Till geochemistry of gold, arsenic and antimony in the Seinäjoki district, western Finland

Till geochemistry of gold, arsenic and antimony in the Seinäjoki district, western Finland

343 Journal of Geochemical Exploration, 39 ( 199 1) 343-36 1 Elsevier Science Publishers B.V., Amsterdam Till geochemistry of gold, arsenic and anti...

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343

Journal of Geochemical Exploration, 39 ( 199 1) 343-36 1 Elsevier Science Publishers B.V., Amsterdam

Till geochemistry of gold, arsenic and antimony in the Seingjoki district, western Finland P. LestinerP, E. Kontasb, H. Niskavaarab and J. Virtasalob “GeologicalSurvey of Finland, P.O. Box 1237, SF- 70101 Kuopio, Finland bGeologicalSurvey ofFinland, P.O. Box 77, SF-96101 Rovaniemi, Finland (Received January 18, 199 1)

ABSTRACT Lestinen, P., Kontas, E., Niskavaara, H. and Virtasalo, J., 1991. Till geochemistry of gold, arsenic and antimony in the Seiniijoki district, western Finland. In: A.J. Bji%lund (Editor), Gold Geochemistry in Finland. J. Geochem. Explor., 39: 343-36 1. The data on Au, As, Sb, Ag, Co, Cu, Mn, Ni, Pb and W concentrations in the tine fraction of till ( -0.06 mm) in the regional geochemical prospecting of Au is dealt with. The study was carried out in the Seinajoki district in western central Finland. The bedrock of the area sampled is composed of variably migmatized volcanic-sedimentary schists and synorogenic granitoids of Middle Proterozoic age. Uneconomic Sb-As-, Pb-Zn-Ag-As-, W- and Sn-occurrences, with Au as a common constituent in the first two types of mineralization, are known to occur in the district. The overburden is composed mainly of sandy basal till deposited during the latest glacial movement from the north. Signs of older till, transported from northwest have also been encountered. The length of chemically detectable dispersal trains in the fraction of till analyzed seems to be not more than some hundred meters in the bottom till and a maximum of about 2 km in the surface till. Preglacially weathered bedrock is common under the till. The 300 km2 study area was sampled on a 500 m by 500 m grid. Percussion drills with flow-through samplers were used to collect composite samples of the C-horizon at l-2-m intervals. Normally, 200-300 g of till per sampling site were taken. Considering the regional geochemical variations typical of Finland, the Seinitjoki study area comprises remarkable anomalies for Au, As and Sb. The factor analysis and distribution patterns suggest that anomalous Au is mostly derived from different sources than anomalous Sb. Anomalous As is partly related to sources of Au and Sb, but also to sources of base metals which, on their part, strongly correlate with each other. The known areas of Au-As mineralization proved to be too small and too low-grade to be seen clearly in till with the sampling method employed. For Au the negative result may be due partly to poor reproducibility of the Au data. In contrast, the Sb pattern corresponds very well to known areas of Sb mineralization and indicates that the mineralization is more widespread than formerly known. Follow-up studies were made in five areas where the regional study revealed anomalous Au. Signs of mineralization were obtained in two targets. In one of them, mineralization, probably hosted in a mica gneiss, contained gold, arsenopyrite and iron sulphides. In another target, Au seemed to be related to a secondary enrichment, the primary source remaining uncertain. In the rest of the targets the source of the anomalies remain unresolved.

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TILL GEOCHEMISTRY OF Au. As AND Sb IN THE SEINtiOKI

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345

INTRODUCTION

The line fraction of till which has been widely used for geochemical prospecting of base metals has come increasingly common material also in gold prospecting (Closs and Sado, 1979; DiLabio, 1982; Stone and Gallagher, 1984; Sopuck et al., 1986; Salminen and Hartikainen, 1986; Toverud, 1987; Gleeson and Sheehan, 1987; Gleeson and Nichol, 1987 ) and the results have been mostly encouraging. In the present paper results based on the use of this material in regional geochemical prospecting for Au and its general pathfinders, As and Sb, are dealt with. The study was carried out in the Seinajoki district in the western central Finland where several Au-bearing Sb-As occurrences are known (Fig. 1). DESCRIPTION

OF THE AREA

The topography of the research area is flat with relief generally not more than 20 m. The ground slopes gently northwestward, being 100 m above the sea level in the southeast and 60 m in the northwest. The bedrock is covered by glacial drift, and outcrops poorly except in the north-central and most eastern parts of the study area. Most of the drift is basal till, which sporadically forms low, north-south-trending drumlinoid ridges. About half of the area is covered by a mire. BEDROCK

