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Application of till geochemistry to gold exploration, Ilomantsi, Finland
Journal of Geochemical Exploration, 39 ( 1991 ) 323-342
323
Elsevier Science Publishers B.V., Amsterdam
Application of till geochemistry to gold exploration, Ilomantsi, Finland A. Hartikainen and M. Damsten Geological Survey of Finland, P.O. Box 1237, SF-70701 Kuopio, Finland (Received January 18, 1990)
ABSTRACT Hartikainen, A. and Damsten, M., 1989. Application of till geochemistry to gold exploration, llomantsi, Finland. In: A.J. Bj6rklund (Editor), Gold Geochemistry in Finland. J. Geochem. Explor., 39: 323-342. The Archean bedrock of the Ilomantsi area consists of narrow greenstone belts between dome-like granitoids. The main tectonic phases are isoclinal folding, dragging of isoclinal folds around subvertical axes and regional faulting. Conspicuous anomalies of As, Au, Mo and W were revealed in the course of regional geochemical mapping of till, which led to more detailed studies in Ilomantsi. The data presented in this study are based mainly on the results of 2960 till samples collected by light percussion drill at local scale ( 16 samples/km 2), but also on the results of regional ( 1 sample/4 km 2) and detailed ( 100-400 samples/km 2) sampling and on rock samples collected from outcrops. The analytical methods most commonly used were emission spectrometry, atomic absorption spectrophotometry and spectrometry. The results indicate that the lowest part of the basal till is dominated by local debris and that the geochemistry of the till reflects well the underlying bedrock. The highest Au values exist near the contacts of tonalite and supracrustal rocks. Arsenic follows intimately the directions of faulting. The anomalous contents of W and Mo are restricted to the southern part of the 15-kin 2 Kuittila tonalite. Potassium, Na and Ca form conspicious anomalies in till, cutting both the supracrustal rocks and the granitoids. Local studies revealed in the Kuittila tonalite a notable network of Mo- and W-bearing quartz veins restricted to tensional zones of main fault directions, which are cut by gold-bearing shear zones and quartz vein nets. These shear zones are ofen echelon type and they are silicified, sericitized, tourmalinized, Na-depleted and partly carbonitized. The hosting tonalite is intensely biotitized. These metasomatic features of the bedrock can be correlated to the wider anomalies of K, Na and Ca in till. The implication is that significant migration of mobile elements has taken place in the bedrock of the study area and that, in places, gold has participated in these processes.
INTRODUCTION
The Archean bedrock of eastern Finland resembles that of Canada, famous for its numerous gold ores (Boyle, 1987; Latulippe, 1982; Kerrich, 1983; Hutchinson, 1986). Although Au occurs unevenly in both bedrock and till, the results of the geochemical studies of till have been encouraging (Fortescue, 1983; Nichol, 1986). 0375-6742/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
324
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APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
325
Since the early 1970s systematic till geochemistry has proved to be a useful tool for prospecting in Finland (Kauranne, 1975; Gustavsson et al., 1979). Because of the lack of precise analytical methods, some elements important for prospecting were not analysed routinely in the early years. However, with the increasing sophistication of analytical methods it has now become possible to make quick, economical and reliable analyses of certain elements, gold in particular (Kontas, 1981; Kontas et al., 1986). The geochemistry of till at different scales has revealed many promising anomalies and anomalous areas in Ilomantsi, eastern Finland (Salminen and Hartikainen, 1986). Knowledge of the mineralizations and their characteristics has been valuable for the studies in this area. The present study focuses on local and detailed features in the geochemistry of till in prospecting at Ilomantsi (Fig. 1 ). Local mapping in particular cover extensive area and permit preliminary models for the behaviour of elements important in gold prospecting to be presented. The geochemistry of till has also been of great importance for bedrock mapping in these areas of sparse outcrops. The feasibility of the method in finding different Au-mineralizations is discussed. GEOLOGICALSETTING Bedrock
The Ilomantsi area is part of the west Karelian granite-greenstone terrain of Archean age (Lavikainen, 1973; Krylov et al., 1984). The greenstones rim the granitoid blocks as narrow but more or less continuous belts (Fig. 1 ). They consist of metavolcanics (mainly dacitic-andesitic), metaconglomerates, metagreywackes and mica schists with interbeds of black schists and banded iron formations (BIF). The granitoids include all types from granite to quartz diorite. Intermediate porphyritic dykes intersect the supracrustal rocks and, as the youngest rocks, the Proterozoic diabases cut the whole Archean suite. Three aspects of particular structural interest are: ( 1 ) The isoclinal folding linked to intrusions of synkinematic granitoids. (2) Dragging of isoclinal phase. During this phase gliding along subvertical S-plane led to folding of the incompetent strata of the supracrustal rocks while the competent strata underwent intense fracturing and mylonitization. (3) Approximately N-S-directed faulting and conjugate tensional shearing and fracturing. The regional character of these is most clearly seen on topographic maps and on aerial photomaps. The Proterozoic diabases follow these tensional directions, showing late activity along these structures. Still later activity is seen as shearing of medadiabases in a NE direction.
326
A. HARTIKAINENAND M. DAMSTEN
Overburden Outcrops are sparse in Ilomantsi area as a whole. The few areas with adequate outcrops are situated outside the Kuittila zone of prospective interest. In the Kuittila area, the overburden is till, concealed between hillocks and morainic forms by bogs. The thickness of overburden varies between 1 and 12 m, the average being 5 m. Weathered bedrock overlain by till is common, especially in the areas of supracrustal rocks. The main direction of ice flow is reflected by drumlinoidal forms trending NW. The striations are mainly in the same direction, although some older striations from west to east have also been observed. Quarternary studies have revealed one till bed only. The transport distance of main part of basal till is short, some hundreds metres or even less, but the boulders on the surface of the till have been transported various distances, many of them at least a few kilometres (figs. 17-19 in Salminen and Hartikainen, 1985 ). The forms of deglaciation encountered are h u m m o c k y and DeGeer moraines and a few eskers. METHODS
Till samples (100-200 g) were collected during regional studies from the surface (average 1.5 m ) with percussion drills. The sampling distance was 100 m on lines 1-2 km apart. 5-15 successive samples were made into a composite for all analyses. Only the finest fraction of the till ( < 0.06 m m ) was analyzed (Tables 1 and 2). During local and detailed studies samples were taken from the till/bedrock interface in order to obtain local information on the bedrock. The local scale mapping covers an area of 185 km 2 including the vicinity of the Kuittila target. The total number of sampling sites was 2960 (Table 1 ). Detailed studies involved till/bedrock sampling, bedrock exposing, and diamond drilling. At local and detailed scales, regular sampling grids of 250 m X 2 5 0 m and 50 m X 50 m were used. To save expenses four neighbouring samples were combined for analyses. Also in these scales the finest fraction was analysed. For TABLEI The size of areas, sampling densities and sampling depth at different scales during till geochemical mapping at Ilomantsi
Region al scale Local scale Detailed studies
Size of area (kin 2)
Sampling density (sample/km:)
Average sampling depth (m)
2000 185 2
0.25-0.40 16 400
I. 5 4.5 5.0
327
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
TABLE2 Analytical methods used for the samples collected during geochemical mapping at different scales at Ilomantsi. I C A P = i n d u c t i v e l y coupled plasma spectrometry, L e c o = s u l p h u r titrator, OES=optical emission spectrometry, spectroph. = spectrophotometry, GFAAS = flameless atomic absorption spectrophotometry, FAAS = atomic absorption spectrophotometry, NAA = neutron activation analysis, EDX = energy dispersive spectrophotometry. For different scales see Table 1
Regional scale Local scale Detailed studies Rock samples 1= 2= 3= 4= 5= 6= 7= 8=
ICAP Leco OES Spectroph. GFAAS FAAS NAA EDX
1
2
x
x
3 x
(x)
4
5
6
7
8
x x x (x)
x x x x
x
x
x
composite of 5-15 subsamples composite of 5-15 subsamples composite of 3-15 subsamples composite of 3-15 subsamples
27 elements S 19 elements W Au,As,Mo Si,A1,Fe,Mg,Ca,K,Na 14 elements Ba
comparison, 50 rock samples were collected from different rock types. The analytical methods used at the different scales are listed in Table 2. The data of local scale are presented in greystone maps. The darkness of each 100X 100 m grid is based on weighted moving median (BjiSrklund and Lummaa, 1983; BjiSrklund and Gustavsson, 1987). The radius of the "'window" used is 800 m for Au, As and W and 500 m for other elements. It occurs about ten sampling sites in each window.
