Dunn Colin E. Biogeochemistry in Mineral Exploration
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The dominant mineralization is contained within magmatic Ni-Cu-PGE (Au)
net-textured massive sulphide, and includes the following:
Violarite (FeNi
2
S
4
) and pentlandite (Fe,Ni)
9
S
8
.
Chalcopyrite (CuFeS
2
), bornite (Cu
5
FeS
4
) and pyrrhotite (Fe
1x
S).
Sphalerite (ZnS).
PGEs: contained mostly in sperrylite (PtAs
2
), moncheite (PtTe
2
) and kotulskite
(PdTe).
Many additional mineral phases, including those with Au, Ag, Bi, Se and Te.
Environment
The survey area experiences a cold continental climate that supports boreal forest
(see Chapter 3), dominated by black spruce, with minor jack pine. Understory shrubs
are mostly alder and Labrador tea.
The area is remote, located 130 km NNE from the town of La Ronge and 55 km
from the nearest all-weather road . It is only accessible by float-plane or by ground
during the winter over frozen ground, bogs and lakes. Consequently, except for some
minor exploration activities during a 30-year period after mining operations ceased in
1968, exploration activities were mostly on a small scale until Uravan Minerals Inc.
commenced a heightened level of exploration in 1999. The remoteness of the area has
permitted natural vegetation to regenerate, and it now comprises an excellent
biogeochemical field laboratory for examining the PGE and multi-element content of
a comprehensive range of common boreal forest plant species. Whereas controlled
laboratory studies are useful for solving some biogeochemical problems, they cannot
fully emulate the natural environment in which the plants are subjected to the rigours
of nature. At Rottenstone winter temperatures of 40 1C are not uncommon, and in
the summer +35 1C may occasionally be reached.
Exploration history
The Rottenstone deposit (formerly the Hall deposit) was discovered in 1928 and
mined between 1965 and 1968. It produced approximately 40,000 tons of high-grade
ore yielding 3.28% Ni, 1.83% Cu and 9.63 g/t PGE. The extremely high Ni-Cu-PGE
grades in association with high-contained sulphides (40–60%) in a small ultramafic
body indicate that the Rottenstone deposit is an extension of a much larger ultramafic
intrusive body hosting a high-grade Ni-Cu-PGE-Au deposit in the area or at depth.
Prior to mining activities the deposit outcropped as a gossanous rust-coloured
hill. Although small (approximately 50 m 50 m 10 m), it was richer in PGEs than
any other Ni-Cu deposit in Canada. During the early stages of mining in 1965, the
deposit yielded 4.79 ppm Pt and 3.92 ppm Pd. Subsequent analyses of scattered
boulders have yielded up to 38 ppm Pt, 13 ppm Pd, 0.27 ppm Rh, 0.18 ppm Ir,
0.2 ppm Os, 0.14 ppm Ru with 4 ppm Au (Hulbert and Slimmon, 2000).
362
Case Histories
Figure 11-10 shows (a) the original orange gossanous hill of friable oxidized
rock prior to mining operations; (b) a view of the partially mined hill taken about
1967; (c) a view of the site taken over 30 years later, with its setting among heavily forested
terrain and abundant lakes; and (d) a view of the forest canopy showing black spruce
crowns that provided the sample medium collected by helicopter in the autumn of 1999.
Biogeochemical surveys
The first small-scale multi-species and multi-tissue vegeta tion sampling pro-
gramme at Rottenstone took place in the early summer of 1983. In the ash of black
spruce twigs maximum concentrations (with dry-weight equivalent in parentheses)
around the old mine site were 1350 (42) ppb Pd, 880 (28) ppb Pt and 240 (7.6) ppb Au
(Dunn, 1983a). Subsequent visits over the following 5 years permitted the collection
of tissues from all common tree and shrub species growing over and adjacent to the
mine tailings, and also from more distant locations to provide an estimate of back-
ground concentra tions of metals in the vegetation (Dunn, 1986b; Coker et al., 1991).
Table 4-VII (Chapter 4) shows elemental concentrations in various tissues collected
in the mid-1980s. The data show that twigs and outer bark of black spruce accu-
mulate PGE more than the other tissues that were examined. Labrador tea is also
(a) Rottenstone Hill − early 1960s (b) During mining ∼ 1967
(c) Mine site 31 years after closure (1999) (d) Canopy − emergent black spruce
Fig. 11-10. Rottenstone Lake. Photos (a) and (b) provided by the late Dr. J.J. Brummer – (a)
discovery outcrop of PGE-Ni-Cu-Au-rich ultramafic rock prior to mining operations; photo
taken in early 1960s; (b) the scene during mining operations; (c) the view 30 years later; (d) the
forest canopy showing black spruce as the primary emergent species.
