Minerály a horniny Slovenska :: prepni na celú stránku
Carbonate mineralization of the Rudnany ore field
zdroj: Acta Geologica Universitatis Comenianae, Nr. 51, Pages: 43-56 Bratislava 1996, Lidia Turanova - Jan Turan
Abstract: The rocks of the Rudnany ore field contain besides siderite also dolomite, Fe-dolomite, ankerite, calcite and virtually all members of the siderite - magnesite isomorphous series. The authors investigated quantity and mineral species of carbonates in the lithostratigraphic units and they characterised their distribution, mode of occurrence, mutual relationships and isotopic composition. The study showed that there is a direct qualitative dependence of the present carbonate species upon the lithological and petrographic rock type. The dependence is very strict in the case of magnesite - breunnerite and calcite, less pronounced for siderite and loose in the case of ankerite and Fe-dolomite. Isotopic composition of the studied carbonates suggests presence of both typical deep-seated source of carbon as well as sedimentary marine carbon. It indicates a marine source for the studied carbonates and their primary rocks.
The Rudnany ore field (hereinafter the ROF), well known for number of carbonate mineral species, bears also significant genetical importance and, until quite recently, an economical importance, too. Several years ago the ROF was not only the largest producer of siderite ore, but it produced also copper, mercury and barite. Nowadays, only barite is exploited.
The Rudnany deposit belongs to the most important object of mineralised faults of the so called north-Gemeric zone. The deposit is an integral part of the arcuate zone running from Dobsina in W to Kosice in E (Poprenak - Ivan - Mihalov 1985). According to the present knowledge of geological structure of the Rudnany region and the stratigraphic division of the present complexes (Bajanik et al. 1983, Grecula 1982, Hovorka et al. 1984, Poprenak - Ivan - Mihalov 1985) the studied region is built by: 1. Lower Paleozoic Rakovec Group, Klatov Group, 2. Upper Paleozoic Dobsina Group (Carboniferous) and Krompachy Group (Permian), 3. Mesozoic and younger units (Fig. 1). The deposit itself is hosted entirely by Paleozoic complexes.
Majority of the previous studies evaluated only carbonate mineralization occurring in epigenetic veins and veinlets (Bernard 1961). However, the carbonates in the ROF are not found on the vein structures only but they are more or less concentrated also in the ambient rocks, often without visible carbonate veinlets (Gubac 1969, 1977, Mandakova et al. 1971, Vancova - Turan 1980, 1983, Hovorka et al. 1984, Ivan 1984, Turan - Turanova 1984a, 1993b). Up to now, the carbonates in rocks were assigned to wall-rock alteration phenomena accompanying formation of the ore veins.
The aim of our research was to determine qualitative and quantitative representation of carbonates in the present lithostratigraphic units and to characterize their distribution, mode of occurrence and mutual relationships. The gathered data should help to solve the genetical questions concerning the deposit.
This paper summarizes results of carbonate mineralization studies performed as a part of metallogenetic research of the Central Western Carpathians in 1980 - 1990. Minor work was done in 1994 - 1996 on a grant project 1/1808/94.
More than 1000 samples were collected in mining works and from 27 deep drill-holes in the vicinity of the mining works (Fig. 1). In order to determine the present carbonates, the samples were analysed by manometric method (analyst M. Hacurova) described in detail by Turanova - Turan (1993a). The samples were collected exclusively in rocks without any visible macroscopic carbonate veinlets. This allowed to gain a basic picture about distribution, qualitative and quantitative representation of carbonates in rocks.
Element content and distribution in carbonates were determined by OES (analyst M. Vanco), exceptionally also by X-ray microanalysis on JEOL-XA-5a (Dr. D. Jan-cula), Edax PV 9100 (Dr. J. Stankovic) and scanning electron microscope JSM-840 (Dr. J. Stankovic).
Isotopic composition of carbon and oxygen in carbonates was determined by Ing. V. Smejkal, CSc. (UUG Praha).
The ROF rocks contain, except for siderite, disse-minated dolomite, Fe-dolomite, ankerite, calcite and practically all members of the siderite - magnesite isomorphous series. The members of the siderite - magnesite isomorphous series are defined according to Meixner (1953). The members of the dolomite - ankerite isomorphous series are defined by the Mg:Fe ratios 9:1, 3:1 and 1:4, i.e. limiting FeO content up to 3.83% for dolomite, 9.34% for Fe-dolomite and 27.41% for ankerite.
