Iron Ore Deposits in Paleokarst

Iron Ore Deposits in Paleokarst

419 IRON ORE DEPOSITS IN PALEOKARST György Bardossy, Yves Fuchs and Jerzy Glazek Introduction Iron ore deposits in paleokarst structures are frequen...

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419

IRON ORE DEPOSITS IN PALEOKARST György Bardossy, Yves Fuchs and Jerzy Glazek

Introduction Iron ore deposits in paleokarst structures are frequent but only a few of them are of economic interest at the present time. During past centuries, however, such deposits were intensively mined all over Europe. High iron contents, occurring frequently in this type of deposit, were welcome for metallurgical reasons during Middle Ages and until the second half of the 19th century. There are some exceptions, such as North-East Bavaria in Federal Republik of Ger­ many and Alapaevsk and Akkermanovsk in the U.S.S.R., where paleokarst iron ore deposits with important reserves are still mined. Probably the best prospects for the discovery of new, rich iron deposits in paleokarst structures are in ancient and recent tropical domains. The latter have been poorly investigated up to the present. The Qui-xa deposit in North Viet­ nam may serve as an example.

Examples of Deposits A great number of karst deposits that are classified as lead-zinc or baryte deposits also contain important quantities of iron even if they are uneconomic. At Bled Zelfane, Tunisia (AMOURI, 1977), paleokarst structures are filled with mixed ore containing halloysite, saucconite clays, goethite, smithsonite, etc. The zinc content ranges from 2 to 10 %, the Fe from 15 to 25 %. In Belgium, Luxembourg and France a great number of small iron ore deposits in paleokarst structures are known because they were mined during the past. In the region of Nismes and Couvin in Belgium (SwYSEN, 1971) paleokarst forms (locally called abannets) developed in Givetian reefal limestones. These are large depressions. Their diameter can be as great as 100 m and their depth, 50 m. They are filled with limonite lenses and Tertiary sands. In Luxembourg and in the southern part of Belgium (LUCIUS, 1952; DEJONGHE, 1985) P. Bosâk, D. C. Ford, J. Glazek and I. Horäcek (Editors): Paleokarst. A Systematic and Regional Review. - Elsevier and Academia. Amsterdam and Praha. 1989.

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Bardossy G., Fuchs Y. and Glazek J.

paleokarst is developed in Bajocian limestones (Longwy Limestone). The karst depressions are filled with reworked lateritic products (locally called bolus) and iron-rich pisolitic concretions. The age of this filling is Eocene according to Lucius (1952), or Oligocène following DEJONGHE (1985). The pedological alteration of limestone and marls of Early Jurassic age formed residual clay deposits which then experienced lateritization under hot climatic conditions. Mobilisation of iron in these particular conditions induced the formation of pisolitic concretions in the paleokarst. These ores were mined from the protohistoric period onwards and were rich in iron (35 to 48 %) while depleted in phosphorus. The deposits were small in size. Mining activity ceased in 1852. Similar deposits are known from France. STEINBERG (1970) reports pisolitic and detrital ores, locally called siderolithique, frequently filling karst depressions in Mesozoic limestones of Poitou in the West of France. Such karst depressions can also be observed in the southern part of the Massif Central (Causses). They are filled with clays, detrital quartz and iron pisoids. Caddis worm (phryganeid) tubes indicate a fluviatile environment. The most important deposits of this type were described by ROUSSET (1967) in the Haut-Var at Beausoleil (SE France). Karst sinkholes filled with iron ore were mined here at the end of the 19th and the beginning of the 20th century. The total quantity of extracted ore was 350 000 tons with an average Fe content of 55 %. The karst is located within Bathonian and Bajocian limestones. The vertical depth of the sinkholes reached 200 m! The material filling the sinkholes consists mainly of goethite accompanied by lesser amounts of hematite and dolomite. Marcasite and pyrite nodules were observed in the lower part of the sinkholes, their quantity increasing with depth. Concerning the source of the iron, ROUSSET (1967) emphasizes the importance of pedological alteration of glauconitic marl of Early Cretaceous age during an emergence as the most plausible origin. All of the deposits described above show some characteristics in common. The karst depressions are filled with goethitic or hematitic ores mixed with con­ tinental sediments. Sulphides can be present in the deeper part of the paleokarst, but minerals like siderite are not present and the phosphorus content is very low. Their vertical extension below the surface can be important. The ore reserves are in the range of tens of thousands of tons. These deposits were formed in continental environments beneath high plains surfaces. A distinctly different type of iron-filled karst structure is known in the Federal Republic of Germany and in the U.S.S.R. Sedimentary iron ores filling karst depressions accumulated in a continental brackish or marine environment. Such karsts are known at Saint-Dizier (France), in Oberpfalz (Bavaria, FR.G.) and in the U.S.S.R. Near Saint-Dizier the karstification occurred at the top of the Jurassic lime­ stones. The karstified surface is overlain by sediments of Neocomian age de-

