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Drift Composition
and Surficial Geology of the Trutch Map Area (94G), Northeastern British
Columbia
Geological Survey of Canada Open File D3815 |
1J. Bednarski, Terrain Sciences Division, Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, Alberta, T2L 2A7 (Email: jbednars@nrcan.gc.ca)
The surficial materials sampling program was designed to characterize the regional till cover and to identify possible anomalies related to mineralization and glacial transport of heavy mineral indicators. This report compliments a 1: 250 000 scale map of the surficial geology of Trutch (Bednarski 2000). An overview of the Quaternary history of the area is given by Bednarski (1999). The mapping procedure consisted of airphoto interpretation of surficial materials followed by ground verification using helicopter, truck and foot traverses during the summers of 1998, 1999 and 2000. One hundred and sixty-five bulk samples of surficial materials were analyzed for grain size, carbon content and a suite of elements. The surficial materials were extracted from shallow pits, natural exposures and road cuts, providing an average density of one sample per 80 km². Element concentrations were measured by INAA (Instrumental Neutron Activation Analysis) in the silt and clay fractions. Anomalies were identified and statistically derived contour maps describe the spatial distribution of elements in the surficial materials. Cluster analysis was used to identify tills of montane versus continental provenance. The heavy minerals were counted in 48 samples. All results are presented as interactive Surview maps (Grant, 1999) and html documents on this CD-ROM.
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Fig. 2. Sample locations |
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The mapping procedure consisted of interpretation of aerial photographs and ground verification by helicopter, truck and foot traverses undertaken during the 1998, 1999 and 2000 field seasons. Till and other surficial materials were extracted from shallow pits, natural exposures and road cuts providing an average density of one sample per 80 km² (Fig. 2). This report compliments a 1: 250 000 scale map of the surficial geology of Trutch (Bednarski, 2000). An overview of the Quaternary history of the area given by Bednarski (1999).
The surficial materials sampling program was designed to characterize the regional till cover and to identify possible anomalies related to mineralization and glacial transport of heavy mineral indicators. The working hypothesis developed during the course of mapping was that surficial deposits derived from three glacial sources influenced the study area. The mountainous region in the west was dominated by Cordilleran and local alpine ice, whereas the east was dominated by continental Laurentide ice. A zone along the mountain front was complicated by the interaction of mountain and continental ice.
Surficial materials, comprising 165 bulk samples of till, glaciofluvial
gravel, and lacustrine fines (Fig. 2) were analyzed for grain size, carbon
content and a suite of elements. The concentrations of 34 elements were
measured by INAA (Instrumental Neutron Activation Analysis) in the combined
silt and clay fractions (<0.063 mm). Attempts are made to identify anomalous
concentrations and statistically derived contour maps describe the spatial
distribution of elements in the surficial materials. Cluster analysis was
used in an attempt to group the glacial deposits. Heavy minerals were counted
for 45 samples and compared to the drift geochemistry.
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Thirty-six mineral occurrences are listed in the Trutch map area the Geological Survey Branch, Energy and Minerals Division of British Columbia MINFILE database. An abridged version of this database can be viewed with Surview on this CD. Note that a number of these showings are found in drill core, deep below the surface, and have no bearing on the surficial materials. Most of the mineral occurrences in the map area are found in the west associated with the underlying carbonates. These rocks were deposited along the continental shelf of ancestral North America as platformal reefs and turbidites. The Robb Lake Mississippi Valley-type Pb-Zn deposit is located within Paleozoic rocks immediately adjacent the southwest corner of the map area (Nelson et al., 1999a & b, Paradis et al., 1999). Within the map area, lead-zinc deposits are hosted within the Middle Devonian Dunedin Formation, Middle to Lower Devonian Stone Formation, and the Silurian Nonda Formation. Mississippi Valley-type lead-zinc deposits are found around the Redfern Lake area and lead-zinc-copper deposits are found in the Richards Creek area. Significant amounts of germanium within brick-red sphalerite are also found north of Richards Creek. The Besa River Formation, a Devonian to Carboniferous transgressive shale, hosts massive barite deposits near Petrie Creek. Other mineral occurrences include phosphate-bearing strata in Mesozoic to Paleozoic rocks underlying the Rocky Mountain Foothills and native sulphur and coal within Cretaceous sediments underlying Alberta Plateau.
