Normal versus Metal-rich Black Shales in the ...

Vestnik Ceskeho geologickeho ustavu 75, 3, 2000

Normal versus Metal-rich Black Shales in the Barrandian Neoproterozoic of the Tepla-Barrandian Unit: a Summary with New Data JANPASAVA

Czech Geological Survey, Klarov 131/3, 118 21 Praha 1, Czech Republic, e-mail: [email protected]

A b s t r a c t . Normal and metal-rich black shales can be recognized in the Barrandian Neoproterozoic of the Tepla-Barrandian Unit. Normal black shales form, together with greywackes, thick flyschoid sequences. They have been deposited mostly under normal marine conditions. They are generally poor in trace elements, with concentrations very close to other world normal black shales. Conversely, metal-rich black shales mostly occur in close association with basic volcanogenic rocks (similar to recent MORB) and were formed in lower energy environment - semiisolated or isolated basins. They bear anomalously high metal concentrations derived mostly from occasional volcanogenic-hydrothermal vents. These outcropping fades together with numerous waste dumps and pit lakes which resulted from their past mining represent a serious danger for human living especially through the generation of acid waters. A b s t r a k t . V oblasti svrchniho proterozoika Tepelsko-barrandienskeho bloku je mozno rozlisit normalni a kovonoske cerne bfidlice. Normalni cerne bfidlice jsou spolecne s drobovymi sedimenty soucasti mocneho flysoidniho komplexu a vznikaly pfevazne v normalnim morskem prostfedi. Jedna se o chemicky nezrale sedimenty prevazne chude stopovymi prvky, ktere maji prumerny obsah C,,rg-0.88 % a S-0.66%. Kovonosne facie tvorene prevazne metaprachovci jsou lizce spjaty s vyskyty bazickych vulkanitu a vznikaly v prostfedi (semi-)izolovanych bazenii s dostatecnou komunikaci s otevfenym proterozoickym oceanem. Tyto horniny se vyznacuji anomalne vysokymi obsahy kovu i vyssimi prumernymi obsahy Corg-2.4% a S4.9%. Vychozy kovonosnych cernych bfidlic, jakoz i odvaly a jezirka po tezbe sulfidickych cernych bfidlic jsou zejmena prostfednictvim uvolnovani kyselych vod vaznym nebezpecim pro zivotni prostfedi. Key w o r d s : Barrandian Neoproterozoic Formation, Tepla-Barrandian Unit, Normal black shale, Metal-rich black shale, Geology, Geochemistry, Mineralogy, Stable Isotopes, Radiogenic Isotopes, Fluid Inclusions, Environmental impacts

Introduction This paper is a contribution to a group of various papers devoted to the Barrandian Neoproterozoic of the Tepla-Barrandian Unit (also known as the Barrandian Upper Proterozoic - for explanation see Kfibek et al. in this volume), edited by Z. Pouba and V. Skocek. The paper summarizes and also brings new geological, mineralogical, geochemical, stable and radiogenic isotope, fluid inclusion and environmental data from normal black shales (NBS) and metal-rich black shales (MBS) of the Barrandian Neoproterozoic in the Tepla-Barrandian Unit. Detailed stratigraphy (see Masek, this volume, see Rohlich, this volume) and the evolution of the Barrandian Neoproterozoic within the Cadomian erogenic belt (see Kfibek et al., this volume) are included in this volume.

Geological Setting The Barrandian Neoproterozoic of the TeplaBarrandian Unit represents a Cadomian Unit incorporated in the Paleozoic mobile zone within the region of low-grade metamorphism of the Central European Variscan Belt (Zoubek et al. 1988) and is composed of a complex of eugeosynclinal clayey and greywacke sediments and abundant products of volcanogenic and pyroclastic rocks of various chemical composition (ranging from acid through intermediate to basic volcanites). The Proterozoic

Formation exhibits a simple structure, with alternating anticlinal and synclinal NE-SW-trending belts (Holubec, 1966) - Fig. 1. Detailed stratigraphy is outlined in the paper by Masek and Zoubek (1980), Masek (this volume) and Rohlich (this volume). A new geodynamic model based on the assumption that the Tepla-Barrandian Unit represents a fragment of an active margin which was formed at the northen margin of the Paleogondwana was suggested by Kfibek et al. (this volume). This model suggests: 1 - the formation of oceanic arc and back-arc basin, 2 - closing of the basin and the formation of synorogenic flysch and, 3 - the late- and post-orogenic igneous activity contemporaneous with the formation of intramontane basins. The formation of isolated and semi-isolated basins locally over 500 m deep with limited seawater circulation which resulted from volcanic and tectonic activities and which was documented in the Upper Proterorozoic Formation by Kukal (1988, 1990), Kukal et al. (1989), Pasava (1990), Pasava et al. (1989) and Suchy (1992) played an important role in the deposition of black shales. Generally, two types of black shale can be identified, based on geological position and geochemical characteristics. Normal black shales form, together with greywackes, thick flyschoid sequences. They have been deposited mostly under normal marine conditions. They often contain clastic material, show typical macro-slump structure and are generally poor in trace elements. 229