RESULTS
Earlier studies
Peat investigations conducted in the 1970s indicated that parts of the area are anomalous for As (Minkkinen and Yliruokanen, 1978). Prospecting for Mo, W and Au in the study area was the subject of a recent report, which describes mainly the methodology of till geochemistry in prospecting (Salminen and Hartikainen, 1986 ). In that paper the authors emphasize that excellent pathfinder for Au in regional scale in Ilomantsi is As, the anomalies of which crosscut the lithological boundaries and follow the directions of tensional shearing (Fig. 2 ).
328 ! °
A. HARTIKAINEN AND M. DAMSTEN
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Results in local scale Correlation between till geochem&try and bedrock. The rock samples collected in the Ilomantsi area for correlation purposes are classified as granites, granodiorites, metabasalts, metagreywackes and metadiabases. The metagreywackes c o m m o n l y have interlayers o f tuffitic material. The essential geochemistry is listed in Table 3. In this study, Ni reflects best the geochemistry of the bedrock (Table 3 and
329
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
TABLE 3 Average contents of certain elements in barren rocks at Ilomantsi, n = number of samples. Analysed by ICAP and AAS methods Rock type
n
K20 %
Na20 %
CaO %
Au ppb
As ppm
Mo ppm
Cu ppm
Ni ppm
Granites Granodiorites Metabasalts Metagreywackes Metadiabases
9 16 6 14 5
3.6 2.3 0.8 2.2 0.8
4.6 4.4 2.4 3.0 2.6
1.7 4.4 10.8 2.9 8.8
0.3 0.6 0.8 1.5 1.2
0.0 0.3 5.5 12.0 0.7
1.5 0.5 3.7 1.0 0.3
12 18 74 48 120
6 16 45 66 30
Lsi
till 0 6 mm
)thed data Ni ppm
31" 10' Fig. 3. D i s t r i b u t i o n o f Ni in till in the study area. Analysed by OES.
330
A. HARTIKAINEN AND M. DAMSTEN
Fig. 3). In general, the anomalous patterns are above supracrustal units, whereas the areas of low contents refer to granitoids. Copper has similar patterns to Ni, although there are weak positive anomalies above the granitoids, too (Fig. 4). The Kuittila tonalite within the schists can easily be detected by these elements, the tonalite being surrounded by distinctive anomalies of Ni and Cu. Problems with interpretation are aggravated by the intense migmatizing effect of the granitoids at the eastern contact of the greenstone belt.
Geochemistry related to ore-forming processes. The elements that are related to the mineralizations at Ilomansi exhibit the following features: Gold follows mainly the contact zone of the supracrustal rocks and biotite tonalite, but high Au contents in till occur above banded iron formations, too. High K is bound to the Au-anomalous contact zone especially in the Kuittila area (Figs. Lsi
till 0 6 mm
}thed data Cu ppm
31 ° 10'
Fig. 4. Distributionof Cu in till in the study area. Analysedby OES.
3 31
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
1 and 5 ). Molybdenum (not shown) and W (Fig. 6) are closely associated with the Kuittila tonalite. A weaker, separate W anomaly without Mo southeast of Kuittila inside the supracrustal rocks may indicate hydrothermal activity of underlying granitoids. Arsenic forms distinct angular anomalies in till parallel the main faults and shears (Fig. 7 ). However, As content seldom exceeds 100 ppm in the till of the study area. Metasomatic alteration features typical of Archean gold mineralizations can be examined by the behaviour of K, Na and Ca (Kerrich, 1983; Studemeister, 1985 ). Silicification, sericitization, tourmalinization, Na-depletion, carbonatization and, in places, sulphidization and biotitization are common in the mineralizations of this area. Hence, high K and low Na in till could show areas of metasomatic alteration. In the Kuittila area there is an eUipsoidal ring of high K anomaly in till
i
ill 3 mm
:1 d a t a ,u ppb 20 11 6
3 2 1
31 ° 1 0 ' Fig. 5. D i s t r i b u t i o n o f Au in till in the study area. Analysed by GFAAS.
332
A. H A R T I K A I N E N A N D M. D A M S T E N
Ilomantsi
W
in till
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smoothed data W ppm E 1
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38 2B 21 1 B
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31 ° 10' Fig. 6. Distribution of W in till in the study area. Analysedby spectrophotometer. (Fig. 8 ). The 5 × 10 km anomaly intersects both the granitoids and the supracrustal rocks. Its position and shape bear a close resemblance to that of W (Fig. 6 ). At least in Kuittila the K anomaly can be explained by intense biotitization of the Kuittila tonalite. The distribution of Na (Fig. 9 ) in till is quite different from K and W. Low Na contents in till are partly above the supracrustal rocks, which have low Na contents (Table 3), but the most conspicious feature is the straight NWtrending positive and negative anomalies cutting different rock types. The distribution of Ca is not related to any particular rock type (Fig. 10 ), in spite of the fact that main rock types regionally have differences in Ca contents (Table 3 ). High Ca contents in till form a circle independent of rock types with a diameter of 10 km in Kuittila and east of Kuittila. It is apparent that there has been substantial migration of mobile elements
333
APPLICATION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, 1LOMANTSI, FINLAND
|tsi
till ).06 mm
)othed data As ppm 38.6 25.9 17.1
11.3 7.5 5.0 3.3 2.2
31 ° 10' Fig. 7. Distribution of As in till in the study area. Analysed by GFAAS.
in the bedrock of the study area. It seems that in spite of the fact that K, Na and Ca are main elements of the bedrock, they reflect metasomatic features better than lithological units in this area. It is evident that the heat of the granitoid intrusives caused the migration of certain elements and is, at least partly, responsible for metasomatic alteration in local scale in the bedrock. Detailed survey The local studies showed that the Kuittila area is a promising exploration target. Bedrock mapping of this area gave poor results because of the lack of outcrops, and geophysical methods did not give the results expected either. Gold, M o and W in till. Detailed till sampling was carried out on a regular
334
A. H A R T I K A I N E N A N D M. D A M S T E N
tsi
till 0 6 mm
~thed d a t a K
%
31 ° 10'
Fig. 8. Distribution of K in till in the study area. Analysedby OES. sampling grid of 50 m. The strongest anomalies of this scale were still followed-up at 5-m intervals along profiles. Samples were taken from the till/ bedrock interface in order to procure both till samples and small fragments of bedrock. The two samples were analysed separately (Salminen and Hartikainen, 1986). On the 50-m grid the rather uniform local Au anomaly split into smaller anomalies (Fig. 11 ) that follow the trend of Au-mineralized shears. The highest Au value on the detailed sampling profiles was 400 g/t; values above 100 ppb were common. In detail, the Au values in till denoted precisely the locations of the mineralizations. Molydbenum and W anomalies follow those of Au, but are considerably wider. The highest values for both elements were above 1000 ppm, the average being 14 (Mo) and 27 ppm (W), respectively.
