Biogeochemistry as an aid to exploration for gold, platinum and palladium in the northern forests of Saskatchewan, Canada

Journal of Geochemical Explorat~'on, 25 ( 1 9 8 6 ) 2 1 - - 4 0

21

Elsevier Science Publishers B.V., A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s

BIOGEOCHEMISTRY AS AN AID TO EXPLORATION FOR GOLD, PLATINUM AND PALLADIUM IN THE NORTHERN FORESTS OF SASKATCHEWAN, CANADA

C O L I N E. D U N N *

Saskatchewan Geological Survey, 201 Dewdney Avenue East, Regina, Sask. S4N 4G3, Canada (Received May 25, 1 9 8 4 ; revised a n d a c c e p t e d J u n e 28, 1 9 8 5 )

ABSTRACT D u n n , C.E., 1986. B i o g e o c h e m i s t r y as an aid to e x p l o r a t i o n for gold, p l a t i n u m and pall a d i u m in t h e n o r t h e r n forests of S a s k a t c h e w a n , Canada. In: C.E. Nichols ( E d i t o r ) , Exp l o r a t i o n for Ore D e p o s i t s o f t h e N o r t h A m e r i c a n Cordillera. J. Geochem. Explor., 25: 21--40. Twigs o f s h r u b alders (Alnus crispa a n d Alnus rugosa) t e n d to c o n t a i n m o r e gold t h a n o t h e r c o m m o n species in t h e n o r t h e r n forests o f S a s k a t c h e w a n . Alders are n o t cyanogenic, h e n c e samples can be ashed to p r e c o n c e n t r a t e gold w i t h o u t loss o f t h e m e t a l as the volatile gold cyanide. Samples f r o m m i n e r a l i z e d z o n e s c o m m o n l y have over 50 p p b Au in the ash of the o u t e r m o s t 50 cm of alder twig. B a c k g r o u n d values are a b o u t 10 p p b Au. In the a b s e n c e of alder f r o m a given locality, o t h e r species m a y p r o v i d e useful i n f o r m a t i o n o n gold in the s u b s t r a t e . E x a m p l e s are given o f surveys using balsam fir (A bies balsamea), w h i t e spruce (Picea glauca) a n d black spruce (Picea mariana). A r e c o n n a i s s a n c e scale survey in w h i c h alder twigs were collected at 2-km intervals has o u t l i n e d a n area, c o i n c i d e n t w i t h a m a j o r l i t h o s t r u c t u r a l d o m a i n , w i t h i n w h i c h ashed twigs c o n t a i n e d f r o m 20 to 130 p p b Au (~ = 45 p p b Au). In sharp c o n t r a s t , alders in a n e i g h b o u r i n g area c o n t a i n e d less t h a n 10 p p b Au. It appears t h a t this regional a p p r o a c h to b i o g e o c h e m i c a l s a m p l i n g in glaciated terrains m a y provide a q u i c k appraisal of the gold potential of underlying bedrock. P l a t i n u m a n d p a l l a d i u m show a t e n d e n c y to c o n c e n t r a t e in twigs a n d t r u n k o f black spruce (Picea mariana) a n d jack pine (Pinus banksiana), and in stems o f l a b r a d o r tea (Ledum groenlandicum). Spruce was sampled close to a w o r k e d o u t n i c k e l - c o p p e r d e p o s i t t h a t c o n t a i n e d 3 0 0 0 p p b Pt a n d 6 0 0 0 p p b Pd. T h e ashed twigs yielded up to 8 8 0 p p b Pt a n d 1 3 5 0 p p b Pd, c o m p a r e d to b a c k g r o u n d levels of below 10 p p b Pt and 2 p p b Pd.

INTRODUCTION

AND RATIONALE

FOR USING BIOGEOCHEMICAL

METHODS

Several factors have contributed to the recent revival of interest in biogeochemical methods of prospecting. These include: (a) the increasing need to find ways of detecting subtle indications of deposits; (b) the great improvement in analytical instrumentation, with concomitant lowering of de*Present address: Geological Survey o f Canada, 601 B o o t h Str., O t t a w a , Ont., K I A 0E8, Canada. 0375-6742/86/$03.50

© 1986 Elsevier Science P u b l i s h e r s B.V.

