Organic matter and the concentration of metals in precambrian stratiform deposits of the bohemian massif

Precambrian Research, 33 (1986) 225--237

225

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ORGANIC MATTER AND THE CONCENTRATION OF METALS IN PRECAMBRIAN STRATIFORM DEPOSITS OF THE BOHEMIAN MASSIF

Z. POUBA and B. KR~BEK Department of Economic Geology, Charles University, Prague (Czechoslovakia)

ABSTRACT Pouba, Z. and K~fbek, B., 1986. Organic matter and the concentration of metals in Precambrian stratiform deposits of the Bohemian Massif. Precambrian Res., 33: 225- 237. Organic matter is preserved in unmetamorphosed and low-grade metamorphic pyrite and/or Mn-bearing black shales, pyrite-Mn stratiform ores, cherts and siliceous stromatolites of Proterozoic age in the Barrandian area of the Bohemian Massif. This organic matter consists mostly of kerogen with an isotopic composition and relative concentration of free radicals which depend on the metamorphic grade of the host rock (6 '3CpDB= 30.7% ,, to 36.9"/0u;Nrel. = 0 . 1 6 1 t o 0 . 1 3 2 ) . Enrichment of the kerogen in the light isotope of carbon, a marked isotopic shift towards more positive ~ ~C values with increasing metamorphic grade, and the very low crystallinity of the incipient graphite lattice reflect the original aliphatic (alicyclic) character and algal origin of the kerogen; its composition is similar to that of shungite. Chiefly aliphatic and alicyclic hydrocarbons were identified in the solvent extracts of these Proterozoic rocks. Alkane patterns differ according to host rock type, indicating their possible syngenetic origin. Other identifiable organic compounds (amino acids, carboxylic acids, phenols and aromatic aldehydes) are probably a result of later contamination. The concentrations of U, Th, V, Cr, Sb, Ag, Au, Zn and Cu in pyrite and pyrite-Mn ores are relatively low. In contrast, stratiform concentrations of U, V and chiefly Au, Ag, Sb and Zn are confined to siliceous stromatolites rich in kerogen, or to adjacent jaspilites. From a palaeogeographic viewpoint, the highest concentrations of metals occur at the boundary between the oxidic facies (stromatolites, jaspilites and intercalations of other rocks) and the reduced facies (black shales) where a syndiagenetic geochemical barrier developed. Mineralization was significantly influenced by subalkaline volcanism. Stromatolites of an intertidal type apparently formed where volcanoes rose to sea level. Stromatolitic breccias are intruded by chromium-rich green quartz veins and locally covered by tufts.

INTRODUCTION A f t e r t h e d i s c o v e r y o f s t r o m a t o l i t e s in t h e C e n t r a l B o h e m i a n B a r r a n d i a n Proterozoic, sedimentary and structural relationships between the stromat o l i t e s a n d l o c a l i r o n ores w e r e s t u d i e d ( P o u b a , 1 9 7 3 ) . S i l i c e o u s s t r o m a t o -

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~ 1986 Elsevier Science Publishers B.V.