363Biogeochemistry in Mineral Exploration
sensitive to the presence of most of these elements. Since the first collection in 1983
several more visits have confirmed these values and in the summer of 2006 newly
collected samples added further weight to the long-term stability of the biogeo-
chemical signature, yielding concentrations in the most recent 10 years growth of
black spruce twigs that were very similar to those obtained 23 years earlier.
On a regional scale the terrain is difficult to cover without air support, because of
the abundance of lakes, streams and bogs, and moderately dense forest. Small-scale
detailed biogeochemical surveys over three geophysical conductors, primarily using
outer bark of black spruce, were conducted by Uravan Minerals in 1999 and yielded
anomalous concentrations of PGE, Au and Ni. Maximum concentrations in ash were
46 ppb Pd, 107 ppb Pt, 52 ppb Au and 218 ppm Ni. These anomalous levels were
substantiated by analysis of twigs from the same trees.
The same year, armed with the knowledge that black spruce is an effective sample
medium, black spruce tops of 1000 trees within an area of 130 km
2
were collected
from a helicopter. The primary grid involved 794 trees, most ly at 500 m spacing on
offset lines with infill sampling at 250 m over and around the old mine site. Figures 11-11
and 5-6 (Chapter 5) show the sample coverage that was achieved during a period of
approximately 5 days and, once procedures had been established, at a collection rate
of more than 30 samples per hour of flying.
After samples had been dried the tissues were separated and twigs were reduced to
ash by controlled ignition at 475 1C. Ashing was necessary in order to concentrate the
PGEs and other ultra-trace elements to detectable level s. A bulk sample of vegetation
was reduced to ash for use as a control on the analytical precision. Determinations
were made by ICP-MS for 61 elements after an aqua regia digestion, resulting in
more 60,000 data bits.
The predominant strike of the bedrock in the area is northeast–southwest, and
the direction of glacial transport was primarily from the northeast. Consequently, it
was anticipated that biogeochemical patterns might exhibit similar trends. However,
the tree top data revealed strong northwest trends of many elements (i.e., normal
to the an ticipated direction) suggesting a structural and lithological control to the
element distributions. Control samples and sequence of analyses were carefully
scrutinized to ensure that these trends were not analytical artefacts.
Figure 11-11 shows concentrations of Ni and Pd as kriged plots, with the maximum
value set at the 98th percentile in order that undue weight is not given to outlier values,
and also that the structure of the bulk of the data can be seen. Therefore, outlier values
greater than the 98th percentile are all shown as the same intensity of colour.
The plots show that Ni is concentrated around the mine site (up to 1730 ppm Ni)
and toward the southeast, with an additional zone in the north near Albert Lake
where 705 ppm Ni in ash was recorded. The Pd data indicate a 4 km by 6 km elliptical
ring of anomalous concentrations with highest values on the south wes tern side of the
annulus, cen tred upon the Rottenstone mine site. Plots of Pt, Te, Au, Fe, Th and Eu
are coincident with this pattern around Rottenstone, and with slightly elevated levels
of Pt, Te and Au near the zone of Ni enrichment at Albert Lake. Plots of Be, Li and
364
Case Histories
0
50
100
150
200
250
300
350
400
Ni ppm
Max. 1730
0 5000
metres
NICKEL IN ASH OF
BLACK SPRUCE TOPS
Albert
Lake
0
6
12
18
24
30
Pd ppb
Max. 77
PALLADIUM IN ASH OF
BLACK SPRUCE TOPS
0 5000
metres
Albert
Lake
Rottenstone
Mine
Rottenstone
Mine
Fig. 11-11. Rottenstone, Saskatchewan. Patterns of Ni and Pd distribution in twigs from the
tops of black spruce trees collected from a helicopter. Concentrations in ash determined by
ICP-MS on an aqua regia digestion. Data released with permission of Uravan Minerals Inc.
365Biogeochemistry in Mineral Exploration
Bi indicate sub-parallel zones, displaced to the northeast, which might define either
shear zones or structure. Additional elements indicate metal zoning (e.g., Re peripheral
to Pd, as shown in section on Re in Chapter 9).
Post survey exploration
In a late 2004 company report by Uravan Minerals, it was stated that up to 2002
reconnaissance drilling of a number of targets that were based on a combination of
geological, geophysical and biogeochemical results had encountered ultramafic
apophyses, some of which were mineralized. Shallow drilling of a coincident biogeo-
chemical and gravity anomaly remains unresolved, although elevated Ni, Au and Cr
concentrations in pelitic rocks suggest an ultramafic intrusive nearby. Petrophysical
determinations of the net-textured ore indicate that it will not provide a good con-
ductive EM geophysical response but a good IP response should be expected. A drill
programme in 2003 intersect ed numerous 1–4-m-thick sulphi de-bearing ultramafic
dykes(?) or apophyses yielding high levels (maxima in pa rentheses) of Ni (1%),
Pt+Pd (1341 ppb) and Cr (7300 ppm). It was considered that the results wer e highly
supportive for a larger intrusive body nearby or at depth.