The following description is based on earlier work done in the area (Neuvonen, 196 1; Paakkijnen, 1966; Oivanen, 1982; Nurmela, 1985; Karkkiinen, 1985 ) . The simplified bedrock map presented in this paper (Fig. 1) was compiled by the senior author. The area sampled lies within a variably migmatized volcanic-sedimentary schist zone of Middle Proterozoic age (Simonen, 1980). Metasediments are mostly biotite-plagioclase gneisses (possibly metagraywackes ) with intercalations of mica schists and phyllites which in places are graphite-bearing. Some mica schists are quartz and feldspar rich and are at least partly volcanoclastic in origin. Metavolcanites are talc-alcaline and mainly intermediate to acid in composition. Most of the intermediate variants are subvolcanic plagioglase or plagioclase-hornblende porphyries, the acid volcanites being mostly tuffites. Diopside and garnet-bearing skams are also common as narrow intercalations or small pockets in the metavolcanites. The intrusive magmatic rocks of the area constitute a complex of synorogenie granitoids, including quartz diorites, granodiorites, granites and pegmatites. The rocks were formed during the main phase of the metamorphism

346

and deformation that occurred 1800- 1900 Ma ago (Simonen, sions of metasediments and meta volcanites are common.

P. LESTINEN ET AL.

1980). Inclu-

MINERALIZATION

Four different kinds of mineralization have been encountered in the Seinajoki district. They comprise Sb-As, Pb-Zn-Ag-As, W and Sn occurrences. Gold is a common constituent in the two first types of mineralization. Arsenic is also anomalous in W occurrences and Sb in Pb-Zn-Ag-As occurrences (Piakkiinen, 1966; Oivanen, 1982, 1983; Alviola, 1986). Most of the known Sb-As occurrences are situated in a 9-km-long and at least 1 km-wide mineralized fault and fracture zone which strikes northwest from Marttalanniemi to Kyrkiisjarvi (Fig. 1). The host rock is metavolcanite or less commonly mica gneiss, and silicilication and sericitization are most common alterations associated with the mineralization. Individual mineralized bodies are mostly small, the suboutcrops being less than 50 m long and 10 m wide. The largest known occurrence is situated in Kalliosalo where surface extension of the body is 150 m long and maximally 30 m wide. In Sikakangas individual mineralized zones are only 2-3 m thick, but because there are many of them and the dip of the zones is very low, they form an almost coherent, northeast-striking mineralized area measuring 200 m by 700 m. Native antimony, pyrrhotite and arsenopyrite are the primary ore minerals in the Sb-As occurrences. Gold has been found as native nuggets in fractures of arsenopyrite and as aurostibite inclusions in native antimony (Aho, 1980; Borodaev et al., 1983 ). The mean Sb content of the mineralized bodies averages typically 0.5%, but in some of them it can reach values up to l-3%. Arsenic contents are mainly under 0.3% and Au contents less than 1 ppm. However, in some parts of the occurrences Au may reach values of several ppm (Oivanen, 1982 ). According to PEikkiinen ( 1966) the Sb-As-mineralization is genetically connected to the porphyries of the area. The known Au-bearing Pb-Zn-Ag-As occurrences are all located in the southern part of the Marttalanniemi-KyrkSsjIrvi mineralized zone, southeast of Tervasmlki. The mineralization is hosted by shear and fracture zones or skam horizons in metavolcanites, the individual mineralized suboutcrops typically being lo-20 m long and l-2 m wide. In Marttalanniemi, where the mineralization has its largest extent, the zones cover an area of 50-80 m by 700 m. Galena, sphalerite and arsenopyrite are the most common ore minerals. Gold seems to follow the As-rich horizons, and according to the drillcore data from Marttalanniemi, the Au content is in the range 0.6-5.1 ppm. Arsenic content of the horizons averages 0.3% and the Sb content less than 0.01% (Oivanen, 1982). Amongst the two remaining mineralization types, the skam-hosted scheelite mineralizations include arsenopyrite in some occurrences. Unfortunately

TILL GEOCHEMISTRY OF Au, As AND Sb IN THE SEINiiJOIU DISTRICT

no data on As contents have been published. The pegmatite-hosted mineralization is practically devoid of Au, Sb and As.

347

cassiterite

OVERBURDEN

The glacial drift is mainly sandy basal till which in the uplands is generally not more than 2 m thick. In the lowland areas the drift is thicker, but only in a few places is it more than 5 m deep. In the peatbog-covered lowland areas, glaciolacustrine deposits commonly underlie the peat. Generally they are not more than 2.5 m thick, but may reach a thickness over 5 m. The peat layer is mostly less than 2.5 m thick. Studies of till stratigraphy made in the area and its surroundings revealed two till beds of different age (Hirvas, 1980; Nenonen and Huhta, 198 1; Nenonen and Johansson, 198 1; Nenonen, 1982,1983; Nenonen and Hakala, 1982,1984; Nenonen and Liippo, 1984). The younger till was transported from the north, the older from the northwest. In most of the area, bedrock is covered only by the younger bed, and the older till occurs very erratically as isolated pockets in bedrock depressions. In some places, sand and silt layers have been noticed between till beds, the observed maximum thickness of layers being 6 m. At many sampling sites, preglacial weathered bedrock was encountered beneath the till. The soft part of this layer is generally under 0.3 m thick, but was found to extend as much as 4 m below the bedrock surface. The dispersal distance of the line fraction of till is not very well known. In the Routakallio district (Fig. 1), where only the younger till covers the bedrock as quite a thin layer, the chemically detectable dispersal train at the bottom of the layer seems to be traceable for not more than 100 m (Nenonen, 1982 ). Detailed till geochemistry sampling in the Marttalanniemi-Routakallio Sb-mineralized zone supports this result (Kontas, 198 1a; Oivanen, 1982). However, the results of our study indicate that the dispersal distance could be 1.5-2 km in places, when the surface till is taken into account. METHODOLOGY