Fig. 1 Geological map of the Rudnany ore field with localization of veins and drill-holes RHV in the vicinity of the deposit (after Hudacek - Fabian in Fabian - Hrusovsky 1988). Explanations: 1 alluvium, loams, debris (Quaternary), 2 basal conglomerates (Paleogene), 3 dark dolomites, 4 light limestones (3, 4 Middle Triassic), 5 variegated clayey and sandy shales and conglomerates, 6 basal conglomerates (5, 6 Lower Triassic), 7 clayey, clayey-sandy shales and conglomerates (Krompachy Group, Permian), 8 sericite - chlorite clayey and sandy shales, 9 conglomerates of the Bindt - Rudnany type, 10 metabasalts with beds of metatuffs and metatuffites (8, 9, 10 Dobsina Group, Upper Carboniferous), 11 basalt metatuffs, metatuffites and metabasalts, 12 sericite-chlorite phyllites (11, 12 Rakovec Group, Upper Devonian - Lower Carboniferous), 13 ore veins, 14 surface drill-holes.
Carbonate minerals in the ROF rocks form independently occurring carbonate associations: 1. siderite - ankerite, 2. dolomite (Fe-dolomite) - breunne-rite, 3. calcite - dolomite.
The study showed that there is a direct qualitative dependence of the present carbonate species upon the lithological and petrographic rock type. The dependence is very strict in the case of magnesite - breunnerite and calcite, less pronounced for siderite and loose in the case of ankerite and Fe-dolomite. It was confirmed by the older, partly published data of Vancova - Turan (1983) listed in Table 1 as well as by newer analytical data about distribution of carbonates in rock types from 27 deep drill-holes realized in the northern vicinity of the ROF (Fig. 2) and the mining work Demag II (Fig. 3).
Fig. 2 Basic statistical characteristics (median, 25 % and 75 % percentiles, variance, extreme values) of carbonate contents in different rock types from deep drill-holes in the ROF. Explanations: The rock types numbered 1 to 14 are listed in the Table 1, 15 - evaporites, 16 - Paleogene conglomerates.
Occurrence of characteristic carbonate association in lithostratigraphic units
The rocks of the Rakovec Group host two separately occurring carbonate mineral associations: the siderite - ankerite one and the calcite - dolomite (Fe-dolomite) one.
Light laminated chlorite - sericite phyllites of the upper parts of the Rakovec Group contain the siderite - ankerite association. In contrary, the deeper parts of the Rakovec Group contain only the calcite - dolomite (Fe-dolomite) association (Table 1).
Siderite and ankerite form light bands in the chlorite - sericite phyllites. Their mutual ratio varies but it usually corresponds to quantitative proportions of sericite and chlorite in the rocks. If there is more sericite than chlorite in the rock, the siderite content increases, and vice versa. It seems that this feature is not valid only for the phyllites but generally. If chlorite dominates in the rock, the Fe content in carbonates rapidly decreases and the only present carbonate association is the calcite - dolomite (Fe-dolomite) one. It clearly demonstrates that the lithology of the rocks played decisive role during formation of the carbonate associations. Vertical changes in the deposit, namely different development of the siderite mineralization, may be explained on the basis of the host rock types.
The deeper parts of the Rakovec Group are represented mostly by metamorphosed products of basic volcanism. These rocks contain typical calcite - dolomite (Fe-dolomite) carbonate association (Table 1) found not only in the rock but also in veinlets. The affinity of the calcite - dolomite (Fe-dolomite) association to this rock type is very strong. "Siderite" veins entering these rocks change their composition, more Fe-dolomite appears instead of siderite.
Fig. 3. Basic statistical characteristics (median, 25 % and 75 % percentiles, variance, extreme values) of carbonate contents in different rock types from the mining work Demag. Explanations: The rock types numbered 1 to 14 are listed in the Table 1, 15 - evaporites, 16 - Paleogene conglomerates.
Metamorphosed basic volcanic rocks with calcite - dolomite (Fe-dolomite) carbonate association build 70% of deeper portions of the Rakovec Group. Therefore, the absence of siderite in these rocks may be attributed not only to lack of suitable tectonic structures for fluid circulation but also to inappropriate lithological environment.
