GEOLOGICAL SURVEY OF CANADA

Open File D3919

C.E. Dunn, D. Smith and D.E. Kerr

A contribution to the Yellowknife EXTECH program and the
Targeted Geoscience Initiative

2001

TABLE OF CONTENTS

Introduction

Regional Setting

• Bedrock Geology
  
• Glacial History, Physiology and Surficial Geology

Biogeochemical Surveys

Sample Collection, Preparation and Analysis

Discussion

Results

• Mud Lake Area
  • Gold
  • Kimberlite
• Drybones Kimberlite
• North of Drybones Bay

Conclusions

Acknowledgements

References

Appendix A
Appendix 1 Spruce bark ash B all analytical data
Appendix 2 Controls on accuracy
Appendix 3 Controls on precision
Appendix 4 Spruce bark ash B analytical data with field coordinates (Zone 12)
Appendix 5 Spruce bark ash B Statistics (Mud Lake area)
Appendix 6 Average concentrations by area

Appendix B
Element plots (dots proportional in size to percentile values)
Elements arranged in alphabetical order by chemical name

LIST OF FIGURES

Figure 1. Location Map

Figure 2. Locations of survey areas

Figure 3. Detail of Mud Lake area with bark sample sites

Figure 4. Contoured bark data for Au, As, Sb and Te

Figure 5. Locations of till samples and count of mantle garnets (after Kerr et al., 2001)

Figure 6. Proportional dot plots of W, Sr, Be and mantle garnet counts

Figure 7. Proportional dot plots of Rb, Cs, Zn and Ni

INTRODUCTION
Black spruce is the most common tree species throughout large areas of the boreal forests of Canada, and many elements concentrate in its outer bark. As part of the Yellowknife EXTECH program, black spruce bark was selected as the sample medium for biogeochemical surveys in three areas near Drybones Bay, on the east side of Great Slave Lake, N.W.T. (Fig. 1). This study complements other on till studies in the same area (Kerr et al., 2000, 2001a) and provides new insight to the chemistry of the underlying rocks and their potential for hosting mineralization. Much of the report focuses on an area centred 4 km southeast of Drybones Bay, approximately 45 km southeast of Yellowknife, in a region informally called Mud Lake (Fig. 2). Two smaller areas with more limited sampling are located on the north and south sides of Drybones Bay, near the diamondiferous Drybones Bay kimberlite.

The second author collected samples on an opportunistic basis during prospecting traverses. Sampling was to a degree dictated by the occurrence of lakes and bogs, but where possible along traverses a consistent spacing of 100 m between sample sites was maintained. However, the resultant sampling pattern is not an even sampling grid (Fig. 3).

Please note that sample site no. 84 to 89 which plot north of Sipper Lake (figures and Appendix B) should, in reality, plot 1 km due south, on the south side of Sipper Lake. The UTM coordinates in Appendix 4 are correct.

REGIONAL SETTING

Bedrock Geology
The survey area lies within the southern Slave Structural Province of the Canadian Shield. Although there are many lakes, bogs and areas with glacial drift cover, outcrops occur at many locations. The dominant rocks are Archean granite, granodiorite and tonalite overlain locally by metasedimentary schists of the Yellowknife Supergroup (Henderson, 1985). A diamond-bearing kimberlite pipe discovered in 1994 in a small bay on the southern side of Drybones Bay (Fig. 2) remains the only kimberlite discovered to date along the western margin of the Slave Craton. It intrudes Archean granitoids and subcrops beneath 67-77 m of lake bottom sediments and a water depth of 38 m. Preliminary dating of the 900 m by 400 m Drybones kimberlite gives an age of 441-485 Ma (Kretschmar, 1997). It is a complex intrusion with 7 lithotypes forming crater facies, diatreme facies and thin basal facies. It contains abundant olivine, ilmenite, pyrope and chromite in a matrix of serpentine clays, calcite and chlorite (Kretchsmar, 1997)

Glacial History, Physiology and Surficial Geology
The Yellowknife region was ice-covered during the Late Wisconsin glaciation until about 10 000 BP (Dyke and Prest, 1987). Ice flow indicators show that ice movement was toward the southwest, with minor divergences related to local topography as the ice sheet thinned and receded (Kerr and Wilson, 2000; Kerr et al., 2001b)Glacial Lake McConnell occupied a large area along the western margin of the retreating ice up to an elevation of 280 m (Craig, 1965), and as water levels fell extensive reworking of sediments took place. Great Slave Lake reached its present level of 157 m a.s.l. by about 8 500 BP (Vanderburgh and Smith, 1988).