226 lites c o m m o n l y occur in association with jaspilites, from which haematitelimonite ores were formed by weathering, most probably during the Carboniferous and Tertiary. These ores have served as a basis for the steel industry at several sites in Bohemia. In a subsequent study of the geochemistry of Proterozoic stromatolites and accompanying rocks, it was found that stromatolitic rock sequences are enriched in V and U (Mr~izek and Pouba, 1977), and that jaspilites and several stromatolite beds have also increased contents of Sb, Ag and Au (Mreizek, 1984). Chemical analyses of Proterozoic basic rocks in the Barrandian revealed that two major geochemical types are present: primitive rocks of the oceanic type (Bou~ka et al., 1974; Jelfnek et al., 1974; Jake~ et al., 1976, 1979) and rocks more characteristic of island arcs and continental margins. Next to volcanic rocks of the subalkalic type and black shales, siliceous stromatolites were found to have the greatest concentrations of heavy metals. Originally, it was assumed that the partial pressure of sulphur and the adsorption by organic materials were the main cause of this concentration. However, chemical analyses of rocks richest in organic matter (pyritic black shales) revealed metal contents of Th, U, V, Cr. Sb, Ag, Au, Zn and Cu lower than those found in siliceous stromatolites. Meanwhile, Pertold et al. (1975) demonstrated that the Fe and Cu sulphide ore in the Palaeozoic stratiform Tisovei deposit was concentrated in phyllites containing the highest proportion of extractable organic compounds. These phyllites are locally light-coloured and siliceous, whereas dark-coloured 'graphitic' phyllites have lower contents of metals and solvent extractable organic compounds. The aim of the present investigation was to determine the composition, origin and function of organic matter in the concentration of metals in Precambrian rocks. The possible contamination of these rocks by deep circulation of meteoric waters was considered. The scanning electron microscopy of the stromatolites had previously demonstrated the presence of micro-organisms which most probably infiltrated the stromatolites after their formation (Pacltov~i, 1976). At the same time, work was carried out on the reconstruction of facies of Proterozoic sediments, and on the processes controlling the distribution of elements derived from submarine volcanism (Bou~ka et al., 1974; Jeli'nek et al., 1974; Mr~izek and Pouba, 1977). This paper summarizes the results of research carried out by a team of workers at the Faculty of Science, Charles University, and in the Isotope Laboratory and Organic Geochemistry Laboratory of the Geological Survey, Prague. The analytical methods used are described elsewhere (Kl'~ek et al.. 1976, 1977; Jehlieka, 1984; Jehlicka et al., 1984). GEOLOGY AND GEOCHEMISTRY OF LOW GRADE METAMORPHICPRECAMBRIAN ROCKS OF THE BOHEMIAN MASSIF The distribution of Precambrian rocks in the Bohemian Massif is not completely defined. A connection of Proterozoic rocks with the Brioverian

227 in B r i t t a n y , F r a n c e , is assumed; and a possible c o n n e c t i o n with the Proterozoic units o f F e n n o s a r m a t i a has been suggested (Fig. 1). In s t u d y i n g the g e o c h e m i s t r y o f r o c k s o f t h e B o h e m i a n Massif it was f o u n d t h a t t h e P r o t e r o z o i c r o c k s are enriched in certain metals (e.g., Au) relative to o t h e r r o c k units (Fig. 2). More detailed g e o c h e m i c a l studies o f the P r o t e r o z o i c r o c k s s h o w e d t h a t the d i s t r i b u t i o n o f Au and o t h e r metals {including Th, U, V, Cr, Sb, Ag, Zn and Cu) in these r o c k s is very i n h o m o g e n e o u s and t h a t the m i n e r a l i z a t i o n d e p e n d s on the rock t y p e , facies c o n d i t i o n s and organic c o m p o n e n t s . T h e u n m e t a m o r p h o s e d and slightly m e t a m o r p h o s e d P r o t e r o z o i c in the B o h e m i a n Massif consists p r e d o m i n a n t l y o f clastics ( g r e y w a c k e s and shales) and basic igneous rocks, chiefly o f the tholeiitic t y p e . Jake~ et al. ( 1 9 7 6 , 1 9 7 9 ) f o u n d t h a t sediments o f the oceanic t y p e f o r m e d the greater part of the P r o t e r o z o i c r o c k sequence. F u r t h e r investigations led t o t h e r e c o g n i t i o n o f a b i m o d a l volcanic belt, p r o b a b l y an island arc, in t h e eastern part o f the B a r r a n d i a n P r o t e r o z o i c and an increase in c o n t i n e n t a l elements f a r t h e r to t h e east. 101

