Journal of Geochemical Exploration, 21 (1984) 95 117 95 Elsevier Science Publishers B.V., Amsterdam - -Pr in ted in The Netherlands
T I L L G E O C H E M I S T R Y I N F I N L A N D A N D C A N A D A
W.W. SHILTS
Geological Survey of Canada, 601 Booth Street, Ottawa, Ont. KIA OE8 (Canada)
(Received January 25, 1984)
ABSTRACT
Shilts, W.W., 1984. Till geochemistry in Finland and Canada. J. Geochem. Explor., 21: 95--117.
Glacial geology has been much more thoroughly integrated into mineral explorat ion programs in Finland than in Canada. In Finland, the composit ional properties of glacial sediments are utilized to find buried ore, whereas in Canada glacial deposits have been regarded more often than not as a hindrance to exploration. The application to mineral explorat ion of the latest theories on ice-sheet dynamics, sedimentation from ice, and till composit ion is presently regarded as somewhat more important in Canada than in Finland.
Three specific geochemical aspects of till have been investigated recently by the author and should have considerable relevance to techniques of explorat ion:
(1) Mineral and chemical partit ioning in till is marked because of the tendency of minerals to crush to certain specific sizes during glacial comminution. For most metals investigated, highest concentrations are in the clay-sized fraction, presumably held within the structure of phyUosilicates or scavenged by secondary oxides, both of which phases occur preferentially among particles finer than about 10 ~m.
(2) The more labile ore minerals, such as sulphides, are destroyed by weathering to depths several metres below the postglacial solum. This destruction is often accompanied by a concomitant increase in metal concentration in the clay-sized (< 2 ~zm) fraction.
(3) A till sample is a composite of composit ions inherited from over-lapping dispersal trains at several different scales. The sizes of trains range from continental scale, covering tens of thousands square kilometres, to local scale which can be mapped for distances of no more than a few hundred metres. It is essential in mineral explorat ion to be able to differentiate between the rare, but sometimes distinctive or easily concentrated clasts from far away and those that might have been generated by a small, but economic, local source.
INTRODUCTION
T h e g l ac i a l b o u n d a r y in N o r t h A m e r i c a a n d E u r o p e d i v i d e s t w o s ign i f i - c a n t l y d i f f e r e n t t e r r a i n s w i t h r e s p e c t t o e x p l o r a t i o n g e o c h e m i s t r y . S o u t h o f t h e b o u n d a r y , t h e n o n - g l a c i a l so i l s have b e e n f o r m e d l a r g e l y in s i t u b y c h e m i c a l d e c o m p o s i t i o n o f u n d e r l y i n g b e d r o c k , a n d s e d i m e n t s d e r i v e d f r o m t h e m c o n t a i n c o m p o n e n t s f r o m w i t h i n ea s i l y i d e n t i f i a b l e d r a i n a g e bas ins . N o r t h o f t h e g l ac i a l b o u n d a r y , g l ac i a l s e d i m e n t s a n d so i l s ( a n d s e d i m e n t s d e r i v e d f r o m t h e m ) h a v e b e e n f o r m e d l a r g e l y b y t h e p h y s i c a l p r o c e s s e s o f
glacial abrasion and crushing of fresh bedrock, chemical processes generally playing only a minor role in the development of the relatively weak post- glacial solum. Furthermore, components of the unconsolidated mantle of glacial sediment have been transported over topographic barriers along relatively straight lines across one or many drainage basins, except in areas of alpine glaciation. For these fundamental reasons, many of the classical methods of exploration geochemistry, developed in areas outside the gla- ciated higher latitudes, are ineffective or, at best, inefficient in these environ- ments.
In glaciated terrain, exploration geochemistry has been applied according to two broad philosophies: (1) geochemical exploration strategies should be designed to circumvent the effects of glaciation; and (2) exploration strategies should be designed to take advantage of the effects of glaciation, particularly the results of glacial dispersal. Adherents of the former philoso- phy have traditionally used sampling and analytical techniques adapted from those employed in areas of residual overburden. Adherents of the latter philosophy have developed geochemical methods based on more traditional glacial geologic techniques, such as boulder tracing. Although sampling and analytical methods developed specifically for glaciated terrain are potentially more efficient than those imported from more temperate areas, they require some understanding of glacial processes and sedimentation. Because there are few exploration geologists with this training and few Quaternary geologists or geomorphologists with an interest in mineral exploration, the glacial geological approach to drift geochemistry is not nearly as widely employed as it should be, particularly in North America.
It is only in the past 20 years that a growing awareness of the fundamental contrasts between glaciated and unglaciated terrain has sparked an increasing interest in combining the principles of glacial geology with those of explo- ration geochemistry so that more effective ways of mineral exploration might be found.
