Dunn Colin E. Biogeochemistry in Mineral Exploration
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Post survey exploration
Subsequent drilling of some of these anomalous Ni zones has resulted in the inter-
section of several intervals with elevated Ni levels. At this time details are confidential.
Anglo American Exploration (Canada) Ltd. is gratefully acknowledged for
permission to release the information contained within this summary.
TABLE 11-IV Continued
Zone Elements over conductor Interpretation
Enrichment Depletion
11 Ba, Sr, Mo, Cr Cs, Rb No discrete Ni anomaly.
Probable carbonate at
conductor.
12 Pb, Zn, Ag, Sr, (Ba), (S) Mn Possibly Pb/Zn with
carbonates. No Nickel.
13 Ni, Co, Mn, Hg, Rb, Li,
Mg, (Fe, REE, P, Pb, &
Au, Ag, Cd, Cu)
Ba, Sr, Cs, As Good Ni, Co, Mn signatures.
Fe-rich, carbonate poor.
14 S, Zn, (Mn) Co, Au, Cd, REE, Li,
Mo
No nickel. Conductor may
reflect carbonates with Fe
sulphide and minor Zn/Pb.
15 No nickel. Possible
stratigraphic change south of
1600 m (more carbonate?)
and some Pb and Zn.
16 Ni, Co, Hg Mo, Pb, Ag, Mn, Cr,
Cs, Cu
Both conductors indicate Ni-
enrichment – especially that to
S. Ni-rich zone between
conductors. Different lithology
(probably carbonate) north of
conductors.
17 Little of significance. No
obvious mineralization.
18 Ni, Co, Mn with some
Ag, Zn, Cd
Low Ni, but positive signatures
over conductor suggest some
enrichment. Probably
stratigraphic change at
1200–1400 m along lines (Fe-
rich rocks to south).
372 Case Histories
URANIUM
Canada: Athabasca Basin, Northern Saskatchewan
Geology and mineralization
World-class U deposits occur at the unconformity between Helikian (Pre-
cambrian) clastic deposits of the Athabasca Group (Athabasca Sandstone) and
underlying crystalline basement rocks (Aphebian). Outcrop of Athabasca Sand-
stone is extremely sparse because of the extensive blanket of sandy glacial tills that
are commonly about 1 m thick. Peat bogs (muskeg) and lakes fill depressions, and
soils are thin podzol.
Locally, the mineralization is complex, but the primary minerals are
Uraninite (UO
2
) as massive pods, veins and disseminated aggregates.
The massive variety of uraninite, pitchblende (UO
2
), fills extensional features
in reactivated fault zones and locally replaces the clay-mineral matrix of the
Athabasca Sandstone.
Associated elements include all or some of Ni (notably gersdorffite – NiAsS), Co,
Cu, As, B and REE.
Exploration history
In 1935, two small pitchblende-bearing veins were discovered in northernmost
Saskatchewan, near Uranium City. The first recognition of U associated with the
Athabasca Sandstone was by Kermeen (1956), but it was not until the mid-1960s
that a major exploration programme commenced in the Athabasca Basin. Initial ly,
exploration took place in the west of the Athabasca Basin in and around a large
circular structure (the Carswell Dome) considered to be a meteorite impact crater. In
1968, Gulf Minerals Ltd. discovered the Rabbit Lake U deposit at the eastern edge of
the Athabasca Basin prompting extensive staking. The 1970s witnessed an intensive
period of mineral explora tion resulting in the discovery of several more U deposits
near the eastern margin of the Athabasca Basin. A more detailed account of the
exploration activity is given by Beck (1977) and Schiller (1978).
By the end of the 1970s many exploration techniques for U had been tested with
varying levels of success, and in 1979 the Geological Survey of Saskatchewan and the
Geological Survey of Canada initiated a joint study to assess the relative effectiveness
of all known U exp loration methods by concentrating studies on an area encom-
passing 1450 km
2
on the eastern side of the Athabasca Basin, to be designated the
‘Athabasca Test Area’ (Fig. 11-16). Cameron et al. (1983) wrote that
The discoveries of the Midwest and McClean Lake deposits y were not ‘easy’ discov-
eries, but required, in equal measure, prescience, skill, determination and money, plus a
373Biogeochemistry in Mineral Exploration
little bit of luck. The technical difficulty of discovering these deep deposits was stimulus
for the program of comparative studies of exploration methods y
Included in the gamut of techniques were some orientation surveys to test the
sensitivity of biogeochemical exploration methods to the presence of U miner aliza-
tion in the Athabasca environment, because Walker (1979) had reported positive
indications from over the Key Lake U deposit, located 150 km to the southwest.