Major gas reservoirs
have been found in the eastern Foothills in Triassic strata and in Carboniferous
Debolt Formation (e.g. Bubbles and Jedney fields, and Sikanni and Pocketknife
fields, respectively, Fig. 4). Active exploration is targeting Devonian
strata in the western Foothills.
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Longitudinal fold and fault controlled valleys trending northwest characterize the mountains and foothills. The transition from foothills to the plains is abrupt although elements of the northwest trending structural trend are still evident in the western plains. Generally, the mountainous western part has been extensively modified by alpine glaciers and the intensity of glacial erosion decreases eastward across the foothills. The highest peaks in the southwest currently harbor small glaciers in north-facing cirques. Large east-trending valleys crosscut the longitudinal valleys through the foothills every 15 to 25 kilometres where major rivers such as the Muskwa, Prophet, Sikanni Chief, and Halfway flow eastward onto the Alberta Plateau from headwaters in the Rocky Mountains. It is evident that the rivers and creeks have experienced several phases of stream piracy because many of the upper tributaries change direction several times, as they exploit the crosscutting pattern of the valley systems.
Both the eastern foothills and Alberta Plateau bear remnants of several eastward-dipping erosional surfaces. The oldest surface may be Miocene in age (Williams 1944), however, there has been only one reported occurrence of preglacial gravel, located north of the Sikanni Chief River near the Alaska highway (Denny 1952). These deposits could not be confirmed in this study, but an isolated diamicton underlying Laurentide till near the Sikanni Chief River may be preglacial in age (see below). Oxidized basal gravels of unknown age are exposed along incised rivers on the the Alberta Plateau (see below). Larger deposits of preglacial gravels lie southeast of Trutch in the Charlie Lake area (Mathews 1978). On the plains, the drainage swings to the northeast, into the Liard River system, part of the Mackenzie River drainage basin. The topography is characterized by low plateaus and cuestas dissected by the major rivers (Fig. 3). Resistant sandstone units of the upper Cretaceous Dunvegan Formation form prominent cliffs, whereas, most low areas and major valley bottoms are underlain by recessive shales of the Fort St. John Group. In the extreme northeast part of map area, erosion by the Muskwa, Prophet and Sikanni Chief rivers formed the Fort Nelson Lowland, very flat, poorly-drained muskeg terrain that is less than 600 metres above sea level.
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The Alberta Plateau is covered by various thicknesses of till containing shield erratics deposited by the Laurentide Ice Sheet during the last glaciation. Most of the Plateau is sparsely covered by till, although Hage (1944), reported glacial drift tens of metres thick along the Prophet River. More commonly, Dunvegan Formation sandstone outcrops on the Plateau uplands, but thicker till is found in lowlands and to the west near the mountain front. Hage (1944) described glacial drift up to 222 metres thick from well data near Fort Nelson town site. The predominant flow direction by the Laurentide Ice Sheet over the Alberta Plateau is shown by southwest oriented flutings on the east side of the map area.
The Laurentide Ice Sheet did not leave moraines marking its advance or retreat to the mountain front, however, meltwater drainage was blocked to the northeast by the ice sheet and many channels and spillways were cut. Consequently, extensive glaciolacustrine sediments were deposited throughout the lowlands and valleys. Today, many valleys have very flat bottoms filled with glaciolacustrine sediments. Near the Alaska Highway, the Sikanni Chief River is deeply incised, and exposes up to 15 metres of bedded clay overlain by gravels. Similar thicknesses of glaciolacustrine deposits were reported along the Beatton, Halfway, Minaker and Prophet River valleys (Hage 1944; Denny 1952). Varved silts were found near the surface along the mountain front in the Muskwa River drainage. Hage (1944) described at least 100 varves in the Beatton River valley. Denny (1952) reported varves along the Alaska Highway near the Sikanni Chief and Prophet rivers.
