Imrni manstveni EZanak The article wasprepared in the pame of Project 1-09-072 entitled

'BarnuUtes and terra rossa in Croatia and their relationship to carbonate pla4onn" financed by the Minktry of Science and techno log^ of Republic of Croatia

- Rudarsko-geoldko- naftni zbornik

THE BAUXITES DISCOVERED ON Mt. RAVNA GORA IN HRVATSKO ZAGORJE (Geological Setting, Composition, Origin)

Boris SINKOVEC') and Antun SIMUNIC~) I ) Faculry of Minin Geology and Petroleum Engineering Pieroffrj'eva 6,10000 Zagreb, Croatia

2fInstitute of Geology, Suchova 22,10000 Zagreb, Croatia

Key-words: Bauxite,Eocen, Diaspore, Hrvatsko zag0 j e

Vol. 8

Kgu2ne r@jeZi: Boksit, Eocen, Dijaspor, Hrvatsko zago j e ,

Clayey bauxites originally underlain by Ule Anisian dolomites U Ravnoj Gon 1992.g. otkriveni su glinoviti boksiti koji se and overlain by Middle Eocene sediments was discovered on Mt. nalaze izmedu anizitkih dolomita i srednjoeocenskih sedimenata. Ravna gora in 1992. in overturned position. The bauxite is of Boksiti su kaoiinitnodijasporsko-bemitnog sastava i sliEni su kaolinitic- diasporic-boehmitic composition and resembles the oligocenskim boksitima Slovenije. Na temelju strukture boksita, Oligocene bauxites of Slovenia. On the basis of the bauxite sad&ja mikroelemenata i ostalih pokazatelja zakljutuje se da su textures, the contents of microelements and other indicators it was ovi boksiti nastali pretaloZavanjem trijaskih boksita. tentatively concluded that this bauxite has been resedimented from bauxites of Triassic age.

str. 67-76

Introduction In Croatia bauxite deposits are mainly situated in

the Outer Dinarides, that is in the SW part of the country. The bauxite deposits positioned farther north, so far known in Croatia, were those of Triassic age in the vicinity of Vojni6, and those of Senonian age in the vicinity of Duga Resa and Karlovac. However, on the southern slope of Mt. Ravna gora, north of Prebukovje village, 25 km WSW from Varafdin, a bauxite deposit was discovered in 1992. The bauxite occurs at the contact between the Triassic and Eocene limestones. The deposit is mainly covered with talus, but can laterally be followed in a length of several hundred meters. This finding is important not only because this bauxite deposit is situated further north than the other bauxites so far known in Croatia, but also because it can be related to deposits having similar geological features, situated some 70 km westward in Slovenia.

The authors thank dr. Ivan HeCimoviC, Radovan AvaniC and Stjepan MarkoviC for their help in the field work. Their thanks also go to Vlasta JuriSiCMitroviC and Mr. Slobodan Miko for performing chemical analyses of the bauxite.

Zagreb, 1996.

Geology of the study area Mt. Ravna gora is composed by Triassic clastites

and carbonates, Eocene limestones, Oligocene marls, Egerian and Eggenburgian clastites and a Neogene clasti~carbonate sequence. Significant amounts of tuffs, blocks and pebbles of andesites are also present. Triassic

Triassic rocks make up the bulk of Mt. Ravna gora. The Lower Triassic rocks are situated in the central

part, while the Middle Triassic ones on both sides of Mt. Ravna gora.

According to the lithological characteristics and the fossil assemblages, the Lower Triassic sediments can be divided into (1) a lower part composed by clastites and (2) an upper part composed by carbonates.

In the lower, clastic part, repeated alternation of reddish-brown, grey and yellow micaceous arenites, quarzites, siltites and shales was recognised. Reddish-brown oolithic limestones and marls appear only sporadically. In these sediments the following shell species were determined: Myacites (Anodonto- phora) fassaensis WISSMANN, Pseudomonotis (Claraia) clarai EMMRICH and Naticella costata MUNSTER. These clastites are equivalents of the Seisian, namely Lower Werfenian (Scythian) sedi- ments.