Boulder-strewn outcrop interspersed with pockets of sandy till dominate the low relief of the Drybones area. Organic sediments in the topographic lows between the outcrops support a typical boreal vegetation of stunted black spruce (Picea mariana), alder (Alnus spp.), Labrador tea (Ledum spp.) and other small shrubs and sedges. More detail of the glacial history and surficial geology is provided in Kerr et al. (2001b)

BIOGEOCHEMICAL SURVEYS
The roots of a single large tree extract elements from many cubic metres of soil, overburden, groundwater and sometimes bedrock. These elements are then transferred to aerial parts of the tree where they may concentrate. Data derived from the analysis of an appropriate vegetation sample medium permits geochemical mapping, with enhanced background to anomaly contrast of certain elements, which may outline broad geochemical patterns, and assist both in mapping bedrock and in the search for concealed zones of mineralization.

Whereas till geochemistry provides valuable information on the elemental composition of surficial sediments and their provenance, consideration of analytical data from vegetation penetrating the till provides additional insight to the chemical nature of the substrate. Locally, comparison of till and biogeochemical data can assist in determining vectors and distance from a mineralized source (Dunn, 1998). There are commonly some similarities in element distribution patterns derived from the two sample media, but also some significant differences that require some explanation.

Firstly, analysis of a till sample involves sieving and selection of one size fraction from a bulk sample commonly weighing 5 - 10 kg, dug from a single small pit. The tree roots, however, may extend downwards and outwards through several cubic metres of till (all soil horizons), and on occasion reaching and penetrating joints and fractures in bedrock. The tree, therefore, extracts elements from a large volume of material of diverse composition, including groundwater. Some elements that are dissolved in groundwater can be readily extracted by the tree roots, but may not precipitate in till and on soil particles.

A second factor of importance is the barrier mechanism established at the root/sediment interface by some plants for some elements (Kovalevskii, 1979). Because each species of plant has a different requirement for, and tolerance to, a range of chemical elements, some partitioning of elements takes place and there is selective absorption and transference into the plants. For biogeochemical exploration, conifers are good sample media because they are primitive plants that have a wide tolerance to many trace elements. The outer bark may, by analogy, be equated with biotite in rocks, in that it is something of a repository for many elements that do not fit elsewhere or are not required for the metabolic function of the tree.

A third factor is that slight enrichments of metals in till samples are unlikely to be reflected in the vegetation as weak biogeochemical anomalies. This is especially true in the ppb (Au) and ppm ranges of element concentrations common in till samples. Some elements may not be present in a chemical form that is available for uptake (e.g. Cr structurally bound in chromite). Some may be excluded from uptake at the roots or only partially absorbed, and some may be taken up but dispersed among tree tissues to the extent that inter-site variations are so small that they cannot be detected.

The net result of these factors is that the geochemical information supplied by the vegetation is different from that of the till. Just as two methods of a geophysical survey will provide totally different information, so will two methods of a geochemical survey. A high correlation between distribution patterns of two geochemical sample media is the exception rather than the rule. In geological environments where there is sufficient concentration of metals to form a mineral deposit, such a critical mass of elements may be sufficient to generate biogeochemical anomalies above (by upward diffusion) or close to (by movement, for example, in electrochemical cells) the mineral source. Tills, however, usually have geochemical anomalies displaced down-ice from the mineralized source. Such factors need to be taken into consideration when interpreting geochemical results.

SAMPLE COLLECTION, PREPARATION AND ANALYSIS
Samples of the loose outer bark scales from black spruce were collected from 165 sites in the Mud Lake area, 12 sites on the north side of Drybones Bay, and 5 sites from 0.5- 1 km down-ice (southwest) from the Drybones kimberlite, described above, which lies beneath a bay on the south side of Drybones Bay (Fig. 2).