Bulletin of the Czech Geological Survey 75, 3, 2000

Permo - Carboniferous platform sediments IV+'+l Cadomian and Variscan granites lljjj'llj'j Neoproterozoic calc-alkaline igneous rocks Neoproterozoic tholeiitic basalts PVH Cherts 25km

|::::::. | Neoproterozoic clastic sediments

Fig. 1.

Conversely, metal-rich black shales are mostly closely related to basic volcanogenic rocks (subalkaline series volcanites similar to recent MORE) and were mostly formed in lower energy environment - semiisolated or isolated basins with occasional volcanogenic-hydrothermal vents. They contain anomalously high concentrations of metals and the highest metal concentrations have been detected in metal-rich black shales of the Central volcanic belt. It has been suggested that this belt indicates advanced spreading stage during which back-arc basin was floored by a newly formed oceanic-type crust (Chab 1993, Jakes et al. 1979, Pouba and Zoubek 1986).

Samples And Methods Altogether 89 samples of normal (n=42) and metalrich black shales (n=47) covering the whole region of the Barrandian Neoproterozoic of the Tepla-Barrandian Unit have been involved in this study. The location of the samples is shown in Fig. 1 and their brief geological description is as follows: 1. Rousinov section - R-8, 10, 345, 350 boreholes - both

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normal and metal-rich black shales; Central volcanic belt. 2. Kamenec near Radnice locality (NE of Pilsen section) KA-5 borehole - both normal and metal-rich black shales; Central volcanic belt. 3. Hromnice-Bykov near Pilsen locality (close to Pilsen section) - HRM-3 borehole - metal-rich black shales and HRM-1 normal black shales; Central volcanic belt. 4. Plzen (Pilsen) section - P-2520 and 2580 boreholes both normal and metal-rich black shales; Central volcanic belt. 5. Manetin section - M-1010 borehole - both normal and metal-rich black shales; Stfibro-Plasy volcanic belt. 6. Stfibro section - S-890 and 900 boreholes - both normal and metal-rich black shales; Stfibro-Plasy volcanic belt. 7. Pfestice section - D-30, 60, 100 and 120 boreholes (Dnesice) - both normal and metal-rich black shales; Central volcanic belt. 8. Chynin locality (SE of Rokycany section) - CHY-1 and 2 boreholes - both normal and metal-rich black shales (at the contact with the Central Bohemian Variscan granite pluton); Nepomuk-Pfibram volcanic belt. 9. Kolovec section - K-640, 660, 770 and 820 boreholes -


both normal and metal-rich black shales; Central volcanic belt. 10. Kolovec Section (Lazne Chudenice) - LCH-2 borehole - both normal and metal-rich black shales; Central volcanic belt. 11. Kolovec section (Andelice) - A-1 borehole-both normal and metal-rich graphitic hornfels about 1.4 km from the Central Bohemian Variscan granite pluton. 12. Mezholezy - UM-2 borehole - both normal and metal-rich black shales; Central volcanic belt. The samples were analyzed for selected major oxides and trace elements. Major oxides were analyzed using classic wet chemistry in laboratories of the Czech Geological Survey (M. Huka et al. and V. Sixta et al. analysts) and X-ray fluorescence method on a Phillips TW 1404 instrument in laboratories of the UNIGEO Enterprise in Brno (Janackova et al., analysts). Trace elements were determined by the X-ray fluorescence method on a Phillips TW 1404 instrument in laboratorie of the UNIGEO Enterprise in Brno (Janackova et al., analysts). A detailed description of major and trace elements determinations, mineralogical, petrologic, fluid inclusion and stable and radiogenic isotopes is listed in Hladikova et al. (1986), Pasava (1990), Pasava et al. (1986, 1993, 1996) and Pasava and Amov (1993).