22 tection limits and improvements in accuracy and precision; and (c) the steadily accumulating evidence that biogeochemical methods really can help in locating concealed deposits, provided such methods are carefully developed prior to conducting large surveys. Nature has taken hundreds of millions of years to develop, in the form of a plant, an efficient geochemical sampling tool. A highly corrosive microenvironment occurs around the myriad of roots and rootlets which feed a plant. Elements dissolved and absorbed by the plants include n o t only those essential to growth, b u t traces of other elements which can be safely tolerated in cell structures. In glaciated terrain extensive areas are covered with overburden which could be blanketing mineralized bedrock. In such areas biogeochemical exploration may be the only feasible method for detecting metals such as gold, that accumulate in low concentrations. The studies reported here were carried out to determine which common species are sensitive to the presence of precious metals in soils and bedrock, and which parts of these plants most readily concentrate the metals. A prime consideration was to identify a useful sample medium that could be collected easily. In addition, a project was initiated to determine regional patterns of gold concentrations in plants, and thereby tried to outline relatively auriferous areas worth closer investigation. This paper constitutes an interim report of an on-going research study. The dynamic nature of biological systems necessitates repeated sampling of sites to determine seasonal and annual variations in element content (cf. Schiller et al., 1973; Konstantinova, 1981). PREVIOUS WORK There is a substantial volume of literature concerning both biogeochemical methods of exploration for gold and measurements of gold uptake by plants. Valuable reviews by Shacklette et al. (1970), Jones (1970), Boyle (1979), Kovalevskii (1979), and Brooks (1982) provide sound surveys of the literature. A recent compilation of case histories of biogeochemical exploration for gold and precious metals is by Carlisle et al. (in press). The only papers on platinum biogeochemistry are those by Fuchs and Rose (1974), and Riese and Arp (in press). Biogeochemical exploration for gold in Western Canada was first reported over 30 years ago (Warren and Delavault, 1950). However, to the author's knowledge no work on gold or platinum biogeochemistry in the boreal forests of central Canada has been published, although similar environments in the USSR have been studied and gold data reported on several species (e.g., Konstantinova, 1981). Consequently, orientation surveys for each metal were the first phases of the studies reported here.

23

GEOLOGY, PHYSIOGRAPHY AND VEGETATION

In central Canada coniferous forest occupies a zone between latitudes of approximately 54 ° and 62 ° N. Glacial deposits of variable thickness and composition are present t h r o u g h o u t the area. Soils are mainly immature podzols and regosols, and peat bogs (muskegs) are c o m m o n . I 10 ° 60<

108 °

~--\

4

Northwest

104 °

Territories .

. . . . . . .

102 ° 647 60°

Uranium C ty

Lake Athabasca

.~

~

~k~onG,

74

74 J c~Lz

~

24

>~z~vollaston

i %

o,

I

z,.,,.,..

; d Si

b." ~arwater R

•

.,~,,

,

~

~. \

,.." .',~'.

t 0 o NORTHERN

~Lake --

50 I 50

100 km lOO Mi

~

I

8#'#l~l~ ~

-~,~ 57

.~

-~ ~--/-'~x~P~

I

J"'"

F on I I

SASKATCHEWAN

Fig. 1. L o c a t i o n map. 1 = R o t t e n s t o n e Lake ( p l a t i n u m , palladium, gold); 2 = Waddy Lake (gold); 3 = S u l p h i d e Lake (gold); 4 = Nicholson Bay (gold, platinum); stippled area = regional gold s t u d y .

24 In northern Saskatchewan (Fig. 1) the bedrock is Precambrian Shield composed of Archaean and Aphebian metamorphic rocks with some igneous intrusions. Younger Precambrian (Helikian) unmetamorphosed clastic sediments o c c u p y the Athabasca Basin. The topography is dominated by forested plains and hills of low to moderate relief. The continental climate gives temperatures b e t w e e n - 5 0 ° and +35°C with 40--60 cm of precipitation, and sustains a largely coniferous forest of black spruce (Picea mariana [Mill.] B.S.P.), and jack pine (Pinus banksiana Lamb), with lesser numbers of white spruce (Picea glauca [Moench] Voss), balsam fir (Abies balsamea [L.] Mill.) and tamarack (Larix laricina [Du Roi] K. Koch). The forest has a deciduous c o m p o n e n t represented by birch (Betula sp.), poplar (Populus sp.), willow (Salix sp.) and alder (Alnus sp.). The latter two occur as tall shrubs (usually 2--3 m). L o w shrubs include labrador tea (Ledum groenlandicum Oeder), leather leaf (Chamaedaphne calyculata [L.] Moench), blueberry (Vaccinium sp.), and juniper (Juniperus sp.). Numerous additional small species are present, b u t have low practical significance for conducting a biogeochemical survey because of their irregular distribution. Mosses and lichens are extremely c o m m o n , but there are difficulties in separating the organic matter from the adhering inorganic particles, hence they are less practical to collect than the other ubiquitous species. METHODS

Choice of sample medium For meaningful inter-site comparisons of biogeochemical data it is important to collect n o t only the same species, but vegetation of similar growth and appearance at each sample station. This is because of the extremely uneven distribution of most elements within a given plant. Gold in jack pine can be used as an example. A 5-m-tall (25-year-old) tree from locality 1 (Fig. 1) was dissected into a number of components and analyzed for gold by neutron activation. Table 1 shows a range in ashed samples from 14 ppb in the inner w o o d (13--25 year old wood) to 140 ppb in scales of the outer bark. Clearly, the outer w o o d has more gold than the inner w o o d , and the outer bark more than the inner bark. Also the y o u n g twigs are richer in gold than the needles and older branches. Each plant organ yields a different amount of ash. There is more ash from the needles than the twigs, and from the bark than the wood. If values of gold in ashed material are converted to a dry weight basis these variations of gold within the jack pine example are emphasized (Table 1). Black spruce (Table 1) shows basically the same gold distribution as the jack pine, except the spruce needles are consistently depleted in gold. Young twig growth commonly has more gold than the old, reflecting the acropetal (i.e. tipward migration) tendency of gold (cf. Girling and Peterson,

25 TABLE 1 Gold content and ash yield of various parts of jack pine and black spruce from locality 1 (Fig. 1) Dry matter (ppt) a

Ash (ppb)

Ash (%)

2100 610 720 360 360 150 90 60 80 40

140 32 42 15 24 17 34 22 32 14

1.5 1.9 1.7 2.4 1.5 0.88 0.26 0.27 0.25 0.28

900 620 220 90 < 160

50 28 24 19 <5

1.8 2.2 0.93 0.47 3.2

Jack pine Outer bark Inner bark Total bark Needles Young twigs Old twigs Wood + bark Wood (no bark) Outer wood Inner wood Black spruce Bark Twigs Wood + bark Wood Needles

aThe gold content of the dry matter is expressed in parts per trillion, calculated from the analysis of gold in ash.