A paint scraper was used to remove 50-100 g of outer bark from around the circumference of black spruce. The scrapings were transferred into bags, and sent to Activation Laboratories Ltd. (Ancaster, ON) where they were reduced to ash by controlled ignition at 470C for 24 hours. They were then digested in strong acid (aqua regia) and the solution analyzed for approximately 60 elements by inductively coupled plasma mass spectrometry (ICP-MS). This technique provided data on the total or near total concentrations of most elements. Data for most of the elements reported are presented in Appendix 1, which includes the detection levels for each element (second row) and all quality control samples. Appendix 2 is a listing of data obtained on the control sample V7a - a vegetation ash of known composition used to assess analytical accuracy. Analytical precision was monitored by repeat analysis of selected samples (Appendix 3).

All data reported are concentrations in ash remaining after controlled ignition at 470C. For bark of black spruce, the ashing process concentrates the elements with little or no losses except for a few of high volatility (e.g. Br and Hg). Appendix 1 includes only data for those elements for which accuracy and precision were within acceptable limits. These limits varied for each element according to concentration levels, but were generally within +/- 10%. Rather than defining a tight statistical control on the acceptability of data, each element was reviewed and evaluated. As a result of this evaluation B, Si, Ge, Se, Zr, Sn and Ba were excluded from all data listings. Most K values were above the detection level (approximately 10%) and so the laboratory coded them as >999999. Although the Ba data were mostly well within acceptable limits, data for the final batch were substantially lower, probably because of precipitation as BaSO4 during the digestion procedure. Data for Cu are included although it should be noted that for the control V7a, there appears to be some instrumental drift toward the end of the last batch. Copper concentrations for other samples in this batch appear to be of the right order, and so it could be just a problem with the control. The digestion of ash appears to have extracted only a small portion of the contained Cr; however, the precision (Appendix 3) is good. Concentrations below detection are recorded as half the detection level. Appendix 4 lists the ICP-MS geochemical data and location by UTM (Zone 12) for each sample. Appendix 5 represents the statistical data for each element as plotted in Appendix B, whereas Appendix 6 presents the average concentration of elements in the three study areas.

DISCUSSION
The interpretation of biogeochemical data should be undertaken with due consideration to chemical requirements and tolerances of plants. Plants require certain elements for their survival, and they have the ability to concentrate metals by scavenging them from the substrate. Zinc, for example, is needed for plant metabolism. Therefore, subtle differences in Zn concentrations between sample sites are more likely to reflect the health of the plant than significant differences in the chemistry of the substrate. However, major differences in Zn concentrations may reflect the presence of Zn mineralization. By contrast, plants also have the ability to exclude those elements that would have a detrimental effect on their growth or health. The 'barrier' mechanism (Kovalevskii, 1979) mentioned above might result in only weak enrichment of an element in a tree from an environment where that element may be enriched in the substrate. As a consequence, for some elements there may be no simple relationship between the chemistry of a tree tissue and the chemistry of the soil and underlying parent material. A brief discussion to assist in data interpretation is given in Dunn et al. (1996) on the role of each element in plant function. Biogeochemistry is a complex science involving the interaction of many organic and inorganic processes. However, careful and systematic collection and preparation of plant samples can provide cost-effective new insight into the chemistry of the substrate and its groundwater.

In light of the occurrence of one diamondiferous kimberlite at Drybones Bay and anomalous concentrations of kimberlite indicator minerals 3 km to the southeast (Kerr et al. 2001a), of particular interest in this study is the possible occurrence of additional kimberlitic bodies. There is unusual abundance of many trace elements in kimberlite (Dawson, 1980). In general, kimberlites are characterized by high content of elements compatible with ultramafic rocks (Mg, Cr, Ni, Co), and concentrations of many other incompatible elements to levels that are higher than usual for ultramafic rocks (especially K, P, Zr, Nb, Sr, Ba, Rb, Cs and the light REE).

A number of case studies address the issue of biogeochemical response to kimberlite:

- Enrichment of Sr, Rb, Ni, Cr, Nb, Mg and P (and depletion in Mn and Ba) was noted in a study of 48 elements in several species growing over the Sturgeon Lake kimberlite in Saskatchewan (Dunn, 1993).