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Fig. 1. Proterozoic geosynclinal development in North, East and Central Europe. After Salop (1960 -- in Tochinin and Goryainov, 1964) and modified and complemented by Pouba (1970). 1 -- pre-Proterozoic platform; 2 -- Proterozoic geosyncline; 3 -- Proterozoic with quartz-iron formation (jaspilites, cherts, magnetite quartzites, etc.). Outside the Bohemian Massif the lower Proterozoic is represented and within the Bohemian Massif predominantly the upper Proterozoic is postulated; 4 -- ore deposits /Fe, in the Bohemian Massif Mn and FeS: deposits are also deposed; 5 -- magnetic anomalies; 6 wider area of banded~re deposits in NE part of the Bohemian Massif. Fig. 2. Gold content in rocks of the Barrandian Precambrian (I), Palaeozoic (II), Precambrian of Jflov6 Zone -- predominantly volcanic (III) and Moldanubian -- highly metamorphosed Precambrian with Palaeozoic relics (IV). (Based on analyses of 1200 samples.) 1 -- faults.

228

From the presence of two sedimentary and volcanic facies it can be deduced that the eastern part of the Proterozoic basin became shallower as a result of extensive volcanic activity and that submarine volcanic rocks evidently accumulated up to sea level in the basin. This interpretation is supported by the discovery of intertidal type stromatolites (Pouba, 1973, Pacltov~ and Pouba, 1978) (see Figs. 3, 4, 5). / -2' " ~,

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Fig. 4. Types of laminar-columnar siliceous stromatolites from the Bohemian Proterozoic. (a) Schematic; (b) colony of stromatolites.

229

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The Proterozoic black shales contain many pyrite (and some pyrite-Mn) ore deposits which have been intensively exploited. The black shales of these deposits contain up to 1.9% organic carbon (average 0.64% Corg)- Most of the organic matter is thermally altered kerogen (atomic H/C = 0.07 to 0.001). Both the carbon isotopic composition of the kerogen (513CpDB = - 3 0 . 7 to -36.9%o) and the number of free radicals as determined by the E.P.R. method (JehliCka, 1984), reflect a westward increase in metamorphic grade in the Barrandian area (Fig. 6). An important shift in the isotopic composition of kerogen with such a relatively minor increase in the metamorphic grade (prehnite--pumpellyite to chlorite facies) indicates preferential loss of a substantial portion of the isotopic light fraction in the original organic matter. This suggests an aliphatic, algal and/or bacterial origin of the organic matter in the sediment (Galimov, 1980; Schidlowski et al., 1983). Similar conclusions followed from X-ray diffraction investigations which revealed no increase in the crystallinity of the amorphous organic substance with increasing metamorphic grade. These results can be explained by the primary aliphatic character of the organic precursors or by lack of shearing stress during the process of graphitization (Oberlin et al., 1980). In its isotopic composition, petrological characteristics and low crystal-

230

linity, the kerogen in these Proterozoic black shales can be compared to the substance of supposed algal origin called shungite (Khavari-Khorasan] and Murchison, 1979). In addition to kerogen, solvent extractable material (Cbit, 30--830 ppm) was found in black shales of the Kamenec borehole (Fig. 7). The extract consists of n- and iso-alkanes (90%), cycloalkanes (6%) and aromatic hydrocarbons {4%). The predominant components are C-13 to C-20 n-alcanes, C-14 to C-19 isoalkanes and the isoprenoid hydrocarbons, pristane and phytane. Aromatic hydrocarbons are represented mainly by biphenyl which may have been produced by thermal alteration of the kerogen (cf. e.g., Scott, 1970). •~ i