335
APPLICA! ION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, ILOMANTSI, FINLAND
l$i
till 06 mm
)thed data Na % 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5
31" 10' Fig. 9. Distribution of Na in till in the study area. Analysed by OES.
As a whole, the till geochemistry shows that the main Au anomalies are situated at the western side of the W and Mo anomaly.
Mineralizations in bedrock. Exposed bedrock revealed two kinds of mineralization within the tonalite in Kuittila. Tectonics plays a decisive role in controlling the position of both. The best example is a lensoidal entity of NWtrending tensional quartz veins along the controlling fault with a NE orientation. The ore minerals are scheelite and molybdenite; the contents of Au are low. In this fault zone there are also later both tensional and parallel quartz veins without important ore minerals. Gold is controlled by shear zones and quartz veins trending NW. These cut the Mo-W-bearing quartz veins at a low angle. In shear zones the tonalitic host rock has completely altered into quartz and sericite with minor biotite, tourmaline, secondary K-feldspar, futile and apatite. The contacts with bio-
336
A. HARTIKAINEN AND M. DAMSTEN
:lata %
3.4 3.1 2.9 2.6 2.4 2.2 2.0
1.8
31 ° 10' Fig. 10. Distribution of Ca in till in the study area. Analysed by OES.
titized tonalite are sharp. Gold exists as native mainly among tellurides and sulphides. There are also some molybdenite, pyrite, galena and calcopyrite. The lithogeochemistry of one of these shear zones is presented in Figure 12. High peaks of Au and Si are accompanied by depletion of Na and Ca. The tonalitic host rock of this shear zone is strongly enriched in K because ofbiotitization. There are high contents of W in the eastern side of the shear zone in tonalite because of older Mo-W-bearing quartz veins. Comparing the contexts of the immobile elements A1, Sc, Hf and Zr in the host rock and in the altered Au-enriched shear it is evident that about two-thirds of the material in this shear has an external source (Table 4 ). To the west of the aforementioned Au occurrence the strongest Au anomaly was checked by diamond drilling. Wide zones of quartz-sericite-rock were found, the m a x i m u m width being 10 m. The Au content of these zones varies;
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
KUITTILA Au Till fraction-0.06 mm
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Fig. 11. Distribution of Au in Kuittila, Ilomantsi, Analysed by GFAAS.
m
337
338
A. HART1KAINEN AND M. DAMSTEN
Tonalite K
3% K No Co
Shear zone
T o n a l i t e with q u a r t z veins
_ o ~
o
~Na
Ca
1%
z,O % 36 % 1000 ppb
32 % Si
28 %
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Fig. 12.Lithogeochemistry of a gold-beating shear-zone, Kuittila, llomantsi. Analysed by GFAAS (Au,As),FAAS(aqua regia and HF leaching, K,Na,Ca,Si)and spectrophotometer (W) methods. some are completely barren, whereas others have evenly distributed Au contents of a few g/t. The Au values of 2 g/t are common, the best being 4.5 p p m / 4 m core. Typical of the latter mineralization is low sulphide content, average < 0.1% S. The Au-As anomaly northwest of Kuittila was checked also by diamond drilling. Narrow quartz-tourmaline veins with some arsenopyrite associated with porphyry dykes of tonalitic composition were found in the schists. The Au content of these dykes is up to 7 g/t.
APPLICATIONOF TILL GEOCHEMISTRYTO GOLD EXPLORATION,ILOMANTSI,FINLAND
339
TABLE 4
Average contents of elements in the Ilomantsi batholite ( 1 ), the Kuittila tonalite (2) and the goldbearing shear zone in tonalite (3) at Kuittila. For abbreviations see Table 2 1 n= 1 58.6 14.2 4.0 1.8 2.9 5.8 2.2
2 n = 6 or_8
3 n = 3 or 12
SiO2 A1203 FeO MgO CaO Na20 K20
(%) (%) (%) (%) (%) (%) (%)
59.5 14.2 3.8 2.2 2.7 3.0 4.2
79.2 6.4 2.3 0.5 0.1 0.1 2.8
FAAS FAAS FAAS FAAS FAAS FAAS FAAS
Cs Hf Sc Zr Rb
(ppm) (ppm) (ppm) (ppm) (ppm)
-
9.8 3.2 13.0 108 104
1.8 1.2 4.6 44 43
NAA NAA NAA NAA NAA
Sr V
(ppm) (ppm)
530 -
514 86
20 140
ICAP ICAP
W Ba
(ppm) (ppm)
0 1000
266 982
16 375
Spectrophotometer EDX
Cu Pb Mo As
(ppm) (ppm) (ppm) (ppm)
12 16 0 i
24 16 141 2
19 127 197 52
FAAS FAAS FAAS FAAS
Au
(ppb)
0
33
5460
GFAAS
Not noteworthy differences: Ti,Li,Co,Cr, Ni,Zn,Ag, Bi,Sb,Cd,P,Ta,Th,Y
CONCLUSIONS
The geochemistry of till reflects well the bedrock of the study area, and the following features can be recognized: ( 1 ) The lowest part of the basal till is local; the transport distance is negligible. Nickel and Cu particularly give reliable indications of the underlying bedrock. They reflect better the geochemical differences of the main rock units in the study area than for example main elements (Figs. 1, 3 and 4). (2) The most prominent Au anomalies follow the contact zone between the tonalite and the supracrustal rocks. The eastern anomaly area is connected with basic metavolcanites not yet studied. In the areas of iron formations the Au values are extensively anomalous because the contents in the bedrock are clearly above background values (Figs. 1 and 5 ).
340
A. H A R T I K A I N E N A N D M. D A M S T E N
(3) The ellipsoidal ring of high K is associated with barren areas of Na ( Fig. 13 ). The connection between high K contents in till and biotitization in the environment of mineralizations is clear (Fig. 8 ). At Kuittila, both a positive K anomaly and negative Na (Fig. 9 ) anomaly in till and in bedrock are connected with the Au anomaly. This evidently reflects hydrothermal activity related to ore-forming processes. Also, Ca forms large-scale anomalies intersecting the rock contacts, thus indicating a migration of this element (Fig. 10). (4) Molybdenum and W are restricted to the southern half of the Kuittila tonalite (Fig. 6 ). The erosional level in the northern half of the tonalite obviously differs from that in the southern half. ( 5 ) Arsenic is a good pathfinder for Au in regional and local surveys (Figs. 2 and 7 ). At Ilomantsi it seems to be associated with supracrustal rocks but also with tectonic zones of NW and NE orientation. Faults and shears in these directions cut all the rock types of the study area.