In the hangingwall of this clastites interlayers of dark-grey platy limestones occur. They gradually increase upwards and at the end they become prevailing. They correspond the Upper Werfenian, namely Campilian sediments. These fine-grained limestones (biomicrosparites) often contain particles of auartz and limonite-clayey material. They also conkin irregularly oriented shells of m&lluscs, crinoid plates and remnants of the microfossils Meandrospira pussila (HO) and Ammodiscus cf. incertus (d' ORBIGNY) (S i m u n i C et al., 1979). In the upper part of the dark-grey limestones there are interlayers of black shales, marls and dolomites. Due to the fact that their importance is increasing in this part, during the geological mapping, the geological boundary between the Lower and Middle Triassic was tentatively placed here.

The Lower Triassic rocks of Mt. Ravna gora are tectonically very disturbed. However, according to

68 Rud.-geol.-naft. zb., Vol. 8, Zagreb, 1996.

0 50km - -. + L.

Varatdin a . 0 L.

V I C3

ZAGREB 0 .-. .+

+

e. Bjelovar \.

1-T,

$inkovcc, B.& &nunid, A.: The bauxites of Mt.

B

Dug; vrh

Fig. 2. Geological cross sections through the SW part of Mt. Ravna gora Legend: 1.- Ottnangian, 2.- Egerian, 3.- Oligocene, 4.- Eocene, 5.-' Aniisian, 6.- Scythian, 7.- Palaeozoic, 8.- bauxite, 9.- erosional

boundary, 10.- vertical fault, 11.- thrust boundary.

superposition relationships one can tentatively suppose that their real thickness is approximately 200 m.

During the Middle Triassic, the area of Mt. Ravna gora was part of a huge carbonate platform on which shallow-marine carbonate sedimentation prevailed while sedimentation of deep water sediments associated with volcanics took place only temporary. A characteristic of the Anisian in Hrvatsko zagorje is that carbonate sediments occur at the base, in the middle part fine-grained clastites with volcanites prevail, while in the upper part carbonate sediments appear again. Due to long-lasting erosion, in the area of Mt. Ravna gora only the Lower Anisian and part of the Middle Triassic has been preserved. The well-known member of the lower part of the Anisian is dark-grey, fine to medium-grained, calcitic dolomite which has developed continuosly from Campilian limestones. Sedimentological studies indicate that it was formed by dolomitization of crinoidal limestones. Except crinoid stems, no other fossil remnants have been found in this dolomite. At places it contains mm to cm thick interlayers of chert, which may be of diagenetic origin. The thickness of the dark-grey dolomite in the area of Mt. Ravna gora ranges from 10 to 20 meters, while in other mount- ains in Zagorje area it reaches up to 100 meters. This dolomite is overlain by grey limestones, recognised as algal biomicrites and algal biosparites. At a few localities microflora and microfauna was found, which document the stratigraphic affiliation of these

limestones. The following species have been recognised: Meandrospira dinarica KOCHANSKY - DEVIDE & PANTIC, Pilammina densa PANTIC, Diplotremmina austrofirnbriata KRISTAN - TOL- MAN, Frondicularia woodwardi HOWCHIN, and others. On the northern side of Mt. Ravna gora the dolomites are much more common than on the southern one. The dolomites have been formed by dolomitization of algal limestones and, consequently, transitions are quite possible between them. They are fine to medium-grained calcitic, stromatolitic and algal dolomites (5 i m u n i C & S i m u n i C, 1980). In the same area, but outside the limits of our map (Fig. I), andesitobasalts, tuffs, siltites and shales are tectonically wedged in dolomites. This is only a small occurrence which is but a remnant of an originally much more extended formation, as it is a case in the area of the neighbouring mountains Ivan3Cica and StrahinSBca. These clastites with volcanites may have taken part in the formation of bauxite.