Iron ores

421

posited in a brackish environment. The deposit consists of iron hydroxides and celestite. In the North-Eastern part of Bavaria important iron ore deposits are located stratigraphically at the boundary between the Jurassic and Cretaceous. Sixty five karst water table

3-28. Stratigraphy in the iron deposit zone of Northeast Bavaria, Federal Republic of Germany (after GUDDEN, 1984). 1. Cardium clay, 0-30 m; 2. Cretaceous, Cenomanian-Coniacian-? Santonian, 0-150 m; 3. ore-bearing horizon, 0-60 m; 4. Upper Jurassic dolostone and 5. Upper Jurassic limestones, both more than 150 m thick; 6. Dogger γ-ζ, 5-15 m; 7. Dogger Sandstones, 55-100 m; 8. Opalinus clay, 50-60 m; 9. Lower Jurassic, 25 m; 10. Rhaetian to Lower Jurassic sandstones, 20 m; 11. refractory claystone, 45-55 m; 12. Upper Burgsandstone, 30-35 m

million tons of ore with an average Fe content of about 45 % have been mined since exploitation began. The ore fills troughs of a karst surface formed on Upper Jurassic limestones (Fig. 3-28). Their depth may be as great as 150 m and they can be several kilometres wide. The earliest ore-containing layers within them are of Early Cretaceous age (GUDDEN, 1984). The ore district forms a NNW extending tongue (Fig. 3-29) that was a marine gulf during the Late Cretaceous (Cenomanian-Coniacian). This paleogeography is probably related to a reactivated feature of the basement. The existence of NNW/SSE-trending structures in the deposits is related to some reactivation during the Early

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Bardossy G., Fuchs Y. and Glazek J.

Jurassic-Cretaceous period. Flexure was active during the sedimentation and faulting occurred after deposition of the ore and before the deposition of Coniacian marls. This tectonism induced the formation of a trough in which previously developed karst features were preserved from erosion and where the supply of dissolved iron in surface waters was important.

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3-29. Paleogeographic situation of iron ore deposits, Northeast Bavaria, F.R.G. (after GUDDEN, 1984). 1. main ore deposits of the Amberg Ore Unit; 2. occurrences of Cretaceous iron ores, partly reworked; 3. ancient iron ore mining near Painten, Neuessing and Kelheim; 4. approximate outer boundary of the Upper Cretaceous (Cenomanian to Santonian) depositional area; 5. approximate boundary of the depositional area of the Amberg Ore Unit; 6. extension of Cretaceous Neukirchen Ochres; 7. approximate outer boundary of Late Cenomanian glauconitic sandstone; H. G. Hahnbach uplift; E. G. Eibenstock uplift.

Iron ores

423

There is some controversy concerning the nature of the environment during deposition of the iron ore beds (continental, brackish or marine?). It seems obvious to us that this area was a trough with submergence of the karst surface. The ore deposits are interbedded with clastic layers. There are three main types of ore: (1) white ore (Weisserz) containing mainly siderite with some accessory chamosite; (2) brown ore (Braunerz) with goethite and limonite; and (3) limonitic ochre ore (Ockererz). During the deposition of the ore, variations in the Eh caused the alternation of siderite and limonite. In many cases, erosion planes can be observed and might be related to post-depositional oxidation of the sideritic ore? Other changes, particularly limonitization, are locally asso­ ciated with late circulation of oxidising water. There are two paleokarst iron ore deposits in the U.S.S.R., described by SOKOLOV and GRIGOREV (1977). The first is near Alapaevsk, Sverdlovsk district, in the Northern part of the Ural Mountains. A paleokarst surface of Car-