Presence of calcite - dolomite (Fe-dolomite) association in metamorphosed products of basic volcanism may be elucidated by the so called spilite reaction (Eskola 1937 in Hejtman 1957) fluids with Na, Si decompose basic plagioclases to albite and calcite:
CaAl2Si3O8 + Na2CO3 + 4 SiO2 NaAlSi3O8 + CaCO3
Lower Paleozoic - Klatov Group
The rocks of the Klatov Group defined by Hovorka et al. (1984) contain calcite - dolomite (Fe-dolomite), siderite - ankerite and dolomite (Fe-dolomite) - breunne-rite carbonate associations.
The calcite - dolomite (Fe-dolomite) association is the dominating one. It is characteristic mainly for thinly bedded, several m thick layers of stratiform carbonates described by Kusak - Hurny (1981) from the 26th horizon of the Zlatnik vein. Calcite - dolomite (Fe-dolomite) association is present also in gneisses and amphibolites of the Klatov Group. Nevertheless, these strongly metamorphosed sediments comprise the siderite - ankerite association, too.
The interesting rock type from the viewpoint of carbonate mineralization showed to be the metasomatites of gabbroid rocks (Mandakova et al. 1971) with carbonate - serpentine - talc, carbonate - talc - chlorite and carbonate - talc - quartz - chlorite facies with fuchsite (the latter one being designated as listvenites). Mandakova et al. (1971) interpreted this rock complex as a group of plutonic (gabbroid to granitoid) rocks and their metamorphic and hydrothermal derivates up to Carboniferous in age. Poprenak et al. (1973) claim that the rocks are not intrusive but they are metamorphosed rocks of the Rakovec Group, Carboniferous sediments and magmatic rocks altered by metasomatic processes. According to Hovorka et al. (1979) the rocks are stronger metamorphosed Carboniferous rocks.
Fig. 4 Chemical composition of breunnerite plotted in the CaO - MgO - FeO diagram (analyses from Vancova - Turan 1980)
The quartz - chlorite rocks with fuchsite bear only the siderite - ankerite carbonate association. The talc - chlorite rocks comprise the dolomite (Fe-dolomite) - breunnerite association characterized in detail by Vancova - Turan (1983). The spatial separation of the two carbonate associations questions the derivation of both of them from a single source.
Intensively altered ultrabasic rocks or their clastic derivates with higher Mg content occurred in the exploration adit Demag II (the 13th horizon) in interval 2320 - 2350 m. These rocks contained about 50% of carbonates - intermediate members of the isomorphous series dolomite - ankerite and magnesite - siderite, most often Fe-dolomite and breunnerite to mesitine.
Alteration of the original ultrabasic rocks was accompanied by steatitization, especially of breunnerite that is in large scale replaced by talc. The only evidence of the primary ultrabasic character of the rocks are dark chromspinelids visible by unaided eye. Mg-carbonates found in the altered ultrabasic rocks point at the importance of the lithological factor. Their presence cannot simply be ascribed to the vertical zonality, moreover because except the 35 m long section of altered ultrabasic rocks the surrounding metapelites contain Fe-carbonates siderite and ankerite.
Causes of different alteration pathways of the ultrabasic rocks were found in the beginning of the exploration adit Demag II (the 9th horizon). Several bodies rich in talc and chlorite and very poor in carbonates, namely Fe-dolomite, occur in 370 - 400 m. Talc probably completely replaced serpentine minerals and actinolite since they are not preserved in the rock. Similarly altered ultrabasics as in the exploration adits Demag I and II were detected also in several drill-holes, e.g. RHV-7.
Carbonates of the dolomite (Fe-dolomite) - breunnerite association form individual grains or aggregates disseminated in the rocks. No vein structures were found even if other members of the isomorphous series - pistomesite and sideroplesite occur frequently in veinlets. Presence of breunnerite was proved in 80% of analysed samples of talc - chlorite rock, Fe-dolomite in 98%. Breunnerite appears usually with Fe-dolomite, exceptionally also separately. Fe-dolomite, in contrary, occurs mostly separately.