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231 T h e a l k a n e d i s t r i b u t i o n p a t t e r n s o f b l a c k shales d i f f e r f r o m t h o s e of s t r o m a t o l i t e s , cherts (lydites) a n d h y d r o t h e r m a l q u a r t z (silexite) (Figs. 7 a n d 8). Similarly, c o n s i d e r a b l e d i f f e r e n c e s w e r e f o u n d b e t w e e n t h e distribut i o n s o f alkanes in black shales a n d in Q u a t e r n a r y soil at the s a m e locality. T h e soil e x t r a c t s , in c o n t r a s t to the P r o t e r o z o i c e x t r a c t s , c o m p r i s e m o s t l y an u n r e s o l v e d c o m p l e x m i x t u r e o f n a p h t h e n e s (see c u r v e B, Fig. 7); p h e n o l is also a m a j o r c o m p o n e n t . T h e higher c a r b o x y l i c acid c o n t e n t s o f t h e shales vary within t h e range 0 . 1 5 0 - - 2 . 4 1 p p m , with a p r e d o m i n a n c e o f p a l m i t i c a n d stearic acids, as in Q u a t e r n a r y soil ( J e h l i c k a et al., 1 9 8 4 ) . T h e h e a v y m e t a l c o n t e n t s o f t h e P r o t e r o z o i c b l a c k shales in B o h e m i a are o n t h e w h o l e in g o o d a g r e e m e n t w i t h d a t a p u b l i s h e d by S o z i n o v ( 1 9 8 2 ) f o r t h e u p p e r P r o t e r o z o i c b l a c k shale facies. H o w e v e r , t w o facies c o u l d be d i s t i n g u i s h e d -- o n e w i t h a n d t h e o t h e r essentially w i t h o u t Mn. B o t h are c h a r a c t e r i z e d b y a d e f i c i e n c y o f t h o r i u m ( t h e T h / U ratio in s u l p h i d e - b e a r i n g b l a c k shales is 2 . 5 - - 3 . 8 ; cf. t h e Chvaletice d e p o s i t , 0 . 6 - - 1 . 0 ) a n d by a primitive a s s o c i a t i o n o f m i n o r e l e m e n t s . T h e c o n t e n t s o f selected e l e m e n t s in t h e sulphide facies are given in T a b l e I. TABLE I Facies of sulphidic shales with Mn Geometric mean

V ppm

Chvaletice deposit 295

U ppm

Cr ppm

Sb ppm

Ag ppm

Au ppm

3.6

189

9.2

n.d

1

Zn ppm

Cu ppm

Corg %

65

112

0.9

Cot~ %

Facies of sulphidic black shales without Mn contents Geometric mean

V ppm

U ppm

Cr ppm

Sb ppm

Ag ppm

Au ppm

Zn ppm

Cu ppm

Kamenec various other localities

134

3.6

62

3.2

1.7

1

112

32

1.1

198

3.2

90

3

1

35

124

0.64

11

ORGANIC MATTER AND MINERALIZATION OF PRECAMBRIAN STROMATOLITES, CHERTS AND JASPILITES (OXIDIC FACIES) T h e P r e c a m b r i a n s t r o m a t o l i t e s investigated c o n t a i n 0 . 6 - - 0 . 8 % Corg in t h e f o r m o f s t r o n g l y t h e r m a l l y altered k e r o g e n ( H / C = 0.06). T h e c a r b o n isot o p i c c o m p o s i t i o n varies in t h e range 513CpD B = - 3 2 . 2 to - 3 4 . 2 % o and d o e s n o t d i f f e r significantly f r o m t h e i s o t o p i c c o m p o s i t i o n o f equally m e t a m o r p h o s e d k e r o g e n in the b l a c k shales. T h e s t r o m a t o l i t i c k e r o g e n d o e s n o t

232

show any incipient graphite structure and, like that in the black shales, is similar to shungite. The a m o u n t of extractable matter is very small (Cbi t = 60 to 80 ppm). The Corg/Cbi t ratio, however, is much lower in stromatolites (10.6) than in black chert (90.9). The low Corg/Cbi t ratio in stromatolites may be due to differences in either the nature and lipid content of the microbial organic matter which survived early diagenesis or the catalytic effect of the rock mineral matrix on cracking reactions during catagenesis and early metamorphism. The largest fraction of the extract is formed of aliphatic hydrocarbons whose distribution patterns (Fig. 8) differ markedly from those of shales (Fig. 7) and therefore apparently depend on the lithology of the host rock. This is an argument for their syngenetic origin (McKirdy and Hahn, 1982).