Au-anomaly K - anomaly , Na-anomaly Jrrence
in Kuitti!.a
jstal r o c k s
/+2* -
/+5'
3 km I
31°~0 ' Fig. 13. Sketch map of anomalies of Au and K and negative anomalies of Na.
I
APPLICATION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, ILOMANTSI, FINLAND
341
Gold in the bedrock at Ilomantsi seems to be similar to Archaean gold described from many deposits in recent years (Boyle, 1987; Valliant and Hutchinson, 1982; Colvine, 1983; Colvine and Stewart, 1984; Groves et al., 1985; Meyer and Saager, 1985 ). Typical features of the mineralizations in Kuittila are: ( 1 ) A tonalitic environment; especially contacts with supracrustal rocks are mineralized. (2) Strong synplutonic tectonism, which created suitable voids for fluids and gases to migrate and concentrate ore-forming and ore-related minerals. (3) Metasomatic alteration of the host rocks with high K and low Na in particular. Increased SiO2 values follow these alteration features. (4) Gold and W do not exist in the same phase; Mo is typical to both mineralization phases. Gold exists as native between silicate grains and among sulphides and tellurides. Arsenopyrite and other sulphides are insignificant in most mineralizations intersected by drilling. In terms of the potential for further Au occurrences within the study area, the following can be concluded: ( 1 ) The occurrence of Au mineralizations similar to the one at Kuittila is improbable because of the restricted Mo and W anomalies typical of this kind of mineralizations. There are, however, some interesting Au anomalies inside the supracrustal rocks surrounding the Kuittila tonalite. The metasomatic alteration features cutting both the Kuittila tonalite and these rocks may indicate Au mineralizations of another type. (2) Gold anomalies in the eastern part of the study area are associated with basic metavolcanites. This area has not yet been studied, but the numerous analogies in Canada and Australia related to basic volcanism show that we can expect some interesting targets in that area, too. (3) Gold anomalies related to iron formations can easily be attributed to the high background of Au in those formations. Nevertheless, it is necessary to study them in greater detail to establish whether there are any targets worth of prospecting among those anomalies. ACKNOWLEDGEMENTS We are indebted to the staff of the Geochemistry and Exploration Departments of the Geological Survey of Finland for the assistance they gave us with this study. We are particularly grateful to Seppo Lavikainen for his valuable comments and good advice. We also thank Gillian H~ikli for correcting the English text. REFERENCES Bj(Srklund, A.J. and Gustavsson, N., 1987. Visualization of geochemicaldata on maps: New options. J. Geochem.Explor., 29: 89-103.
342
A. HARTIKAINEN AND M. DAMSTEN
Bjrrklund, A.J. and Lummaa, M., 1983. Representation of regional, local and residual variability of geochemical data by means of filtering techniques. In: Proc. 2nd Int. Symp. on Methods of Geochemical Prospecting. Irkutsk, USSR, pp. 25-34 (translated to Russian). Boyle, R.W., 1987. Gold: History and Genesis of Deposits. Van Nostrand Reinhold, New York, N.Y., 676 pp. Colvine, A.C. (Editor), 1983. The Geology of Gold in Ontario. Ont. Geol. Surv., Misc. Pap. 110, 278 pp. Fortescue, J.A.C., 1983. Geochemical prospecting for gold in Ontario. In: A.C. Colvine (Editor), The Geology of Gold in Ontario. Ont. Geol. Surv., Misc. Pap., 110:250-271. Groves, D.I., Phillips, G.N., Ho, S.E. and Houstoun, S.M., 1985. The nature, genesis and regional controls of gold mineralization in Archean greenstone belts of the western Australian Shield: a brief review. Trans. Geol. Soc. S. Afr., 88:135-148. Gustavsson, N., Noras, P. and Tanskanen, H., 1979. Seloste geokemiallisen kartoituksen tutkimusmenetelmist~i. English summary: Report on geochemical mapping methods. Geol. Surv. Finl., Rep. Invest., 39: 1-20. Hutchinson, R.W., 1986. Metallogeny of Precambrian Gold Deposits: Space and Time Relationships. Terra Cognita, 6 (3): 533. Kauranne, L.K., 1975. Regional Geochemical mapping in Finland. In: M.J. Jones (Editor), Prospecting in Areas of Glaciated Terrain 1975. I.M.M., London, pp. 71-81. Kerrich, R., 1983. Geochemistry of gold deposits in the Abitibi Greenstone belt. Can. 1.M.M. Spec. Pap., 27: 1-75. Kontas, E., 1981. Rapid determination of gold by flameless atomic absorption spectrometry in the ppb and ppm ranges without organic solvent extraction. Atomic Spectr., 2 (2): 59-61. Kontas, E., Niskavaara, H. and Virtasalo, J., 1986. Flameless Atomic Absorption Determination of Gold and Palladium in Geological Reference Samples. Geostandard Newslett., 10(2): 169-171. Krylov, I.N., Levchenkov, O.A., Lobach-Zhuchenko, S.B. and Chekulajev, V.P., 1984. Heterogeneity of structure and development of Archean lithosphere of the Karelian granite-greenstone terrain. 27th J. Geol. Congr., 5: 233-246. Latulippe, M., 1982. An overview of the Geology of Gold Occurrences and Developments in Northwestern Quebec. In: R.W. Hodder and W. Petruk (Editors), Geology of Canadian Gold Deposits. C.I.M. Spec. Vol., 24: 9-14. Lavikainen, S., 1973. Kallioper'~ikartta- Pre-Quarternary Rocks. Lehti-Sheet 4244 Ilomantsi. Suomen geologinen kartta - Geological map of Finland, 1:100 000. Meyer, M. and Saager, R., 1985. The gold content of some Archean rocks and their possible relationship to epigenetic gold-quartz vein deposits. Miner. Deposita, 20 (4): 284-289. Nichol, I., 1986. Geochemical exploration for gold deposits in areas of glaciated overburden: problems and new developments. In: W.J. Phillips (Editor), Prospecting in Areas of Glaciated Terrain 1986 in Kuopio, Finland. I.M.M., London, pp. 7-16. Salminen, R. and Hartikainen, A., 1985. Glacial transport of till and its influence on interpretation of geochemical results in North Karelia, Finland. Geol. Surv. Finl. Bull., 335:48 pp. Salminen, R. and Hartikainen, A., 1986. Tracing of gold, molybdenum and tungsten mineralization by use of a step by step geochemical till study in Ilomantsi, eastern Finland. In: W.J. Phillips (Editor), Prospecting in Areas of Glaciated Terrain 1986 in Kuopio, Finland. I.M.M., London, pp. 201-210. Studemeister, P.A., 1985. Gold-bearing veins around a felsic stock near Wava, Ontario: implications for gold exploration. C.I.M. Bull., 78 (874): 43-47. Valliant, R.I. and Hutchinson, R.W., 1982. Stratigraphic Distribution and Genesis of Gold Deposits, Bousquet Region, Northwestern Quebec. In: R.W. Hodder and W. Petruk (Editors), Geology of Canadian Gold Deposits. C.I.M. Spec. Vol., 24: 27-40.