Large areas of glaciolacustrine sediment form the uppermost surficial unit and probably date from recession of the Laurentide Ice Sheet. Nevertheless, the stratigraphic relationship of some glaciolacustrine exposures outside the mountain front may be more complicated. Hage (1944) noted that lake deposits exposed along the Halfway and Minaker rivers, and for 160 kilometres south of Fort Nelson in the Prophet River valley, all rest on montane till. This was not confirmed by this study, however, the same stratigraphic relationship was found within several valleys in the foothills. Sections along upper Sikanni Chief River typically show this sequence (Figs.3, 8). Outside the mountain front, glaciolacustrine deposits are usually found overlying continental till. When the Laurentide Ice Sheet retreated from the foothills, the highland between the Buckinghorse and Sikanni Chief rivers became a drainage divide separating meltwaters draining northward to the Liard River system (Lemmen et al. 1994), from meltwaters draining southward to glacial Lake Peace (Mathews, 1980).
The extent of the Laurentide Ice Sheet during the last glaciation can
be inferred from the westernmost extent of surface till containing abundant
shield erratics. The ice sheet reached the mountain front and was thick
enough to overtop some of the mountains, as indicated by shield erratics
deposited up to 1588 m asl. The westernmost extent of the Laurentide Ice
Sheet was probably inhibited by the presence of the Cordilleran Ice Sheet.
East of the mountain front, the relative abundance of shield erratics varies
along the Alaska Highway. Pink granite and gneiss pebbles and boulders
are abundant in till north of the Buckinghorse River, but are uncommon
from Pink Mountain to Sikanni Chief. This boundary at the Buckinghorse
River coincides with the westernmost extension of Laurentide till into
the mountain front (Fig. 6).
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This study mapped the western limit of till containing abundant shield erratics along the upper Buckinghorse River where Laurentide ice apparently pushed into the mountain front (Figs. 3, 6). On the north side of the valley mouth, the uppermost level of granite and gneiss-rich till lies at 1380 m asl. Sandy till and outwash containing granite pebbles crops out about 5 kilometres upvalley on the valley bottom at 1234 m asl. For about 12 kilometres upvalley, shield erratics are abundant on the surface up to an elevation of 1360 m, but granite till is absent. In these cases, the shield erratics may have been ice-rafted up the valleys. Erratics were also redeposited by stream transport. For example, the upper Buckinghorse River has been pirated by Nevis Creek, which transported shield erratics northward to Besa River, albeit to lower elevations.
The relative decrease in shield erratics south of Buckinghorse River,
led Mathews (1980) to suggest that there may have been a late readvance
of the Cordilleran ice onto the plains, effectively diluting the pre-existing
Laurentide till with Cordilleran till. According to Mathews (1980), the
Buckinghorse River also coincides with the southern end of local Rocky
Mountain glaciation. Mathews zone of local glaciation extends northward
for 245 kilometres to the Liard River (Fig.1). Till
within this zone is composed of only local rocks, with no diagnostic Cordilleran
erratics. Nevertheless, this study shows that Cordilleran ice covered this
area during the last glaciation. North of the upper Buckinghorse River,
striations measured along mountain crests, up to 1860 metres above sea
level, suggest that Cordilleran ice inundated the foothills in the Trutch
map sheet area. Moreover, the measured east-northeastward flow runs across
the major topographic trend, implying that a significant thickness of ice
flowed across the ranges, unimpeded by topography. Striated bedrock with
the same flow direction also occurs at lower elevations within the zone
of Laurentide erratics in the upper Buckinghorse River valley. If the upper
and lower striations are of the same age, this implies that the Laurentide
erratics were deposited after the Cordilleran advance. Samples of the upper
striated bedrock were analyzed for cosmogenic 36Cl exposure
dating (Philips 1999). The results suggest that Cordilleran ice retreated
from the mountain summits as late as 14 020±760 to 13 100±1560
calendar years ago.
Glacial features mapped in western Trutch map sheet show that glaciation of the mountains progressed with initial growth of local alpine glaciers which were later overtopped by the Cordilleran Ice Sheet flowing from the west as glaciation progressed (cf. Clague 1989). During the onset of glaciation, there was undoubtedly growth of local cirque glaciers, which eventually fed valley systems. At later stages, however, this system was overwhelmed by expanding Cordilleran ice from the west and the combined ice mass was thick enough to flow across the foothills unimpeded by topography. Thus, high-level striae on the summits of foothills were made during the last glacial maximum by Cordilleran ice, although the tills at lower elevations are predominantly of local origin. Moreover, diagnostic erratics of Cordilleran till may not have been preserved everywhere. These are predominantly friable pebbles of slate and schist, derived from Hadrynian exposures along the Rocky Mountain Trench (Mathews 1980), and they may have been extensively comminuted during glacial transport.