Results And Discussion Lithology Normal black shales are represented by a group of Corg.-containing siltstones and shales often with admixture of sandy material (metasandstones, clasts of grey wacke etc.) and typical with macro-slump structure. Locally, they can contain abundant pyrite (e.g. pyritic black shale which used to be exploited at Hromnice). They usually are part of thick flyschoid sequence where normal black shales alternate with greywackes. The thickness of the normal black shale sequence at Hromnice, found in the HRM-1 borehole reaches 163 m (Pasava 1990). Metal-rich black shales are represented by black mudstones which are locally regionally metamorphosed to graphitic schists and at the contact zone of Variscan granitic intrusions to hornfels. Mudstones with a laminated structure are typical with an irregular alternation of dark layers rich in Corg and framboidal pyrite, with lighter layers composed predominantly of volcanoclastic material. There are also numerous younger veinlets and microslump-fragments of quartz, carbonate and quartz-carbonate. The thickness of metal-rich facies ranges from several tens of centimetres to the maximum continuous thickness of 45 m encountered in the HRM-3 borehole at HromniceBykov.

Geochemistry of Major and Trace Elements Major elements The distribution of selected major oxides in normal and metal-rich black shales from the Barrandian Neoproterozoic is given in Table 1. The chemical composition of shales is a significant indicator of their origin (Potter et al. 1980; Kukal, 1984). Most of the studied normal and metal-rich black shales revealed the Al2O3/Na2O ratios to be significantly lower than 15 (NBS = 5.48 and MBS = 7.33) as is characteristic of chemically immature sediments. Normal black shales show the average value of SiO2/Al2O3 ratio similar to that given for greywackes by Kukal (1985). Table 1 - Distribution of selected major oxides in normal and metal-rich black shales from the Barrandian Neoproterozoic of the TeplaBarrandian Unit. Type of rock

SiO 2 (wt.%) Ti02 A1203 Fe2O3 FeO Na2O K20 P205 Corg

Su SiO2/Al2O3 Al203/Na20 K2O/ Na2O

Normal black shale n = 42 average 65.02 0.70 15.15 5.32 3.11 2.76 2.60 0.14 0.88 0.66 4.29 5.48 0.94

Metal-rich black shale n = 47 average 63.40 1.5 10.34 5.43 4.61 1.41 2.45 0.14 2.43 4.92 6.1 7.33 1.73

Higher average value of SiO2/Al2O3 ratio (Table 1) in metal-rich black shales reflects higher content of siliciclastic material. Kfibek (1991) suggested that high K2O/Na2O ratios in Proterozoic black shales provide evidence for the absence of basic tuffaceous material, whose existence in sedimentary rocks is expressed by high Na2O values and the variability of the K2O/Na2O ratio. This would suggest that normal black shales which do not contain appreciable amount of volcanogenic material should have the values of this ratio higher than those of metal-rich shales closely associated with basic volcanogenic rocks. Normal black shales, however, show apparently lower average K2O/Na2O = 0.94 when compared to metal-rich facies (avg. K2O/Na2O = 1.73). Similar trend was also documented by Pasava et al. (1996) on the example of metalrich black shales closely associated with basic volcanites from the Barrandian Neoproterozoic (Kamenec and Hromnice-Bykov locality) and other regions of the world.

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Bulletin of the Czech Geological Survey 75, 3, 2000 Table 2 - Distribution of selected trace elements in normal and metal-rich black shales from the Barrandian Neoproterozoic of the Tepla-Barrandian Unit compared to other black shale standards. Normal black shale this work n=42 average 0.21 27 48 12 93 6 35 12 4 99 82 5 212 112 2.3

Type of rock

(ppm) Ag As Cu Co Cr Mo Ni Pb Sb Sr Rb U V Zn V/Cr

Metal-rich black shale this work n=47 average 4.6 71 275 22 190 58 183 40 20 110 72 26 691 608 3.6

SDO-1 normal black shale Kane et al. (1990)

SDO-1 metalliferous black shale Kaneetal. (1990)

Average siliceous black shale Yudovich and Ketris (1997)

average

average

median 1.2 30 100 11 86 29 63 17 8.8 140 47 13 250 160 2.9

69 60

138 120

66 134 99.5 28

132 134 199 56

75 126 49 160 64 2.4

49 320 128 2.4

These authors suggested the importance of diagenetic and metamorphic processes in the redistribution of alkalies. Normal and metal-rich black shales from the Barrandian Neoproterozoic differ significantly in the Corg and S contents. In metal-rich black shales the Corg content ranges from 0.4 (graphitic hornfels from the Andelice locality) to 5.51 wt.% (with average value of 2.43 wt.%) and S from 0.32 (graphitic hornfels from the Andelice locality) to 14.29 wt.% (with average value of 4.92 wt.%). Normal black shales possess much lower values (avg. Corg = 0.88 wt.% and S = 0.66 wt.%). Kovalova and Litochleb (1992) noted average values of Corg = 0.68 and S = 1.61 for a set of 54 samples of the Barrandian Neoproterozoic black shales. Much higher values of Corg. and S in metal-rich facies most likely reflect the biological overproduction and subsequent decay around higher temperature (up to 300 °C) volcanogenic-hydrothermal vents which were evidenced in the Barrandian Neoproterozoic by Pasava (1990) and Pasava and Dobes( 1992).