1978). Consistent with this observation is the tendency for the deciduous plants to have higher gold levels in their leaves than twigs. It should be noted, however, that this tendency is not universal. For example, douglas fir has more gold in sapwood than in bark (Erdman et al., in press; and J.A. Erdman, pers. commun., 1984). Twigs have proved to be the most practical sample medium, and a hierarchy of c o m m o n species has been established according to their sensitivity to gold. Alders top the list, followed by balsam fir, jack pine, birch, spruce, and willow. Labrador tea appears to be moderately sensitive, but insufficient data are available for confirmation. Bark could be a useful sample medium but it is more tedious to collect from deciduous trees than twigs. Some conifers have flaky outer bark which may prove to be an effective and practical sample medium. Forest litter has been used extensively in gold exploration in recent years, providing useful information which can be related to gold deposits (Curtin et al., 1968; Hoffman et al., 1979; Dunn, 1980, and in press; Hoffman and Brooker, in press). Forest litter, however, contains a mixture of plant organs from several species in various states of decay, and there are commonly adhering inorganic particles. As a result, the composition of material collected

26

is rarely known and, as shown in Table 1, gold in the vegetation is unevenly distributed. In effect, therefore, the forest litter may give a good broad indication of gold mineralization, but by sampling specific parts of a particular species much "cleaner" data should be obtainable. Two species of aider are c o m m o n in northern Saskatchewan; Alnus rugosa [Du Roi] Spreng. (river, speckled or tag alder) which is c o m m o n in wet areas, particularly along stream banks; and Alnus crispa [Ait.] Pursh (green or mountain aider) which is common in drier areas. These species are rarely found together, b u t similar gold concentrations were present in twigs of the two species collected within the same general area. In a preliminary compilation of gold biogeochemicai data the aiders were misidentified as being all Alnus rugosa (Dunn, 1983). A re-evaluation of reference samples and field notes has ascertained that the more ubiquitous Alnus crispa was collected, and this species represents over 90% of the samples reported in the studies dealing with alder in this paper. Sample collection

A 200-g composite sample of twigs (with needles or leaves) of similar appearance, length, diameter, and age was collected at each site. Tests to determine if gold accumulations vary with the age of twig, in the same manner as uranium (Dunn, 1982), indicate that there are variations in a single twig, but TABLE 2 Variations in the gold c o n t e n t o f twigs f r o m four c o m m o n species in the boreal forest (each c o m p a r i s o n based o n a single sample) Sample m e d i u m

P o r t i o n sampled

Black spruce (Picea mariana) twigs

1--3 yr. old 4--6 yr. old 7--9 yr. old +9 yr. old

Alder (Alnus twigs

crispa)

Birch (Betula sp.) twigs

(I~nus banksiana) twigs

Jack pine

Ash (ppb)

Dry matter (ppb)

Ash (%)

26 22 31 31

0.60 0.60 0.68 0.53

2.3 2.7 2.2 1.7

y o u n g : --3 m m thick older: 3--7 m m thick y o u n g : o u t e r 25 cm older: n e x t 25 c m y o u n g : o u t e r 25 cm older: n e x t 25 cm

42 61 140 386 19 7

1.13 1.22 2.73 5.37 0.57 0.21

2.7 2.0 2.0 1.4 3.0 3.0

y o u n g : --5 m m thick older: 5--10 m m thick

32 19

0.47 0.17

1.5 0.9

y o u n g : --1 cm thick older: +1 cm thick

24 17

0.36 0.15

1.5 0.9

27 patterns are not consistent (Table 2). It is, therefore, advisable to collect twigs of similar age at each sampling site in order to minimize problems of heterogeneity within the sample medium. In theory, this procedure is straight forward, but in practice problems arise. For example, annual growth nodes on spruce twigs are easy to discern, but on other conifers and deciduous species they may be less apparent. In order that biogeochemical surveys may remain practical, it is necessary to compromise between the careful time-consuming methods, and the less precise y e t rapid and effective methods of sample collection. A practical solution to sampling alder, willow, and birch is to collect the outermost 50 cm of twig growth: in northern Saskatchewan this usually constitutes three years growth. A convenient amount for spruce twigs is the latest 10 years growth; whereas jack pine and balsam fir have growth nodes that are less clearly defined, hence it is easier to collect the outermost 50 cm (usually 5--7 years growth). The important point is that for any survey the same a m o u n t of growth of a given species should be collected at each site. The distance between sampling stations varied from one area to the next, according to the objective of the study, the geology, and the type of mineralization. Each area will be discussed separately.