Sampling

The 300-km2 study area was sampled on a 500 m by 500 m grid by using portable percussion drills and flow-through samplers. At every sampling site a composite sample was taken. It comprised samples collected at l-2-m intervals from the top to the bottom of C-horizon of till. At least two subsamples were collected at each site, one from the top and one from the bottom of the till sheet. Normally 200-300 g of till were collected from one site. One site in thirty was sampled in duplicate to estimate sampling variability. The du-

348

P. LESTINEN ET AL.

plicate points were not more than 5 m apart at each site. In all, 124 1 samples were collected. Follow-up sampling at a more detailed scale - mostly on a 200 m by 200 m grid - was carried out at live targets. In this phase, samples from different levels in the till as well as from the weathered and fresh bedrock were collected using the same equipment as used in the regional phase. In all, 1626 till samples and 752 weathered or fresh bedrock samples were taken. Sample

preparation and analysis

The samples of till and weathered bedrock were dried at 70-80” C and sieved into four fractions: 20-2 mm, 2-0.5 mm, OS-O.06 mm and minus 0.06 mm. For till, the chemical analyses were made on the finest fraction and for weathered bedrock on the finest and coarsest fraction. Samples for Au analysis ( 1 g) were digested with aqua regia at room temperature. Separation and preconcentration was made using a coprecipitation method with SnCl, serving as reductant and Hg as the coprecipitating agent. The precipitate was dissolved with HCl-H202. Concentrations were determined using a graphite furnace-AAS equipped with D,-background corrector. The results were reported from 1 ppb, the actual detection limit being l-2 ppb. The methods is described by Kontas ( 198 1b) and Kontas et al. ( 1986). Antimony and As were digested in aqua regia at 90°C. Measurement was made with a graphite furnace-AAS equipped with Zeeman-background corrector using Pd as matrix modifier (Niskavaara et al., 1985). The results of both elements have been reported from 0.1 ppm, the actual detection limit being about 0.3 ppm. The contents of Ag, Co, Cu, Mn, Ni, Pb, W and Zn are briefly discussed in this paper, too. Of them, W was leached with 10 M HCl from a 0.20-g laboratory subsample at a temperature of 85 ‘C. Tungsten was determined as 3,4toluendithiol complex in organic white spirit phase by spectrophotomerer. For all other elements mentioned above, a 0.50 g laboratory sample was digested in aqua regia at 90°C and analyzed by flame-AAS. For Ag and W the determination limit was 0.1 ppm, for other elements 1 ppm. All chemical analyses, unless otherwise noted, were made in the laboratories of the Geological Survey of Finland. RESULTS AND DISCUSSION

Precision ofAu, As and Sb measurements

The precision of Au, As and Sb measurements on till at the 90% confidence level is presented in Table 1. The values are based on relative standard devia-

TILL GEOCHEMISTRY OF Au. As AND Sb IN THE SEINtiOKI

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DISTRICT

TABLE 1 Precision at 90% confidence level of the elements analyzed in the minus 0.064 mm fraction of till (data are based on the relative standard deviation of field duplicate samples) Element

Precision (% )

42 As Au co cu Mn Ni Pb Zn Sb W

Range’

Number of duplicates

<0.5-0.6 55 104 42 32 22 25 43 19 105 -

1.6-27.2 l-46 3-6 9-41 101-597 IO-38 6-20 13-58 0.1-1.2
23 16 23 23 23 23 23 23 17 -

‘The unit for Au is ppb and for other elements ppm.

tions of determinations on the collected field duplicate samples. For Au and Sb the precision is quite poor, for As and the base metals analyzed reasonably good. The poor precision of the Au data was expected because of the very low abundance and frequent heterogeneous occurrence of this element as particulate form in natural materials (Harris, 1982; Nichol, 1986). To achieve a more representative estimate of the Au contents would have required larger samples than it was possible to use in this study. The anomalous Au contents quite probably are real indications of mineralization, whereas the lack of the anomaly does not necessarily mean that the area is not mineralized. For Sb contents the calculated precision probably does not give a correct indication of reliability, because most of the duplicate analyses were near the lower detection limit of the analytical method employed. The coherent pattern of the Sb anomalies clearly indicates that the precision for the anomalous contents could be better (Fig. 5 ) . Factor analysis The orthogonal rotated factor analysis with quartimax method (Table 2) on Au, Sb, As, Ag, Co, Cu, Mn, Ni, Pb, Zn and W indicated that both As and Pb follow Au. Antimony clearly creates its own factor and so do W, too. In the Sb factor, only As has a weak significant loading. Factor 1, which explains far more of the total variance (41%) than any other factor, shows that the base metals strongly correlate with each other. Arsenic is also included in this factor with a slightly significant loading. Factor analysis indicates that most Au originates from other sources than

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P. LESTINEN ET AL.