Deglaciation likely occurred in a similar fashion, which implies that the late glacial presence of local ice does not preclude a previous ice cover by the Cordilleran Ice Sheet during the last glaciation. During deglaciation, as the Cordilleran ice thinned, the underlying topography exerted greater control on glacial flow. This is evidenced by drumlin and fluting fields within Rocky Mountain valleys (Fig. 7). In some areas, drumlin and fluting fields crosscut one another, suggesting large shifts in flow directions as the ice thinned. Eventually, the diminished Cordilleran ice would acquire the character of alpine glaciation as a system of cirque-fed valley glaciers. At this stage, western till, originating from the Rocky Mountain Trench and beyond, was no longer transported to the map area. In the final stages of deglaciation, the valleys became ice-free and only cirque glaciers remained. During the Holocene most of the cirque glaciers have completely ablated.
Most of the stratigraphic sections within mountain valleys relate to
the advance and retreat of montane ice. In general, these involve thick
sequences of outwash, ice-contact stratified drift, till and glaciolacustrine
sediments. For example, thick sections along the upper Sikanni Chief River
show a lower dark, indurated gravel, overlain by a dark silty diamicton,
in turn, overlain by a bedded diamicton (Fig. 8). The whole sequence is
capped by laminated silt. It is apparent that the upper Sikanni Chief River
valley was dammed to the east after mountain ice retreated from the area.
Several sections within crosscutting valleys are topped by glaciolacustrine
silt, suggesting that Laurentide ice was blocking drainage at or near the
mountain front at this time. As noted above, shield erratics overlying
Cordilleran striations suggest that the Cordilleran ice advanced prior
to the Laurentide ice reaching the mountain front. Nevertheless, stratigraphic
sections showing both Laurentide and mountain glaciations, as reported
by Hage (1944), could not be found.
Thick sections of glaciofluvial outwash of mountain origin lie at the
mouths of major rivers at the mountain front. These sediments are usually
overlain by glaciolacustrine deposits forming the uppermost unit. In places
within the valleys dissecting the Alberta Plateau, stratigraphic sections
of Quaternary sediments tend to be poor because of excessive slumping.
Much of this area is underlain by thick units of recessive shale and the
banks of most rivers are covered by colluvium. This is particularly true
of the deeply incised Buckinghorse and Sikanni Chief rivers, where whole
forests are slumping into the river (Fig. 9).
Large catastrophic failures have also occurred in the area. For example,
a particularly large landslide occurred in a tributary valley to the Halfway
River (Fig. 10). When visited in the summer of 1998, the freshness of the
feature suggested that the landslide occurred sometime in the spring of
that year, however, there are unconfirmed reports from local residents
that the failure occurred in 1994. The side of the valley failed along
a steep slope (25°) and sent debris, including huge sandstone blocks
for distances up to 1.25 kilometres, filling the valley bottom. The landslide
covers an area of 0.4 km² and currently dams a small lake upvalley.
Should this lake drain catastrophically, its small size would probably
not pose a risk, but landslide events like these obviously pose a significant
hazard and their occurrence unpredictable. Aerial photographs taken in
1958 and 1985 do not show any signs of the impending event. An objective
of this project is to try to evaluate areas of potential landslide hazards.
Large areas of landslides are mapped as colluvium on the 1: 250 000 scale
surficial geology map.
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Samples weighing ~2 kg were recovered by pick from below any pedogenic
or oxidized horizons usually from a depth of 50 cm. The samples were stored
in plastic freezer bags and shipped in metal pails. In Calgary, wet samples
were air-dried prior to shipment for analysis. The location of each sample
site was fixed on 1:60 000 aerial photographs or by averaged GPS readings.
Sample locations are given in Latitude/Longitude and UTM (NAD83)
coordinates (Appendix 1).
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The geochemistry of sediment samples was determined with INAA (Instrumental Neutron Activation Analysis), by Intertek Testing Service, Val-dOr Quebec, for 34 trace elements (Table 1). The silt-plus-clay fraction, was used in each analysis (Appendix 2). The analytical precision of the INAA was assessed using laboratory duplicates. For the most part the results are consistent but occasional discrepancies arise when dealing with trace amounts. The accuracy of the technique was determined by submitting a standard reference till with each batch to be analyzed. In this case a GSC in house till standard (TCA8010) was submitted. The mean, standard deviation and range of the reference till measured by Intertek Testing Services during 1999 and early 2000, is shown in Table 2.