The way in which these disciplines have been combined, however, has differed between North America and Northern Europe, largely for demo- graphic and economic reasons. In Canada, for instance, with its vast, largely unexplored, lightly populated areas, geochemical research has focused on developing methods to be applied at a regional scale, with the objective of covering large areas and producing diffuse targets to which other more traditional methods of exploration can be applied. In Fennoscandia, geo- chemical reconnaissance is carried out at a more local scale, because of the detailed geological information available and because of lower costs resulting from the more evenly distributed populations. Drift-geochemical exploration is also more important in Europe than in North America because there are no longer any "easy-to-find" orebodies in Europe, and serious research into the obscuring effects of the glacial cover had to be undertaken decades ago. In contrast, drift prospecting is just beginning to gain some acceptance in North America and is still looked on with suspicion by many exploration companies.
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TILL GEOCHEMISTRY IN FINLAND
In August, 1983 I was invited to carry out an intensive two-week exami- nation of drift-prospecting techniques employed in Finland by both the Geological Survey of Finland and by the three other agencies actively involved in mineral exploration, Ou tokumpu Oy, Rautaruukki Oy, and Lapin Malmi. It was obvious from this excursion and from available litera- ture that the style and extent of integration of Quaternary geology with geochemical exploration differs greatly between Finland and Canada. In Finland and Scandinavia an appreciation of Quaternary geology is generally regarded as essential in mineral exploration programs, and anomalous prop- erties of the glacial drift are both sought and carefully evaluated. In Canada, the glacial drift is more of ten than not regarded as a hindrance to explo- ration and, because the drift of ten is not unders tood to differ significantly from residual soils, glacial geology has not been regarded, traditionally, as of any great importance to exploration programs. I should say, however, that this a t t i tude is changing as exploration targets become less obvious in the early 1980's.
Three styles of till sampling are now being carried out routinely in Fin- land:
(1) Percussion drills with "f low-through" bits are used to obtain samples at close (typically 100 m) intervals along lines oriented at right angles to glacial f low and separated by a distance about 10 times that of the sample spacing along the lines (Kauranne, 1975). The idea of this sort of anisotropic pat tern is to form a "grating" that will maximize the chance of intercepting narrow dispersal trains with a minimum number of samples. In the 1980's a regularly spaced grid has, however, been preferred.
(2) Reconnaissance till-geochemical sampling of the whole country using equidistant sample points at a density of about one site per 3 or 4 km 2 has started recently. The northernmost part of Finland has already been sampled in this fashion as part of the "Nordka lo t t " project in which an integrated, multimedia sampling program is being carried out jointly by the geological surveys of Finland, Sweden, and Norway (Bergstrom et al., 1983). In these reconnaissance projects, other sampling media, principally humus, aquatic moss and organic and inorganic stream sediments, are collected where possible.
(3) Pits are dug with backhoes (tractor-excavators) to depths of 4 to 5 m near the up-ice ends of known ore-boulder trains or over geochemical or geophysical anomalies. On-site examination of heavy minerals panned from till and of mineralized clasts are used to guide the sampling pattern. All agencies in Finland make extensive use of the latter technique in the final stages of mineral exploration with spectacular (by North American stan- dards) success. One till-sampling group of the Geological Survey of Finland found the bedrock source of 6 ou t of 10 boulder trains studied in the summer of 1983, alone. This technique is particularly applicable in Finland
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because of the generally good vehicular access to areas of interest, relatively thin till sheets, and general lack of a significant cover of late- or postglacial lacustrine or marine clays.
Two particularly t roublesome problems beset those involved in till-explo- ration geochemistry in Finland, however. In northern Lapland much of the terrain is underlain by a thick mantle of unconsolidated, deeply weathered bedrock which is covered by one or more till sheets. The origin of this material, its compositional relationship to till, and the reason for its preser- vation after repeated glaciations have been little studied. It is known that conventional till geochemistry is not so successful in this region as elsewhere, and plans are being made to address the problem of the "weathering crust", as the residuum is called locally.
A second problem is the presence of as many as five thin, compositionally distinct tills in many parts of Finland. The stratigraphy and ice-flow history of these tills have been studied systematically in the 1970's in Lapland and elsewhere by examining mine excavations through thick overburden and by studying thousands of backhoe pits (Hirvas, 1977). Nevertheless, local and regional correlations of till sheets are still problematic, and the large- scale glacial history of the country is open to considerable debate.