In the area of Mt. Ravna gora no rocks of Upper Triassic, Jurassic or Cretaceous age have been recognised. On the basis of resedimented rock fragments, one can tentatively suppose that sedimentation continued on the carbonate platform during the Middle and Upper Triassic, Mt. Ravna gora still being part of it. Many authors who had been working in NW Croatia came to the conclusion that in the Middle Liassic a disintegration of the carbonate platform started. From that time until the Middle Eocene, Mt Ravna gora represented an

Fig. 1. Geological map of the SW part of Mt. Ravna gora Legend: 1.- Ottnangian; sands, mark and clays, 2.- Egerian- Eggenburgian; tuff, 3.- Egerian; marls and sands, 4.- Oligocene; marls, 5.-

Eocene; limestones, 6.- Anisian; limestones and dolomites, 7.- Scythian; sandstones, siltites, shales and limestones, 8.- bauxites; 9.- geological boundary, 10.- overturned erosional boundary, 11.- vertical fault, 12.- thrust boundary, 13.- fossil finding place (microfossils, macrofossils), 14.- major spring, 15.- line of cross section.

70 Rud.-geo1.-naft. zb., Vol. 8, Zagreb, 1996.

isolated geological unit, on which, due to longlasting emersion, bauxite could develop. Eocene

On the slopes of Mt. Ravna gora there are several outcrops of Eocene limestones. These represent erosional remnants and technically wedged blocks of, formerly, wider extended sediments (3 i k i C et al., 1976). At their base two entirely different sediment types occur. This indicates that important differences in paleorelief existed prior to the Middle Eocene trasgression. These are; monomictic limestone breccia and reddish-brown clayey bauxite. The bauxite was found only at the contact between the Middle Triassic and Middle Eocene limestones, above the village PintariCi, on the southern slope of Mt. Ravna gora. Due to tectonic movements the rocks have been overturned, and the bauxites are now situated below the Eeocene limestones, while the Triassic limestones are situated above the bauxite (Fig. 2).

Palaeontologic analyses have shown that the limestones situated immediately below the bauxite are Middle Eocene age while the Upper Eocene limestones are located a little lower. The Middle Eocene age is based on the following microfossil community: Nummulites cf: sniatus BRUGNIERE, N.c$ rotularius DESHAYES, Operculina cf: granulosa (ARCHIAC), Actinocyclina radians (ARCHIAC), Archeolithothamnium sp. and others. It the lower part of the limestones the following microfauna was discovered: Nummulites fabianii (PREVER), Chapmanina gassinensis (SILVESTRI), and numerous miliolids, coral fragments and husks of small shells (3 i k i C et al., 1976). This fossil community indicates Upper Eocene age.

On the basis of sedimentological analyses several types of limestones have been recognised which were formed in warm and shallow sea, next to coral reefs. The following limestone types have been discovered: coral biolitites, algal-foraminifera1 biomicrites and biomicrudites. The outcrop of the Eocene limestones is covered by talus and luxuriant vegetation, but it can be followed in a length of 800 m. The thickness of the Eocene limestones ranges from 30 to 40 m. Oligocene

In NW Croatia outcrops of Oligocene rocks are known only on the southern slope of Mt. Ravna gora. They are represented by greyish-green marls. According to L. 5 i k i C (1985) they are equivalents of

"Kiscelln clays in Hungary and "sivice" marine clays in Slovenia (K u S C e r, 1967), respectively. They contain the following microfossils: Trituxia szaboi (HANTKEN), Nodosana acuminata HANTKEN, Uvigerina farinosa HANTKEN and other. The only macrofossil is the species Chlamys duodecim- lamellatum (BRONN). Due to the vegetation cover and tectonic disturbance the primary relation between the Oligocene marls and Eocene limestones could not be established. Because there is no large stratigraphic gap, continued transition between these rocks was tentatively postulated (5 i k i C et al., 1985). However, geological mapping has shown that the Eocene and Triassic limestones have been trusted over Oligocene marls which have been trusted on Egerian clastites (Fig. 1). Egerian (Oligo-miocene)