10

Organic carbon vs. sulfur plots have proved useful in characterizing modern and ancient sedimentary depositional environment. Normal marine environment, with periods of euxinic conditions (anoxic-sulfidic water column) are documented on Fig. 2. Majority of normal black shales (avg. S/C = 0.7) plots in the area of the Holocene ,,normal marine" sediment field (sediments deposited in oxic environments but with sufficient organic matter to allow pore waters to go anoxic after deposition and with average S/C = 0.4). Majority of metal-rich facies fall into the field of marine environments with anoxic bottom waters where ironsulfide formation occurs in the water column resulting in excess sedimentary sulfur and a higher S/C ratio in sediments with low C01-g. contents. The average value of the S/C ratio in the Barrandian Neoproterozoic metal-rich black shales (2.0) is much higher than those in recent and ancient normal marine sediments, suggesting an additional source of sulfur rather than only a simple C-S relationship. Trace elements

1

2

3 Corg (wt%)

4

5

6

A metal-rich black shale ° normal black shale Fig. 2. Corg /Slm plot for normal and metal-rich black shales from the Upper Proterozoic Formation of the Tepla-Barrandian U n i t ; refernce lines for normal marine and euxinic environment from Berner and Raiswell(1984).

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It is apparent from Table 2 that the Barrandian Neoproterozoic metal-rich black shales from the TeplaBarrandian Unit are strongly enriched in many metals (e.g. Zn, Ni, Cu, Mo, As, Cr, V, Ag, Au, Pd and Pt) when compared with normal black shales and also average normal black shale of Huyck (1991) and average siliceous black shale of Yudovich and Ketris (1997). Pasava et al. (1996, 1997) have noted the following maximum concentrations of selected metals: Zn-0.42 wt.%, Cu-0.16 wt.%, Ni-0.12 wt.%, Cr-0.12 wt.%, V-0.21 wt.%, Mo0.03 wt.%, As-0.06 wt.%, Ag-15 ppm, Au-132 ppb, Pd 102 ppb, Pt - 25 ppb, Rh - 1.7 ppb, Ru - 1.3 ppb and Ir 0.34 ppb.


In metal-rich black shales, important correlations have been established between Corg/V and Corg/Mo (Figs. 3 and 4) while Sb exhibits strong link with S (Fig. 5 - also confirmed by the identification of berthierite). The distribution of Co and Ni in normal and metal-rich black shales is shown on Fig. 6. It should be emphasized that the highest metal concentrations in metal-rich black shales were found in facies closely associated with basic volcanogenic rocks in the region of the Central volcanic belt (e.g.HRM-3 borehole at Hromnice, KA-5 borehole at Kamenec, LCH-2 borehole at Lazne Chudenice). Pasava (1990) have suggested that this metal enrichment is a result of volcanogenic hydrothermal processes. Newly established very close relationship between the distribution of selected PGE (platinum group elements) in metal-rich black shales and adjacent basic tuffitic rocks from the Hromnice-Bykov locality support this mechanism of metal enrichment (Fig. 7)

70 A

60 50-

A

1.40-

A

Q.

*.AA

20-

A

A

A A A •* A

10

A O

O

A

A

A

At A o AOA"joen>an>GDOoca> as 1 1 1

n0

2

4

6

°

o 1

1

8 10 Stot. (wt.%)

A metal-rich black shale

i

12

oo i. ..

14

16

° normal black shale

Fig. 5. Stot/Sb plot in normal and metal-rich black shales from the Upper Proterozoic Formation of the Tepla-Barrandian Unit. 600

2500 2000 £1500 a

>1000500

20 Co(ppm) 40 6C A metal-rich black shales o normal black shales

0

dP -H

0 0

1

2

3

4

5

o 5

6

Corg. (wt.%)

Fig. 6. Co/Ni plot in in normal and metal-rich black shales from the Upper Proterozoic Formation of the Tepla-Barrandian Unit.

* metal-rich black shale normal black shale 0.1

Fig. 3. Corg/V plot in normal and metal-rich black shales from the Upper Proterozoic Formation of the Tepla-Barrandian Unit. Please note significant correlation between Corg and V in metal-rich facies.

basic tuff metal-rich black shale E 0.01 Q

normal black shale

Z O I o

150

g 0.001

3100 O

DC

S

50 -£-

0

1

2

3 4 5 6 Corg (wt%) * metal-rich black shale ° normal black shale

Fig. 4. Corg/Mo plot in normal and metal-rich black shales from the Upper Proterozoic Formation of the Tepla-Barrandian Unit.