All in all, glacial geology, till geochemistry in particular, plays a major role in modern mineral exploration in Finland. The Quaternary section of the Geological Survey is its largest group, and understanding of the extensive glacial cover has been recognized as critical in Finnish mineral exploration since its earliest stages. Ore-boulder prospecting was proposed as early as 1740 by Daniel Tilas (Sauramo, 1924). The entire population of Finland is involved in exploration through an organized boun ty program in which various rewards are offered for discovery and donation of ore- bearing float boulders. Much detailed till geochemistry is carried out in the vicinity of ore-boulder trains discovered by amateur prospectors partici- pating in the bounty program.
TILL GEOCHEMISTRY IN CANADA
Canada, a country with glaciated terrain of high mineral potential that covers an area many times that of Finland, has taken quite a different approach to integrating Quaternary expertise with mineral exploration. Because mineral resources are abundant and have been relatively easy to find by conventional prospecting, geophysics, or bedrock geology mapping, Quaternary studies have not received high priority in universities or in mining companies. This situation has changed recently because changing world economic conditions render many mineral deposits in frontier areas uneconomic. Consequently, exploration is increasingly concentrated around mining camps or populated areas where "convent ional" exploration methods have already been tried. Till geochemistry has gained increasing prominence because it is one of the few techniques that has not been utilized systemati-
99
cally in the past. In effect, mineral exploration in Canada is becoming increasingly restricted to relatively small areas around existing populat ion centres, leading to exploration philosophies that are increasingly close to those popular in Finland.
Till geochemistry in Canada is carried out in a much more haphazard fashion than it is in Finland. Nevertheless, till-geochemical research is in many respects more advanced than that practised in Finland because, para- doxically, of the relatively low priority it has had in mineral exploration. Efficient methods of sampling and sample processing have been developed to overcome (1) the generally low level of funding for Quaternary studies in mineral exploration, and (2) the problems of thick overburden, particu- larly lacustrine and marine clays, that cover many of the prime mineral belts of the country.
Till geochemistry is carried out in a number of ways in Canada, but the general phi losophy has been, with some notable exceptions in uranium exploration, to employ geochemical exploration techniques that circumvent the effects of glaciation. Sampling is increasingly carried out by reverse circulation drilling, a technique popularized by a Geological Survey of Canada emergency employment project carried out in 1971 over the deeply clay-covered mining belt of the Timmins-Val d 'Or area of Ontario and Quebec (Skinner, 1972). In this technique sand-sized heavy minerals sepa- rated from washed till samples are analyzed, commonly from the base of the lowest till over bedrock. Percussion drills with "f low-through" samplers are also extensively used in basal-till sampling, but stratigraphic control of these devices in the generally bouldery till of the Canadian Shield and Appalachians is difficult. In addition, their small sample volume allows only a limited range of analytical tests, mostly on the fine fraction. In the Canadian Cordillera till sampling is rarely employed, being supplanted by stream-sediment geochemistry. Other than these sampling techniques, till sampling is carried out by hand excavation of shallow pits or by sampling man-made or natural exposures.
The Geological Survey of Canada has carried on several research programs designed to provide private industry both with case-history studies of appli- cation of till geochemistry to mineral exploration and to point out new sampling and analytical rationales that may be transferred to the private sector. In the arctic, starting in 1970, w e have carried out experimental reconnaissance till-sampling programs very similar in scale and objectives to those presently starting in Finland and northern Fennoscandia (Ridler and Shilts, 1974; Klassen and Shilts, 1977). We have studied in some detail the effects of weathering on the labile ore minerals in till and have carried out chemical partitioning studies in till. In the following section, I will discuss, briefly, what I consider to be some important principles and prob- lems of glacial geology and till geochemistry in Canada. Many of these problems have not been addressed widely in Finland, just as many of the philosophies and exploration techniques employed in Finland have not yet been widely applied in Canada.
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CHANGES IN UNDERSTANDING OF SOME PRINCIPLES OF GLACIAL GEOLOGY
Although the theory of continental glaciation has been accepted for over eighty years, the popular conceptions of the history and configuration of both the Fennoscandian and North American ice sheets have recently undergone major revisions that have direct bearing on mineral exploration in glaciated terrain. Studies of the glacial history of the Canadian arctic and subarctic, for example, have shown that the Laurentide Ice Sheet, which covered most of that port ion of the Canadian Shield with high poten- tial for mineralization, was a much more complex and dynamic body of ice than had been supposed previously (Shilts, 1980; Dyke et al., 1982; Andrews et al., 1982). Rather than consisting of a monolithic mass of ice centred on Hudson Bay during each of four major glaciations, it was found to com- prise several independent, confluent centres of out f low toward which ice fronts may have shrunk one or more times during each major glacial stage. The history and configuration of centres of out f low has considerable bearing on interpretation o f geochemical dispersal patterns, even at a local scale.