Sample preparation Samples were left in paper bags to partially air dry prior to removal of all moisture in a microwave oven. The dried samples were separated (leaves or needles from twigs, etc.) and about 50 g of twigs was weighed into aluminum trays. Samples were then ashed overnight at 470°C in a p o t t e r y kiln, and the ash weight recorded. Table 2 shows that younger growth tends to have a higher percentage of the ash than the older. This is because of the relatively high ratio of bark to w o o d in the thin outer growth of the twigs; bark has a higher ash yield than wood. Two alder samples have high ash yields because they were collected near old mine workings where dust contamination was probable. In general the ash yield of the latest 50 cm growth of alder twig is from 1.8--2.2%; higher yields are usually attributable to adhering dust particles. The same ash yield is obtained from spruce twigs, whereas balsam fir twigs yield 2.6--3.0% ash. Jack pine and birch twigs have only 1.2--1.5% ash.

Analytical methods Ashed samples of 0.5--1 g were preweighed into polyethylene vials, and submitted for gold analysis by instrumental neutron activation analysis (INAA) at Nuclear Activation Services Ltd. Depending upon the level of gold concentration, precision obtained on laboratory duplicates of ashed samples (that were n o t carefully homogenized) was c o m m o n l y perfect, and rarely greater than + 10%. Field replicates generated precision of better than

28

+ 20%. The detection limit was 10 p p b Au, with 5 ppb achieved on some batches. Samples of about 1 kg were required for platinum and palladium determinations, since 5--10 g of ash was needed for analysis by fire assay prior to plasma emission spectrometry. Duplicate ashes yielded precision of between + 5--30%. The detection limits were 2 ppb Pd and 10 ppb Pt. Gold volatilization

Gold may volatilize during ashing, particularly if the plant species is cyanogenic (Girling et al., 1979; Hoffman and Brooker, in press). To determine if gold and arsenic are lost from alder twigs during ashing, nineteen samples were divided into two portions: one portion was submitted for analysis by INAA as a briquette of dry material, and the other portion ashed prior to analysis by INAA. Data received from the ashed portion were recalculated to a dry-weight basis, and compared with data on gold and arsenic contents of the unashed material (Table 3). Results are similar over a wide range in concentrations, and evidently for alder there is little or no loss of gold or arsenic from the ashing process. The small variations between methods can be attributed to sample heterogeneity and the precision to be expected at these low levels of concentration. TABLE 3 Gold and arsenic content of alder twigs from several localities (see Fig. 1) Site

Cluff Lake a (Carswell dome) Box Granite a (Uranium City) Key Lake Road (SE of Cree L.) Anglo-Rouyn Cu Mine (N. Lac La Ronge) S. of La Ronge (Cretaceous sandstone)

n

Mean gold content (ppb)

Mean arsenic content (ppm)

Ash

Ash (recalc.)b

Dry

Ash

Ash (recalc.)b

Dry

4

152

2.38

1.53

0.95

0.06

0.06

6

79

1.17

0.98

0.92

0.02

0.02

3

38

0.83

0.58

0.60

0.01

0.03

6

17

0.52

0.57

1.45

0.04

0.05

1

15

0.40

0.40

0.80

0.02

0.04

aKnown gold mineralization (low grade). bConcentration in ashed sample recalculated to a dry weight basis, for comparison with data in " d r y " column which were obtained by direct analysis (INAA) of and irradiated 8-g briquette of dry material.

29 In the light of these tests it was felt that ashing of samples was preferable because: (a) a relatively large sample could be submitted for analysis since 50 g of dried twig produces 1 g of ash, compared to the 8-g briquette of dried material that is analyzed; thus the ashed sample should be more representative than the dried sample of the rather heterogeneous twig; and (b) preconcentration of the sample by ashing commonly elevates the metal concentration appreciably above detection limits. Hence, all values quoted are from ashed material unless otherwise stated. GOLD SURVEYS An orientation study, one case history, and a regional survey, all using alder twigs, are outlined in this section. In addition, results of three surveys using other sample media are presented in order to show that, although alder is preferred, other types of vegetation can provide useful information. The biogeochemist should enter the field armed with knowledge of the ideal sample medium, but should be prepared to use another medium in the event that the former is absent. Orientation study Alder was known to be more sensitive than other c o m m o n species to the presence of gold (Dunn, 1983). Hence, as part of the orientation phase, samples of alder twigs were collected in late May, 1983, from above auriferous soils and bedrock, and from areas where no gold mineralization was known or suspected. Table 3 summarizes the gold concentrations found in the ashed alder twigs. The data show that in the Cluff Lake (Carswell structure) and Box granite (Uranium City) vicinities {Fig. 1), the mean gold concentrations are appreciably higher than in the other three test areas. Results from these few alder samples were sufficiently encouraging to experiment further with local and regional surveys. Local survey Waddy Lake {Fig. 1, locality 2) - - A l d e r In the Waddy Lake area gold occurs in andesite close to the contact with intruded granodiorites. Visible gold is present in quartz veins and in shear zones containing chlorite and carbonate. Several zones of auriferous soils have been outlined, and one of these (the Riddle Zone) was selected as a test site. Early in August, 1983, alder twigs were collected at 21 sites set 50 m apart along three cut lines over the Riddle Zone (Fig. 2). Background gold concentrations in alder for that area are less than 10 ppb Au, hence several of the samples from the Riddle Zone are highly anomalous. There is not a good correlation with gold concentrations recorded previously from nearby soil sites, but this is to be expected because the gold is present as discrete

30

GOLD (ppb)

103 ° 53' 50"

Fig. 2. Gold concentrations (ppb) in ashed alder twigs from the Riddle Zone of goldenriched s o i l s - Waddy Lake (locality 2, Fig. 1 ).

gains. The soil sample comprises a handful from one spot, whereas the root system of the alder is sampling several cubic metres of soil and integrating in its aerial parts the geochemical signature of all the soil horizons plus dissolved gold in groundwaters. In this area, both soil and alder samples contained gold concentrations of sufficient magnitude to warrant closer investigation of the area. The biogeochemical m e t h o d has the advantage that it may give a more representative indication of gold in the environment than a spot soil sample, and a survey can be easily conducted in winter when there is snow cover.