Sediments of Egerian age are surrounding Mt. Ravna gora and other mountains in NW Croatia as a narrow and interrupted belt. They contain brown coal and were known as "Sotzka-beds" (G o r j a n o v i C - K r a m b e r g e r, 1904). They are transgressive sediments but their contacts with older rocks are tectonically disturbed. They consist of alternating sands, silts, mark and clays. They were formed on low nearshore plains which were intersected with channels and estuaries protected by sand barriers from the action of waves (so called %atw). The smallest particles were sedimented near the shore and in inter- channel positions, while sand and gravel was deposited in channels. Among the rock fragments in sands and gravels, metamorphic ones prevail. This indicates transport from the area of the Alps. Palaeontologic analysis indicates an alternation of marine, brackish water and fresh water sediments. In the nearer environs of Mt. Ravna gora only marine sediments are known. This explains why no brown-coal has been discovered in the area. According to A v a n i t et al. (1990) marine sedimentation is proved by the following species: Ammodiscus incertus d' ORBIGNY, Ciclarnmina acutidorsata (HANTKEN), C. placenta (REUSS) and others. Tuffs

At the end of the Egerian, in the NW part of the Hrvatsko zagorje, strong volcanic activity took place. Along fault zones (one of them extends along the southwestern slopes of Mt. Ravna gora), small masses of andesite and large quantities of tuff

Plate 1 Fig. 1) Fragment of resedimented diasporic bauxite in bauxite. Length of phot. is 3.3 mm. Fig. 2) Detail from fig. 1. Length of phot. is 1.36 ma. Fig. 3) Resedimented diaspore mid. Length of phot. is 0.86 mm. Fig. 4) Resedimented fine diaspore mids. Length of phot. is 1.36 mm. Fig. 5) Authigenic diaspore (enhanced relief) in kaolinitic- hematitic bauxite. Length of phot. is 0.86 mm. Fig. 6) Same as fig. 5, crossed niwls (whitediaspore). Length of phot. is 0.86 mm. Fig. 7 and 8) Diaspore veinlets in kaolinitic-hematitic bauxite. Length of phot. is 3.3 mm.

~inkovec, B.& &muni~, A.: The bauxites of Mt.

Rud.-geo1.-naft. zb., Vol. 8, Zagreb, 1996.

accumulated (S i m u n i C & P a m i 6, 1993). Vitroclastic, vitrocrystaloclastic and lithic tuff types could be distinguished (5 i m u n i C et al., 1979). Besides tuffs, on the southern slopes of Mt. Ravna gora, big pebbles of andesites were discovered, but they do not figure on the geological map (Fig. 1). Ottnangian

In the southwestern part of the geological map Ottnangian clastites are in fault-contact with Egerian sediments. The Ottnangian rocks are composed by repeated alternation of sand, marls and clays. They also contain lenses and layers of tuff. The marls .contain numerous planktonic and benthic fora- minifera indicating that this was part of a major sea basin. It is interesting to mention that in the same time other parts of Croatia were emerged. The marine facies of the Ottnangian sediments is proved by the species Elphidium crispum (LINNE), E. ungeri (REUSS), Globigerina praebulloides BLOW, G. ciperoensis ottnangensis ROGL and others. Tectonic framework ,

Mt. Ravna gora is part of a thrust which developed from a so called knee fold. Due to this fact, its central part assumes the shape of an anticline (Fig. 2). This is expressed by the arrangement of Triassic rocks rather then by the position of layers. The b-axis of the anticline extends E-W being parallel to the orographic axis of Mt. Ravna gora. In addition to the geological indications, an evidence which indicates that this structure is of thrust origin, rather than an anticline, is the hydrogeological setting of Mt. Ravna gora. Namely, all major springs are situated in the central part of the mountain. If the structure were an anticline, all major springs would be situated at the boundary between the Triassic limestones and the Oligocene marls. On the basis of the stratigraphic affiliation of rocks, it can be tentatively concluded that the tectonic movements started at the end of the Egerian. Due to the N-S oriented stress, a knee fold developed which was later transformed to a thrust. In a similar way were formed thrusts in other mountains of the Hrvatsko zag0 rje region (3 i m u n i C & H e C i- m o v i 6,1979). However, there is a difference in the direction of movement of the thrust on Mt. Ravna gora which was shifted southwards, and the thrusts on Mt. IvanSCica and Mt. StrahinSEica which were shifted northwards.