The 63 µm to 250 µm size fraction of 45 sediment samples
were separated for ferromagnetic and heavy minerals (heavy liquid separation,
3.3 g cm-³) by the Terrain Sciences sedimentology lab (Appendix
3). Consorminex Inc., Gatineau, Quebec, analyzed the heavy mineral
fraction of the 45 samples which were mounted in epoxy on 1 x 3 inch glass
slides by the staff of the sedimentology lab. A Zeiss, Stemi SR, stereoscopic
microscope with polar and Nicol, in conjunction with a petrographic microscope,
were used to count the heavy minerals. The ribbon method was used to count
300 grains. A computer program was used to enter the data by keyboard in
the same manner as a Swift blood cell counter. A preliminary scan of all
the slides was done before the classification for counting was prepared.
The data is presented in (Appendix 4, Paré
2000).
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Populations of elemental concentrations are usually positively skewed giving lognormal distributions typical in spatial surveys (Appendix 5; Appendix 6; Levinson, 1974; Govett, 1983). The frequency histogram of molybdenum is a typical positively skewed distribution common for many elements (Fig. 11). In order to highlight anomalous values, log-transformations were not done on these distributions because this would have the effect of minimizing the importance of high values. In contrast, sodium appears to be normally distributed (Fig. 12). Normal distributions are symmetrical with skewness near zero and a kurtosis (peakiness) near three. Other histograms are multimodal suggesting a mix of more than one population (Sinclair, 1976). This may be expected because of the region was affected by ice transport from two different directions. Cluster analysis utilizing all elements was used to determine if geochemistry of the drift could identify two or more distinctly different populations. The results are discussed below.
Fig. 11. Histogram of molybdenum concentrations (n=165). |
Fig. 12. Histogram of sodium concentrations (n=165). |
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A limitation of grouping all the sediment types together in this analysis is
that the background elemental concentrations may be different for each sediment
type. Table 4 presents the results of two separate analyses for gravels (n=28)
and for tills (diamictons, n=117). The anomalies are sufficiently high
that the same samples emerge as anomalous in the categorized data as for the
combined data. This further confirms that the Surview plots, using all the
sediment types combined, effectively displays the concentration patterns.
Nevertheless, some elements are no longer significant anomalies when the
sediment types were separated. However, other elements became significant when
the separate analysis was done (see below).
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Sand, silt and clay ratios for all the samples are presented in Fig. 13 and Appendix 1. The sand-silt-clay fractions of lacustrine fines, glaciofluvial gravels, and tills range from sandy to very silty, with less than 10% clay, despite their varied genesis. The ternary plot shows that most of the surficial materials have very low clay contents, except for three till samples. Thick shale units found throughout the area likely contributed to the high clay contents in these samples. Nevertheless, low overall clay contents in the tills seems unusual. Although this may be due to the nature of the source bedrock or unconsolidated material, large numbers of till samples usually show more scatter on such a the plot. This suggests that some of the samples may not have been completely disaggregated prior to grain size analysis. Pebble contents (> 2 mm) in the tills were as high as 79% by weight and boulders up to 1 m in diameter are common in the tills and glaciofluvial gravels. Most of the till is composed of local bedrock with varying amounts of far-travelled lithologies. Generally, tills from the northeast half of the map area contain many rocks from the Canadian Shield. Local lithologies dominate the composition of mountain tills covering the southeast half of the map area.
Fig. 14. Contour map of percent of total carbon content measured by LECO. |
Fig. 15. Contour map of calcite/dolomite ratios. |
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The anomalous values listed in Table 3 correspond well to the pattern of bulls eyes on the contour maps and provide an independent assessment of the contouring procedure. Some individual samples stand out as having anomalously high values for several elements. For example, sample 99BJB0081 had values significantly above background in nine elements (Table 3). When grouped with diamictons only, the sample was still anomalous in eight elements (Table 4). This sample is unusual because it was taken from an oxidized diamicton of unknown origin exposed in along a road cut north of the Sikanni Chief River. The level of oxidation is much greater than any glacial sediment in the area and the sample probably predates the last glaciation because it is overlain unconformably by about a meter of till. Although the concentrations reported here are anomalous, the absolute values of the elements fall within the range of and mean values reported in soils and surficial deposits elsewhere (Table 3; Levinson, 1980; Gough et al., 1988).