323
Elsevier Science Publishers B.V., Amsterdam
Application of till geochemistry to gold exploration, Ilomantsi, Finland A. Hartikainen and M. Damsten Geological Survey of Finland, P.O. Box 1237, SF-70701 Kuopio, Finland (Received January 18, 1990)
ABSTRACT Hartikainen, A. and Damsten, M., 1989. Application of till geochemistry to gold exploration, llomantsi, Finland. In: A.J. Bj6rklund (Editor), Gold Geochemistry in Finland. J. Geochem. Explor., 39: 323-342. The Archean bedrock of the Ilomantsi area consists of narrow greenstone belts between dome-like granitoids. The main tectonic phases are isoclinal folding, dragging of isoclinal folds around subvertical axes and regional faulting. Conspicuous anomalies of As, Au, Mo and W were revealed in the course of regional geochemical mapping of till, which led to more detailed studies in Ilomantsi. The data presented in this study are based mainly on the results of 2960 till samples collected by light percussion drill at local scale ( 16 samples/km 2), but also on the results of regional ( 1 sample/4 km 2) and detailed ( 100-400 samples/km 2) sampling and on rock samples collected from outcrops. The analytical methods most commonly used were emission spectrometry, atomic absorption spectrophotometry and spectrometry. The results indicate that the lowest part of the basal till is dominated by local debris and that the geochemistry of the till reflects well the underlying bedrock. The highest Au values exist near the contacts of tonalite and supracrustal rocks. Arsenic follows intimately the directions of faulting. The anomalous contents of W and Mo are restricted to the southern part of the 15-kin 2 Kuittila tonalite. Potassium, Na and Ca form conspicious anomalies in till, cutting both the supracrustal rocks and the granitoids. Local studies revealed in the Kuittila tonalite a notable network of Mo- and W-bearing quartz veins restricted to tensional zones of main fault directions, which are cut by gold-bearing shear zones and quartz vein nets. These shear zones are ofen echelon type and they are silicified, sericitized, tourmalinized, Na-depleted and partly carbonitized. The hosting tonalite is intensely biotitized. These metasomatic features of the bedrock can be correlated to the wider anomalies of K, Na and Ca in till. The implication is that significant migration of mobile elements has taken place in the bedrock of the study area and that, in places, gold has participated in these processes.
INTRODUCTION
The Archean bedrock of eastern Finland resembles that of Canada, famous for its numerous gold ores (Boyle, 1987; Latulippe, 1982; Kerrich, 1983; Hutchinson, 1986). Although Au occurs unevenly in both bedrock and till, the results of the geochemical studies of till have been encouraging (Fortescue, 1983; Nichol, 1986). 0375-6742/91/$03.50
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APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
325
Since the early 1970s systematic till geochemistry has proved to be a useful tool for prospecting in Finland (Kauranne, 1975; Gustavsson et al., 1979). Because of the lack of precise analytical methods, some elements important for prospecting were not analysed routinely in the early years. However, with the increasing sophistication of analytical methods it has now become possible to make quick, economical and reliable analyses of certain elements, gold in particular (Kontas, 1981; Kontas et al., 1986). The geochemistry of till at different scales has revealed many promising anomalies and anomalous areas in Ilomantsi, eastern Finland (Salminen and Hartikainen, 1986). Knowledge of the mineralizations and their characteristics has been valuable for the studies in this area. The present study focuses on local and detailed features in the geochemistry of till in prospecting at Ilomantsi (Fig. 1 ). Local mapping in particular cover extensive area and permit preliminary models for the behaviour of elements important in gold prospecting to be presented. The geochemistry of till has also been of great importance for bedrock mapping in these areas of sparse outcrops. The feasibility of the method in finding different Au-mineralizations is discussed. GEOLOGICALSETTING Bedrock
The Ilomantsi area is part of the west Karelian granite-greenstone terrain of Archean age (Lavikainen, 1973; Krylov et al., 1984). The greenstones rim the granitoid blocks as narrow but more or less continuous belts (Fig. 1 ). They consist of metavolcanics (mainly dacitic-andesitic), metaconglomerates, metagreywackes and mica schists with interbeds of black schists and banded iron formations (BIF). The granitoids include all types from granite to quartz diorite. Intermediate porphyritic dykes intersect the supracrustal rocks and, as the youngest rocks, the Proterozoic diabases cut the whole Archean suite. Three aspects of particular structural interest are: ( 1 ) The isoclinal folding linked to intrusions of synkinematic granitoids. (2) Dragging of isoclinal phase. During this phase gliding along subvertical S-plane led to folding of the incompetent strata of the supracrustal rocks while the competent strata underwent intense fracturing and mylonitization. (3) Approximately N-S-directed faulting and conjugate tensional shearing and fracturing. The regional character of these is most clearly seen on topographic maps and on aerial photomaps. The Proterozoic diabases follow these tensional directions, showing late activity along these structures. Still later activity is seen as shearing of medadiabases in a NE direction.
326
A. HARTIKAINENAND M. DAMSTEN
Overburden Outcrops are sparse in Ilomantsi area as a whole. The few areas with adequate outcrops are situated outside the Kuittila zone of prospective interest. In the Kuittila area, the overburden is till, concealed between hillocks and morainic forms by bogs. The thickness of overburden varies between 1 and 12 m, the average being 5 m. Weathered bedrock overlain by till is common, especially in the areas of supracrustal rocks. The main direction of ice flow is reflected by drumlinoidal forms trending NW. The striations are mainly in the same direction, although some older striations from west to east have also been observed. Quarternary studies have revealed one till bed only. The transport distance of main part of basal till is short, some hundreds metres or even less, but the boulders on the surface of the till have been transported various distances, many of them at least a few kilometres (figs. 17-19 in Salminen and Hartikainen, 1985 ). The forms of deglaciation encountered are h u m m o c k y and DeGeer moraines and a few eskers. METHODS
Till samples (100-200 g) were collected during regional studies from the surface (average 1.5 m ) with percussion drills. The sampling distance was 100 m on lines 1-2 km apart. 5-15 successive samples were made into a composite for all analyses. Only the finest fraction of the till ( < 0.06 m m ) was analyzed (Tables 1 and 2). During local and detailed studies samples were taken from the till/bedrock interface in order to obtain local information on the bedrock. The local scale mapping covers an area of 185 km 2 including the vicinity of the Kuittila target. The total number of sampling sites was 2960 (Table 1 ). Detailed studies involved till/bedrock sampling, bedrock exposing, and diamond drilling. At local and detailed scales, regular sampling grids of 250 m X 2 5 0 m and 50 m X 50 m were used. To save expenses four neighbouring samples were combined for analyses. Also in these scales the finest fraction was analysed. For TABLEI The size of areas, sampling densities and sampling depth at different scales during till geochemical mapping at Ilomantsi
Region al scale Local scale Detailed studies
Size of area (kin 2)
Sampling density (sample/km:)
Average sampling depth (m)
2000 185 2
0.25-0.40 16 400
I. 5 4.5 5.0
327
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
TABLE2 Analytical methods used for the samples collected during geochemical mapping at different scales at Ilomantsi. I C A P = i n d u c t i v e l y coupled plasma spectrometry, L e c o = s u l p h u r titrator, OES=optical emission spectrometry, spectroph. = spectrophotometry, GFAAS = flameless atomic absorption spectrophotometry, FAAS = atomic absorption spectrophotometry, NAA = neutron activation analysis, EDX = energy dispersive spectrophotometry. For different scales see Table 1
Regional scale Local scale Detailed studies Rock samples 1= 2= 3= 4= 5= 6= 7= 8=
ICAP Leco OES Spectroph. GFAAS FAAS NAA EDX
1
2
x
x
3 x
(x)
4
5
6
7
8
x x x (x)
x x x x
x
x
x
composite of 5-15 subsamples composite of 5-15 subsamples composite of 3-15 subsamples composite of 3-15 subsamples
27 elements S 19 elements W Au,As,Mo Si,A1,Fe,Mg,Ca,K,Na 14 elements Ba
comparison, 50 rock samples were collected from different rock types. The analytical methods used at the different scales are listed in Table 2. The data of local scale are presented in greystone maps. The darkness of each 100X 100 m grid is based on weighted moving median (BjiSrklund and Lummaa, 1983; BjiSrklund and Gustavsson, 1987). The radius of the "'window" used is 800 m for Au, As and W and 500 m for other elements. It occurs about ten sampling sites in each window.