TABLE 2 Orthogonal rotated (quartimax) Fl Co(ppm) Cubpm) Mn(ppm) Ni(ppm) PWppm) Znbwm) A&pm) AQwm) SWwm) Au(ppb) Wbpm) Pet of var’

0.87 0.77 0.73 0.89 0.66 0.89 0.70 0.36 0.02 0.10 0.03 41.1

factor matrix of till data from the Seintijoki study area F2 -0.03 0.16 - 0.09 - 0.02 0.25 -0.01 0.15 0.58 -0.03 0.86 -0.01 10.9

F3

F4

0.06 -0.01 0.07 -0.10 0.17 -0.09 0.1 I 0.30 0.95 -0.10 0.01 9.7

0.02 -0.04 -0.01 0.03 -0.07 0.00 0.13 -0.04 0.01 -0.00 0.99 9.2

‘Percentage of the total variance.

those containing Sb. The meaning of the weak but significant loading of Pb in the Au factor is not well understood, but could imply that part of the Au in till is associated with Pb-Zn-Ag-As-type Au occurrences. Unlike Au and Sb, As has several different sources and is widely anomalous in the Seinajoki district. Though observed to be anomalous in some W occurrences, As does not follow W to any significant degree. Distribution patterns Gold. Almost half of the Au data are at or below the lppb-detection limit which means that it is not possible to study the background variation of Au. Nevertheless, for prospecting purposes the detection limit should be low enough. However, because of quite poor precision of the Au analysis, better sensitivity of the analytical method might have provided a better possibility to delineate anomalies. Comparison of the Seinajoki till data to the so-called Atlas till data from the whole country (Koljonen, 199 1) shows that Au concentrations in till of the Seinajoki area form nationwidely a faintly anomalous population (Fig. 2). In the area1 distribution pattern of Au in till, known Au-bearing Sb-As occurrences are poorly reflected (Fig. 3 ). Only the Sikakangas area is clearly indicated (max. 300 ppb Au), where Au mineralization seems to extend even farther northeast and southwest than was known before. In the main Sb-mineralized Marttalanniemi-Kyrkisjarvi zone, Marttalanniemi mineralization is indicated by one strongly anomalous sampling site (220 ppb Au) which, according to its location, probably reflects Pb-Zn-Ag-As-type occurrences. The nearest known occurrence of this kind is located about 200 m in the up-ice

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direction. In Kalliosalo, the faint Au anomaly probably has the same kind of source. Throughout the rest of the main Sb zone, only in Puolivalinkallio is Au mineralization very weakly manifested in till. Both in Kalliosalo and Puolivalinkallio the anomaly seems to be very local. The poor manifestation of known Au occurrences in the fine fraction of till could be caused by a coarse average grain size of gold. However, only a few microscopically identified gold nuggets have been found in samples of the Aubearing Sb occurrences (Aho, 1980), which means that most gold from these sources should be fine enough to occur in the analyzed fraction of till. More probably the size and grade of the mineralized suboutcrops have in most cases been too small to introduce enough gold into the till to allow these targets to be detected with the sample method employed. This is supported by the fact that the Sikakangas mineralization, which has largest surface extension, is also reflected best of the known mineralized areas. Though the distribution pattern of Au is incoherent, it is still possible to delineate anomalies, in places surprisingly large, for example in the Viitalankyli-PerWM&elGrloukko area, in Rajaneva and in Amm5l5nkyli. Until now a follow-up till sampling is carried out only in the western areas, in Mustakorpi (max. 32 ppb Au), Honkakyli (max. 940 ppb Au), TupamPki (max. 130 ppb Au), Savusmaki (max. 34 ppb Au) and SudenkylH (max. 33 ppb Au) (Fig. 3 ). Clear indications of Au mineralization were found in two of them, in Mustakorpi and Sudenkyla.