Distribution of REE (rare earth elements) in metal-rich black shale and associated basic tuff from the HromniceBykov locality (the HRM-3 borehole) is shown on Fig. 8. Both lithological types are characterized by similarly shaped REE curves with negative Ce and Eu anomalies. Such

0.0001

Ir

Ru

Rh

Pt

Pd

Fig. 7. Chondrite normalized PGE values in metal-rich black shale and associated basic tuff from the HRM-3 borehole (Hromnice-Bykov locality, Central volcanic belt) compared with the average chondrite normalized curve for the Upper Proterozoic normal black shale. Note very close distribution of PGE in both metal-rich black shale and basic tuff which reflects the same source of PGE - most likely hydrothermal-volcanogenic vents. In contrast, average normal black shale shows different distribution curve.

REE curves have been reported by Piper and Graef (1974), Marching et al. (1986) and others from metallife233


rous sediments of the East Pacific Rise and are very similar to those of seawater and differ only in absolute REE abundances. Similarly shaped chondrite normalized REE curves, are taken as indicators of a seawater source for REE. In contrast to the latter, strong enrichment in LREE and Eu is typical for pure hydrothermal precipitates, which suggests their hydrothermal origin (Rona 1984). The hydrothermal sediments possess distinctly higher total REE contents (ppm levels) than seawater or hydrothermal fluids (ppb levels), and a REE pattern of metalliferous sediments can be influenced, by the degree of mixing between seawater and hydrothermal solutions. Possible examples of such a process might be the LREE enriched and the depleted REE patterns of HREE, Eu and Ce within hydrothermal pyrite concretions, from the Romanche fracture zone of the equatorial Atlantic region (Bonatti et al. 1976). Similar mechanism might be suggested for REE accumulation in metal-rich black shales and associated basic tuffs at Hromnice-Bykov. 1E3

1EO La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu -*-tuff -*- metal-rich black shale Fig. 8 - Chondrite normalized REE values in basic tuff and associated metal-rich black shale from the HRM-3 borehole (Hromnice-Bykov locality - Central volcanic belt). Note similarly shaped chondrite normalized curves in both different lithologies which reflect very likely a similar source of REE.

It should be noted that rather flat REE distribution curves from both metal-rich black shale and basic tuff from the Hromnice-Bykov locality (part of the Central volcanic belt) are close to those of geochemically primitive tholeiitic basalts and basaltic andesites of the Tepla-Barrandian Precambrian published by Waldhausrova (1997). Ore Mineralogy Based on a detailed mineralogical studies of metalrich black shales from Hromnice and Kamenec, Pasava (1990) and Pasava et al. (1993, 1996) have identified the following mineral associations: Framboidal and grainy pyrite, sphalerite, chalcopyrite, millerite, pentlandite, sporadically molybdenite and galena. Very rarely, native gold, berthierite, clausthalite, and possible new Mo mine-

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rals - a Mo selenide (Mo3Se4) and a Mo telluride (Mo3Te4), uraninite and cassiterite were recognized. Recent mineralogical studies in the samples of metalrich black shales from the UM-2 borehole (Mezholezy locality - no. 12 on Figure 1) have resulted in the identification of abundant grainy aggregates of pyrrhotite (FegSg) which dominates over pyrite. Less common sulfides are arsenopyrite (FejAso.gvS 1.073), cobaltite (Co,Ni,Fe)o.96AsSo.9? and chalcopyrite. Inclusions of berthierite (FeSb2S3.8v) in pyrite, microscopic grains of galena and most likely a mineral from the joseite group Bi|2Pb2.i2Feo.88 (Te5.46Si.52) are sparse. Ilmenite belongs to relatively common accessory minerals. Organic Geochemistry Kribek (1997) has pointed out that the degree of graphitization of organic matter in black shales of the Barrandian Neoproterozoic and Domazlice crystalline complex is controlled by the degree of metamorphism which widely varies in this regions. Organic extracts from Proterozoic normal black shales at Kamenec consist of n- and isoalkanes (90 %), cycloalkanes (6 %) and aromatic hydrocarbons (4 %). C-13 to C-19 n-alkanes are the most common alkanes; isoalkanes C-14 to C-19 and the isoprenoid hydrocarbons pristane and phytane were also identified (Jehlicka et al. 1984). The presence of biphenyl as well as the overall association of hydrocarbons suggest that organic extract derived from normal black shale could have resulted from metamorphic pyroh -is at the condition of prehnite-pumpellyite facies (Jehlicka et al. 1984). The results of Rock-Eval pyrolysis (IH = 10.30-27.17; IO = 12.33-95.42; Pasava, 1990) from metal-rich black shales at Kamenec reflect a high degree of thermal maturation of organic matter. This was confirmed by Pasava et al. (1996) who have studied laser Raman spectra in two samples of metal-rich black shale from Kamenec. It was shown that the type of spectrum (larger peak at 1361 cm" 1 than at 1600 cm"1 and the position of the centre of the 1600 cm"1 peak at 1589-1592 cm"1 ) is typical for samples of Precambrian black shales with very low H/C values, about 0.1, and reflect very mature samples, at the stage prior to well-crystallized graphite. Sulfur Isotopes The first data on the isotopic composition of sulfur in sulfides from black shales of the Barrandian Neoproterozoic was published by Smejkal et al. (1974). Later, bulk of new data from black shales occurred in papers by Hladikova et al. (1986), Hladikova et al. (1987), Hladikova and Mrazek (1991), Pasava (1990) and Pasava etal. (1996). The range of 34S data in sulfides from both normal and