RESULTS
Earlier studies
Peat investigations conducted in the 1970s indicated that parts of the area are anomalous for As (Minkkinen and Yliruokanen, 1978). Prospecting for Mo, W and Au in the study area was the subject of a recent report, which describes mainly the methodology of till geochemistry in prospecting (Salminen and Hartikainen, 1986 ). In that paper the authors emphasize that excellent pathfinder for Au in regional scale in Ilomantsi is As, the anomalies of which crosscut the lithological boundaries and follow the directions of tensional shearing (Fig. 2 ).
328 ! °
A. HARTIKAINEN AND M. DAMSTEN
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Results in local scale Correlation between till geochem&try and bedrock. The rock samples collected in the Ilomantsi area for correlation purposes are classified as granites, granodiorites, metabasalts, metagreywackes and metadiabases. The metagreywackes c o m m o n l y have interlayers o f tuffitic material. The essential geochemistry is listed in Table 3. In this study, Ni reflects best the geochemistry of the bedrock (Table 3 and
329
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
TABLE 3 Average contents of certain elements in barren rocks at Ilomantsi, n = number of samples. Analysed by ICAP and AAS methods Rock type
n
K20 %
Na20 %
CaO %
Au ppb
As ppm
Mo ppm
Cu ppm
Ni ppm
Granites Granodiorites Metabasalts Metagreywackes Metadiabases
9 16 6 14 5
3.6 2.3 0.8 2.2 0.8
4.6 4.4 2.4 3.0 2.6
1.7 4.4 10.8 2.9 8.8
0.3 0.6 0.8 1.5 1.2
0.0 0.3 5.5 12.0 0.7
1.5 0.5 3.7 1.0 0.3
12 18 74 48 120
6 16 45 66 30
Lsi
till 0 6 mm
)thed data Ni ppm
31" 10' Fig. 3. D i s t r i b u t i o n o f Ni in till in the study area. Analysed by OES.
330
A. HARTIKAINEN AND M. DAMSTEN
Fig. 3). In general, the anomalous patterns are above supracrustal units, whereas the areas of low contents refer to granitoids. Copper has similar patterns to Ni, although there are weak positive anomalies above the granitoids, too (Fig. 4). The Kuittila tonalite within the schists can easily be detected by these elements, the tonalite being surrounded by distinctive anomalies of Ni and Cu. Problems with interpretation are aggravated by the intense migmatizing effect of the granitoids at the eastern contact of the greenstone belt.
Geochemistry related to ore-forming processes. The elements that are related to the mineralizations at Ilomansi exhibit the following features: Gold follows mainly the contact zone of the supracrustal rocks and biotite tonalite, but high Au contents in till occur above banded iron formations, too. High K is bound to the Au-anomalous contact zone especially in the Kuittila area (Figs. Lsi
till 0 6 mm
}thed data Cu ppm
31 ° 10'
Fig. 4. Distributionof Cu in till in the study area. Analysedby OES.
3 31
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
1 and 5 ). Molybdenum (not shown) and W (Fig. 6) are closely associated with the Kuittila tonalite. A weaker, separate W anomaly without Mo southeast of Kuittila inside the supracrustal rocks may indicate hydrothermal activity of underlying granitoids. Arsenic forms distinct angular anomalies in till parallel the main faults and shears (Fig. 7 ). However, As content seldom exceeds 100 ppm in the till of the study area. Metasomatic alteration features typical of Archean gold mineralizations can be examined by the behaviour of K, Na and Ca (Kerrich, 1983; Studemeister, 1985 ). Silicification, sericitization, tourmalinization, Na-depletion, carbonatization and, in places, sulphidization and biotitization are common in the mineralizations of this area. Hence, high K and low Na in till could show areas of metasomatic alteration. In the Kuittila area there is an eUipsoidal ring of high K anomaly in till
i
ill 3 mm
:1 d a t a ,u ppb 20 11 6
3 2 1
31 ° 1 0 ' Fig. 5. D i s t r i b u t i o n o f Au in till in the study area. Analysed by GFAAS.
332
A. H A R T I K A I N E N A N D M. D A M S T E N
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31 ° 10' Fig. 6. Distribution of W in till in the study area. Analysedby spectrophotometer. (Fig. 8 ). The 5 × 10 km anomaly intersects both the granitoids and the supracrustal rocks. Its position and shape bear a close resemblance to that of W (Fig. 6 ). At least in Kuittila the K anomaly can be explained by intense biotitization of the Kuittila tonalite. The distribution of Na (Fig. 9 ) in till is quite different from K and W. Low Na contents in till are partly above the supracrustal rocks, which have low Na contents (Table 3), but the most conspicious feature is the straight NWtrending positive and negative anomalies cutting different rock types. The distribution of Ca is not related to any particular rock type (Fig. 10 ), in spite of the fact that main rock types regionally have differences in Ca contents (Table 3 ). High Ca contents in till form a circle independent of rock types with a diameter of 10 km in Kuittila and east of Kuittila. It is apparent that there has been substantial migration of mobile elements
333
APPLICATION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, 1LOMANTSI, FINLAND
|tsi
till ).06 mm
)othed data As ppm 38.6 25.9 17.1
11.3 7.5 5.0 3.3 2.2
31 ° 10' Fig. 7. Distribution of As in till in the study area. Analysed by GFAAS.
in the bedrock of the study area. It seems that in spite of the fact that K, Na and Ca are main elements of the bedrock, they reflect metasomatic features better than lithological units in this area. It is evident that the heat of the granitoid intrusives caused the migration of certain elements and is, at least partly, responsible for metasomatic alteration in local scale in the bedrock. Detailed survey The local studies showed that the Kuittila area is a promising exploration target. Bedrock mapping of this area gave poor results because of the lack of outcrops, and geophysical methods did not give the results expected either. Gold, M o and W in till. Detailed till sampling was carried out on a regular
334
A. H A R T I K A I N E N A N D M. D A M S T E N
tsi
till 0 6 mm
~thed d a t a K
%
31 ° 10'
Fig. 8. Distribution of K in till in the study area. Analysedby OES. sampling grid of 50 m. The strongest anomalies of this scale were still followed-up at 5-m intervals along profiles. Samples were taken from the till/ bedrock interface in order to procure both till samples and small fragments of bedrock. The two samples were analysed separately (Salminen and Hartikainen, 1986). On the 50-m grid the rather uniform local Au anomaly split into smaller anomalies (Fig. 11 ) that follow the trend of Au-mineralized shears. The highest Au value on the detailed sampling profiles was 400 g/t; values above 100 ppb were common. In detail, the Au values in till denoted precisely the locations of the mineralizations. Molydbenum and W anomalies follow those of Au, but are considerably wider. The highest values for both elements were above 1000 ppm, the average being 14 (Mo) and 27 ppm (W), respectively.