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In Mustakorpi, highly anomalous Au contents were detected in three nearby sampling sites at the eastern end of the anomaly. At two of the sites high Au concentrations were encountered in the weathered surface layer of the bedrock (0.1 ppm Au in both sites) and at one site in till (0.6 ppm Au). The lithology of samples from these sites suggests that the bedrock is mica gneiss or acid metavolcanite with pegmatite. Heavy-mineral studies were made on the anomalous till sample and on one of the anomalous weathered bedrock samples. They revealed that the > 2.8 s.g. fraction contained more than 95% mica. In the heavy-mineral fraction of the till sample, the rest was composed of garnet and in that of the weathered bedrock sample of limonite. No gold nuggets or grains of sulphides were found. It seems that the host of gold is ether erratic native nuggets or more probably mica and possibly also limonite. A repeated analysis of the till sample yielded 0.5 ppm Au, indicating that the “nugget effect” is not important. This leaves the possibility that the anomaly is a secondary enrichment of gold connected with pre- or postglacial weathering. More detailed studies are necessary to support this hypothesis. In Sudenkyla follow-up studies revealed an area covering about 600 m by 600 m, where several highly Au-anomalous sites occurred in till. In most of these sites Au-mineralized bedrock was found roughly to underly anomalous till. In bedrock, partly weathered as much as one meter deep, maximum Au concentration was 3.5 ppm and in till, which is 1.8 m thick on average, it was 4.1 ppm. Mineralization contains arsenopyrite (max. 5900 ppm As) and iron sulphides and seems to be mica gneiss hosted. Although Au and As are both present in the mineralization, their maximum anomalies infrequently coincide. Antimony was not analyzed, but according to the results of the regional study, it is not a characteristic element. More detailed prospecting, including diamond drilling, is going on at this target. Arsenic. Contents of As are well above the lower detection limit of the analytical method employed, and the frequency distribution is lognormal. Nationwide, the Seinajoki As data are more clearly anomalous than the Au data collected from that area. Almost half of the As results are above the content level which corresponds 90th percentile in the nationwide Atlas data (Fig. 2). The area1 distribution pattern of As (Fig. 4) resembles that of Au, but is more scattered. In places it clearly differs from the pattern of Au, indicating that As is only partly related with same sources. The As-mineralized Marttalanniemi-Kyrkosjgirvi zone is not well delineated by As in till. Only some highly anomalous As concentrations above the zone itself (55-83 ppm) indicate the mineralization. On the contrary, the Sikakangas area, particularly the western part of it, is clearly manifested (max. 270 ppm). Many samples with highly anomalous As are located around the small Sb occurrence at Viitalankyla (max. 486 ppm, northwest from the occurrence). This implies, together with the elevated Au and Sb contents, that

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mineralization occurs here over a wider area than was known before. Antimony contents are only slightly anomalous, however, indicating that the chemistry of the source is different from that encountered in the Marttalanniemi- KyrkosjPrvi Sb-zone. Highly anomalous As contents also occur with anomalous Au contents in Sudenkyla (max. 3 1 ppm As, 33 ppb Au), Pojanluoma (max. 89 ppm As, 53 ppb Au), Honkakyla (max. 42 ppm As, 940 ppb Au), and Perala (max. 68 ppm As, 18 ppb Au). The source of these anomalies is not exactly known, except in Sudenkyla where the detected Au mineralization was noticed to contain arsenopyrite. Anomalous As contents also seem to follow patterns of base-metal enrichment (Fig. 6). This is most pronounced in the eastern end of the area sampled and in Pojanluoma in the western part of the area. Bedrock clasts from till samples and samples from bedrock indicate that this kind of As could be related to iron-sulphide and graphite bearing mica schists and gneisses. Antimony. About 20% of the Sb contents of the till samples are at or below 0.1 ppm (the reporting limit of the results), making the frequency distribution of the data more positively skewed than it should be. It is probably near log-normal. The Seinajoki population does not be clearly distinguished from the nationwide Atlas population. However, it has a clear anomalous tail which lacks in the Atlas data and implies Sb-mineralized bedrock to occur in the Seinajoki district (Fig. 2 ). The areal distribution pattern of Sb in till differs significantly from that of Au (Figs. 3 and 5), the fact that factor analysis of the till data clearly predicts. Unlike the pattern of Au and As, the pattern of Sb nicely reflects the Marttalanniemi-Kyrkiisjarvi Sb zone, the highest contents (max. 8.7 ppm) occurring in the Kalliosalo area, where the mineralization was strongest. Only in KCmesneva and ViitalankylH was the mineralized bedrock not clearly indicated. Highly anomalous areas occur also west and south of Kalliosalo and Marttalanniemi and south of Sikakangas. It is hard to believe that these anomalies are part of glacial dispersal trains derived from the known Sb-mineralized areas. It is more likely that the patterns indicate that mineralization comprises a larger area than was known before (bedrock outcrop is poor). Follow-up studies, made in Honkakyla, in the southernmost end of the Sb anomaly, extending south from Sikakangas, support this idea. The results there indicate that most of the till anomalous in Sb is probably derived from a local, faintly Sb-mineralized pegmatite. The dispersal train from this source is traceable in surface till for about 2 km. New Sb-anomalous target was encountered in Pojanluoma, in the West central part of the sampled area. The anomaly seems to cut across the contact of mica gneiss and granodiorite, occurring mostly within the area where the granodiorite outcrops. Because bedrock is poorly exposed, the true relation-