In the middle of the Badenian, in the frame of the Styrian orogenetic phase, Mt. Ravna gora was formed as a separate mountain. At the beginning of the transgression, Mt. Ravna gora was an island in the Badenian sea. On the basis of the distribution of erosion remnants, it can be assumed that at the end of Badenian Mt. Ravna gora was completely submerged. It acquired its present day shape in the Late Pliocene and the Quaternary due to uplifting along marginal faults.

Bauxite The only bauxite outcrop is situated on the

southern slope of Mt. Ravna gora, above Pintaridi village, approximately 450 meters above sea level. The slope is very steep, mainly covered by talus, and there are no paths. This is why the bauxite was only recently discovered. The width of the outcrop ranges from 0.75 to 3 meters. Its length is not known because it is mainly covered by talus. From the geological map and the cross section (Figs. 1 and 2) it is clear that the hanging wall is composed by Anisian limestones while in the footwall there are Eocene limestones. The plane which separates the Anisian limestones and the bauxite is smooth and is dipping 22' N. It contains striae which are parallel with the direction of dip of the plane itself. Because of this it wass supposed that this surface represents a napped plane along which Triassic rocks were thrusted over Eocene ones (S i k i C et al., 1976). However, during the geological mapping it was established that the Triassic and the Eocene rocks are overturned. The fact that the Middle Eocene limestones which are situated immediately under the bauxite are underlain by .limestones of Eocene age is in favour of the previous statement. The absence of breccia at the contact between the bauxite and the Eocene rocks may indicate that there was no pronounced relief in the vicinity at the beginning of the transgression. Sedimentological studies indicate that the limestones were formed close to coral reefs in a shallow and warm sea.

The bauxite is pelitomorphic with rare detrital grains and mainly of red color. Often, especially in the upper and lower parts of the deposit, the bauxite is pale-grey due to deferrification.

The bauxite consists of boehmite, kaolinite, diaspore, hematite, goethite and anatase. The main mineral constituents are boehmite and kaolinite. They are cryptocrystalline. There is more hematite than goethite which is a secondary mineral. On the basis of the shift of the reflection (111) of the goethite it was concluded that goethite contains 8 mol. % of Al00H. Detrital accessory minerals, tourmaline and zircon, were also observed.

The diaspore is microcrystalline and occurs in several modes. It was found in redeposited ooids (Plate 1,3 and 4). Some of them are solely composed of diaspore which is uniformly grained. The grains are very small (2 to 15 pm) and display mosaic twins. More often the ooids are of pronouncedly husk-like appearance. The outer shells are composed of diaspore while the central husks consist of boehmite and kaolinite or, more often of hematite. In this case the diaspore grains are slightly coarser. The second mode of occurence of diaspore is very seldom found in deferrificated bauxites as tabular grains of uniform size (around 30pm). It may be a result of secondary diasporization of the kaolinite-boehmite ground-

&dwvec, B.& $imuni6, A. : The bauxites of Mt. Ravna Gora

mass. The third mode of diaspore appearance, diaspore veinlets which intersect the bauxite, is the most comon one, especially in the lower part of the bauxite deposit (Plate 1 ,7 and 8). Also observed were micro fold^ and intralayering, and the cavities formed in them are filled with diaspore. The diaspore veinlets are up to 0.2 mm thick. At the contact between the bauxite and the footwall limestones the diaspore veinlet is 1.2 mm thick. Diaspore also fills cavities between the hard detrital bauxite and the matrix. The size of diaspore grains in veinlets range from 10 to 80 pm. The grains display a mosaic and fan-shaped texture. Fine detrial diaspore grains, ranging in size from 20 to 200 pm are also very often observed. Coarser grains are round-shaped and display mosaic twins. Smaller grains represent fragments of diaspore crystals.