Similarly, in Fennoscandia, stratigraphic work carried ou t in conjunction with mineral-exploration programs has shown that deposits of multiple glacial events with divergent directions of ice flow complicate interpretation of glacial geochemical dispersal, particularly in Finland (Hirvas, 1977). This recent discovery of major flaws in our concepts of the basic glacial geological f ramework of North America and Northern Europe suggests that the science of glacial geology is still at a rudimentary stage. It is, then, no surprise that the sophisticated analytical and statistical techniques of exploration geochemistry are of ten unsuccessful when applied to terrain where both glacial sediments and glacial history are poorly understood.
Along with the changing ideas about the North American and Fennoscan- dian ice sheets, a revolution in understanding of glacial sedimentation has been taking place. In the past 15 years glacial sedimentologists, led princi- pally by Boul ton (1970a, b) of Britain, Shaw (1977) of Canada, and Lawson (1981} of the United States have presented models of glacial and near-glacial sedimentation, based largely on observations and experiments made around modern glaciers. As a result of the work of these and other Quaternary geologists, the genesis of glacial and associated sediments is more easily understood, and better geochemical sample control is possible than pre- viously.
METAL PARTITIONING
In order to understand better how various metals are distributed among the mineralogically diverse size fractions of till, the author has recently begun experiments to determine metal partitioning in fractions from 6 mm to ~ 1 pro. The ultimate goals of this and related research are to determine how and where metal is held and what is the opt imum size range for geo-
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chemical analysis for metals commonly sought in geochemical exploration. Although many partitioning experiments have been carried out in the past, very few have dealt with the reactive, clay-sized (< 4 /am) fraction. Our research and research carried out independently at the Geological Survey of Finland in Kuopio (Nikkarinen et al., 1983) indicate that most metals are concentrated and greatly enriched in till size fractions finer than 4 pm, regardless of the mineralogy of the source rocks. In slightly weathered or unweathered till this fraction, although commonly referred to as the "clay" fraction, is actually composed largely of minerals that were easily reduced to these sizes by the physical processes of glacial grinding and abrasion. Although quartz, feldspars, carbonates, and other minerals can be present in the clay-sized fraction, it is commonly dominated by well-crystallized phyllosilicates (mainly chlorite and micas). Locally it may contain significant amounts of less common "soft" minerals, such as kaolinite and other true clay minerals, serpentine, graphite, and hematite.
Twenty-six samples of drift and two of postglacial gossans from the Canadian Shield were selected from among samples that passed through our laboratories in 1980. The drift samples included oxidized (from below the postglacial solum) and unoxidized till and ice-contact gravel, with known anomalous and background metal concentrations for each of the elements normally analyzed in our projects. Each sample was fractionated into six size grades from 6 mm to < 1 /am and into a bulk sample < 6 mm. All fractions were analyzed by AAS (except U by fluorimetric techniques and As by colourimetric techniques) after a "total" (perchloric acid) leach. The 4/am to 1 /am size range was also analyzed using two selective leaches, ammonium citrate and sodium dithionate, to remove weakly bound or adsorbed metal from phyllosilicate phases and from secondary oxide phases, respectively.
The results of this study were nearly identical to those reported for a similar study of Cu and Zn by Nikkarinen et al. (1983) in Finland; for the elements investigated, metal" enrichment in the clay-sized fraction is signifi- cant, being particularly strong for anomalous samples known to be related to mineralization (Fig. 1). In our study, the selective leaches usually removed only a small percentage of the total metal available, except for Mn and Fe (Fig. 2), suggesting that the metal in these samples is largely held in the structure of the clay-sized minerals and is not adsorbed or incorporated into secondary mineral phases. Most oxidized samples yielded significant per- centages of Fe and Mn from secondary oxide-hydroxide phases, but usually yielded little other metal, suggesting either that the secondary phases have not generally been efficient scavengers or that little metal was released from the clay-sized phases during weathering.
The implications of these results are significant in evaluating till-geo- chemical data, particularly in reconnaissance surveys. They suggest that metal levels in standard - 1 7 7 /am ( -80 mesh) or - 7 4 #m (--200 mesh) analyses are strongly related to the amount of < 4 ~m material in those
Fig. 1. Partitioning of Cu, U, and As among different grain sizes in till. Samples in right-hand figures considered to be anomalous; those on left represent background. Concentrations are from perchloric acid leach and are considered to represent "total" metal.