metal-rich black shales is quite broad (from -31.2 to +8.5 %o). Prevailing negative values indicate that sulfide sulfur was predominantly derived through bacterial reduction of the Upper Proterozoic marine sulfate. It is important to note that differences in the distribution of sulfur isotope values in sulfides from normal and metal-rich black shales have resulted from various sedimentary conditions. For example, sulfides from metalrich black shales with the highest metal concentrations at Hromnice-Bykov (HRM-3 borehole, 534S from -31.2 to -2.0 %0) and Kamenec (KA-5 borehole, 634S from -27.7 to -19.2 %o) were formed in (semi)isolated basins with limited sediment/water interaction and with basalt bodies being a source of ferrous ions (Pasava et al. 1996). These basins have communicated with the open Proterozoic sea during the whole period of sulfide accumulation. Conversely, bacterial reduction of seawater sulfate in a restricted basin with most likely limited communication with the open Proterozoic sea was suggested for the origin of pyrite in normal black shales of the former pyrite open pit mine at Hromnice (HRM-2 borehole - absence of volcanogenic rocks, 534S from -6.3 to +8.5 %o). Significant contamination of mafic magma by sedimentary sulfur was evidenced by both petrography and sulfur isotope studies at Kamenec (Pasava et al. 1996). Mrazek and Hladikova (1991) have presented a model of interaction between seawater, sediments and basalt at the Trnci locality, suggesting the enrichment of basalt in sedimentary sulfur through the water-rock interaction. This contamination have resulted in the origin of negative 34S data in sulfides from basalt (Kamenec - 834S from -24.1 to -14.4 %0, Trnci -534S from -26.5 to -19.4 %0). Carbon Isotope Composition of Sedimentary Organic Matter Carbon isotopic composition of sedimentary organic matter was studied in normal and metal-rich black shales from Kamenec and Hromnice localities. The 513C values in normal black shales varies from -28.0 to -31.9 %oand in metal-rich facies from -24.2 to -37.5 %0 Pasava et al. (1996) have concluded that very low and variable 5I3C values found for sedimentary organic matter from the Barrandian Neoproterozoic of the Tepla-Barrandian Unit were the result of variability of carbon isotopic composition of dissolved HCOi, its higher concentration and type of organisms. Carbon and Oxygen Isotope Compositions of Carbonates Carbonates from oblique and stratiform veinlets in metal-rich black shale from Kamenec (KA-5 borehole) and Hromnice-Bykov (HRM-3 borehole) as well as carbonates from different types of veinlets in normal black shales at Hromnice (HRM-2 borehole)were studied by Hladikova et al. (1986), Pasava et al. (1996).

It is important to note that these carbonates could be formed during several processes of which sedimentation, diagenesis, sub-sea floor metamorphism, regional metamorphism and post-metamorphic alterations were the most important events in this region. Each of these processes could have contributed to the isotopic record of carbonates. Carbonates from Normal black shales The 613C values in diagenetic or metamorphic carbonate veinlets from normal black shales (the HRM-2 borehole at Hromnice) are very homogeneous and range from -11.6 to-14.4 %c. The 518O values of carbonates fall into a relatively wide range (+14.7 to +26.7 %0 SMOW ) which correspond with the values typical for sedimentary limestones. Carbonates from Metal-rich black shales The 5 I3 C values of carbonates from metal-rich black shales at Hromnice-Bykov and Kamenec which are closely associated with volcanogenic rocks are very heterogeneous, ranging from -5.8 to -22.2 %c. Pasava et al. (1996) suggested, that the great variability of 513C values in the studied carbonates is most likely a result of mixing of at least two sources of carbon (organic matter with an involvement of deep-seated carbon or carbon from sedimentary carbonates). The 18O values range from +11.9 to +17.9 %o. Based on calculated I8 O values (+2.6 to +7.5 %oSMOW) it was suggested that such oxygen isotopic composition of fluids was the result of the interaction between fluids and both volcanics and metasediments rich in 818O. The interaction could have passed during sub-sea floor metamorphism as well as during regional metamorphism (Pasava et al. 1996). Lead Isotopes Pasava and Amov (1993) have shown that normal black shales are less radiogenic than metal-rich facies (Table 3). There is no correlation between the presence of radiogenic lead and the distance from Variscan granitoids since Table 3. Comparison of lead isotopic composition in normal and metalrich black shales from the Barrandian Neoproterozoic of the TeplaBarrandian Unit (data from Pasava and Amov 1993). rock