A boulder gravel at the base of a section near the confluence of Trutch Creek and Sikanni Chief River (99BJB0044; Fig. 3) had the highest gold and one of the highest silver concentrations found in the study (46 ppb and 3 ppm, respectively). This sandstone and quartzite gravel was strongly indurated with an imbrication showing ESE paleocurrent. In contrast to the modern stream bed, no shield rocks where present in this gravel unit, which suggests that they may also be preglacial in age. In general, 56 out of 165 samples had gold concentrations above the detection level (>2 ppb) with a mean value of 2.3 ppb, which is slightly elevated compared to typical values in soils (Levinson, 1980).
In places, gravels deposited during and after the last glaciation had anomalous concentrations of elements. A sample of gravel at the top of a section above Green Creek (99BJB0053) had elevated levels of arsenic and iron. Arsenic is a useful pathfinder for gold, although the values reported here fall within reported range for soils (1-50 ppm, Levinson, 1980). The gravel was cemented and orange stained suggesting enrichment by ground water may have occurred. When the gravels were grouped separately (Table 4), the arsenic level was no longer anomalous but the barium concentration became anomalous.
A coarse gravel unit above upper Chipesia Creek, just off northern edge of map area (Fig. 3; 99BJB0023) has anomalous silver levels. The gravels are 3 m thick, and overlie interbeds of silt and ripple laminated sand, which are about 5 m thick. These gravels are underlain by a 1 m thick diamicton, lying over bedrock at the base to the section. All these units contain granite clasts.
Other anomalous values in gravels include: i) anomalous bromine (98BJB0046) in glaciofluvial outwash forming a large terrace along the north side of Muskwa River, ii) a cobalt anomaly of 98 ppm (98BJB0020), which is above the normal range for soils, in thick channel-fill gravels along Petrie Creek, and iii) anomalous hafnium from outwash gravel in a kame terrace above Nevis Creek (99BJB0001; Fig. 3; Table 3).
Several diamicton samples, mostly till, also had element concentrations
well above background (Table 3). A sample from 15
m thick upper till along the south side of the Besa River (99BJB0077) provided
anomalous values in four elements, molybdenum, cadmium, antimony and
selenium. A diamicton, possibly slumped till from a hillslope north
of Mount Withrow (99BJB0052, Fig. 3), also had anomalous
molybdenum
and antimony concentrations. In general, these elements have the
highest values along the foothills (see Surview plots). In contrast, the
rare earth elements samarium and terbium, which are anomalous
in a Laurentide till north of the map area (98BJB0001) and the gravel sample
noted above (99BJB0081), have highest values outside of the mountain front.
This may be due to the abundance of intrusive lithologies in the Laurentide
till.
Fig. 16. Histogram of zinc concentrations (n=165). |
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Till sample 99BJB0077 also has a high zinc value of 410 ppm (the average and range reported in soils is 50 ppm and 10-300 respectively, Levinson 1980), but not anomalous according to the criteria noted above. In the study area, zinc values are normally distributed with only a slight positive skewness with a clustering around 400 ppm (Fig. 16). Cordilleran till samples 98BJB0010 and 98BJB0037 have high zinc values as well as other anomalies. Sample 98BJB0037, silty till from the core of a drumlin near Trimble Lake, has anomalous gold and barium concentrations. Sample 98BJB0010, from a very thick till deposit on a mountain slope, has anomalies in cadmium and uranium. Cadmium has very similar geochemistry to zinc (Levinson 1980). Overall, cadmium was the third most common anomalous element and was only found in association with other anomalies confined to the foothills (4 occurrences, Table 3). Sample 99BJS0013, an upper till marking a readvance of mountain ice at the mountain front near the Prophet River, also has an anomalous uranium concentration and the highest cadmium level recorded by this study (18 ppm).
Some tills found at the contact between Laurentide and Cordilleran ice had anomalous values. Sample 99BJS0001 was high hafnium and zirconium. Both elements occur together in nature (Boyle 1974). Till sample 99BJS0004 was anomalous in arsenic and selenium levels. Anomalous selenium concentrations were also found in Laurentide till near the mountain exit of the Prophet River (samples 99BJS0014 and 99BJB0014; see Surview plots). The largest Se anomaly recorded in the study area comes from a massive landslide deposit west of Sleeping Chief Mountain (99BJB0101). Anomalous arsenic levels were also found in Cordilleran till above upper Bat Creek (Fig. 3; see Surview plots).