The results of XRD analysis of selected samples are presented in Table I. The following samples (and their parts) were analysed: (1) defferificated sample in lower part of the deposit (la), limonitized part (lb) and diaspore veinlets with neighbouring bauxite (lc); (2) sample of red bauxite from the middle part of the deposit; and (3) sample of bauxite of the Oligocene age from St. AndraZ, west of Celje, Slovenia. The analyses show that the main minerals in the bauxite are boehmite and kaolinite, followed in lower amounts by hematite, goethite, diaspore and anatase. The thermic analyses are in concordance with the XRD analyses and point out that the kaolinite content is higher in the upper part of bauxite deposit than in the middle one.

TAB. I. X-ray diffractometric analyses* of the Mt. Ravna gora bauxites

Sample Boehmite Diaspore Kaolinite Hematite Goethite Anatase

la P m P a lb P m P c a lc m d m m a 2 P P c a 5 P m P c a

'Analyses obtained on the Faculty of Mining, Geology and Petroleum Engineering, Zagreb

Legend: >50 5 100%(d) - dominant >25 c 50%(p) - plentiful >10 1r 25%(c) -common > 5 5 10%(m) - minor > 1 5 5%(a) - accesory

l a lower part of the bauxite deposit, pale part l b lower part of the bauxite deposit, reddish-brown part l c lower part of the bauxite deposit, whiteveinlets 2 middle part of the bauxite deposit 5 St. AndraZ, Slovenija

Chemical analyses was performed on bauxite from the middle and the upper part of the deposit (Table 11.). The analyses show that the bauxite is clayey.

TAB.11. Chemical composition* of the Mt. Ravna gora bauxites (wt.%)

1. 2. 3. SiO, 20.82 14.63 '19.31 TiO, 0.80 0.23 0.34 a 0 3 45.95 49.72 51.08 Fe20, 17.56 18.39 9.89 MnO 0.03 0 Q M g o 0.20 0.88 1.81 CaO 1.12 2.74 2.35 Na,O 0.08 n.d. n.d. %o 0.03 n.d. n.d. p205 093 0 0 L.i. 13.15 12.49 14.29.

C 99.77 G.08 99.08

*Analysis performed by ~. '~uf iXik~i t rovi6 , Institute of Gsofogy, Zagreb

1,2 middle part of bauxite deposit 3 upper part of bauxite deposit On the basis of chemical analyses and XRD

analyses the approximate mineral composition of the bauxites has been determined (Table 111).

TAB. 111. Mineralogical mmpositioh of the Mt. Ravna gora bauxites

1. 2. 3. Kaolinite 44.7% 31.5% 41.5% Boehmite & diaspore 33.3% Hematite 17.6% Calcite 2.0% Anatase ~ 0.8% 0.2% 0.3%

C 98.7% 99.0% 96.8%

The contents of eight microelements were deter- mined in four bauxite samples from the lower, upper, and middle (two samples) parts of the bauxite deposit. In order to compare the content of microelements in the bauxite of Mt. Ravna gora with the Oligocene bauxites of Slovenia (St. Andraf and Aifernik) two samples from Slovenia were also analysed. The analyses were performed using ICP (Inductively Coupled Plasma; JY-SOP), and are shown in Table IV. For comparison the contents of microelements in bauxites from some bauxite horizons in Croatia, Herzegovina and Hungary are also shown.

TAB. N. Content of trace elements in bauxites (in ppm)*

Ca Ni Cr V Zn Cu Ga Mn

1 12 98 66 98 550 73 8 30 I1 26 96 110 74 289 28 1858 I11 21 108 212 109 340 59 223 IV 12 29 130 292 6 24 V 22 75 150 500 42 VI 27 265 1200 959 54 VII 31 441 780 566 210 38 91 VIII 35 427 l i n 1072 IX 47 528 738 519 418 47 89 X 17 188 254 694 60 90 XI 19 152 226 380 340 39 31

Rud.-geol.-naft. zb., Vol. 8, Zagreb, 1996.