206pb/2()4pb

2()7pb/2()4pb

208pb/204pb

NBS

18.053-19.552 (avg. 18.651) 18.657-26.488 (avg. 21.542)

15.567-15.667 (avg. 15.613) 15.630-16.105 (avg. 15.811)

38.219-38.583 (avg. 38.433) 37.704-38.788 (avg. 37.704)

MBS

NBS=normal black shale (n=4) MBS=metal-rich black shale (n=12)

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metal-rich black shales are always closely associated with volcano-sedimentary sequences and are almost always located far from granitoids. The Late Proterozoic age of metamorphism was confirmed by the slope of secondary isochrone of the uranogenic lead, t = 635 ± 45 Ma (Fig. 9), and by the model age of the thorogenic lead in the samples with low thorium/lead ratios, 540-590 Ma (Pasava and Amov, 1993). It was also suggested by Pasava and Amov (1993) that metal-rich black shales were most probably enriched by uranium during the Cadomian metamorphism, dated 550-700 Ma (van Bremmen et al., 1982), particularly at 635 ± 45 Ma (Pasava and Amov 1993). Most of the lead isotopic ratios (207Pb/204Pb) reflect, according to the plumbotectonic models (Doe and Zartman, 1979; Amov, 1983) an "orogene"- type lead, which indicates sedimentary rocks of oceanic-type crust as the most probable source. Fluid Inclusions

riable densities of CO2 have been interpreted as a result of heterogeneous trapping of two, originally unrelated fluids under the conditions of partial mixing of CO2 rich and H2O-rich fluids (T 4Pb plot of uranogenic lead with the growth curves for ,,upby Pasava (1998) and Posmourny (1998). per crust", ,,orogene" and ,,mantle" of the Plumbotectonic model (Doe and Zartman Similar problems can be expected at many ot1979) and the growth curves at T = 2.8, 3.0 and 3.2 Ga and isochrones at tmi)d. = 0, 200, 400 and 600 Ma (Amov 1983a). 1 - normal black shale, 2 - metal-rich black shale, 3 - her places of the Barrandian Neoproterozoic of basic volcanogenic rocks (metabasalts, metatuffs). the Tepla-Barrandian Unit. 236


Conclusions Normal and metal-rich black shales have been distinguished in the Barrandian Neoproterozoic of the Tepla Barrandian Unit. Normal black shales are represented by Corg -containing siltstones often with admixtures of sandy material and typical macro-slump structure. They are a part of a thick flyschoid complex. Their thickness reaches 163 m at Hromnice. These facies are usually poor in trace elements whose distribution is well comparable with that in other world normal black shales. The average Corg value is 0.88 wt.% and S = 0.66 wt.%. Thermal maturity of organic matter reflects well de- Photo 1 - At Hromnice, exploitation of pyritic black shales has resulted in an open pit (~190xl30x50m) gree of metamorphism. The re- with a 15 m deep acid water lake. Acid mine waters leaking from the open pit cause serious environsults of sulfur isotopes have mental hazard in the Hromnice village (Photo by J. Pasava). shown that pyrite in normal black shales originated most likely by bacterial reduction of seawater sulfate in a restricted basin with limited communication with the open Proterozoic sea (e.g. the former pyrite open pit at Hromnice). The 518O values of carbonates studied at Hromnice correspond with the values typical for sedimentary limestones. The results of lead isotopes have revealed that normal black shales are less radiogenic than metalrich facies. Conversely, metal-rich black shales (siltstones) are always closely associated with submarine mafic volcanic rocks and were formed in (semi)isolated basins under anoxic conditions. The black shales are characteri- Photo 2 - At Dolni Bela - Lite quarrying of pyritic black shales ended up with the formation of an open zed by an average of 2.4 wt.% pit ( 60x50x20m) with so called ,,emerald lake" (Photo by Z. Pouba). Core, and 4.9 wt. ( o S and carry anomalous metal concentrations ly identified: pyrrhotite, arsenopyrite, cobaltite and micrelated to volcanogenic-hydrothermal vents. The distriburoscopic grains of a mineral from the joseite group tion of REE indicate mixing of seawater and hydrothermal Bi]2Pb2.i2Fe0.88 (Te5.46Si.52)- Sulfur isotope results sugsources. Their thickness varies from about 1 to 45 m. gest the dominant source of sulfur to have been seawater Abundant framboidal pyrite widely occurs in Corg-rich sulfate, reduced by bacteria in (semi)isolated basins with layers. Beside grainy pyrite, sphalerite, chalcopyrite, millimited sediment/water interaction. These basins have lerite, pentlandite, molybdenite, galena, native gold, bertcommunicated with the open Proterozoic sea during the hierite, clausthalite, Mo - selenide, Mo - telluride, uraniwhole period of sulfide accumulation. The 5 I3 C values of nite and cassiterite the following minerals have been new-