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A considerable a m o u n t of amino acids (up to 23 ppm) was found in quartzose rocks and, in contrast to the hydrocarbons, their distribution pattern was identical in all the samples studied. This fact, as well as the identification of very unstable serine and threonic (K]h'bek et al., 1977), suggests an epigenetic origin. Phenols and aromatic aldehydes (including vaniline) are present in the rocks in low concentrations (0.17 to 0.39 ppm). They are probably of epigenetic origin, considering the absence of a Precambrian lignin source. It is most likely that t h e y are products of the b i o chemical activity and decomposition of recent lichens (K~]%ek et al., 1976). Although siliceous stromatolites contain approximately the same a m o u n t of organic matter as black shales, they have increased contents of V, U, As, Au, Sb, Ag, Cu and Zn. The U/Th ratio is also higher (3.2--4.6). The average contents of metals in Proterozoic siliceous rocks from the area of subalkaline volcanics (Fig. 3) are given in Table II. Dark and light laminae from the siliceous stromatolites at the Kokgin locality were analysed and differences in the contents of many elements

233 T A B L E II Facies o f siliceous s t r o m a t o l i t e s a n d a c c o m p a n y i n g r o c k s

Geometric mean

Stromatolites loc. Kok~fn Stromatolites loc. B o r o v n o Cherts loc. Kok~fn Cherts var. loc. JaspUites loc. Kok~fn Jaspilites var. loc. Cr-bearing q u a r t z loc. Kok~fn

V ppm

U ppm

Cr ppm

Sb ppm

Ag ppm

1218

78

28

65

5

936

56

32

48

196

12

59

76

10

268

Au ppm

Cu ppm

Zn Coa~ ppm %

2.4

14

520

0.78

4

1.3

9

438

0.66

30

0.1

1.2

72

238

0.00

86

29

0.2

1.8

63

185

0.00

28

87

24

0.8

1.4

21

248

0.01

235

32

40

15

0.9

32

33

210

0.02

132

14

386

5

0.1

3

10

130

0.00

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Fig. 9. C o n c e n t r a t i o n s o f s e l e c t e d elements in laminae of a stromatolite (oncolite) from t h e Kok~fn locality (see Fig. 3). (X-ray fluorescence and I N A methods--laboratories GPUP.)