Legend: I Bauxite of Mt. Ravna gora, Hrvatsko zago je , Croatia I1 St. AndraZ, Slovenia, Oligocene bauxites I11 iifernik, Slovenia, Oligocene bauxites IV Lika, Upper Triassic bauxites V Kordun, Upper Triassic bauxites VI Dalmatia-Imotski, Early Paleogene bauxites VII Siroki brijeg, Herzegovina, Early Paleogene bauxites VIII Dalmatia, Late Paleogene bauxites IX SobaE, Herzegovina, Late Paleogene bauxites X Dalmatia, Miocene bauxites XI Transdanubian Central Range - Eocene bauxites IV, V, VI, VIII, X after 3 i n k o v e c (1976) VII, IX after 3 i n k o v e c et al. (1989) XI after M a k s i m o v i 6 et. al. (1991). *Analytics for I, I1 and 111: S. Miko, Institute of geology, Zagreb

The bauxite has pelitic texture with slight transition to detrital one. In the cryptocrystalline groundmass, reworked small ooides of diaspore- boehmite and hematite composition, fragments of cryptocrystalline bauxite and grains of diaspore can rarely be found. In the sample of deferrificated bauxite a fragment of undeferriificated bauxite was observed, which is, according to its texture essentially different from the other investigated bauxites. This fragment represents a bauxite of oolitic texture with a dense package of ooids. The individual ooids which are found in bauxite matche those of the reworked fragment. In the lower part of the bauxite deposit which is deferrificated, seconday limonitization with crystallisation of goethite along fissures was obser- ved. From the fissures bauxite was limonitized frontally, and irregular "islands" of pale bauxite remained.

Discussion According to the present knowledge of the

geological setting, in the wider area of Mt. Ravna gora two major emersion phases occured: (1) between the Middle Liassic and the Tithonian, and (2) between the Turonian and the Middle Eocene. The Tithonian rocks transgresively overlie Liassic and, in major part, Triassic sediments. The Middle Eocene rocks are situated on Anisian limestones and dolomites. Due to the small extent of Eocene rocks (they were formed only in the lowest parts of the paleorelief) the Mesozoic sediments are mainly transgressively overlain by Oligo-miocene (Egerian) rocks.

The stratigraphic gap between the hangingwall and the footwall of the bauxites in Mt. Ravna gora is very large (Anisian to Middle Eocene). This raises the question of the age of bauxite formation. The bauxite is exclusively situated below Middle Eocene sediments, and is never found below Tithonian and Oligocene rocks. One may tentatively conclude that the bauxite was formed during the Early Eocene in the lower parts of the paleorelief.

The texture and structure of the bauxite clearly show that the Mt. Ravna gora deposit was formed by reworking of already formed bauxite. The bauxite was resedimented from primary deposit(s) (Plate 1, 1 and 2). In a new environment sedimentation partly reductive conditions prevailed. Due to this pheno- menon some parts of the deposit, especially the lower and the uppermost parts were affected by deferri- fication. It has to be stressed that the harder and coarser resedimented fragments of bauxite were not affected by deferrification. In the postgenetic phase, the deposit was exposed to pressure and numerous fissures were formed in bauxite. Slightly acid water of probably negative redox potential circulated along these fissures partly dissolving aluminium from bauxite. As a result of neutralisation of the solution in the vicinity of the carbonate footwall rock, diaspore was formed, filling fissures and voughs (Plate f , 5 and 6). Sparse diasporisation of boehmite and kaolinite probably also took place at that time. Organic matter, which might be the cause of the negative redox potential (which is prerequisite for the cristallyzation of diaspore), was not observed in the bauxite. This may indicate that the processes mentioned took place in greater depth, where ground water are oxygen deficient, but without increase of pressure and temperature. The appearance of secondary diaspore veinlets (as distinguished from the appearance of gibbsite veinlets which are often find in younger bauxites) is a very rare feature and consequently, very interesting. In the second phase, probably after certain new tectonic strains, formation of veinets filled by goethite and partial limoniti- tization of the deferrificated bauxite took place.