335
APPLICA! ION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, ILOMANTSI, FINLAND
l$i
till 06 mm
)thed data Na % 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5
31" 10' Fig. 9. Distribution of Na in till in the study area. Analysed by OES.
As a whole, the till geochemistry shows that the main Au anomalies are situated at the western side of the W and Mo anomaly.
Mineralizations in bedrock. Exposed bedrock revealed two kinds of mineralization within the tonalite in Kuittila. Tectonics plays a decisive role in controlling the position of both. The best example is a lensoidal entity of NWtrending tensional quartz veins along the controlling fault with a NE orientation. The ore minerals are scheelite and molybdenite; the contents of Au are low. In this fault zone there are also later both tensional and parallel quartz veins without important ore minerals. Gold is controlled by shear zones and quartz veins trending NW. These cut the Mo-W-bearing quartz veins at a low angle. In shear zones the tonalitic host rock has completely altered into quartz and sericite with minor biotite, tourmaline, secondary K-feldspar, futile and apatite. The contacts with bio-
336
A. HARTIKAINEN AND M. DAMSTEN
:lata %
3.4 3.1 2.9 2.6 2.4 2.2 2.0
1.8
31 ° 10' Fig. 10. Distribution of Ca in till in the study area. Analysed by OES.
titized tonalite are sharp. Gold exists as native mainly among tellurides and sulphides. There are also some molybdenite, pyrite, galena and calcopyrite. The lithogeochemistry of one of these shear zones is presented in Figure 12. High peaks of Au and Si are accompanied by depletion of Na and Ca. The tonalitic host rock of this shear zone is strongly enriched in K because ofbiotitization. There are high contents of W in the eastern side of the shear zone in tonalite because of older Mo-W-bearing quartz veins. Comparing the contexts of the immobile elements A1, Sc, Hf and Zr in the host rock and in the altered Au-enriched shear it is evident that about two-thirds of the material in this shear has an external source (Table 4 ). To the west of the aforementioned Au occurrence the strongest Au anomaly was checked by diamond drilling. Wide zones of quartz-sericite-rock were found, the m a x i m u m width being 10 m. The Au content of these zones varies;
APPLICATION OF TILL GEOCHEMISTRY TO GOLD EXPLORATION, ILOMANTSI, FINLAND
KUITTILA Au Till fraction-0.06 mm
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m
337
338
A. HART1KAINEN AND M. DAMSTEN
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Fig. 12.Lithogeochemistry of a gold-beating shear-zone, Kuittila, llomantsi. Analysed by GFAAS (Au,As),FAAS(aqua regia and HF leaching, K,Na,Ca,Si)and spectrophotometer (W) methods. some are completely barren, whereas others have evenly distributed Au contents of a few g/t. The Au values of 2 g/t are common, the best being 4.5 p p m / 4 m core. Typical of the latter mineralization is low sulphide content, average < 0.1% S. The Au-As anomaly northwest of Kuittila was checked also by diamond drilling. Narrow quartz-tourmaline veins with some arsenopyrite associated with porphyry dykes of tonalitic composition were found in the schists. The Au content of these dykes is up to 7 g/t.
APPLICATIONOF TILL GEOCHEMISTRYTO GOLD EXPLORATION,ILOMANTSI,FINLAND
339
TABLE 4
Average contents of elements in the Ilomantsi batholite ( 1 ), the Kuittila tonalite (2) and the goldbearing shear zone in tonalite (3) at Kuittila. For abbreviations see Table 2 1 n= 1 58.6 14.2 4.0 1.8 2.9 5.8 2.2
2 n = 6 or_8
3 n = 3 or 12
SiO2 A1203 FeO MgO CaO Na20 K20
(%) (%) (%) (%) (%) (%) (%)
59.5 14.2 3.8 2.2 2.7 3.0 4.2
79.2 6.4 2.3 0.5 0.1 0.1 2.8
FAAS FAAS FAAS FAAS FAAS FAAS FAAS
Cs Hf Sc Zr Rb
(ppm) (ppm) (ppm) (ppm) (ppm)
-
9.8 3.2 13.0 108 104
1.8 1.2 4.6 44 43
NAA NAA NAA NAA NAA
Sr V
(ppm) (ppm)
530 -
514 86
20 140
ICAP ICAP
W Ba
(ppm) (ppm)
0 1000
266 982
16 375
Spectrophotometer EDX
Cu Pb Mo As
(ppm) (ppm) (ppm) (ppm)
12 16 0 i
24 16 141 2
19 127 197 52
FAAS FAAS FAAS FAAS
Au
(ppb)
0
33
5460
GFAAS
Not noteworthy differences: Ti,Li,Co,Cr, Ni,Zn,Ag, Bi,Sb,Cd,P,Ta,Th,Y
CONCLUSIONS
The geochemistry of till reflects well the bedrock of the study area, and the following features can be recognized: ( 1 ) The lowest part of the basal till is local; the transport distance is negligible. Nickel and Cu particularly give reliable indications of the underlying bedrock. They reflect better the geochemical differences of the main rock units in the study area than for example main elements (Figs. 1, 3 and 4). (2) The most prominent Au anomalies follow the contact zone between the tonalite and the supracrustal rocks. The eastern anomaly area is connected with basic metavolcanites not yet studied. In the areas of iron formations the Au values are extensively anomalous because the contents in the bedrock are clearly above background values (Figs. 1 and 5 ).
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A. H A R T I K A I N E N A N D M. D A M S T E N
(3) The ellipsoidal ring of high K is associated with barren areas of Na ( Fig. 13 ). The connection between high K contents in till and biotitization in the environment of mineralizations is clear (Fig. 8 ). At Kuittila, both a positive K anomaly and negative Na (Fig. 9 ) anomaly in till and in bedrock are connected with the Au anomaly. This evidently reflects hydrothermal activity related to ore-forming processes. Also, Ca forms large-scale anomalies intersecting the rock contacts, thus indicating a migration of this element (Fig. 10). (4) Molybdenum and W are restricted to the southern half of the Kuittila tonalite (Fig. 6 ). The erosional level in the northern half of the tonalite obviously differs from that in the southern half. ( 5 ) Arsenic is a good pathfinder for Au in regional and local surveys (Figs. 2 and 7 ). At Ilomantsi it seems to be associated with supracrustal rocks but also with tectonic zones of NW and NE orientation. Faults and shears in these directions cut all the rock types of the study area.