Summary notes
In this terrain of difficult access the spruce tops were acquired in less than a week
for a cost of about two-thirds of that required to conduct a regional lake or stream
sediment sampling programme. The multi-element data provide substantiation of the
concept that signi ficant undiscovered Ni and PGE mineralization may be present in
the Rottenstone area. The biogeochemical anomalies are under investigation as ex-
ploration continues.
Uravan Minerals Inc. is gratefully acknowledged for permission to release these data.
NICKEL
Canada: Thompson Nickel Belt, Northern Manitoba
Geology and mineralization
In the survey area Precambrian metasediments of the Ospwagan and Burntwood
Groups host Ni-beari ng ultramafic intrusions within barren Archaean gneiss.
Mineralization is dominated by massive sulphides that are locally Ni-bearing. These
are expressed as geophysical conductors.
Environment
Boreal forest – black spruce with minor jack pine and tamarack. The understory
is dominated by shrub alder and Labrador tea. The area is flat with low rolling hills
interspersed with many lakes, streams and bogs (Fig. 11-12). There is extensive forest
366
Case Histories
cover with open areas of bogs and patches of trees burnt by forest fires. There are no
all-weather roads into the area; access is by winter trail over frozen ground or by
float-plane in the summer.
Biogeochemical survey
During a 5-day period toward the end of winter in Apri l, 2003, prior to complete
spring thaw, a helicopter was used to undertake a tree top sampling program
involving 670 black spruce tops. Targets were geophysical conductors representing
17 separate zones within an area of 600 km
2
. Total helicopter time was 20 h, including
all ferrying and a reconnaissance flight, generating an overall sampling rate of almost
35 samples per hour. The all-inclusive average sample collection (i.e., sample col-
lection and flight to the next sample site) time was 1 min 48 s, although during the
course of the main part of the survey this was reduced to 40 s. The sampl e design
involved flights normal to the strike of the conductors. Typically, there were two
flights over each conductor with sample spacing at 250 m, closing to 100 m at a few
sites over the conductors (Fig. 11-13). In a few areas with more complex groupings of
conductors addition lines were flown.
A practical divergence from the planned flight line was set at 30 m in order to
obtain a suitable sample. This tolerance was required because of lack of coverage of
black spruce and/or appropriate canopy structure (i.e., uniform height to the stand
canopy which would preclude safe collection of a sampl e). Averaged across all
Fig. 11-12. Typical mixture of open bog, lakes, rivers and forest covering the survey area.
Water bodies frozen at the time of sample collection.
367Biogeochemistry in Mineral Exploration
samples, the departure from planned flight line was about 6 m. Each sample consisted
of about 30 cm from the top of each tree.
While in the air there was no time for the two-person sampling crew to undertake
reduction of sample material to the size and consistency required for analysis. This
step was undertaken immediately after each flight (Fig. 11-14). The material needed
to be trimmed down to a consistent composition with respect to age of grow th and,
while still fresh, notes made on the appearance of the sample (e.g., signs of chlorosis)
and the number of years of growth collected. A greater volume of each sample was
collected than was likely to be needed for analysis in order that there was plenty of
material for any additional or repeat analyses that may be required.
Samples were oven-dried at 70 1C for 24 h, and the latest 3 years growth of twig
was separated for milling and analysis of dry tissue by ICP-MS for 53 elements (after
a nitric acid/aqua regia digestion). This consistency in years of growth was critical to
the meaningful interpretation of the analytical data. Tests of twig chemistry of three
years of growth co mpared to samples of random age (mostly a greater number of
years of growth) demonstrated that for some elements concentrations were similar
(e.g., Cd, Cu, Ni), but for other elements there were moderate to substantial differ-
ences. For this suite of samples, three years proved to be a practical amount to process.
Whereas five years of growth would have produced some different concentrations, the
Black Spruce Tops
ZONE 33
Sample Sites
Conductor
Zone 33
L-1
L-2
Odei River
0 250
Weak
Conductor
metres
Fig. 11-13. Typical design of flight lines and spacing of samples over geophysical conductor.
368 Case Histories
patterns of element distribution would have been the same. Consistency of sample
medium is of paramount importance.
Over all, element concentrations were not unusual except for Ni values that were
considerably higher than typical background levels for conifer tops. Elsewhere in the
boreal forests of Canada median concentrations of Ni in spruce tops are usually
between 0.5 and 1.5 ppm Ni. In the survey area the median concentration was
6.2 ppm Ni, reaching a maximum of 24 ppm Ni over some conductors.