Several samples had anomalous concentrations of only a single element.
The most common anomalous element in was silver (Table
3), with most of the anomalies in till in the northeastern quadrant
of the map area, outside of the mountains (see Surview plots). Samples
of lacustrine silt and outwash gravel also had significant silver anomalies
(99BJB0010 and 99BJB0023, respectively). Selenium, the second most
common anomaly, was found in five till samples, although it was the only
anomaly in three of those samples. Bromine was the sole anomaly
in two out of three anomalous samples. In general, bromine anomalies are
concentrated in the upper Muskwa River and the Sikanni Chief River near
the mountain front. Of the three samples with arsenic anomalies,
there was only one sample with arsenic as the sole anomaly (99BJB0032).
Likewise, barium was the sole anomaly in two samples. An isolated
barium anomaly comes from one of three distinct till units exposed in a
drumlin situated southwest of Trimble Lake (98BJB0032; Fig.
3). A sample from one of the other till units at this site (98BJB0037)
was anomalous in barium and gold. Barium was also anomalous in sand at
the mouth of the Prophet River (99BJS0012). Hafnium was the only
anomaly in a sample of gravel (99BJB0001). Finally, gravel sample 98BJB0020
was the only sample anomalous in cobalt.
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Fig. 18. Contoured clusters including all the samples with all INAA geochemistry as variables. Agglomeration was to four groups using Ward's method. |
Fig. 19. Contoured clusters including only diamicton (till) samples with all INAA geochemistry as variables. Agglomeration was to four groups using Ward's method. |
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Fig. 20. Dendogram of heavy mineral grain data clustered using Ward's method. |
Fig. 21. Contoured clusters of percentage heavy mineral grains (n=45). Agglomeration was to four groups using Ward's method. |
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Background levels of geochemistry can be estimated from the summary statistics (Appendix 5). The mode or median of a geochemical distribution is often used as a measure of background value (Levinson, 1974) but if trends or multiple populations exist in the data set this may not be a valid procedure. Nonetheless, several samples in this study were found to have element concentrations well above the normal background levels, although most fall within the range of values reported in other areas (Table 3). To determine the significance of these requires more detailed sampling. For example, there may be trends imparted by the underlying bedrock or dilution effects by geomorphic processes that should first be subtracted from the data set.
Carbon analysis of the drift samples shows regional variability related to the physiography and presumed glacial history, like the drift geochemistry. Of particular interest is the distinct pattern of high calcite/dolomite rations between the Sikanni and Buckinghorse rivers (Fig. 15). The significance of this pattern is not known, but it should be noted that the absolute values producing the ratios are quite small.
In conclusion, this study shows distinct differences in drift composition and several anomalous samples. The significance of the anomalies as mineral indicators requires more detailed study, especially with the high variability expressed in mountainous areas. To utilize surficial materials their erosional and depositional history must be known. For glacial drift this means an understanding of glacial processes and Quaternary history. Eastern parts of the Trutch map area were covered by the Laurentide Ice Sheet, which not only transported abundant material from the Precambrian shield, but affected the regional drainage as it approached and later receded from the mountain front. Similarly, in the west, the Cordilleran Ice Sheet was able to transport material, from the Rocky Mountain Trench, to beyond the foothills onto the the Alberta plateau. During the retreat phase of both ice sheets extensive meltwater reworking of sediment took place which would diffuse the geochemical patterns.
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Boyle, R.W. 1974: Elemental associations in mineral deposits and indicator elements of interest in geochemical prospecting. Geological Survey of Canada, Paper 74-45, 40 p.
Burwash, R.A., and Culbert, R.R. 1976: Multivariate geochemical and mineral patterns in the Precambrian basement of Western Canada. Canadian Journal of Earth Sciences, v.13, p.1-18.
Clague, J.J. 1989: Cordilleran Ice Sheet. in Chapter 1 of Quaternary Geology of Canada and Greenland, R.J. Fulton (ed.); Geological Survey of Canada, Geology of Canada, no.1.
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