Au-anomaly K - anomaly , Na-anomaly Jrrence
in Kuitti!.a
jstal r o c k s
/+2* -
/+5'
3 km I
31°~0 ' Fig. 13. Sketch map of anomalies of Au and K and negative anomalies of Na.
I
APPLICATION OF TILL GEOCHEMISTRY TO G O L D EXPLORATION, ILOMANTSI, FINLAND
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Gold in the bedrock at Ilomantsi seems to be similar to Archaean gold described from many deposits in recent years (Boyle, 1987; Valliant and Hutchinson, 1982; Colvine, 1983; Colvine and Stewart, 1984; Groves et al., 1985; Meyer and Saager, 1985 ). Typical features of the mineralizations in Kuittila are: ( 1 ) A tonalitic environment; especially contacts with supracrustal rocks are mineralized. (2) Strong synplutonic tectonism, which created suitable voids for fluids and gases to migrate and concentrate ore-forming and ore-related minerals. (3) Metasomatic alteration of the host rocks with high K and low Na in particular. Increased SiO2 values follow these alteration features. (4) Gold and W do not exist in the same phase; Mo is typical to both mineralization phases. Gold exists as native between silicate grains and among sulphides and tellurides. Arsenopyrite and other sulphides are insignificant in most mineralizations intersected by drilling. In terms of the potential for further Au occurrences within the study area, the following can be concluded: ( 1 ) The occurrence of Au mineralizations similar to the one at Kuittila is improbable because of the restricted Mo and W anomalies typical of this kind of mineralizations. There are, however, some interesting Au anomalies inside the supracrustal rocks surrounding the Kuittila tonalite. The metasomatic alteration features cutting both the Kuittila tonalite and these rocks may indicate Au mineralizations of another type. (2) Gold anomalies in the eastern part of the study area are associated with basic metavolcanites. This area has not yet been studied, but the numerous analogies in Canada and Australia related to basic volcanism show that we can expect some interesting targets in that area, too. (3) Gold anomalies related to iron formations can easily be attributed to the high background of Au in those formations. Nevertheless, it is necessary to study them in greater detail to establish whether there are any targets worth of prospecting among those anomalies. ACKNOWLEDGEMENTS We are indebted to the staff of the Geochemistry and Exploration Departments of the Geological Survey of Finland for the assistance they gave us with this study. We are particularly grateful to Seppo Lavikainen for his valuable comments and good advice. We also thank Gillian H~ikli for correcting the English text. REFERENCES Bj(Srklund, A.J. and Gustavsson, N., 1987. Visualization of geochemicaldata on maps: New options. J. Geochem.Explor., 29: 89-103.
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Bjrrklund, A.J. and Lummaa, M., 1983. Representation of regional, local and residual variability of geochemical data by means of filtering techniques. In: Proc. 2nd Int. Symp. on Methods of Geochemical Prospecting. Irkutsk, USSR, pp. 25-34 (translated to Russian). Boyle, R.W., 1987. Gold: History and Genesis of Deposits. Van Nostrand Reinhold, New York, N.Y., 676 pp. Colvine, A.C. (Editor), 1983. The Geology of Gold in Ontario. Ont. Geol. Surv., Misc. Pap. 110, 278 pp. Fortescue, J.A.C., 1983. Geochemical prospecting for gold in Ontario. In: A.C. Colvine (Editor), The Geology of Gold in Ontario. Ont. Geol. Surv., Misc. Pap., 110:250-271. Groves, D.I., Phillips, G.N., Ho, S.E. and Houstoun, S.M., 1985. The nature, genesis and regional controls of gold mineralization in Archean greenstone belts of the western Australian Shield: a brief review. Trans. Geol. Soc. S. Afr., 88:135-148. Gustavsson, N., Noras, P. and Tanskanen, H., 1979. Seloste geokemiallisen kartoituksen tutkimusmenetelmist~i. English summary: Report on geochemical mapping methods. Geol. Surv. Finl., Rep. Invest., 39: 1-20. Hutchinson, R.W., 1986. Metallogeny of Precambrian Gold Deposits: Space and Time Relationships. Terra Cognita, 6 (3): 533. Kauranne, L.K., 1975. Regional Geochemical mapping in Finland. In: M.J. Jones (Editor), Prospecting in Areas of Glaciated Terrain 1975. I.M.M., London, pp. 71-81. Kerrich, R., 1983. Geochemistry of gold deposits in the Abitibi Greenstone belt. Can. 1.M.M. Spec. Pap., 27: 1-75. Kontas, E., 1981. Rapid determination of gold by flameless atomic absorption spectrometry in the ppb and ppm ranges without organic solvent extraction. Atomic Spectr., 2 (2): 59-61. Kontas, E., Niskavaara, H. and Virtasalo, J., 1986. Flameless Atomic Absorption Determination of Gold and Palladium in Geological Reference Samples. Geostandard Newslett., 10(2): 169-171. Krylov, I.N., Levchenkov, O.A., Lobach-Zhuchenko, S.B. and Chekulajev, V.P., 1984. Heterogeneity of structure and development of Archean lithosphere of the Karelian granite-greenstone terrain. 27th J. Geol. Congr., 5: 233-246. Latulippe, M., 1982. An overview of the Geology of Gold Occurrences and Developments in Northwestern Quebec. In: R.W. Hodder and W. Petruk (Editors), Geology of Canadian Gold Deposits. C.I.M. Spec. Vol., 24: 9-14. Lavikainen, S., 1973. Kallioper'~ikartta- Pre-Quarternary Rocks. Lehti-Sheet 4244 Ilomantsi. Suomen geologinen kartta - Geological map of Finland, 1:100 000. Meyer, M. and Saager, R., 1985. The gold content of some Archean rocks and their possible relationship to epigenetic gold-quartz vein deposits. Miner. Deposita, 20 (4): 284-289. Nichol, I., 1986. Geochemical exploration for gold deposits in areas of glaciated overburden: problems and new developments. In: W.J. Phillips (Editor), Prospecting in Areas of Glaciated Terrain 1986 in Kuopio, Finland. I.M.M., London, pp. 7-16. Salminen, R. and Hartikainen, A., 1985. Glacial transport of till and its influence on interpretation of geochemical results in North Karelia, Finland. Geol. Surv. Finl. Bull., 335:48 pp. Salminen, R. and Hartikainen, A., 1986. Tracing of gold, molybdenum and tungsten mineralization by use of a step by step geochemical till study in Ilomantsi, eastern Finland. In: W.J. Phillips (Editor), Prospecting in Areas of Glaciated Terrain 1986 in Kuopio, Finland. I.M.M., London, pp. 201-210. Studemeister, P.A., 1985. Gold-bearing veins around a felsic stock near Wava, Ontario: implications for gold exploration. C.I.M. Bull., 78 (874): 43-47. Valliant, R.I. and Hutchinson, R.W., 1982. Stratigraphic Distribution and Genesis of Gold Deposits, Bousquet Region, Northwestern Quebec. In: R.W. Hodder and W. Petruk (Editors), Geology of Canadian Gold Deposits. C.I.M. Spec. Vol., 24: 27-40.