Θ 6 ^ 7 B e ΕΞ39 E 3 o 3-30. Geological sketch-map and cross-section (A-B) of the Akkermanovsk deposit, U.S.S.R. (after KISELEV, 1963 fide SOKOLOV and GRIGOREV, 1977). 1. brown and grey loams; 2. rubbly-pebbly redeposited iron ores; 3. yellowish-grey sandy clays, frequently with gypsum; 4. mottled clays, ferruginized sands and pebble beds; 5. brown ironstone, upper horizon; 6. sideritic clay ores, lower horizon; 7. Visean limestones; 8. siliceous shales; 9. outlines of brown ironstone, upper horizon; 10. outlines of sideritic ores, lower horizon.

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Bardossy G., Fuchs Y. and Glazek J.

boniferous limestones is overlain by a stratiform ore-bearing sequence of Mesozoic age. The overburden consists of Cenozoic sands and Quaternary loam. The thickness of the ore sequence varies from 0.5 to 70 m, being largest in the karst depressions. The deposit has a length of 5 to 10 km and width of 0.5 to 1 km. The upper part of the ore-bearing sequence consists essentially of goethite and the lower part (located below the groundwater table) of strigovite and hydrogoethite accompanied by siderite. The ore has a nodular and concretionary structure and is associated with ochreous clays. The average Fe 2 0 3 content of high quality ore is 55 % (38.5 % Fe). The reserves of the deposit are estimated to be 42 million tons. The other deposit is located in SE Ural 20 km west of Orsk at Akkermanovsk. A paleokarst surface of Visean limestones is covered by Jurassic continental sediments. At their base are stratiform iron ore deposits. The maximum thick­ ness of the ore sequence is 35 m. Laterally, the ore passes into a sequence of mottled clays and ferruginous sands underlain by sideritic ore and clay. The ore consists of a mixture of goethite and hematite within a clayey groundmass. It contains 45-50 % Fe 2 0 3 (31.5 to 35 % Fe). The structure of the ore is partially oolitic-pisolitic, partly blocky-rubbly. The reserves of the entire deposit are estimated at 280 million tons; about the half of them belong to the paleokarst type (Fig. 3-30). Important iron ore deposits were discovered in the northeastern part of Viet­ nam, in the Phang-xi-pang (Fan-si-pan) Mountains (HOFFET, 1939). There, bet­ ween two belts of Proterozoic (?) gneisses, there is a narrow synclinorium of crystalline schistose rocks (OsiKA, GLAZEK and JUSKOWIAK, 1967) among which dolomitic marbles classified as Middle Devonian occur (GLAZEK and JUSKOWIAK, 1964). All rocks have been metamorphosed to greenschist facies and were strongly folded during the Indosinian Orogeny (Late Triassic) (Fig. 3-31). The planated mountain ridges are covered by thick latente enriched with limonite and containing martite blocks that are without economic importance. However, in some places on the karstified marbles huge accumulations of limonite ore were found. The weathering and erosion processes that produced and concentrated them probably started in the Early Jurassic (GLAZEK,1966). They were accelerated by the Himalayan uplift which caused the deep incision of the present meandering river pattern. The planated Qui-xa Hill (Fig. 3-32) in the axial part of the synclinorium probably represents a fragment of pre-Late Tertiary surface. The ore-bearing sequence consists of porous limonite with intercalations of latente, weathered gravels and detrital blocks from surrounding rocks. The ores compose 85 % of it. The thickness of these deposits reaches 150 m in karst depressions. The length of the deposit is 2 km and the width is a little less than 1 km; the average thickness is about 100 m. The ore consists of goethite, hydrogoethite and hydrohematite, with some manganese hydroxides. Relicts in the ore sequence in

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elude oxidized pebbles and blocks of magnetite quartzites and pyrite-bearing rocks. Particularly interesting are intercalations and lenses of post-carbonate, rhombohedral limonite enriched with manganese oxides, called turgite. Average

3-31. Geologic sketch-map of the ore bearing area in Vietnam (inset shows the location), modified after OSIKA, GLAZEK and JUSKOWIAK (1967). Proterozoic (?) : 1. gneisses, 2. Lang Lech Group of mica schists; Paleozoic (?) : 3. Coc-xan Group of phyllites, 4. Devonian (?) marbles; Upper Tertiary: 5. conglomerates and brown coal deposits; Plutonic rocks: 6. metamorphosed gabbroids, Iron ore deposits: 7. primary magnetite-bearing quartzites, 8. karst limonitic ores; 9. faults and overthrusts; 10. mogotes.