Environment
The continental climate of the Athabasca Basin experiences a temperature range of
50+35 1C, with 40–60 cm of precipitation that occurs mainly during summer. Winds
are generally light (8 kph) and dominantly from the west. These conditions sustain a
boreal forest in a region of discontinuous permafrost. Dominant tree species are black
spruce and jack pine, with an understory of mainly shrub alder and Labrador tea.
Biogeochemical surveys
The primary area for testing the biogeochemical methodology was in the vicinity
of the McClean Lake mineralization (Fig. 11-16) that had been discovered only a few
weeks prior to the first field investiga tions. The discovery occurred at the end of the
1978/1979 joint partnership between Canadian Occidental Petroleum Ltd. and Inco
Metals Inc. when the last hole of the winter drilling programme intersected remark-
ably high-grade mineralization (up to 27% U
3
O
8
) at the basal unconformity beneath
150 m of unmineralized sandst one (Dunn, 1981, 1983b,c).
At the time that the biogeochemical surveys commenced in the late spring of 1979
an open pit comprising the Rabbit Lake U mine, located 10 km southeast from
McClean Lake (i.e., downwind), was the first and only operating mine within a radius
Fig. 11-16. Location map: Uranium deposits in northern Saskatchewan (as known in 1982)
and limit of Athabasca Basin Test Area (Cameron et al., 1983).
374 Case Histories
of several hundred kilometres. Except for a rough road that reached Rabbit Lake from
the south, there were no roads and access was by floatplane. Consequently, the
environment was pristine except for some local disturbance by drilling operations.
The first surveys found that samples of black spruce twigs contained unusually
high concentrations of U at McClean Lake and elevated levels at Midwest Lake.
Background concentrations of U in plant ash are typically less than 1 ppm U,
whereas these first samples locally yielded over 100 ppm U. These results provided
the impetus for a more thorough investigation of other species and of a br oader area
during subsequent visits over the following three summers. These studies sought
to establish variations in U content that might be attributed to a wide array of
variables, including the following.
Local variation in U content by sampling all 10 black spruce trees growing within
an area of 200 km
2
. This is a typical tree density for this area.
The effect of tree height; tree health (as noted from any visible differences, such
as chlorosis of the needles); live versus dead twigs; any variation in sampling
procedures (two people collected samples).
Variations around individual trees (e.g., twigs from the north side of the trees
versus those from the south side; twigs from chest height versus those near the top).
Seasonal variations in U content by re-sampli ng the same individuals in the spring
and summer.
Annual variations in U content by re-sampling the same individuals over three years.
Differences in age of twig growth (by dissecting twigs into their respective last
10 years of growth).
Differences in U content of trees from boggy areas compared to dry ground.
Comparisons of U content of twigs with the U content of the underlying soils (see
Table 1-V in Chapter 1)
Comparisons of analytical determinations on dry tissue versus those on ash.
Whereas some variability in the data was observed for many of these parameters,
over all it was evident that the patterns were robust and reproducible, attesting to the
validity of the method to delineate U distribution patterns that provided focus for ex-
ploration activities. The fundamental tenets of maintaining consistency in sampling
methods and not mixing plant species were the underlying protocols that were of
paramount importance. Details of these studies are summarized in Dunn (1983b,c).
Among the results obtained were data from a comprehensive set of samples that
permitted an evaluation of the relative sensitivity of each of the common species and
tissues from a small area in the vicinity of the McClean Lake deposits. By determining
the relative concentrations of U in each sample medium it was confirmed that black
spruce twigs represented the sample medium yielding the highest concentrations of U
(Table 9-VIII).
By subdividing 10 years growth of spruce twig tissues according to their age it was
determined that highest U concentrations occurred in the 2–6-year-old growth.