Local survey Sulphide Lake (Fig. 1, locality 3) -- Balsam fir At this locality gold occurs in association with arsenopyrite in iron formation of exhalafive origin, beneath 1--5 m of fill. Here the d o m i n a n t species are balsam fir and black spruce, with very few alder. Balsam fir twigs were chosen as the principal sample medium. Sample spacing was at 12.5-m intervals and crossed trenched occurrences of arsenopyrite (near arsenic peaks, Fig. 3). The survey was conducted by Saskatchewan Mining and Development Corporation personnel in late February, 1984, at a time when there was a thick snow cover.

31 GOLD

(ppb) i2O-

I00~ ARSENIC (ppm) 40--8030--60-

\'\

20--4O"

,~.

i0--20.

GOLD (ppb) IOOq ARSENIC / (ppm) [ 40--801

30--601

x

/

1

2O--4O-4

FO--20 l

GOLD (ppb) 80ARSENIC (ppm) 30--60.

/

i/"

~

~!:::~' •

~ ~

GOLD ) 3 TIMES ARSENI 'BACKGROUND

20--40. 10--20

[00 m

Fig. 3. Gold (ppb) and arsenic (ppm) concentrations in ashed balsam fir twigs -- Sulphide Lake (locality 3, Fig. 1). Shaded areas represent those where concentrations are greater than 3 times background. Results (Fig. 3) suggest that background levels are about 10 ppb gold and 2 ppm arsenic in ash. The shaded areas indicate zones where concentrations of the two metals are greater than about three times background. Along each profile there is a pronounced gold anomaly, with an associated displaced arsenic anomaly. Some of the highest gold values occur at upland sites, and are n o t therefore attributable to down-slope drainage. This pattern may reflect either the greater mobility of arsenic away from the source of gold mineralization, or perhaps the arsenopyrite may prove to be displaced from

32 the main centre of gold mineralization in the bedrock. The zone of high biogeochemical gold has y e t to be trenched. These results are encouraging in that from a series of biogeochemical profiles it may be possible to outline a mineralized zone and effect considerable savings on trenching and drilling costs. Local survey Nicholson Bay (Fig. 1, locality 4) -- White spruce

On the east side of Nicholson Bay there is a 1-km 2 ultramafic body of possible komatiitic affinity (Sibbald et al., 1983). Field relationships are insufficiently detailed to establish whether the complex is intrusive or extrusive, and serpentinization and metamorphism to amphibolite facies have obscured primary internal structures. Chemically, the rocks are rich in magnesium, water, nickel and chromium, with gold values of up to 180 ppb in adjacent sulphide rich rocks (Sibbald et al., 1983). Gold occurs also in pitchblende veins of complex mineralogy and geochemistry. The vegetative cover is dominated by white spruce, with some black spruce, jack pine, ground juniper, alder, birch, and poplar. The most recent 10 years growth of white spruce twigs was sampled in mid-June, 1983, at 50 sites. Ashed samples yielded 10--15 ppb Au in most samples, but 25--30 ppb Au was present in several samples from near the contacts with sulphide-rich

o

o

59 ° 27I IO'L

LEGI

~ULT

~ ~

MAI ME" MIN

x

SA~ IN ASH OF WHITE

SPRUCE TWIGS 500

LAKE

m

ATHABASCA

Fig. 4. Gold (ppb) concentrations in ashed twigs of white spruce from the Nicholson Bay

ultramafic complex (locality 4, Fig. 1).

33 quartzites which comprise part of the metasedimentary sequence (Fig. 4). The marked exception was from the vicinity of the western mineral occurrence (Fig. 4), where the spruce twigs yielded 280 ppb Au. At that site poplar twigs contained 30 ppb Au; birch twigs 110 ppb Au; alder twigs 120 ppb Au, and the newly grown alder leaves yielded 1100 ppb Au. Local survey R o t t e n s t o n e Lake (Fig. 1, locality 1) --Black spruce

This locality was selected as a test site for platinum and palladium uptake by vegetation, and reference should be made to the section entitled "platinum biogeochemical studies" for details of the geology. The old mine working yielded small amounts of gold, hence samples were analyzed for this too. The gold distribution in the ashed twigs of black spruce (Fig. 5) shows anomalous levels down drainage to the southeast of the railings area. Most other samples contained low levels of gold, except for two samples in the northeast of the map with 76 and 68 ppb Au. These sites were near the base of a steep south-facing hill which contains a gossan.