tO
"•100"
E
g 60- E
~ 4o-
20-
o
0 0 ~ • • • OCQ D 0 • 0 0 , •
% iron (I-4)Jm fraction) after treatment with total (HF) leach
Se lec t i ve Leach ing o f 1 -4 ~m Frac t ion o f Glacial Sed iments
ioo
80 o
E6o o
40- o
~ 2o-
o
o • •
o • • [] •
~ o o
% Iron (I- 4#m fraction ) after treatment with total (HF) leach
I00-
o 8o
.~ 6 0 g I= E ~ 4 o
a. 'r_ 2 0
N
.,= I00 0 unoxidized till ou • oxidized till [ ] unoxidized gravel _~ • oxidized gravel o 80 A gossan o
"o ,~ 60
40
~ 2 0
0 "4 • • • 0 •
200 400 600 800 IdO0 12'00 1400 16'00 0 200 400 660 e()o I0'00 i~00 PPM Zinc (l-4)Jm fraction) after treatment with PPM Zinc (I-4~um fraction) after treatment with
total (HF) leach total (HF) leach
14bo 1600
Fig. 2. R e l a t i o n b e t w e e n Fe and Z n c o n t e n t s in d i f f e r e n t select ive l eaches o n the 1- -4 u m f r a c t i o n o f till a n d r e l a t ed Q sed imen t s , co
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fractions, the coarser sand and silt-sized fractions acting essentially as dil- uents. This is why the ~ 2 ~m fractions of most of the drift samples passing through the drift prospecting laboratory of the Geological Survey of Canada have been separated by centrifugation and analyzed directly since 1973 (Shilts, 1975). A question that remains unresolved is why massive sulphide and other types of mineralization are indicated by geochemical patterns derived from a size fraction that appears to reflect structural metal enrich- ment in phyllosilicates. Careful search of the < 2 pm fraction of anomalous samples using SEM with EDS* capabilities has not revealed any identifiable ore-mineral particles that could be contributing to the metal enrichment.
WEATHERING OF TILL
Postglacial weathering can have a radical effect on the geochemistry of drift to considerable depths, depending on the configuration of the local groundwater or permafrost table (Rencz and Shilts, 1980). Furthermore, the effects of weathering on the geochemistry of relatively impermeable till are quite different than those on some of its more permeable derivatives, such as esker or other ice-contact gravels. In an oxidizing environment, i.e., on weU-drained slopes, labile minerals such as sulphides and carbonates are generally dest royed above the water or permafrost table, their chemical consti tuents being carried away in solution or scavenged locally by clay- sized phyllosilicates and by secondary oxides/hydroxides, depending on the element and the local geochemical environment. In poorly drained sites, where the water table is at or close to the surface and/or the surface is covered with an organic mat, very little destruction of primary labile minerals o c c u r s .
In permeable glacial sediments, particularly well-sorted sands and gravels, destruction of labile components also takes place, but weathering of some of the more labile phases produces a fine debris of mixed-layer or other clays, oxides, etc. which can be physically translocated from the surface downward through the deposit (Shilts, 1973). This is especially important in esker or similar deposits that stand largely above the groundwater table. The enhanced scavenging ability of these secondary materials gives the fine fractions of glaciofluvial and similar deposits elevated background concen- trations of trace elements relative to the same size fractions of nearby till, the fine fractions of till being produced largely by physical crushing of primary minerals with much lower exchange capacity. Thus, chemistry of clastic fragments above the water table will not usually reflect the presence of some of the more common (i.e., sulphide) economic minerals (Ridler and Shilts, 1974; Shilts, 1975).
For reasons cited above, weathering restricts the use of heavy mineral
*EDS - Energy Dispersive Spec t rome t ry .
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or other coarse-grained fractions of near-surface till and derived sediments except in cases where ore minerals are resistant (cassiterite, gold, chromite, etc.). The contrast between the mineralogy and chemistry of fine fractions of till and sorted sediments makes accurate sample characterization ab- solutely essential to interpreting exploration-geochemistry results in glaciated terrain.
Several natural exposures of till and associated sediments have been sampled in the Appalachian region of Quebec. A detailed sedimentological and geochemical s tudy of one such section has been published by the author (Shilts, 1978), but weathering effects of importance in mineral exploration were not discussed at length.
Section 531, a natural stream bank on Ruisseau Nadeau near Thetford Mines, Quebec reveals 10 m of hard, olive-gray till which weathers to a brown to tan colour to a depth of 2 m below the ground surface. The till contains many cobbles bu t few boulders and, where unoxidized, has a noticeable component of sand-to granule-sized pyrite cubes and fragments. The pyrite is derived from the underlying and surrounding bedrock, quartz- albite-sericite schists with abundant recrystallized pyrite cubes. A northeast- striking belt of chlorite-epidote schists comprising metabasalts and rhyolites with known base-metal mineralization lies less than 15 km northwest (up- ice) from the section (Harron, 1976). The most important glacial dispersal direction, presumably the one that prevailed during deposit ion of the till, was southeastward. The last direction of glacial movement in this region was northward (LaMarche, 1971) in response to development of a calving bay in the St. Lawrence valley to the north, but very little glacial erosion or transport was effected by this event. If northward dispersal had been effective, it should have carried a significant amount of ultramafic debris to this site, which is located only 3 km north of the Thetford Mines--Black Lake ophiolite mass from which a Cr-Ni-Co dispersal train extends for over 80 km southeastward (Shilts, 1976).