The studied rocks contain in average 11.5% of breunnerite and up to 28% of Fe-dolomite. Originally the ratio may have been reversed because breunnerite is intensively substituted by talc while Fe-dolomite remains untouched. Replacement of breunnerite by talc starts already at lower temperatures in the greenschist facies during metamorphic and hydrothermal events in the presence of SiO2 (Winkler 1967, Miyashiro 1975). The main source of Mg bound in talc are Mg-carbonates because breunnerite and talc are seen always together and talc intensively replaces breunnerite.
The FeO content in the studied carbonates varies between 11.72 to 21.92%, in average 14.25% (Fig. 4). According to the Meixners (1953) classification they belong to breunnerite, in a single case to mesitine.
Geochemically interesting information is the elevated content of Cr (510 - 1380 ppm), Ni (540 - 780 ppm) and Co (45 - 69 ppm). It is several times higher than the contents reported from magnesite of the Carboniferous magnesite zone or even the amorphous magnesite from serpentinized bodies (Vancova - Turan 1981).
We suppose that a specific rock environment enriched in Mg already before the metamorphic and hydrothermal events played a significant role in the formation of listvenite mineral association. Mandakova et al. (1971) and Ivan (1984) assumed the presence of ultrabasic rocks. However, it could have been clastic material with fragments of ultrabasic rocks as well. Serpentinized lithoclasts are known from several places in the ROF. Besides antigorite they consist of talc and chlorite, ore minerals magnetite and Cr-spinelids. Their occurrence is rather sporadic but it is indicated by elevated content of Cr, Ni, Co, Fe and Mg.
Quartz - chlorite rocks with fuchsite are much more common in the ROF than the talc - chlorite rocks. Fuchsite usually forms irregular nests of various size, sometimes several mm thick bands parallel to the schistosity of the rock. It is most abundant in the siderite zones of the central part of the Rudnany deposit.
Fig. 5 Intensively limonitised siderite fragments in variegated polymict Permian conglomerates, partly substituted by the surrounding sericite - quartz matrix. Loc. Rudnany, sample No. RHV-2/525 m, magn. 11 x, crossed nicols. Photo L. Osvald.
Spatial link of fuchsite and siderite is unambiguous, much more developed than in the case of other carbonates. Siderite was determined in 78% of analysed samples, occurring either separately, or with ankerite, less Fe-dolomite. The average carbonate content in the fuchsite-bearing rocks was about 41.5% (Table 1). Fig. 2 shows the basic statistical data about carbonates from drill-hole samples.
The studies of lithological rock types and their siderite content showed that siderite is present almost in all rock types. The frequency of siderite occurrence and its average content in the Carboniferous rocks is high. The rocks richest in siderite are the terrigenous sediments, especially metapelites, black and sericite shales (Table 1). Greater thicknesses of epigenetic siderite veins in Carboniferous are not surprising if regeneration of vein siderite from the ambient rocks is assumed.
Siderite and ankerite in sericite and black shales form many times repeating parallel bands, often recrystallized, or bedded and true veins. In spite of high frequency of siderite occurrence in the black shales no thicker siderite veins are found in these rocks. Yet the black shales may be used in the exploration practice as leading horizons because important siderite veins are situated in their proximity although Poprenak (1964) claims that they are not suitable for siderite veins formation. The basic statistical data of carbonates in black shales from drill-holes in the ROF and mining work Demag are depicted in Figs. 2 and 3.
The pelosiderite zones bear greater quantities of carbonates but the mode of occurrence is equal to that of the black shales. The average carbonate content varies around 30% (avg. ankerite = 6.5%, avg. siderite = 22.5%) (Table 1, Fig. 3).
Fig. 6 Variegated Permian pelitic shales with abundant Fe-dolomite concretions. Loc. Rudnany. Photo L. Osvald.
Genesis of the pelosiderite zones, up to 10 m thick, is not clear yet. They were thought to represent a product of wall-rock alteration. If considering the regional distribution of carbonates in these rocks as well as in the black shales, we regard them as the potential source for younger epigenetic siderite and siderite - ankerite veins.
Siderite - ankerite association was found also in clastic, mainly fine-grained Carboniferous sediments, mostly conglomerates and sandstones (Table 1). The basic statistical data of siderite and ankerite in conglomerates and sandstones from the drill-holes and mining work Demag are shown in Figs. 2 and 3. The carbonate minerals are usually part of the matrix, nevertheless, specific cases when they appear only in clasts are reported, too (Fig. 5).
