100
980
2100
1400
1100
900
720
2300
330
210
500
490
220
180
300
110
2966
3466
2579
3249
2306
2164
2418
6355
91
84
91
117
79
62
72
118
1305
669
2360
1760
904
913
629
882
4460
5 400
4230
4430
3 380
3300
3620
> 10000
120
160
620
230
130
160
210
60
130
100
130
140
110
68
78
390
60
96
85
75
50
100
70
70
679
592
727
674
446
406
490
1601
53
41
60
64
40
34
37
104
845
407
1440
1010
538
469
445
1045
477
433
411
436
346
291
344
1405
20
44
4
6
8
34
26
10
Samples collected and donated by Michel Bouchard, UniversitC de Montreal.
NOTES:
"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
the ground-water table. Owing to the enhanced scavenging
ability (exchange capacity) of the secondary mineral phases,
the fine fractions of glaciofluvial and other sandy or gravelly
deposits have elevated background concentrations of trace elements relative to the same size fractions of nearby till (Fig. 6).
In till, the fine fractions were produced largely by the physical
crushing of well-crystallized primary minerals with much lower
exchange capacities than the clay-sized phyllosilicate oxide hydroxide debris formed by weathering of sands or gravels.
Weathering of both near-surface till and glaciofluvial sediments 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 nonprovenancerelated 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 northeaststriking 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
heavy minerals
0
(D
u
o
concentration m c 2 p m
0
clay fraction
z
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 sulphur 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) postdepositional weathering, the real variations of geochemistry, mineralogy, or clast lithology may be contoured to provide maps of
compositional variations related to sediment provenance. These
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., limestone, 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
2An indicator is a glacially transported rock, mineral, or chemical
component that is derived from, and can be traced to, a specific
source area.
SHILTS
'0°
1100
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1200
1300
11
1
Background
I Head of Dispersal I-Tail of Dispersal
Direction of Glacial Flow
Ultrabasic Outcro
( - 2 2 0 0 ppm Ni)
1-
*
*:
-
A
W\
1
----.
N~ckel~n
In Clay
S ~ l tand
Fract~on
Clay Fract~on
(<2pm) (<64pm)
--
Nlckel ~nCoarse Stlt Fract~on( 4 4 - 6 4 p m )
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, displacing, or truncating dispersal curves or trains. Blocking and
diversion are particularly common in parts of the ~ a n a d i a n
and Fennoscandian Shield, the Appalachians, and the Canadian 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~raelacialdebris) is melted out with or without
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
-
~L
u
-
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, erosion 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 "fartravelled" to "local" components in till and its derivatives are
concepts about which it is difficult to generalize, notwithstanding 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 conclusions about dispersal, the conflicts being largely due to the
local nature of the studies with the attendant influence of local
factors on till composition.
344
CAN. 1. EARTH SCI. VOL. 30, 1993
BOREHOCE 29
BOREHOLE 3 0
ft
rn
Ni
As As (heavy
' 6 3 p m ) ((2 p m ) minerals)
% kaolinite
in clay
abundant fresh pyrle,
heterolihic-local shale, volcaniclastii
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and ultrarnafic clasts
right brown colour,
rnweathered pyrite and ultramafic
20
contorted larnhatlons h debris flow,
COW--upward
sequence
olive-green to brown cob,
mimr weathered clasts
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 indistinguishable 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
,I'
'
..
Group erratic isopleth
-weight % of 2-6mm size
granules from tlll or till-like
sediment
Sample point
:
.,
:,
'.,
..
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 limestones (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 propertylevel 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 finegrained 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. 11). 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
CAN. J. EARTH SCI. VOL. 30, 1993
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346
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, nickelcopper 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 Copperneedle 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)
,,
7
zim-Coppr
M l " . r ~ n
GladslFbw DlndKn
FIG. 12. Local-scale dispersal of zinc, Henninga - Turquetil Lake
area, District of Keewatin. Note secondary train of zinc-rich till trailing 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 competing 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 geomorphologically 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
CAN. J. EARTH SCI. VOL. 30. 1993
LEGEND
r--.
>,,-'
200
Copperneedle Esker
Esker sample, Ni (ppm)
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Till sample
Glacial transport direction
.
.
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
Nickel (ppm) in < 6 4 ~ mfraction; contour
Interval lOppm
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). Likewise, several narrow trains of carbonate debris extend southwestward 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,
1
1
I
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.
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I
349
SHILTS
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. Likewise, recent GSC research (Dyke and Dredge 1989; Thorleifson
and Kristjansson, in press) has played a major role in clarifying 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 physical properties of minerals and on their behaviour under the
stresses of glacial abrasion.
Technically, the GSC has pioneered the use of various types
of overburden drilling and logging techniques for geochemical
and stratigraphic studies, particularly the use of reverse circulation (Skinner 1972) and rotosonic drills (Shilts and Smith
1988; Smith 1992). The methodology of physical separation of
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 morphology (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
I0
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 lowtemperature 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 reservoir 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. Fundamental 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
CAN. 1. EARTH SCI. VOL. 30, 1993
Metasedirnentary and metavolcanic rocks
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80
Kilometres
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
provided by the GSC.
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 foundered 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 controversies, 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-
standing of postdepositional alteration of the chemistry of drift
and its influence on the mobilization and concentration of elements like mercury will come increasingly from drift composition 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 Subdivision 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.
SHILTS
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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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