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the organic matter reflect the combination of the carbon isotopic composition of dissolved HCO3, its high concentration and type of organisms. The results of RockEval pyrolysis as well as laser Raman spectroscopy show a high maturity, close to that of well-crystallized graphite. Fluid inclusion and stable isotopic data from quartz and carbonate veinlets of very low-grade regionally metamorphosed metal-rich black shales at Kamenec and more highly metamorphosed facies at Hromnice indicate that they resulted of the interaction between fluids 818O-rich volcanics and metasediments. The interaction could have occurred during sub-sea floor or later regional metamorphism. Lead isotope data evidence that metal-rich black shales were most probably enriched by uranium during the Cadomian metamorphism, particularly at 635 ± 45 Ma. It has been documented that metal-rich black shales can cause through generation of acid water serious environmental harm and that remaining waste dumps and pit lakes after their exploitation represent a potential danger for human living in this region. References Amov, B.C. (1983): Evolution of uranogenic and thorogenic . 1. A dynamic model of continuous isotopic evolution.- Earth and Planetary Science Letters, 65, 61-74. Berner, R. - Raiswell, R. (1984): C/S method for distinguishing freshwater from marine sedimentary rocks.- Geology, 12, 365-368, Boulder. Bonatti, E. - Honnorez-Guerstein, M.B. - Honnorez, J. (1976): Hydrothermal pyrite concentrations from the Romanche Trench (equatorial Atlantic): metallogenesis in oceanic fracture zones.Earth planet. Sci. Lett., 32, 1-10. van Bremmen, O. - Aftalion, M. - Bowes, D.R. - Dudek, A.- Misaf, Z. Povondra, P. - Vrana, S., (1982): Geochronological studies of the Bohemian Massif, Czechoslovakia, and their significance in the evolution of central Europe.- Trans. Roy. Soc. Edinburgh, Earth Sciences, 73, 89-108. Doe, B.R. -Zartman, R.E. (1979): Plumbotectonics. I. The Phanerozoic, in Barnes, H.L. ed., The geochemistry of hydrothermal ore deposits.- New York, Wiley Intersci., N,Y,. p. 22-66. Hladikova, J. - Smejkal, V. - Pasava, J. - Breiter, K. (1986): Isotopes of sulphur, carbon and oxygen at selected localities of the Barrandian Proterozoic.- Vestnik ustredniho ustavu geologickeho, 61, 339-348, (in Czech with English abstract). Hladikova, J. - Mrazek, P. (1991): The origin of sulphides and carbonates in the volcano-sedimentary complex of the West Bohemian Upper Proterozoic (Trnci near Klatovy).- Casopis pro mineralogii a geologii, 36, 39-49. Praha. (in Czech with English summary). Hladikova J. - 2ak, K. - Kfibek, B. (1997): The distribution of carbon, oxygen and sulphur isotopes in rocks and ore deposits of the TeplaBarrandian unit and the Moldanubian zone.- Journal of Geological Sciences, 47, 205-211. Prague. Holubec, J. (1966): Stratigraphy of the Upper Proterozoic in the core of the Bohemian Massif.- Rozpravy Ceskoslovenske Akademie Ved, r. mat-pfir., 76, 43-62. Praha. Huyck, H.L.O. (1991): When is a metalliferous black shale not a black shale ? - In Grauch, R.I. - Huyck, H.L.O. (eds) Metalliferous Black Shales and Related Ore Deposits - Proceedings, 1989 United States Working Group Meeting, IGCP #254, U.S. Geological Survey Circular 1058, 42-56. Chab, J. - Pelc, Z. (1968) Lithology of Upper Proterozoic in the NW limb of the Barrandian area.- Krystalinikum, 6, 141-167. Praha. Chab, J. (1993): General problems of the TB (Tepla-Barrandian)

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