Carboniferous metamorphosed basic volcanic rocks bear, similarly as the older complexes the calcite - dolomite association (Table 1, Figs. 2, 3).
In the Carboniferous rock complex, thin bodies of metamorphosed basic volcanic rocks with the calcite - dolomite association are alternating with beds of metapsammites and metapelites with the siderite - ankerite association. It is suggested, that the movement of hydrothermal fluids along the fractures is limited in both vertical and horizontal direction because the calcite - dolomite association in basic metavolcanics is preserved even in close surrounding of the siderite vein. The fluids did not diffuse outside the vein structures because the carbonate minerals are not replaced by siderite. The most acceptable explanation of the observed phenomena is the remobilization of carbonates from the surrounding rock complexes initiated by tectonometamorphic processes.
Fig. 7 Roc-Tourne twinning in euhedral albite crystals. Loc. Rudnany, sample No. Ry-2/80, magn. 27.5 x, crossed nicols. Photo L. Osvald.
The basal Permian sequence, locally up to 600 m thick comprises the siderite - ankerite association. The carbonate association of calcite and dolomite is almost completely missing.
Frequency of siderite occurrence in the Permian conglomerates from the mining work is rather high (Table 1), appr. 40%, but the average content is only 6%. The frequency was higher in the drill-hole samples (more than 80%) with content 2 - 12% (avg. 7%). The ankerite content is less than 20% (avg. 10%) (Figs. 2, 3). As a rule, the determined amount of carbonates is lower than that in the Carboniferous rocks and therefore ankerite veins dominate over the siderite ones and no thicker siderite veins are developed.
The carbonates of the siderite - ankerite association are most often disseminated in matrix of the Permian sediments. Some samples of polymict conglomerates include up to 0.5 cm large siderite fragments with different degree of subsequent reworking, from margins replaced by quartz and sericite (Fig. 6). Their genesis is unclear. Presence of siderite - quartz clasts and clasts of hematite quartzites allows to conclude that siderite could be locally transported from the underlying complexes. Nevertheless, it is uncertain whether a siderite fragment would be able to survive transport, sedimentation and diagenesis. Genetical significance of occurrence of siderite clasts requires to treat this fact very carefully.
Fig. 8 Chemical composition of the concretions plotted in the CaO - MgO - FeO diagram (analyses Turan - Turanova 1984).
Deepening of the sedimentation basin resulted into sedimentation of shales and furthermore replacement of the siderite - ankerite association by the Fe-dolomite or ankerite association. It can be used as a typical example of relationship between a carbonate association and a facial environment. Concretions composed of siderite were found only very rarely also in violet shales.
The change of carbonate association is often reflected by change of the occurrence mode. In the sha-les, concretions and septaria (Fig. 7) are very common. It is a typical feature for variegated Permian and Scythian shales in the Spissko - gemerske Rudohorie Mts. The concretions are composed of fine-grained to pelitic Fe-dolomite, quartz and sericite, locally also tourmaline. They are 1 - 20 mm large, occasionally larger, isometric in shape. Mineralogy of the concretions and septaria corresponds to the composition of the surrounding host rocks. The matrix of the concretions is usually built by Fe-dolomite or ankerite, sometimes are these minerals replaced by limonite. The insoluble part of the concretions consists of chlorite (more than 50%), quartz, hydromicas, feldspars, tourmaline, hematite, limonite, pyrite and Ti-minerals. Siderite and barite were identified in septaria. The feldspars often form albite, Carlsbad and Roc-Tourne twins (Fig. 8) pointing at their origin in diagenetic conditions. Chemical composition is depicted in Fig. 9.
Fig. 9 Euhedral pinolite magnesite grains (dark) in gypsum. Loc. ROF, sample No. Ry 29/89. Photo L. Osvald.
The determined properties of the concretions (structure, porosity, shape, chemical composition, compaction degree) from the studied rocks but especially studies of macroconcretions from the locality Porac - Nad dubom (described in detail by Turan - Turanova 1984b) enabled to postulate their very early diagenetic origin. The opinions of Miskovic - Varcek (1983) about their hydrothermal genesis is hardly acceptable as well as Drnzikovas and Mandakovas (1961 in Abonyi 1961) findings that the siderite concretions these are clastic rocks of tectonic origin.