Examination of element profiles with respect to each flight line and known
geophysical conductors indicated the presence of several multi-element anomalies.
Some, mostly the commodity metals and their pathfinder elements, were confined
to the immediate vicinity of the conductors, whereas other element assemblages
indicated possible faulting and/or lithological changes. Figure 11-15 provides an
example that compares element profiles along two parallel flight lines that passed
over a conductor, such as shown in the example comprising Fig. 11-13.
Over conductors yielding positive responses in Ni, zones of enrichment varied from
single point anomalies directly over conductors, to dispersed zones of enrichment
several hundred metres wide. Of relevance to the exploration programme was that the
five zones with strongest Ni signatures followed a northeast trend, indicating probable
stratigraphic control to the strata that represent the most prospective zones for Ni
exploration.
Over other conductors, trees yielded no positive response in Ni, but were slightly
enriched in Zn, Pb, Cd and S suggesting that the conductors represented enrichments
of these elements in the substrate. Additional zones contained insi gnificant element
Fig. 11-14. Sample preparation in the field – samples spread out for examination, determi-
nation of age of growth and culling to the appropriate size.
369Biogeochemistry in Mineral Exploration
enrichments, especially of base metals, except for local Fe and S which were inter-
preted to reflect the presence of barren sulphides (pyrite +/pyrrhotite). Barium and
Sr enrichments were considered to indicate the presence of stratigraphic zones with
elevated carbonate contents. A summary of the 17 zones upon which this survey
focussed is given as Table 11-IV. Four of the target zones (5, 9, 13 and 16) exhibited
positive indications of Ni enrichment over conductors. Several additional zones
yielded less definitive signatures. Typically, elements associated with the Ni were Co,
Mn, S, Hg and locally Mg.
The data indicated that the treetop sampling may be of use in differentiating
barren from base metal-rich sulphides, and the primary metal content of the latter.
Used in conjunction with geoph ysical and geological infor mation, the technique was
considered to have the potential for screening conductors and optimizing the chances
of defining which geophysical targets might host Ni-bearing mineralization.
Ni ppm
Co ppm
Mn ppm
Ni ppm
Co ppm
Mn ppm
0
5
10
15
20
-
metres
0
2
4
6
8
10
12
14
- 500 1,000 1,500 2,000 2,500
metres
Conductor
0
0.1
0.2
0.3
0.4
- 500 1,500 2,000 2,500
metres
0
0.1
0.2
0.3
- 500 1,000 1,500 2,000 2,500
metres
Conductor
0
100
200
300
400
500
600
- 500 1,000 1,500 2,000 2,500
metres
0
100
200
300
400
500
- 500 1,000 1,500 2,000 2,500
metres
Conductor
500 1,000 1,500 2,000 2,500
1,000
Fig. 11-15. Profiles of Ni, Co and Mn in dry spruce twigs along two parallel lines (100 m
apart) over a known conductor.
370 Case Histories
TABLE 11-IV
Tabular summary of the element responses in black spruce twigs from each of the 17
conductive zones
Zone Elements over conductor Interpretation
Enrichment Depletion
1 No indications of
mineralization other than
possible trace of ZnS.
2 Traces of Au, Ag, Bi,
Cd, Co, Fe, Hg, Mn, Ni,
Pb and REE over
conductor
Fe, REE, Nb, Ni
REE: Ba, Sr, depleted
in SW
Possible minor precious/base
metal zone coincident with
conductor in SW.
3 Fe, Mn, Hg, Ni, and Co Conductors may delineate Fe-
rich zone (stratigraphic
feature) and have some
sulphide mineralization
associated.
4 S, Sr, Ba, Cs Pb, Cd Probably barren sulphide with
carbonate adjacent to S side of
conductor.
5 Ni, (Co), Mn, REE Ba, Sr, Rb, Cs Ni-rich zone, associated with
carbonate depletion.
6 Co (As, Mo, Cr, Se, Sn) Barren sulphides – only traces
of Ni. Probable change in
stratigraphy at 1000–1100 m.
7 Hg, (Cd) P, Cs, Rb No Ni associated with
conductor. Possible
stratigraphic change S of
conductor.
8 Hg either side of low P, Cs, Rb, Mo, Fe,
Cu, Co, S, Hg
9 Ni, Co, Mn, Fe, S, Pb,
Hg, Ca, Rb, REE
Mo, Ag, Sr (P) Ni-rich zone and /or faulting.
10 Ni, Co, Mn, Pb (Li – Ba
zone)
Hg, Mo, S, Au Ni/Co peak over conductor in
east – less prominent moving
west.
Continued
371Biogeochemistry in Mineral Exploration