ore composition is: Fe 54.8 %, Mn 3.0 %, Si0 2 3.75 %, A1203 2.0 %. P 0.08 %, S 0.025 % (KUBICKI in OSIKA, GLAZEK and JUSKOWIAK, 1967). The reserves of these deposits are estimated at 106 million tons. Similar but lesser deposits were found in Lang Vinh (HOFFET 1939). There, the thickness is up to 40 m (in sinkholes), the average iron content is 40 %, and the Si0 2 + A1203 content is 19.2 %. The reserves were estimated to be 10 million tons (GRZEGORSKI and SAWICKI in OSIKA, GLAZEK and JUSKOWIAK, 1967). This deposit is much more dissected and eroded by the post-Neogene erosion. It is remarkable that both the Qui-xa and Lang Vinh deposits were originally

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Bardossy G., Fuchs Y. and Glazek J.

treated as iron caps upon supposedly primary iron-bearing rocks (HOFFET, 1939; OSIKA, 1961); however, boreholes revealed only barren rocks (mostly marbles) beneath the ores.

3-32. Cross-section of the Qui-xa deposit (after KUBICKI in OSIKA, GLAZEK and JUSKOWIAK, 1967).

Paleozoic substratum: 1. phyllites, 2. marbles; Ore-bearing sequence: 3. latente with weathered gravels and blocks of older rocks, 4. limonites, 5. turgite intercalations, 6. latente and limonite weathering waste.

The Role of Physico-Chemical Processes in the Genesis of the Paleokarst Type of Iron Ore Deposits The existence of monometallic or mainly monometallic, exogene concentra­ tions like the paleokarst iron ore deposits calls for processes which induce selective mobilization, transportation and deposition of iron. The weathering of continental areas under hot and wet climatic conditions is particularly important. The abundance of lignitic fragments in the deposits (Saint-Dizier, Oberpfalz), and the kaolinization of the crystalline basement of the Bohemian Massif during the Early Cretaceous period seems to establish such climatic conditions for the formation of the Saint-Dizier and Bavarian iron ore deposits. The selective mobilization of Fe with respect to other elements, par­ ticularly Al, is one of the most important conditions for the supply of iron in the depositional areas (as defined by RUPPERT, 1984). The iron is present in the soils mainly as goethite and can be taken into solution according to the following equilibrium reaction: a FeOOH + 3 H + + e" where

Fe 2+ + 2 H 2 0

Eh = 0.826 + 0.059 log

+ Ί3 ΙϋΠ 2+

[Fe ]

(1)

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427

and l o5g [LF e 2 + J]

=

2 ^ 6 E h _ 3 pP H 0.059

The solutions which are percolating through soils have pH values close to 4.0 and Eh values about 0.4 V (HERMS, 1982; RUPPERT, 1984). The equilibrium state, solution/goethite, is reached at a value of 10 48 mol · l - 1 (= 0.9 ppm Fe). Decrease of pH to a value of 3 means an increase of 1 000 times in the solubility of Fe 2 + . A decrease of the Eh value of 0.1 V increases the solubility of Fe 2+ some 50 times. Therefore the solubility of Fe 3 + in a domain where the pH is higher than 3 never lies beyond 10~7 mol · 1 - 1 . In soils, however, organic complexing agents are present. They can form chelate boundings with iron. The existence of organic substances resulting from the decomposition processes in soils induces very low pH values if no buffers (alkaline or alkaline-earth elements) are present. Soluble complexes containing Fe 3+ or even Fe 2+ are then formed. At a pH of 4 these Fe m complexes are more important in solution than the free Fe 2+ ions. Therefore, the Fe 3+ ions are more easily complexed. The relation to other elements, principally to Al, shows that the [Fe m ] complexes are selectively mobilized in solution. This process is essential for the selective mobilization of iron with respect to aluminium. The concentration of Al in solution can be determined as follows: 2 Al 3 + + 2 H4SiO° + H 2 0