The Upper Permian to Lower Triassic evaporites and evaporitic conglomerates contain members of the magnesite - siderite isomorphous series with minimal amount of the calcite component. Presence of magnesite s.s. cannot be ruled out.
A characteristic feature of carbonates in evaporites is their uniform distribution in the entire profile of the sulphate sequences. The carbonate contents are low, most commonly about 10% (avg. 5%), only exceptionally higher (Fig. 2).
Mg-Fe carbonates in evaporites crystallised from the residual parent solution in presence of CO2 after precipitation of major amount of calcium as Ca sulphates from the brines.
Majority of the drill-holes situated in the vicinity of the Rudnany deposit between the villages Rudnany and Matejovce penetrated in their upper parts also Mesozoic sequences and locally also Paleogene conglomerates.
Siderite - ankerite mineralization is absent in the Mesozoic limestone and dolomite sequences which are 100 - 200 m thick, occasionally even thicker if the mineralization is abundant in the older underlying complexes. Some authors believe that the variegated Permian and Lower Triassic shales acted as a non-penetrable barrier horizon for the hydrothermal fluids. Other authors state that the absence of Fe-carbonates is due to tectonics of surficial style that is not suitable for the mineralization. They consider post- Cretaceous age of the mineralization and therefore the absence of Fe-carbonates in the Middle Triassic limestones and dolomites cannot be elucidated by the nappe structure of the studied region.
Our study of lithological factors offers another explanation. The Mesozoic rocks are very poor in Fe and other metals and the mobilization events could not collect and concentrate them into siderite or ankerite veins with accompanying ore mineralization.
Paleogene conglomerates rest directly on the Middle Triassic limestones and dolomites. Siderite - ankerite association was detected in 3 samples from a total of 14, the siderite content was max. 1.5%, the calcite - dolomite association was found in 12 samples (out of 14) (Fig. 2).
Siderite and ankerite in the Paleogene conglomerates form individual grains often in quartz clasts found only in sandy pebbles with dark organic pigment. Limestone and dolomite pebbles as well as pebbles of altered basic rocks contained only carbonates of the calcite - dolomite association.
Fig. 10 Isotopic composition (d13C) of carbonates in different rock types in lithostratigraphic units of the ROF. Explanations: The numbered rock types are listed in Table 1.
Fig. 11 Isotopic composition (d18O) of carbonates in different rock types in lithostratigraphic units of the ROF. Explanations: The numbered rock types are listed in Table 1.
There is relatively large number of data on isotopic composition of the carbonates in Slovakia (Varcek et al. 1985, Cambel et al. 1985, Kantor - Misik 1992, Lintnerova - Hladikova 1992, Vozarova et al. 1995) but interpretation of the results is neither simple nor unequivocal. There is no doubt that these values may be influenced by a number of processes. Maynard (1985) states that stable isotopes of S, C and O dont bring important information in the case of sedimentary rocks studies because the per mille deviation of the isotope ratios in the sedimentary systems varies considerably as a result of substantial fractionation with regards to these ratios in the international standards.
Generally, isotope composition of an element in a given material carries information either about its origin or the processes which somehow altered the original material. Isotope composition of carbon in carbonates is dependent on isotopic composition of carbon in bicarbonates dissolved in water which precipitate into the carbonates (Hladikova 1984). Variation of the 18O and 13C values in the carbonates is proportional, i.e. decrease of the 18O value means also decrease of the 13C value, and vice versa (Borshevsky et al. 1981). The carbon isotopic composition changes are smaller than those of oxygen because a solution interacting with carbonate contains more oxygen than carbon (Hladikova 1984).
Increase of Mg content in water leads to observable enrichment of the carbonate by the heavy isotope (Borshevsky et al. 1981). Moreover, the heavier isotope of any element will preferentially enter the chemical compound that binds it stronger. In the case of the siderite - dolomite - magnesite series, magnesite will concentrate more on the heavier carbon isotope than dolomite and dolomite more than siderite. The rule seems really to be working for the carbonates in the Spissko - gemerske Rudohorie Mts. Magnesite concentrates relatively more of the heavier carbon isotope than dolomite and the carbon in dolomite is isotopically heavier than that in siderite.