However, in that cold continental climatic regime there are only small annual
375
Biogeochemistry in Mineral Exploration
increments of growth. For efficient field operations it is impractical for a biogeochemical
survey to isolate small growth increments and so 10 years of growth, a length of ap-
proximately 25 cm in the Athabasca region, was obtained at each sample station thereby
providing an integrated U signature over that time period. Throughout the remainder
of the 3-year biogeochemical survey all spruce twig samples were of consistent age
(10 years) and diameter. The consistency in diameter was important because, as shown
earlier (Fig. 3-5) U is concentrated mostly in the bark, hence for meaningful data
interpretation it is necessary to maintain a consistent twig wood to twig bark ratio.
The optimal analytical method at the time was neutron activation/delayed
neutron counting of 1 g sampl es of ash. This required the collection of approximately
100 g of fresh twig, since the moisture content was about 50% and the loss on
ignition was 98% of the dry twig. Therefore, values reported here are concentrations
in ash. Dry-weight equivalent concentrations can be approximated by dividing the
ash values by 50 (i.e., 2% ash yield ). Details of other analytical procedures involving
multi-species determinations by ICP-ES and INAA are given in Dunn (1981) . For
this case history only the U data are presented, because for most samples in the
subsequent regional survey U was the only element determined.
Detailed surveys
At McClean Lake black spruce twig samples were collected at 30 m intervals (with a
few at 15 m) normal to the strike of the ore bodies. The criterion for the 30 m intervals
was simply one of practicality, since survey pickets for the drilling programme were set
at these intervals. A more conventional spacing of 25 m would have been equally
appropriate. At a distance of 1 km on either side of known mineralization the sampling
interval was increased to 60 m. Most traverse lines were 60 m apart, with a few at 30 m.
Figure 11-17 shows contoured concentrations of U in the ash of the spruce twigs
in relation to known zones of U mineralization , established by later drilling, at a
depth of up to 150 m beneath barren sandstone. The McClean North U deposit
(between McClean and Candy Lakes) has biogeochemical anomalies which follow
the strike of the mineralization, but they are displaced to both sides with relatively
strong anomalies on the north-western flanks of the ore zones, giving rise to a cross
section profile of classic ‘Rabbit’s Ears’ type of geochemical anomaly (i.e., enrichments
on either side of a zone of mineralization). A second sub-parallel zone of mineral-
ization representing the McClean Lake South zone has less intense anomalies in spruce
twigs. An additional zone of strong biogeochemical anomalies occurred at the south-
eastern end of the detailed survey area, south of Bena Lake (see following section on
‘Subsequent Discoveries’).
In the vicinity of the McClean North deposit normal background concentrations
of U are present in surface groundwater, soil and peat. Formation waters exhibit an
upward hydraulic gradient and some artesian flow from drill holes has been re-
ported. These waters locally contain several tens ppb U near mineralization (Dyck,
1983). Relatively permeable zones (flow rates of 10
3
–10
4
cm/sec) in the Athabasca
376
Case Histories
Sandstone give horizontal continuity to water flow, and pathways for upward flow are
provided by an abundance of near vertical fractures. Within the Athabasca Sandstone
ample evidence of upward movement of fluids is provided by the presence of sec-
ondary alteration and oxidation products that locally reach the surface (up to 200 m
above the uranium-rich unconformity), and enrichment of trace elements within the
alteration zones and along steeply dipping faults and fractures (Sopuck et al., 1983).
Possible explanations for the ‘Rabbit’s ears’ include (a) the ore bodies have
created a natural galvanic cell similar to those discussed by others (e.g., Govett et al.,
1976; Smee, 1983); and/or (b) there is a ‘shadow-zone’ of silicified breccia above the
ore in which there are relatively few steeply inclined fractures. In the latter situation
any upward movement of formation waters containing dissolved U would pass
around the silicified zone and be tapped by the plant roots from trees on either side of
the vertical extension of the ore (Fig. 11-18).
Soon after first the biogeochemical survey was carried out at McClean Lake in 1979,
samples were collected also from the vicinity of the Midwest Lake deposits, located
14 km northwest from McClean Lake (Fig. 11-16). At this location U mineralization is
present beneath 200 m of Athabasca Sandstone with secondary U oxidation products
close to the surface, above the ore zone. Spruce twigs were sampled from 69 sites at
mostly 150 m spacing (i.e., only one-fifth of the sample density at McClean Lake) and
concentrations ranged from 20 to 136 ppm U in ash, with highest values occurring
above the southern part of the mineral deposit. Over all, it was considered that the data
from this area were not conclusive, and that a more detailed survey would be required
to establish a clear relationship to U mineralization. However, the situation was com-
plicated in that most of the Midwest mineralization is located beneath a lake.