Figure 3 shows variation of some geochemical parameters among the samples collected at Ruisseau Nadeau. The effects of weathering of labile sulphide minerals are easily discerned from the profiles of trace metals in sand-sized heavy minerals. For all the elements studied there is a sharp decrease in metal concentrat ion at and above the oxidized zone through the upper 2 m of till. There is a corresponding increase in concentrations of metal in the clay fraction, which indicates that clay-sized phyllosilicates and/or secondary oxides and hydroxides have scavenged some of the metal released by weathering. Although this sympathet ic relationship between leached and scavenged metal is not always apparent in this region, it is well- developed at this section. The decrease of sulphur and total iron at the level of oxidation is related mainly to the destruction of pyri te that is abundant in the till. The Fe/S relationship supports the conclusion that other sulphide phases were the hosts for the metal that has been translocated in the zone of oxidation.
Cu ( p i ) m ) Pb (DDm) Zn ( p p m ) Ni ( I ) p m ) C o ( P P m ) C d ( P p m )
2 "~" :~-~- :" ~- - e '--- .Ik.~. . ~ Legend
L ° i ._c i o " ~
101~ii ~ 8 "' i'i :
400 600 800 3 6 9 12 15 0.0 0.2 0.4 1.0 2.0 3.0
Mn (pore) Fe (%) S(%) T o t a l C 0 3 ( % )
( 0 . 1 2 5 - 0 . 2 5 0 ram) < 6 4 ~ m
F i g . 3. Chemistry o f till fract ions in o x i d i z e d and u n o x i d i z e d till, 10-m till s ec t ion 5 3 1 , southeastern Quebec . N o t e that scales for c lay and heavy-mineral fract ions are not the same.
• sand-size heavy mineraJs (s.g. :> 3.3)
• clay-size fraction ( , ~ 2 / Jm)
At this site, one must exercise caution in interpreting the increase of Ni near the top of the section, since this element may have been transported northward from the ophiolite mass to the south during the late-glacial reversal of flow. The low concentrations of Ni this close to the source suggest that this is not the case, however, and that the increase in Ni in clay-sized detritus is a weathering effect.
The conclusion that I have drawn from the geochemical profiles of this and several other similar sections sampled in the Appalachians and in peren- nially frozen tills of the Canadian Shield, is that weathering effects can be important well below the postglacial solum. The effects of most importance in mineral exploration are the destruction of primary labile mineral phases, many of which are glacially dispersed ore minerals sought in mineral-explo- ration programs. Thus, it is of paramount importance to have some under-
107
standing of this source of vertical geochemical variation when evaluating geochemical patterns obtained from splits containing coarser fractions of till, particularly if heavy minerals are preconcentrated and analyzed. The postglacial weathering problem is, of course, not nearly so important in exploration for resistate minerals.
Finally, it is important to be able to differentiate these vertical geo- chemical manifestations of weathering from primary variations due to shifting ice-flow directions during deposition, multiple stratigraphic units, multiple till facies f rom the same glaciation, variations related to undisturbed mel tout tills, etc. Making these distinctions accurately requires the input of a well-trained Quaternary geologist.
PATTERNS AND SCALE OF GLACIAL DISPERSAL
Glacial dispersal trains have been described (Shilts, 1976} as comprising a head or area of very high metal concentrat ion in drift at or near the source, which decays quickly down-ice to a ribbon-shaped tai l of dispersal where metal levels are slightly above background due to dilution by metal-poor debris eroded from the glacial floor of the dispersal area. The tail is many times larger than the head and is generally the part of the dispersal train intercepted by reconnaissance or widely spaced sampling. A major objective of till geochemistry is, simply stated, to detect the tail of dispersal and trace it back to the head. To understand and interpret the significance of the sometimes subtle geochemical tails of dispersal, however, it is necessary to understand, among factors such as those discussed in previous sections, something about the scales of glacial dispersal.
In glaciated terrain, the composit ion of a sample is a composi te of many overlapping dispersal trains. The trains occur at a variety of scales and emanate from sources up-ice from where the sample was collected. For convenience of discussion, I have defined four geologically meaningful scales of dispersal: (1) continental; (2) regional; (3) local; and (4) small-scale.