P. LESTINEN ET AL.

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ship of anomaly to bedrock lithology remains unclear. Also Au and As had anomalous concentrations in till in the Pojanluoma area. A clearly different distribution pattern for Au implies, however, that the source of Au differs from that of Sb. Arsenic is connected, at least partly, with the Sb source. In the eastern part of the study area Sb contents in till are faintly anomalous over large areas, possibly reflecting lithological variation of the bedrock. In Rajaneva the,Sb anomaly is located on and just south of a known amphibolitic horizon indicating that low-level Sb anomalies also elsewhere in the Seinajoki region could be related to similar bedrock. Though most of the areas known to include Sb occurrences are reflected in the distribution pattern of Sb, the contents are low, even near occurrences. This suggests that the actual mineralized bodies are not indicated, but rather the anomalous host rock or a larger faintly mineralized zone comprising several bodies. This idea receives support from the fact that the Seinajoki porphyrite, a general host for the Sb occurrences contains 12.4 ppm Sb (average of 43 samples; Karkkainen, 1985). This corresponds quite well to the highest concentrations found in till. Base metals. Concentrations of base metals in till correlate positively with each other (Table 2) suggesting that they are associated with the same bedrock sources. Accordingly, only factor scores of base-metal-dominated factor 1 of the factor analysis is presented (Fig. 6). The factor score pattern shows anomalous till to occur both in the western and eastern parts of the study area. The central part, including the Marttalanniemi-Tervasmlki Pb-Zn-Ag-As mineralized zone, is mostly at background level. In Pojanluoma and in the most eastern end of the area sampled, base-metal anomalies seem to be related, at least partly, with iron sulphide and graphite-bearing schists, as mentioned earlier. It is noteworthy that Ag clearly follows the pattern of base metals instead of the pattern of Au. CONCLUSIONS

The study on the reliability of Au, As and Sb data obtained from the line fraction of till revealed that the results of As are reasonably good estimates of contents, the precision being roughly as great as for the base metals analyzed. The results of Au proved to be quite poor, which suggests that the absence of an Au anomaly in till does not necessarily means the absence of Au-mineralized bedrock. For the Sb data reliability remained largely open because of a too low Sb content of most of the duplicate samples used for estimation. Nationwide, the Seintijoki As data form a clear anomalous population, and there is a remarkable anomalous component in the Au and Sb data, too. Both factor analysis of the till data and the area1 distribution patterns indicate that anomalous Au in the fine fraction of till is mostly derived from sources different from those for anomalous. Sb. Anomalous As, though partly

TILL GEOCHEMISTRY OF Au, As AND Sb IN THE SEINkJOKI DISTRICT

359

related to Au and Sb sources, is also related to the base metals analyzed, which on their part strongly correlate with each other. Known Au-As occurrences are poorly indicated by the area1 distribution pattern of these elements in the fine fraction of till. On the other hand, there are large Au anomalies in areas where mineralized bedrock is not known. The meaning of these anomalies as an indicator of Au ore-potential areas remains uncertain. Unlike Au and As the distribution pattern of Sb in till indicates the known areas with Sb mineralization. The most anomalous concentrations are found around Kalliosalo where mineralization is strongest. However, with the sampling method used individual occurrences do not seem to be indicated. Probably the Sb anomalies in till reflect more widespread, less anomalous units (mineralized zones) than those comprising actual occurrences. It is further concluded that there is no significant geochemical halo for Au and As. Detailed studies in live areas with anomalous Au in till gave two positive results, in Sudenkyla and Mustakorpi. In Sudenkyhi, the style of mineralization seems to be different from that encountered before this study. According to investigations carried out in the SudenkylP area until now, the host rock seems to be mica schist or gneiss and the mineralization to include gold, iron sulphides and arsenopyrite, but few or no Sb minerals. In Mustakorpi, the anomaly encountered in till and weathered bedrock seems to be caused by a secondary gold reprecipitated in mica and probably also in limonite, but the primary source of gold is unknown. The distribution pattern of Sb indicates that areas with possible ore-grade Sb potential are much larger south of the Kalliosalo-Marttalanniemi zone and south of Sikakangas than was previously known. A promising anomaly was also found in Pojanluoma. A strong As anomaly near Viitalankyla, where Au and Sb are also anomalous, suggests that this area is worth prospecting, as well as those mentioned above. ACKNOWLEDGEMENTS

The field staff of the Geochemistry Department of the Geological Survey of Finland carried out the sampling, and the laboratory staff of the Survey in Rovaniemi and Kuopio made the analyses. Mrs. M. Nikkarinen, M. SC., was responsible for the heavy mineral studies, Mr. M. Partanen for the ADP handling, and Miss H. Moberg for drawings. Professor R. Salminen critically read the manuscript, and Mr. N. KZrkkainen, M. SC., gave valuable comments. To all these the authors wish to express their thanks. REFERENCES Aho, L., 1980. Seintijoki-Nurmon Sb-mineralisaatiosta. (On the Seingjoki-Nurmo Sb-mineralization.) Rep. M 19/2222/-80/l / 10, Arch. Geol. Surv. Finl., 8 pp. (in Finnish).