The most common interval of the 13C values listed in literature for normal marine carbonates is + 4.0 to - 4.0 . Diagenetic changes of primary sedimentary carbonates and diagenetic neoforms (concretions) are characteristic by substantial shift of the 13C values out of the above-mentioned interval (Borshevsky et al. 1981, Veizer 1983, Ustinov et al. 1984).
The 13C values of marine Mg-Fe carbonates lie within the range - 2.5 to - 5.1 (Goroshnikov et al. 1981), i.e. they represent isotopical composition partly enriched by the lighter isotope related to the processes of microbiological decomposition of organic substances and crystallisation of carbonates during contemporaneous transformation of organic substances and carbonates in the stage of diagenesis.
Certain changes of isotope and geochemical features are conspicuous also during the metamorphosis of sedimentary carbonates. Recrystallization processes along with transport in the fluid phase and crystallisation in fractures, especially above 130 C are accompanied by enrichment by lighter carbon isotope by about 5 (Goroshnikov et al. 1981). Ustinov et al. (1984) observed changes in the 13C values by 0.5 to 9.6 in variously intense processes. The reason of such changes is the exchange of oxygen in carbonates for oxygen of isotopically lighter surficial water acting in the metamorphosis (Sidorenko - Borshevsky 1975, Snezhko - Lugovaja 1978) and higher content of organic material (Snezhko - Lugovaja 1978).
Fig. 12 Fields of sedimentary marine and fresh-water carbonates (Keith - Weber 1964) with plotted analyses of the ROF carbonates. Explanations: I - marine sedimentary carbonates, II - fresh-water sedimentary carbonates.
Fig. 13 Fields of genetically different carbonates (Borshevsky et al. 1981) with plotted analyses of the ROF carbonates. Explanations: I - marine sedimentary carbonates, II - fresh-water sedimentary carbonates, III - metamorphosed carbonates, IV - marine concretions, V - fresh-water concretions, VI - magnesite, VII - dolomite, VIII - siderite (VI, VII, VIII - rocks from the Satka and Bakal deposits).
It is important to note that the diagenetic and metamorphic changes can produce carbonates that are isotopically similar to juvenile carbonates (Kuleshov 1978, Goroshnikov et al. 1981, Ustinov et al. 1984).
According to majority of authors siderite originates directly on the boundary sediment - sea water. Maynards (1985) 13C value for siderite is - 10 while the 13C values for co-precipitating calcite and dolomite are between - 2.0 to + 2.0. He assumes that the Fe-carbonates originate below the surface of the sediment but probably before its complete consolidation. He explains the different isotopic value of siderite by decay of organic substances.
Fig. 14 Fields of metamorphosed carbonates (ONeil 1979) with plotted analyses of the ROF carbonates. Explanations: I - marbles, II - hornfelses, III - skarns.
Fig. 15 Field of genetically different siderites (Timofejevova et al. 1976) with plotted isotopical composition of oxygen in the ROF carbonates. Explanations: A - sedimentary - diagenetic siderites from terrigenous complexes, A/1 marine, A/2 saline - lagoonal, A/3 fresh-water lacustrine, boggy, B - siderites from marine volcano - sedimentary complexes, C - metasomatic siderites, D - hydrothermal - vein siderites
All these facts should be kept in mind when evalua-ting the data about isotopic composition of carbonates from the Rudnany deposit (Figs. 10, 11). The 13C value varies widely from + 0.30 to - 8.0. The average value for siderite was - 3.87, dolomite - 3.66 and calcite - 3.43. The 13C values were changing relatively little, the maximal deviation was 1.22. Slightly greater differences can be seen in the case of oxygen isotopes. The isotopically lightest oxygen was determined in the carbonates of the Klatov Group (+ 17.7) and the heaviest oxygen in the carbonates of the Krompachy Group (+ 21.6).
The gathered data are practically identical to those of Cambel et al. (1985). These authors consider mostly a deep-seated source of carbon and ore mineralization in Rudnany.