X<2 X<2

i

X
X(2

X 76

X68

K29

X
X<2 ~'~.

u

X5

X7 Xl4

Llne~

TA,~IIN~SS ~) ~DS', / 24ox\ I )

I-I-

~'~2 O0

Line3 X<2

Xl3 X<2

\

(Jne ~ m 0

I00

200

300m

Fig. 5. Gold (ppb) concentrations in ashed twigs of black spruce from the Rottenstone Lake ultramafic body (locality 1, Fig. 1).

34

Evidently the black spruce is capable of concentrating gold, but it is less sensitive than other species and will only absorb appreciable quantities when a rich or readily leached source is available. Regional survey Figure 1 - - A l d e r An attempt was made to determine if analysis of aider twigs could be used as a regional exploration t o o l to help delineate relatively auriferous areas. In August, 1983, samples were collected at 2-km intervais northeastward from La Ronge (Fig. 1). In this region the rocks are predominantly metavolcanics, metasediments, and iron formations, intruded by felsic plutons. One interpretation of the geology as lithostructurai domains is shown on Fig. 6, although it should be noted that there exists some disagreement on these boundaries. For convenience, the domains will be referred to as out104°30 '

Fig. 6. Gold (ppb) concentrations in ashed twigs of alder: regional survey north of La Ronge (near southern boundary of Fig. 1).

35 lined here. Within the La R'onge domain gold concentrations in the alder twigs range from less than 10 p p b to 20 ppb Au: by taking values below detection limit as 5 ppb, the mean value is 8 p p b Au. By contrast, samples from the Glennie Lake domain range from 20 to 130 ppb Au, with a mean of 45 ppb Au. Possible explanations for this regional pattern are either (a) the extensively sheared rocks characteristic of the Glennie Lake domain are generally more auriferous than rocks of the La Ronge domain; or (b) the traces of gold that are present within the bedrock or soils of the Glennie Lake domain, occur in a form that is more readily accessible to absorption by the plant roots than the gold traces that occur in the La Ronge domain. On the basis of these data, further biogeochemical investigation of the Glennie Lake domain seems warranted. PLATINUM BIOGEOCHEMICAL STUDIES In Saskatchewan platinum has been recorded from several ultramafic bodies. Two occurrences, Rottenstone and Nicholson (Fig. 1, localities 1 and 4) were selected as biogeochemical test sites to determine the distribution of platinum (Pt) and palladium (Pd) in a variety of common plants. Minimal information was obtained from the Nicholson samples because of suspected contamination by the analytical laboratory: duplicate samples and standards indicated that the data were spurious, and the only information for which there is a moderate degree of confidence is from white spruce twigs, collected near a mine shaft at the western mineral showing (Fig. 4), which yielded 80 ppb Pt and 80 ppb Pd. It seems probable that twigs of poplar, birch, juniper and alder from the same site contained between 20 and 80 ppb of the two metals. The Rottenstone (Hall showing) nickel-copper-platinum-palladium deposit consisted (prior to being worked out in 1968) of about 50% sulphides disseminated in a body of pyroxene-rich rock surrounded by gneissose metasediments (Richards and Robinson, 1966). The body formed a rusty hill and had an average thickness of 8 m. It has been interpreted as a r o o f pendant within the migmatitic complex (cf. Gilboy, 1975). Several boulders of oregrade rock containing pyrrhotite (FeS), violarite (Ni~FeS4), and chalcopyrite (CuFeS2) were analyzed for precious metals to provide a general comparison with the vegetation analysis (however, no vegetation sample was found rooted in or close to a boulder of high grade ore). Concentrations in the rocks were 1200--3600 ppb Pt, 3500---6900 ppb Pd, 20--67 ppb Rh, and 210--340 ppb Au. Platinum is reported to occur as minute grains of sperrylite (PtAs~). Late in May, 1983, vegetation samples were collected from 22 sites in the general vicinity of the worked-out deposit. Spruce twigs were collected at all sites, and samples of several species at a selected test site on disturbed ground above the old mine. Data obtained from samples at the test site

36 TABLE 4 Platinum and palladium c o n t e n t o f plants growing in a disturbed site above a disused nickel-copper-platinum-palladium mine ( R o t t e n s t o n e - - l o c a l i t y 1 on Fig. 1). Samples from w i t h i n a 5 × 5 m a r e a

Black spruce twigs Black spruce needles Black spruce trunk Jack pine twigs Jack pine needles Jack pine trunk Labrador tea stems Birch twigs Willow twigs

Pt(ppb)

Pd(ppb)

80 10 70 50 20 70 80 < 10 < 10

120 10 62 52 17 120 170 7 3

i XS0

X
X2o

X3O

v

<40 XeO

Xso

X40

W ~'~

v~.CLE,~F~ING~X30 X3o .]HEADFRAME

I--

X 120

~.~

TAILINGS ~ )

~DS

,

~o x ~I

Line,2

f

J

L ~ne 3

X I0

X~io X2O X30-LIne~

'

Fig. 7. Platinum (ppb) concentrations in ashed twigs of black spruce from the Rottenstone Lake ultramafic b o d y (locality 1, Fig. 1).

37

i

J'L i+r X~

X12 X I0 X8

X9

×6

LU

X 19 X25 ~

Xll X21

- CLEARING ~ "'3o (" ~ ~, "~ XX~ ),'~ XsT X64

Line 2 TAI' INGS NDS-~.\ ~

,,oox')

J

(

~../~'1350 440 L ~'~e 3

X17 X4

XlO

Fig. 8. Palladium (ppb) concentrations in ashed twigs of black spruce from the Rottenstone Lake ultramafic body (locality 1, Fig. 1).