360

P. LESTINEN ET AL.

Alviola, R., 1986. Tinamalmitutkimukset Etell-Pohjanmaan liuskejakson alueella vuosina 19801983: Tutkimukset Seinajoen Pajuluoman alueen ympiristossl. (Tin-ore investigations in the South-Pohjanmaa schist zone in 1980-1983: Studies in the surroundings of Pajuluoma, Seinajoki). Rep. M 19/2222/-86/l /lo, Arch. Geol. Surv. Finl., 9 pp. (in Finnish). Borodaev, Yu.S., Bortnikov, N.S., Mozgova, N.N., Ozerova, N.A., Oivanen, P. and Yletyinen, V., 1983. Association of ore minerals in the deposits of the Seintijoki district and the discussion on the ore formation. Bull. Geol. Sot. Finl., 55 ( 1): 3-23. Gloss, L.C. and Sado, E.V., 1979. Geochemical drift prospecting studies near gold mineralization Beardmore-Geraldton area, Northwest Ontario, Canada. In: J.R. Watterson and P.K. Theobald (Editors), Proceedings of the Seventh International Geochemical Exploration Symposium, Golden, Colorado, 1978. The Association of Exploration Geochemists, Rexdale, Ont., pp. 459-477. DiLabio, R.N.W., 1982. Drift prospecting near gold occurrences at Onaman river, Ontario and Oldham, Nova Scotia. In: R.W. Hodder and W. Petruk (Editors), Geology of Canadian Gold Deposits. CIM, Spec. Vol., 24: 261-266. Gleeson, CF. and Sheehan, D.G., 1987. Humus and till geochemistry over the Doyon, Bousquet and Williams gold deposits. CIM Bull., 80( 898): 58-66. Gleeson, C.F. and Nichol, I., 1987. Workshop 2: Till Geochemistry. In: R.G. Garrett (Editor), Geochemical Exploration 1985. J. Geochem. Explor., 29: 359-373. Harris, J.F., 1982. Sampling and analytical requirements for effective use of geochemistry in exploration for gold. In: A.A. Levinson (Editor), Precious Metals in the Northern Cordillera. Proceedings of a Symposium, Vancouver, British Columbia, Canada, 198 1. The Association of Exploration Geochemists and The Cordileran Section of the Geological Association of Canada, pp. 53-67. Hirvas, H., 1980. Malminetsintla palvelevat maaperitutkimukset Nurmon Kalliosalossa. (Studies of Quarternary deposits as an aid to ore prospecting in Kalliosalo, Nurmo). Rep. M 19/2222/-80/3/90, Arch. Geol. Surv. Fin]., 1 p. (in Finnish). Koljonen, T., (Editor), Suomen Geokemian Atlas, Osa 2: moreeni (The Geochemical Atlas of Finland, Part 2: Till). Geol. Surv. Finl., in print. Kontas, E., 1981a. Geokemialliset kultatutkimukset Seinajoen-Nurmon antimonivyohykkeelll. (Geochemical gold investigations within the Seinajoki-Nurmo antimony zone). Rep. M 19/ 2222/-81/l /3O/Au, Arch. Geol. Surv. Finl., 16 pp. (in Finnish). Kontas, E., 198 lb. Rapid determination of gold by flameless atomic absorption spectrometry in the ppb and ppm ranges without organic solvent extraction. Atomic Spectr., 2: 59-6 1. Kontas, E., Niskavaara, H. and Virtasalo, J., 1986. Flameless atomic absorption determination of gold and palladium in geological reference samples. Geostandard Newslett., 10 (2 ): 169171. Karkkainen, N., 1985. Seinajoen litogeokemiallisen profiilin Sn, W, Li, Mn ja V seka e&den muiden alkuaineiden jakauma. (The distribution of Sn, W, Li, Mn, V and some other elements in the Seinajoki litogeochemical profile). Rep. M 19/2222/-851 l/32, Arch. Geol. Surv. Finl., 20 pp. (in Finnish). Nenonen, K., 1982. Till stratigraphic studies as an aid to ore prospecting in Finland. Striae, 20: 101-105. Nenonen, K., 1983. Satamoluhdan kassiteriittipegmatiittilohkareviuhkan ja tina-anomalioiden selvitys 2 l .-24.9.198 1. (Studies of a boulder train of kassiterite-bearing pegmatite and Sn anomalies in Satamoluhta 2 l.-24.9.198 1). Rep. P 13.02.39, Archives of Geological Survey of Finland, 4 pp. (in Finnish). Nenonen, K. and Hakala, P., 1982. Malminetsintfa palvelevat maaperatutkimukset Seinajoen Pajuluomassa KL. 2222 08. (Studies of Quarternary deposits as an aid to ore prospecting in Pajuluoma, Seinajoki, map sheet 2222 08). Rep. P 13.02.035, Arch. Geol. Sure. Finl., 4 pp. (in Finnish).