In order to interpret the genesis of the studied carbonates we used diagrams of Keith - Weber (1964), Timofejevova et al. (1976), ONeil (1979) and Borshevsky et al. (1981) with delimited field of carbonates of various genesis. The investigated carbonates fall on the Keiths diagram into the field of marine sedimentary carbonates (Fig. 12) as well as in the Borshevskys diagram into the field of siderites from the deposits in Bakal and Satka regions on the border of marine sedimentary carbonates and metamorphosed carbonates (Fig. 13). Borshevsky et al. (1981) consider synsedimentary origin of our carbonates on the basis of their isotopic composition. Similarly, Smolin et al. (1984), regarding the geological position, mineral composition, petrological and geochemical data classify the mentioned deposits as marine ones belonging to a group of lagoonal epicraton deposits of the miogeosynclinal paleoclimatic zone.
ONeil (1979) searched the isotopic composition of carbonates metamorphosed to different degree. The studied carbonates belong to the field of carbonates metamorphosed in the marble facies (Fig. 14).
The Timofejevovas et al. (1976) diagram for siderites of various genesis offers both sedimentary - diagenetic (sea, lagoon) and metasomatic and vein - hydrothermal ways of origin of carbonates from the Rudnany deposit (Fig. 15). However, if we check for their criteria delimiting the field of metasomatic and hydrothermal siderite we find out that the borders were set up according to the deposits Bakal and Satka whose genesis is questionably similar to the genesis of our siderite and magnesite deposits.
Recently, Vozarova et al. (1995) published new data about isotopic composition of carbonates from Upper and Lower Carboniferous of the Gemeric unit. They used the data to set criteria for correlation of undated strata and also for interpretation of genesis of the carbonates and their postdepositional changes.
They propose polygenetic evolution for the Lower Carboniferous carbonates of the Ochtina sequence. Isotopic evidence combined with additional data (content of Na, Sr, Sr/Ca etc.) implies origin in shallow marine to evaporitic environment. They look for potential source of Mg in associated basic and ultrabasic rocks as well as in the intraformation crinoidal detritus with high MgCO3 content. The origination environment of carbonates from the Crmel Group, Zlatnik sequence and Dubrava sequence is explained in a very similar way. They emphasize that the isotopic composition of the carbonates is affected mainly by the metamorphic solutions.
The absolute values of oxygen and carbon isotopic composition of Vozarova et al. (1995) are well compa-rable with ours.
The carbonate mineralization in the Rudnany ore field (ROF) is located not only in the vein structures and their immediate vicinity but carbonates are found disseminated in rocks in regional scale. The latter mode of occurrence was the chief object of our study.
Carbonate minerals in the rocks of the ROF form several separate carbonate associations: 1. siderite - ankerite, 2. dolomite (Fe-dolomite) - breunnerite, 3. calcite - dolomite. The study showed that the quantitative as well as qualitative representation of carbonates depends strongly upon the host rock lithology.
A clear relationship exists between appearance of the calcite - dolomite (Fe-dolomite) association and the presence of basic rocks between the breunnerite - dolomite association and altered ultrabasic rocks. It makes us to believe that the hydrothermal fluids were influenced by topomineral properties of the present rocks so strongly that the siderite - ankerite association could not be formed in these rocks.
Concretions and septaria are typical for the pelitic Permian and Lower Triassic sediments of the ROF. The carbonate minerals concentrated in concretions and septaria reflect truly the disseminated carbonate mineralization in the ambient Permian and Scythian rocks. We suppose that these phenomena are the product of diagenetic processes. Summarising, large number of ankerite veins in the Permian sediments is only a logical consequence of carbonate mobilization from the rocks. Siderite or some intermediate members of the siderite - magnesite series are found in some concretions and septaria, less in vein structures. No concretions are described from the pre-Permian complexes and therefore it is not possible to ascribe similar genesis to the carbonate minerals.
In our opinion, isotopic research of the studied carbonates did not solve the question of their origin. It allows to conclude that the carbonates contain not only typical deep carbon but also sedimentary marine or mixed carbon. We agree with the belief of Kralik et al. (1989) stated for crystalline magnesite, that the data indicate marine source for the studied carbonates or their primary rocks.
Acknowledgements. We are grateful to the colleagues from Faculty of Chemistry and Technology (STU Bratislava), former Dionyz Stur Geological Institute (Bratislava) and UUG (Praha) who performed part of the analytical work used in this publication. We thank our colleagues co-operating in this research. Translated by Juraj Majzlan.
Acta Geologica Universitatis Comenianae, Nr. 51, Pages: 43-56 Bratislava 1996, Lidia Turanova - Jan Turan