234 were found (Fig. 9). The role of biogenic matter in selectively concentrating certain metals {notably V, Cr, U) seems to be demonstrated. PALAEOGRAPHY, VOLCANISM, SOURCE OF METALS AND THEIR REMOBILIZATION Higher contents of vanadium and uranium are found not only in stromatolites but in all the oxide facies (stromatolites, jaspilites and cherts) associated with the volcanics. Sulphide-bearing black shales of the same area are very poor in U and V. Gold (0.3--0.5 ppm) and Ag (over 1 ppm) were found in cherts and some other quartz-rich sediments of the Barrandian upper Proterozoic. Gold occurs predominantly in the banded iron formation (jaspilite and similar rocks) but the highest contents of other metals are in the siliceous stromatolites (Table II). It is suggested that subalkaline volcanics were the source of U and V, as well as of Au, Ag, Zn, Sb, Cu and Cr. These elements were concentrated at the syndiagenetic geochemical barrier between the sulphide and oxide facies, and particularly where organic matter accumulated locally in the oxide-facies through the burial of stromatolites. Stromatolites apparently formed in places where volcanoes rose to sea level. Volcanic explosions gave rise to stromatolitic breccias (Fig. 5) which were intruded by chromium-rich green quartz veins and locally covered by tuffs (see Table II, Cr-bearing quartz). This mechanism for the accumulation of metals has all the essential features of the model proposed by Trudinger and Williams {1982). The authors suggest that tidal algal flats represent an ideal medium for base metal mineralization. The model includes the burial of algal mat, the develo p m e n t of microbially mediated reducing sulphate activity, and the fixation of metals from metal-enriched ground-water passing through the buried mat. The lack of an exact recent analogue of this model {e.g., in Spencer Gulf, South Australia, or in sabkha sediment of the Trucial Coast, Arabian Gulf) can be explained by the absence in these areas of source metals as suitable as the volcanic rocks in the Barrandian Proterozoic. It is assumed that Variscan granitization and anatexis of rocks in the Bohemian Massif could have brought about a further concentration of Au, U and possibly other heavy metals in hydrothermal solutions. This would explain why a spatial connection now exists between metal accumulations in the Proterozoic sediments and the younger ore deposits associated with Variscan granitoids. Organic compounds, and possibly even U, of the upper Proterozoic Series might have been the source for U-bearing anthraxolites in ore veins of Variscan age in the P~fbram ore district (Fig. 3). The isotopic composition of the anthraxolites (5 J3CPDB = --54.2 to -56.8%o) and the presence of isoprenoid hydrocarbons (phytane and pristane) in the bituminous fraction suggest a biogenic origin (Ki~fbek, 1981}. These anthraxolites may have

235 f o r m e d b y r a d i o l y t i c p o l y m e r i z a t i o n o f m i g r a t i n g h y d r o c a r b o n s , in t h e s a m e w a y as was p r o p o s e d for organic m a t t e r ( t h u c h o l i t e ) o f t h e Witwatersr a n d c o n g l o m e r a t e s ( S c h i d l o w s k i , 1981}. CONCLUSIONS (1} S t r a t i f o r m a c c u m u l a t i o n s o f Fe sulphides in t h e r e d u c e d facies o f t h e P r o t e r o z o i c o f t h e B o h e m i a n Massif are c o n t r o l l e d b y organic m a t t e r p r e s e r v e d in black shales. H o w e v e r , t h e c o n c e n t r a t i o n s o f U, Th and num e r o u s o t h e r h e a v y metals in t h e s e shales are relatively low. T h e r e f o r e , organic m a t t e r did n o t p l a y a significant role in t h e s e q u e s t r a t i o n o f t h e s e metals. T h i s can be e x p l a i n e d b y the original lipid-rich algal a n d / o r bacterial c h a r a c t e r o f t h e organic m a t t e r and its p o o r c o m p l e x i n g ability. (2) H i g h e r s t r a t i f o r m c o n c e n t r a t i o n s o f U, T h , V and o t h e r m e t a l s o c c u r in k e r o g e n - r i c h s t r o m a t o l i t e s . H o w e v e r , t h e largest c o n c e n t r a t i o n s o f m e t a l s e v i d e n t l y lie at t h e b o u n d a r y b e t w e e n t h e o x i d i c facies ( s t r o m a t o l i t e s , B I F s , jaspilites) a n d t h e r e d u c e d facies {black shales} w h e r e a g e o c h e m i c a l b a r r i e r d e v e l o p e d d u r i n g diagenesis. (3) T h e original m i n e r a l i z a t i o n was i n f l u e n c e d significantly b y subalkaline v o l c a n i s m . T h e s t r o m a t o l i t e s , being o f an inter-tidal t y p e , m o s t likely f o r m e d w h e r e volcanic r o c k s a c c u m u l a t e d u p t o or j u s t b e l o w sea level. ACKNOWLEDGEMENTS Sincere t h a n k s are d u e t o Dr. P. Mr~zek and J. Jehli~ka for s o m e o f the analyses a n d s t i m u l a t i n g discussions.

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