(1) Dispersal on a continental scale can be measured in hundreds to more than a thousand kilometres (Fig. 4). Although not usually important in mineral exploration, very far-travelled coarse clasts and fine-grained compo- nents, 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 is perhaps the best example of this problem. Although very rare, diamonds are easily identifiable and may come from as far away as kimberlite dikes that cut the Mesozoic and Paleozoic strata of the Hudson Bay Lowlands, from which an immense train of Paleozoic debris has been dispersed southward and westward during repeated glaciations (Fig. 4). Individual gold or other mineral grains concentrated from the large heavy mineral separates panned from till in Finland may be another example of far-travelled debris that can be confused with debris from a local source.
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(2) Geochemical dispersal on a regional scale can be measured in tens or hundreds of kilometres. We have mapped this scale of dispersal in the Dis- trict of Keewatin for till that overlies Archean and younger Precambrian bedrock (Fig. 5). The reasons for the consistently elevated metal levels in some parts of this region are not clearly understood. Some may be caused by the areal homogenization, with distance, of several relatively small bodies of geochemically distinctive rocks. For instance, elevated Ni, Co, and Cr concentrations could result from dispersal from a cluster of komati i te or other ultramafic bodies. In other cases, trace element levels in till are known to be suppressed as a result of far-travelled, metal-poor debris being mixed with more metal-rich debris from local rocks. The large dispersal train of clay-sized hematite and kaolin from the easily-eroded Dubawnt group (Figs. 4, 5) (Donaldson, 1965) has depressed background and anomalous 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 (Fig. 5). Also, within the area of the Dubawnt dispersal train, the geochemical expression of miner- alized outcrop is depressed because of the diluting effects of the Dubawnt detritus.
(3) Local glacial dispersal can be detected by reconnaissance sampling at the scale of one sample per one to four square kilometres. We have sampled over 10,000 km 2 of terrain in southern Keewatin at this scale, and recon- naissance-sample spacing in Finland and on the Nordkalot t project is similar. Geochemical anomalies detected at the local scale are much more easily related to mineralization than are those of larger-scale dispersal. The tails of dispersal trains from potential orebodies are likely to be detected in till, but sample and analytical control (particularly partitioning and weathering effects) must be precise enough to allow them to be differentiated from background or f rom the tails o f trains of regional or continental scale. Figures 6 and 7 illustrate some examples of local dispersal. In Fig. 6, Zn anomalies from known zones of Zn mineralization are superimposed on a regional train of Zn from an unknown source (see Fig. 5 for regional con- text) . Figure 7 shows a "negative" dispersal train in which metal-poor kaolin which cements the poorly-consolidated Precambrian (Dubawnt Group) sediments that underlie Pitz Lake has been dispersed southeastward, depressing regional metal levels so much that a distinct train of metal-poor debris has been formed. This is a local expression of the dilution phenomenon illustrated by Fig. 5.
(4) Small-scale dispersal is usually encountered in the last stages of mineral exploration. The boulder-train tracing routinely carried out in Finland is designed to map this scale of dispersal. While there are many published case histories showing examples of this scale of dispersal, I have chosen a small train extending down-ice from a zone of presently uneconomic Ni-Cu mineralization in District o f Keewatin {Fig. 8). This example illustrates the importance of choosing proper sample-processing and analytical techniques. The train of Ni that is so clearly defined by analysis of the < 2 pm fraction
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~ ~
.MA
NIT
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o
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o
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Fig. 5. Regional dispersal of Zn in central District of Keewatin. Over 2500 samples were collected from each of the outlined areas.
Huds~
Bay
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* Somple si te
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[ ~ 2 nd con tou r 120-1" ;
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~ m , ~ m Oute r l imi t of orec Dubownt t i l l
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o
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• Sample site
I s~ contour 100-120 ppm Zn
2 t~d contour 120-175 ppm Zn
I 5 fd contour > 175 ppm Zn
Late Precambrian redbeds of Dubawnt Group
~ Outer limit of area covered by red Dubawnt ti l l
I0 0 50km
i
92°00 ̀
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of till (AAS after hot HNO3-HCL leach) is very poorly expressed by analyses of sand-sized heavy minerals (s.g. > 3.3) and of < 64 pm ( - 2 5 0 mesh) fractions, 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 pattern.
97°04 s+,0~+
9 7 ° 0 ( 3 61o45+Ji -
• 100-120 ppm
~ 120-17S PPm
~ > 175 ppm
/ J Zinc-Copper MIIterlllizlltion
,o ,6 12 k,,,,
J
ZINC IN TILL (<2jum)
Fig. 6. Local Zn anomaly from Cu-Zn mineralization superimposed on a regional anomaly. Map based o n over 1000 samples collected o n a 1.6 k m × 1.6 k m grid. This area is the southwest pa r t o f t he s o u t h e r n m o s t o u t l i n e d grid o n Fig. 5.