Fig. 11-17. McClean Lake – contoured concentrations of U in spruce twig ash in relation to
known zones of U mineralization.
377Biogeochemistry in Mineral Exploration
Adjacent to the Rabbit Lake open pit operation a maximum of 2270 ppm U was
recorded in spruce twig ash. Nearby sites locally yielded in excess of 1000 ppm U.
Regional survey: Eastern part of the Athabasca Basin
The recognition of U enrichm ent over zones of mineralization prompted increas-
ingly wider investigations in an attempt to delineate the size of the anomal ous zone,
since during the first two years of surveys it was apparent that all trees that were
sampled contained considerably more U than the usual background concentrations
of o1 ppm U in ash. Samples were collected at increasing intervals along the road
leading southward from Rabbit Lake. Spacing was at intervals of 2 km southward
from Rabbit Lake, then increasing to 5 km for about 50 km and finally at intervals of
10 km for a further 150 km (Dunn, 1981). Sample sites to the north were accessed by
boat and to the west, southwest and northwest by float plane. This succeeded in
establishing that the ‘Wollaston Uranium Biogeochemical Anomaly’, defined as the
10 ppm U-in-ash contour, extends over an area of approximately 10,000 km
2
(Fig. 11-19). Within this immense area the 50 ppm U contour encompass es 3000 km
2
and within a central core of 1000 km
2
almost all spruce trees sampled (more than
1000) contained in excess of 100 ppm U in the ash of their twigs.
Fig. 11-18. Conceptual model of U migration from ore beneath the Athabasca Sandstone to
the surface, giving rise to biogeochemical anomalies.
378 Case Histories
Exhaustive checks on analytical precision, accuracy, sampl e preparation, analyt-
ical procedures, and resampling of trees confirmed the valid ity of the data. Although
the Rabbit Lake open-pit U mine was in operation at the time, no other U mine
existed within 300 km of Wollaston Lake. However, the possibility of airborne
contamination was investigated with the following results.
Environmental baseline studies conducted in the Wollaston area at around that
time demonstrated that the air was very pure and had lower than normal levels
of radioactivi ty. For example, the gross beta level near the McClean deposits was
0.22 mCi/km/30 days during the autumn of 1979, several months after first rec-
ognition of U enrichment in the vegetation. This level was less than half of that
measured two years earlier over the Prairi e Pro vinces.
Gross alpha levels of dus t-fall at McClean lake were 0.018 mCi/km/30 days,
representing about one-twentieth of the alpha levels that had recently been recorded
in northern Ontario.
Thirteen air-monitoring stations around Rabbit Lake repeatedly reported U levels
to be less than 6% of the maximum permissible concentrations set by the
Fig. 11-19. ‘Wollaston Biogeochemical Uranium Anomaly’, with U deposits referred to in
the text and outline of Athabasca Test Area.
379Biogeochemistry in Mineral Exploration
International Commission on Radiological Protection (i.e., o6% of 6 10/mCi/cc
of insoluble U). Furthermore, McClean Lake is upwind from Rabbit Lake.
The ‘Wollaston Uranium Biogeochemical Anomaly’ remains the world’s largest
known U biogeochemical anomaly. Discussions with Alexander Kovalevsky in 1984
indicated that the only other comparable biogeochemical anomaly of this size and
intensity was one of Mo from an undisclosed location in the former Soviet Union.
Subsequently, a Mo biogeochemical anomaly of similar magnitude has been defined
around the Endako Mo mine in British Columbia (Dunn, 1999).
Subsequent discoveries
Within the large, zoned Wollaston ano maly there are several areas where intense
local U in twig anomalies were delineated. Some of these local anomalies were
sampled in sufficient detail to determine their relationships to locations of U de posits
discovered since the 1979–1981 surveys. Others were too regional in their spacing to
provide more than a general ‘signature’ to buried miner alization. The following
observations are pertinent:
The JEB deposit (announced in May, 1982) yiel ded ‘Rabbit’s Ears’ anomalies
toward the eastern extent of conc ealed mineralization (Dunn, 1983c). Maximum
concentration was 264 ppm U in ash. The grade announced at the time was 9.72%
U
3
O
8
over 7 m at a depth of 90 m for a tested strike length of 190 m.