^

Al 2 Si 2 0 5 (OH)4 + 6 H +

(2)

with

log

,

{H + J

— = -6.25

[Al3 + ] 2 - [H 4 SiO°] 2

Al 3+ + 3 H 2 0

^± y-Al(OH)3 + 3 H +

(3)

with log ί Η ^ = -8.44 [Al 3 + ] Si(OH)° ^±

Si0 2 + 2 H 2 0

(4)

log Si(OH)4 = -2.74 (amorphous silica), -3.10 (soil silica), -3.94 (cristobalite), —4.00 (quartz). In equilibrium with kaolinite [Al 2 Si 2 0 5 (OH) 4 ], the Al 3+ content of the solutions will be 0.4 ppm, with gibbsite [y-Al(OH)3], 7 ppm. Reactions (2) and (4) are the most frequent and the Al 3+ in the solutions is normally less than 1 ppm (RUPPERT, 1984).

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Iron can be mobilized selectively. Fe 2+ ions in solution do not oxidise or do so very slowly at pH = 4 and p02 = 0.2, so that the transportation of iron as ionic Fe 2+ in solution is very easy if the bacterial activity in this relatively low pH domain does not induce an increase of the oxidation rate (to 106 times). The karst environment of this type of iron deposit includes water that is saturated in the deepest part of karst, particularly with respect to CaC0 3 . The water has a neutral or basic pH. In the equilibrium state, with a pC02 = 0.000 29 MPa, the pH of a solution saturated with CaC0 3 is 8.3. The C 0 2 content of karst water can increase in response to the oxidation of organic matter and the pH for a C 0 2 content of 0.098 MPa will be 6.6. The precipitation reactions of siderite, FeC0 3 (PLUMMER and BUSENBERG, 1982; SMITH and MARTELL, 1976; quoted by RUPPERT, 1984) are: CaC0 3

^± Ca 2 + + CO 2 "

(5)

log [Ca 2 + ] [CO 2 "] = -8.12 Fe 2+ + CO 2 " log

= 2

^

FeC0 3

-

: — = 10.32

^

FeC0 3 + Ca 2 +

(6)

[Fe +] [CO 2 "]

Fe 2+ + CaC0 3

(7)

log [Ca 2 + ] [Fe 2 + ] - 2.20 The precipitation of siderite can begin as soon as Fe 2+ is higher than 1/160 of the molar concentration of calcium. The main part of the iron is mobilized as organic iron comlexes or is bound as colloids to humic compounds. The oxidation of organic matter during the transportation and the rapid change in pH as the solutions enter the karst system have an important influence on the stability of the complexes. Calcium can replace the iron bounded in the organic ligands. Iron hydroxides are then deposited. Such reactions occur in the first type of deposits described (i.e. Luxembourg, South-East France, etc.). On the other hand, in the second type of iron deposits (N. E. Bavaria, U.S.S.R., etc.) some areas of flooded karst were characterized by euxinic con­ ditions. In these areas aqueous oxygen was completely exhausted by the oxida­ tion of organic matter. Eh was very low. Iron hydroxides and the Fe 3+ organic complexes could be reduced and solutions with Fe 2+ ions could appear. These solutions can precipitate siderite (5), (6), (7). All these conditions also favour the deposition of P and Mn, which often accompany siderite in this second type of deposit (RUPPERT, 1984).

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The Qui-xa deposit seems to be a result of both processes: highly oxidizing conditions prevailed during the initial erosion and accumulation of ore, then euxinic conditions within the deposits caused precipitation of siderite and finally, Late Tertiary uplift caused a second oxidizing and enriching of the deposit. Both types are characterized by: (1) high iron concentration; (2) predominant hydroxide mineral composition; (3) low silica admixture which enables cheaper metallurgical processing, and (4) very varied thickness and superficial distribu­ tion caused by the buried karst surface as well as by distribution of carbonate rocks within the substratum. As a conclusion, it seems important to emphasize that these two types of iron deposits in paleokarst developed in quite equivalent environments with regard to the conditions for the mobilization and transportation of iron. The main difference seems to be the morphology of the karst and the tectonic environ­ ment. The first type evolved on stable continental platforms whereas the second type occurred at the borders of more active areas and were inundated because the iron ores were deposited shortly before marine transgression (Saint-Dizier, North-East Bavaria, Akkermanovsk). © György Bârdossy, Yves Fuchs, Jerzy Gtazek, 1989