- In northern Ontario several species growing over three kimberlite pipes exhibited enrichments of Sr, Rb, Be, Mo, Au, Cr, Ni, Cu, Co, Cs, Na, Al, Cd, Zn and REE. Black spruce bark was only notably enriched in Sr, Ba and Zn (McClenaghan and Dunn, 1995).

- Over the Peddie kimberlite in northern Ontario, several species were variously enriched in Sr, Rb, Ni, Co, Cr, As, Cd, Nb, Ta, Zr, K, Mg, Th and Tl (McClenaghan et al., in prep.).

- In Alberta, the Mountain Lake ultramafic intrusion is similar in composition to kimberlite and yielded a biogeochemical response in Rb, Ni, Cr, Co, Mg, Au, Fe, Na, V, W and the light REE (Eccles, 1998).

Clearly, there is a wide range of trace elements that may be enriched in tissues of different plant species growing over kimberlites. Several elements appear consistently in the above studies; Rb, Ni, Cr and many others are noted in 3 of the 4 studies.

RESULTS
In general, element concentrations are similar to those found in black spruce bark from elsewhere in the boreal forests of Canada. There are, however, a few exceptions, the most notable of which are elevated levels of Au, As and locally Sb. Typically, this suite of elements is indicative of gold mineralization. Other elements are quite highly enriched at a few sites: Li, Mo, Cr, Cd, Cs and W. Of greater significance than the absolute concentrations are the spatial relationships of relative enrichment of several elements and subtle enrichment of several elements (e.g. Bi, Te).

There are no obvious indications of either base metal or platinum group metal mineralization. Base metal levels are close to usual background concentrations in spruce bark ash; data for Pt and Pd were not obtained, but elements commonly enriched with PGE mineralization (e.g. Ni) are not enriched. Of note are: a) indications of Au; and b) possible kimberlites, especially when viewed in conjunction with the detailed glacial dispersal and indicator mineral studies of Kerr et al. (2000, 2001).

Mud Lake Area

Gold
Concentrations of Au and As are significantly higher than those typically found in the ash of black spruce outer bark. For example, in the Star Lake area of Saskatchewan, the mean of 560 samples was 8 ppb Au, 5.7 ppm As and 2.2 ppm Sb (unpublished data of the first author; summary information in Dunn et al., 1990). By contrast, trees in the Mud Lake area have average concentrations of 27 ppb Au, 64 ppm As and 4.1 ppm Sb. The closest gold mine operations are near Yellowknife, some 45 km to the northwest. Nickerson (1999) reported Au values in black spruce bark ash ranging from a few hundred ppb to >1000 ppb Au near the Con and Giant mines, Yellowknife. Arsenic concentrations were commonly from 100 to >1000 ppm As. Unpublished data supplied by the third author on metals in humus demonstrate that for 25 to 50 km upwind (northwest) from Yellowknife the average concentrations are 25 ppm As and <8 ppb Au, whereas for 25-50 km downwind (southeast) As decreases from 120 ppm at 25 km to 50 ppm at 50 km, Au from 12 to 6 ppb, and Sb from 4 to 1 ppm. It is, therefore, possible that mining operations around Yellowknife have contributed to the Au, As, Sb signature of the spruce bark from the Drybones Bay area by atmospheric transport of airborne particulates. However, values in the Mud Lake area are quite variable and samples to the west of the Drybones kimberlite, several kilometres closer to Yellowknife, yielded typical average background levels for spruce bark of only 5 ppb Au. If the sole source of the elevated levels of Au, As and Sb was from airborne contamination, then it might be expected that there would be moderate consistency to the concentrations of these metals in the survey area. This is not the case, therefore it appears that at least some of the element signatures in the Mud Lake area are related to local sources of metal enrichment.

Figure 4 shows the contoured data for Au, As, Sb and Te. Relative enrichments are observed:

• at the west end of Fastner Lake (Au, As, Sb)
• to the north and south of Swamp Lake (Au, Te)
• near a small lake in the southeast corner of the survey area (Au, As, Sb)
• south of Chanky Lake As, Sb and Te show relative enrichment, with no associated Au.

It should be noted, however, that the maximum Au value is 86 ppb. This could represent modest, yet uneconomical Au mineralization near surface, or more significant grades at greater depth. The implication from the arsenic is that there is a high regional background in As with which Au is locally associated.