Over the Dawn Lake deposits, spruce twigs collected by the Saskatchewan Mining and
Development Corporation in 1983 yielded maximum concentrations of 80 ppm U.
A linear trend of U enrichment, locally over 400 ppm U in ash, was located 200 m
west of the Sue deposits that were discovered in 1988. The Sue A, B, C, CQ and D
deposits lie on a 2 km long north-trending segment of graphitic gneisses at the west
contact with the Collins Bay dome.
Intense U anomalies occur east and north of the Sue deposits. The strongest of these
is 1360 ppm U (Dunn, 1983c) recorded from a site at the north end of Tut Lake,
located 2 km northeast of the Sue and 5 km northeast of the McClean Lake deposits.
To date the source of these anomalies has not been identified. Re-sampling of several
of the same trees in 1990 confirmed that U enrichments persist.
South of Bena Lake (Fig. 11-17) in 2005 Denison Mines Ltd. reported 0.53% U
3
O
8
over 5 m, and in October, 2006, their website indicated that ‘work during 2006 is
focused on evaluating the recently discovered Bena Lake alteration system’. It
remains to be seen if the zones of anomalous U enrichments in the spruce twigs
shown in Fig. 11-17 could be related to the new discovery.
Since the biogeochemical sampling programme was completed in 1981, significant
new discoveries (>1 million pounds U
3
O
8
and up to 70 million pounds) within
the area outlined by the 100 ppm U in ash contour (Fig. 11-19) include Collins Bay
A-zone, Collins Bay B-zone, Collins Bay D-zone, Eagle Point, Raven-Horseshoe,
Sue A, B, C, D and E and West Bear. Rabbit Lake and the Dawn Lake and
McClean deposits ha d already been discovered, but only Rabbit Lake was being
380
Case Histories
mined. Within the 50 ppm U-in-as h contour, where mineralization is mostly at
greater depth, the Midwest Lake deposits were already known; subsequently, JEB
was discovered and mined, and recently high-grade mineralization has been
reported at La Rocque (up to 31.9% U
3
O
8
over 7 m reported) and Bell Lakes.
Drilling at an additional zone 3 km north of the main Midwest mineralization
known as the Mae Zone has recently intersected 31.3 m of an average weighted
grade of 3.21% eU308 from 191.8 to 223.1 m.
The world-class Cigar Lake and McArthur River deposits are located to the south
of the Wollaston Biogeochemical Anomaly. Any biogeochemical data that might
have been acquired from over these deposits is not in the public record. However,
both deposits occu r at depths greater than 400 m and, unless there is considerable
upward flow of groundwater from the basement unconformity (where the deposits
are located) to the surface, it is considered unlikely that a significant biogeochem-
ical response occurs. From the evidence accumulated to date, it appears that a
cover of 200 m of Athabasca Sandstone is about the maximum through which a
biogeochemical signature from basement-related U deposits can be detected.
Summary notes
These U biogeochemistry case histories recount the results of survey programs
completed more than 25 years ago. Extraordinarily high concentrations of U in
vegetation were encountered, and the world’s largest U biogeochemical anomaly was
delineated. During the intervening years there was a lull in the exploration activities
because of the depressed prices for U. Over the past 2–3 years the pace of U explo-
ration has picked up considerably and old results can start to be put in context of new
activities and discoveries.
It can now be demonstrated that the Wollaston Uranium Biogeochemical Anomaly
encompasses an area containing many significant zones of mineralization with more than
20 deposits outlined that each contain well in excess of one million pounds U
3
O
8
and up
to 70 million pounds. However, the widely spaced sampling of the regional survey is
inadequate to define zones of mineralization. Semi-detailed traverses (200 m spacing)
over large parts of the Athabasca Test Area are sufficient to define trends of U enrich-
ment, but it requires detailed surveys, such as those at 30 m pacing over the McClean
deposits, to establish patterns that might be useful for establishing drill programmes.
KIMBERLITES
Canada: Ekati Trend, Lac de Gras, Northwest Territories
Geology
Eocene kimberlite diatremes (including crater and hypabyssal facies) emplaced in
metamorphosed Archaean sediments, granites and diorites. Minimal dilution of the
kimberlites by country rock xenoliths.
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Biogeochemistry in Mineral Exploration