(Table 4) show that the media most effective in concentrating Pt and Pd are twigs and trunk of black spruce and jack pine, and labrador tea stems. Twigs of birch and willow, and the conifer needles do not concentrate the metals to any significant degree. Concentrations of up to 880 ppb Pt and 1350 ppb Pd are present in ashed twigs of black spruce growing down drainage from the tallings debris (line 3 on Figs. 7 and 8). In addition, there are elevated concentrations of Pt and Pd in plants from a hill immediately north of the old mine (line 2), and two sites with 50 ppb Pt along line 1. SUMMARY A N D CONCLUSIONS

Chemical analysis of c o m m o n plant species in the northern forests of Canada has demonstrated the heterogeneous distribution of precious metals within a single plant, and among plant species. In the conifers studied, gold is more concentrated in bark than wood, and twigs than needles; however,

38 deciduous trees seem to have more gold in the leaves than the twigs, although concentrations may vary within the growing season. In general, twigs provide the most practical sample medium, since: (a) they are sensitive to the presence of gold in the substrate; (b) they are easy to collect; (c) they are more chemically stable than leaves or needles; and {d) they may be collected in the winter. The sensitivity of twigs of common species to gold is, in decreasing order: alder (Alnus crispa), balsam fir (Abies balsamea), jack pine (Pinus banksiana), birch (Betula sp.), spruce (Picea glauca and Picea mariana), and willow (Salix sp.). Gold values obtained from orientation surveys and four small-scale local surveys demonstrate the relationship of biogeochemical anomalies to known mineralization, and in one case outline a previously unknown zone of potential gold mineralization. A regional survey has delineated an area, coincident with a lithostructural domain, of relatively high gold concentrations in alder twigs. Orientation surveys to provide information on platinum and palladium distributions in vegetation have shown relative enrichment of the two metals in twigs and trunk of black spruce and jack pine, and in stems of labrador tea. The data suggest that these sample media might be useful in helping to locate platiniferous bedrock concealed by a veneer of glacial material. Beneath the boreal forests of the world there are extensive areas where a veneer of glacial drift masks the bedrock, and appraisal of the mineral potential of the area is rendered difficult. Similarly, in deeply weathered terrains there is a need for a cheap device for appraising the metal content of the bedrock. Careful development of biogeochemical techniques can establish a framework from which practical sample media can be selected by the exploration geologist, to assist in the increasingly difficult task of locating mineralization. Plants are efficient geochemical sampling tools which may provide " w i n d o w s " through the surface cover to the underlying bedrock. As yet, n o t all the processes are understood which govern the uptake of metals by plants. In many areas of the world biogeochemical techniques are neglected if n o t totally unused. By steadily increasing the data bases and researching the patterns and controls of element uptake by plants, biogeochemistry should become firmly established as an additional exploration tool to the geologist. ACKNOWLEDGEMENTS I thank the Saskatchewan Mining Development Corporation (SMDC), Waddy Lake Resources, and Amok Ltd. for their co-operation in this study, and for permission to publish some of the data. Sampling of vegetation over the Sulphide Lake area was conducted by Dr. V. Sopuck and R. Chapman (SMDC). The study has benefitted greatly from the efficient assistance of P. Schwann and G. Buller.

39

I acknowledge with thanks the useful discussions with, and the critical review by Dr. D.F. Paterson, and Dr. J.A. Erdman. This project was funded by the Saskatchewan Department of Energy and Mines. ADDENDUM

After completion of this manuscript data were received from two recent biogeochemical surveys conducted in the northern forest of Manitoba. Alder twigs were collected by G. Meyer over gold showings, and analyzed for gold by the procedures outlined in this paper. The results obtained are pertinent to this study. (a) Sparky Project, Wekusko Lake (Red Earth Energy Ltd.), near Snow Lake, Manitoba. Gold concentrations in ashed alder twigs ranged from 40 to 590 ppb with a mean of 182 ppb and a standard deviation (S.D.) of 122. The biogeochemical response to high grade gold in two trenched quartz samples (>1.5 oz Au/ton) ranged from 90 to 120 ppb Au. Several alder samples within 100 m of these trenches yielded in excess of 300 ppb Au, with two samples returning values of over 500 ppb Au. No outcrop is present within this biogeochemical anomaly, nor has any trenching taken place. (b) Laguna Mine Project, Wekusko Lake (Wekusko Gold Resources Ltd.), near Snow Lake, Manitoba. Gold concentrations in ashed alder twigs ranged from 30 to 340 ppb. Over one zone the mean was 181 ppb Au (S.D. = 58), and over a second zone the mean was 109 ppb Au (S.D. = 58). The alder samples clearly outlined a 30-cm-wide auriferous quartz vein for a length of 300 m. The biogeochemical anomaly appears to be at least 280 times wider than the gold source. Willow twig samples from the same region contained 20--130 ppb Au but showed no spatial relationship to known mineralization. I thank the above-mentioned companies and Alex Harris for providing me with these data and granting permission to publish the information.