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Nenonen, K. and Hakala, P., 1984. Geokemialliset Au-anomaliat harvapistenlytteenotossa. Mustakorven Au-pitioset lohkareet. (Geochemical Au anomalies in the regional scale study. Au-bearing boulders in Mustakorpi). Report, Arch. Geol. Surv. Finl., 2 pp. (in Finnish) (unpubl.). Nenonen, K. and Huhta, P., 198 1. Malminetsintia palvelevat maaperatutkimukset Perlseinajoen Viitalassa KL. 2221 09. (Studies of Quarternary deposits as an aid to ore prospecting in Viitala, Peraseinajoki, map sheet 2221 09.). Report, Arch. Geol. Surv. Finl., 5 pp. (in Finnish) (unpubl.). Nenonen, K. and Johansson, P., 198 1. Malminetsintia palvelevat maaperlitutkimukset Jalasjlrven sceeliittilohkareaiheella, KL. 222 I 06, 08, 09. (Studies of Quaternary deposits as an aid to ore prospecting in the surrounding of Scheelite boulders in Jalasjlrvi, map sheets 222 1 06,08,09.). Rep. P 13.2.024, Arch. Geol. Surv. Finl., 6 pp. (in Finnish). Nenonen, K. and Liippo, L., 1984. Malminetsintz palvelevat maaperitutkimukset Seinajoen Eskoonmlen scheeliittilohkareaiheella K/73999 ja 7400/-83 KL 2222 07 B. (Studies of Quaternary deposits as an aid to ore prospecting in the surroundings of scheelite-bearing boulders K/7399 and 7400/-83 in Eskoonmaki, Seinajoki, map sheet 2222 07 B.) Rep. Arch. Geol. SUN. Finl., 3 pp. (in Finnish) (unpubl.). Neuvonen, K.J., 1961. Pre-Quarternary rocks, sheet 2222, Seinajoki. Geological map of Finland, 1: 100 000. Geol. SUN. Finl. 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. IMM, London, pp. 201-2 10. Niskavaara, H., Virtasalo, J. and Lajunen, L.H.J., 1985. Determination of antimony in geochemical samples by graphite fumice atomic absorption spectrometry using different matrix modifiers. Spectrochim. Acta, 40B: 12 19- 1222. Nurmela, P., 1985. Seinajoen Pajuluoman alueen pegmatiiteista. (On the pegmatites in Pajuluoma, Seinajoki). M.Sc. thesis, Archives of the University of Helsinki, 115 pp. (in Finnish) (unpubl.). Oivanen, P., 1982. Antimonitutkimukset Seinajoen-Nurmon alueella vuosina 1975-1982. (Antimony studies in the Seinajoki-Nurmo area in 1975-1982). Rep. M 19/2222/-82/ l/ 10, Arch. Geol. SUN. Finl., 123 pp. (in Finnish). Oivanen, P., 1983. Tinamalmitutkimukset Etell-Pohjanmaan liuskejakson alueella vuosina 1980-l 983: Tutkimukset Seinajoen Pajuluomassa. (Tin-ore investigations in the SouthPohjanmaa schist zone in 1980-1983: Studies in Pajuluoma, Seinajoki), Rep. M 19/2222/ -83/ l/ 10, Arch. Geol. SUN. Finl., 25 pp. (in Finnish), Pilkk8nen V., 1966. On the geology and mineralogy of the ocurrence of native antimony at Seinajoki, Finland. Bull. Comm. Geol. Finl., 225, 70 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 llomantsi, eastern Finland. In: W.J. Phillips (Editor), Prospecting in Areas of Glaciated Terrain 1986. IMM, London, pp. 201210. Simonen, A., 1980. The Precambrian in Finland. Geol. SUN. Finl., Bull. 304, 58 pp. Sopuck, V., Schreiner, B.T. and Averill, S., 1986. Drift prospecting for gold in the southern Shield of Saskatchewan, Canada. In: W.J. Phillips (Editor), Prospecting in areas of glaciated terrain 1986. IMM, London, pp. 2 17-240. Stone, P. and Gallagher, M.J., 1984. Mineral exploration in Lower Paleozoic turbidites of south Scotland. In: M.J. Gallagher (Editor) Prospecting in Areas of Glaciated Terrain 1984. IMM, London, pp. 201-2 12. Toverud, O., 1987. Geochemical prospecting for gold in the county of Jamtland, upper central Sweden. 12th lnt. Geoch. Explor. Symposium and 4th Symposium on Methods of Geochemical Prospecting, Programme and Abstracts. BRGM, Orleans, p. 59.