Fig. 7. Negat ive Cu a n o m a l y near Baker Lake , d r a w n f r o m resu l t s f r o m over 2500 samples collected at 1.6-kin in tervals in n o r t h e r n m o s t grid o u t l i n e d o n Fig. 5.
114
OP . " " " .
/ o 0 ~ . ~ • . " ' . L E G E N D
" ~ 6 ' O " ~ L ~ - - ~ ~ . " " ' . • ~ ' ~ - _ _ ' . % Copp . . . . . die Esker
• ~ .~ l ! ~ . . " " • . • " ~ ' ~ Glacial transport direction - • • •
Nickel (ppm) in <64jJm traction; contour Inler val I0 pprn
• o
2 0 0 0 metres o,,,,?oo , o o o , ,
Nickel (ppm)in clay (<2um) fraction of till.
Fig. 8. Small-scale dispersal o f Ni f rom k n o w n z o n e o f Ni mineral izat ion.
OTHER GLACIAL PROBLEMS REQUIRING RESEARCH
Problems that were not discussed here and about which not enough is known, also deserve considerable research by mineral-exploration personnel working in glaciated regions.
(1) Rogen or ribbed moraine, for example, is a common landform around former centres of ice dispersal in Finland, Sweden, Newfoundland, Quebec- Labrador, and District of Keewatin. These short, sinuous, transversely oriented ridges occur over many areas of potential mineralization and are sometimes intimately associated with mineralized boulder trains, particularly in Canada. An understanding of its genesis is essential in interpreting and planning geochemical surveys in areas where Rogen moraine occurs• Presently there
115
is no concensus about whether these features are proglacial or subglacial, although the author and several others feel that they are most likely the products of shearing of the glacial bed with subsequent stacking of the plates of debris during ablation of a largely stagnant ice mass.
(2) Processes of esker sedimentation are also imperfectly known, and the relationships of esker composi t ion to the composit ion of the basal load of an ice sheet, which forms till adjacent to the subglacial channel, must be clarified. The author and others have observed that eskers of ten follow zones of structural discontinuity in the bedrock, such as faults, an observation that may indicate that eskers can be preferentially located over sites with high potential for mineralization.
(3} The relationship of ice-movement history to directions of effective glacial transport is not well established. In many places striae or till fabric may indicate several directions of ice flow during a single glaciation, particu- larly near ice divides or isolated late-glacial ice caps. In many cases, however, dispersal apparently occurred along only one of the azimuths of flow. Although the reasons for this are not clear, the phenomenon is probably related to f low directions prevailing just after the first passage of an ice front across an area, as the glacier is growing. Thus, striation and till-fabric orientation, although reliable indicators of dispersal direction in many instances, do not necessarily bear direct relationship to direction of debris transport.
(4) There are also a number of geochemical aspects of till that are poorly known or described. DiLabio and the author (DiLabio and Shilts, 1979) in Canada and Stephens et al. (1983} in the U.S. have studied the trace element distribution of debris entrained in Alpine-type glaciers. On Bylot Island, off the north tip of Baffin Island, debris bands in glaciers draining Precam- brian metasediments show strong vertical variations in trace-element chem- istry. The geochemical nature of the till that eventually will be formed by these bands will depend on whether the till is formed by basal meltout, preserving the integrity of the vertical variations, or by supraglacial mel tout or basal lodgment, homogenizing the debris bands into a till that varies little geochemically from bo t tom to top.
C O N C L U S I O N
One might conclude from the brief discussion above that geochemical exploration using till or derived sediments is a formidable task. On the contrary, I believe that the recent exponential increase in reliable models of glacial history and sedimentation augur well for the successful application of geochemical techniques to exploration in glaciated terrain. The very processes of glacial erosion and transportat ion produce glacial geochemical dispersal trains that are much larger than the original source, if only we can apply the appropriate glacial geological and exploration-geochemical tech- niques to recognize them. To this end we will have to integrate the disci-
116
plines of glacial geology and explora t ion geochemis t ry more closely and will have to encourage universities to inst i tute academic programs tha t provide the dep th of t raining necessary to mee t this requi rement , par t icular ly in Nor th America.
ACKNOWLEDGMENTS
The original manusc r ip t was improved cons iderably by discussion and cor rec t ions made by R.N.W. DiLabio. I also t hank Alf Bj6rklund and m a n y of his colleagues at the Geological Survey of Finland and on the organizing c o m m i t t e e o f the IGES-SMGP s y m p o s i u m for allowing me to take an in- dep th look at the appl ica t ion of till geochemis t ry in mineral explora t ion in Finland.
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