REFERENCES Boyle, R,W., 1979. The geochemistry of gold and its deposits. Geol. Surv. Can. Bull., 280, 576 pp. Brooks, R.R., 1982. Biological methods of prospecting for gold. J. CJeochem. Explor., 17 : 109--122. Carlisle, D., Berry, W.L. and Kaplan, I.A., in press. Organic Matter, Biological Systems and Mineral Exploration. Rubey Vol. V, Prentice Hall, New York. Curtin, G.C., Lakin, H.W., Neuerburg, G.J. and Hubert, A.E., 1968. Utilization of humusrich forest soil (mull) in geochemical exploration for gold. U.S. Geol. Surv. Circ., 562: 1--11. Dunn, C.E., 1980. Gold biogeochemistry investigations. In: Summary of Investigations 1980, Saskatchewan Geological Survey. Sask. Dep. Miner. Resour. Misc. Rep., 80-4: 81--85. Dunn, C.E., 1982. The massive Wollaston Uranium Biogeochemical Anomaly in the boreal forests of northern Saskatchewan, Canada. In: Uranium Exploration Methods:

40 Review of the NEA/IAEA Research and Development Programme, Paris, France, 1--4 June, 1982; OECD Nuclear Energy Agency, pp. 477--491. Dunn, C.E., 1983. Biogeochemical investigations in northern Sasketchewan: preliminary data on tungsten, gold, platinum, rare earths and uranium. In: Summary of Investigations 1983, Saskatchewan Geological Survey. Sask. Dept. Energy and Mines, Misc. Rep., 83-4: 106--122. Dunn, C.E., in press. The application of biogeochemical methods to mineral exploration in the boreal forests of central Canada. In: D. Carlisle, W.L. Berry and I.A. Kaplan (Editors), Organic Matter, Biological Systems and Mineral Exploration. Rubey Vol. V, Prentice Hall, New York. Erdman, J.A., Leonard, B.F. and McKown, D.M., 1985. A case for plants in exploration: Gold in douglas-fir at the Red Mountain stockwork, Yellow Pine District, Idaho. U.S. Geol. Surv., Open-File Rep. 85-117. Fuchs, W.A. and Rose, A.W., 1974. The geochemical behaviour of platinum and palladium in the weathering cycle in the Stillwater Complex, Montana. Econ. Geol., 69: 332--346. Gilboy, C.F., 1975. Foster Lake area: reconnaissance geological mapping of 74A-6E, -7, -8W, -9W, -10 and - l l E . In: Summary of Investigations 1975, Saskatchewan Geological Survey. Sask. Dep. Miner. Resour., 29--34. Girling, C.A. and Peterson, P.J., 1978. Uptake, transport and localization of gold in plants. Trace Subst. Environm. Health, 12: 105--108. Girling, C.A., Peterson, P.J. and Warren, H.V., 1979. Plants as indicators of gold mineralization at Watson Bar, British Columbia, Canada. Econ. Geol., 74: 902--907. Hoffman, E.L., Brooker, E.J. and White, M.V., 1979. The determination of gold by neutron activation analysis. In: R.D. Morton (Editor), Proceedings of the Gold Workshop, Yellowknife, N.W.T. Hoffman, E.L. and Brooker, E.J., in press. Biogeochemical prospecting for gold with reference to some Canadian gold deposits. In: D. Carlisle, W.L. Berry and I.A. Kaplan (Editors), Organic Matter, Biological Systems and Mineral Exploration. Rubey Vol. V, Prentice Hall, New York. Jones, R.S., 1970. Gold content of water, plants and animals. U.S. Geol. Surv., Circ., 625: 1--15. Konstantinova, I.M., 1981. The influence of plants on gold contents in natural water (Abstr.). Second Int. Symp. on Methods of Applied Geochemistry, Irkutsk. Vinogradov Geochemistry Institute, II: 93. Kovalevskii, A.L., 1979. Biogeochemical Exploration for Mineral Deposits. Amerind Publ. Co., New Delhi, 136 pp. Richards, B.R. and Robinson, B.G.W., 1966. Mining and milling a small ore d e p o s i t Rottenstone Mining Limited. Can. Min. Metall. Bull., 59: 1423--1428. Riese, W.C. and Arp, G.K., in press. Biogeochemical prospecting in the Stillwater (Pt) Complex, Montana. In: D. Carlisle, W.L. Berry and I.A. Kaplan (Editors), Organic Matter, Biological Systems and Mineral Exploration. Rubey Vol. V, Prentice Hall, New York. Schiller, P., Cook, G.B., Kitzinger-Skalova, A. and Wblfl, E., 1973. The influence of the season variation for gold determination in plants by neutron activation analysis. Radiochem. Radioanal. Lett., 13: 283--286. Shacklette, H.T., Lakin, H.W., Hubert, A.E. and Curtin, G.C., 1970. Absorption of gold by plants. U.S. Geol. Surv., Bull., 1314-B: 1--23. Sibbald, T.I.I., Schwann, P.L. and Dunn, C.E., 1983. Uranium-gold metallogenic studies: Nicholson Bay Ultramafic Complex. In: Summary of Investigations 1983, Saskatchewan Geological Survey. Sask. Dep. Energy and Mines, Misc. Rep., 83-4: 75--79. Warren, H.V. and Delavault, R.E., 1950. Gold and silver content of some trees and horsetails in British Columbia. Geol. Soc. Am. Bull., 61: 123--128.