Kimberlite
Figure 5 shows locations of till samples collected by Kerr et al. (2001a) and the total counts of mantle garnets at each sample site. This is clear evidence of a local kimberlite source, presumably up-ice to the east northeast. Figure 6 shows concentrations of W, Sr, and Be in the Black Spruce bark, and a repeat of Fig.5 for comparison. Each of these elements indicates local up-ice enrichments. Tungsten is of particular note with relative enrichment in the spruce bark immediately up-ice from the till samples with high mantle garnet counts, and trailing northeast toward Sipper Lake. Beryllium shows a similar pattern. Close to the west shore of Grass Lake (the small lake immediately south of Swamp Lake) there is enrichment of both Sr, in addition to W and Be. The rare earth elements, with Eu as an example (see plot in Appendix B) show a similar pattern to the Sr. Similarly, these elements show slightly elevated levels at the south end of a small lake in the southeast corner of the study area. Each of these elements has been recorded as being enriched over kimberlites elsewhere in Canada (see 'Discussion', above).

Figure 7 shows plots of Rb, Cs, Zn and Ni. Relative enrichment of these elements occurs near the southeast corner of Swamp Lake and at the same small lake in the southeast corner of the study area. Other elements with elevated levels in the same general areas include Cu, Ag, Co, and Li (Appendix B). Also enriched near Swamp Lake are Mg, U and Mo; several other elements yield elevated levels in the southeast corner (Appendix B).

Previous biogeochemical studies of kimberlites have indicated that most element enrichments are closely confined to the extent of the kimberlite. Given this observation, and the high numbers of mantle garnets, areas that warrant focussed attention in conjunction with geophysical results are:

• the west side of Grass Lake (200 m south of Swamp Lake), and beneath that lake
• the southeast corner of Swamp Lake and beneath the lake
• between Swamp Lake and Sipper Lake
• the southeast corner of the survey area.

Drybones Kimberlite
A half to 1 km west-south-west of the Drybones kimberlite (Fig. 2), five samples of black spruce bark were collected to determine if they yielded a biogeochemical signature that might be diagnostic of the nearby kimberlite. If so, then such information could be used elsewhere to assist in defining a vector toward other possible kimberlite bodies. As noted above, biogeochemical responses seem confined to the extent of the underlying kimberlite. This is because the signature of a few grains of kimberlite smeared down-ice by glacial dispersion is insufficient in concentration for a biogeochemical signature to be recognized. This appears to be the case with the Drybones kimberlite. The 5 bark samples had similar mean concentrations of elements to those in the other two survey areas (Appendix 6) except for enrichment in As, Sb, Mn and Ca. Gold, Pb, Bi, Sr, Tl and U were depleted with respect to the other areas. Only site #DN-118 (Appendix 1) yielded elevated levels of Ni (54 ppm). Although one of the highest values for the entire dataset, a concentration of this magnitude is not unusual, and actually lower than might be expected for mafic rocks.

North of Drybones Bay
On the north side of Drybones Bay, at a distance of 1-2 km from the east shore of Great Slave Lake, 12 bark samples were collected at spacing of 300 to 500 m (sites #DN146 to 157, Appendix 1). No extreme values were found in any sample, and mean values were very similar to those from the much larger population of the Mud Lake survey area. The generally elevated levels of Au and As again appear to attest to a regionally high background concentration for these elements (see discussion under 'Gold'). The range in Au values was from 7 to 44 ppb, with the higher values occurring in the more northerly sites of the traverses (DN-151 to 157).

CONCLUSIONS
The biogeochemical survey of the three study areas has indicated several sites that are worthy of closer investigation for potential gold mineralization and the presence of kimberlites. As with any surficial geochemical dataset, data should not be viewed in isolation, but in conjunction with environmental, geological and geophysical information, and detailed knowledge of the geologist who has studied the local physical and geological environment.

ACKNOWLEDGMENTS
The second author extends thanks for the encouragement provided by Dave Nickerson, and for his time and patience in explaining the sampling methods and refining procedures. I. McMartin critically reviewed the manuscript. Digital formatting for the CD-ROM was prepared by Alalonde Corporation.

REFERENCES