The question is raised about the age of the primary bauxite deposits which were destroyed and rese- dimented, supplying material for the formation of the Mt. Ravna gora bauxite deposit. These primary bauxite deposits might have been of (1) Eocene, or (2) Triassic age. More facts are in favour of their Triassic age: 1) the footwall of Mt. Ravna gora bauxite is exclu- sively composed by Triassic carbonate rocks; 2) younger bauxites are almost regularly of boehmitic (Early Paleogene bauxites), boehmitic-gibbsitic (Late Paleogene bauxites of the Outer Dinarides and Eoce- ne bauxites in Hungary) and gibbsitic compostion (Miocene bauxites) while the investigated bauxites are of diasporic-boehmitic composition, just as the Upper Triassic bauxites are; 3) deposits of the investigated bauxites are very rare. In the area of our research only one deposit was discovered. In the Republic of Slovenia only a few deposits have been found. It can tentatively be connected with the bauxite deposits of Triassic age which are often big, but rare. As an example, in the Lika region, in a large area, only six bauxite deposits of Triassic age were discovered;

&nkovec, B.& &nunib, A,: The bauxites of Mt. Ravna Gora

4) the texture of the preserved fragments of reworked bauxite closely resembles the texture of Triassic bauxites; 5) we consider the contents of microelements in the bauxites investigated to be the strongest evidence for their Triassic origin. It has already been established that the content of microelements is fairly uniform in each particular bauxite horizon. Significant differences were observed for separate elements in different bauxite horizons. This is particularly valid for cobalt, nickel, chromium, gallium and, to a certain degree, vanadium. Compared to the Eocene bauxites of the Outer Dinarides, the contents of certain microelements in the bauxites of Mt. Ravna gora are as follows: (1) the cobalt content is 2 to 3 times lower, (2) the nickel content is 3 to 5 times lower, (3) the chromium content is 10 to 18 times lower, (4) the vanadium content is 5 to 10 times lower, and (5) the gallium content is 12 times lower in the bauxite of Mt. Ravna gora. The contents of these microelements in the bauxite of Mt. Ravna gora are similar to their contents in Triassic bauxites. The concentration of the mentioned microelements in Miocene bauxites is also higher than in the investigated bauxite. Although the contents of these microelements in the Eocene bauxites in Hungary (Transdanubian Range) are lower than in the Paleogene bauxites of the Outer Dinarides, they are still higher compared to the investigated bauxite (Table IV). 6) the Triassic bauxites are geographically wide- spread. They extend from Slovenia, through Lika and Kordun (Slunj and VojniC) in Croatia and BihaC in western Bosnia and Herzegovina to Crna Gora and Albania. The distance between these bauxites and the bauxite of Mt. Ravna gora is approximately 100 km. The nearest Paleogene bauxites of the Outer Dinarides and the Eocene bauxites of Hungary are approximately 160 km away. However, it has to be stressed that these distances were considerably different in Eocene.

If we accept the idea that the Mt. Ravna gora bauxite deposit was formed from eroded and resedimented primary bauxites of Triassic age, we have to suppose an emersion phase in Triassic. This phase of subarea1 exposure probably existed between the Middle and Upper Triassic and was widespread in the area of the Outer and Inner Dinarides. Since the Triassic rocks in the northern part of Croatia are mainly composed by dolomites, it is hard to observe in them the interruption of sedimentation. There- fore, we may tentatively suppose that in the investigated area the subaerial exposure phase between the Middle and Upper Triassuc existed only in some places. However, due to the small number of Triassic bauxite deposits, this assumption is not easy to confirm. In the area of Mt. Ravna gora it is not possible to ascertain the existence of a subaerial

exposure phase in the Triassic because the Triassic rocks have been eroded, probably during the Late Cretaceous-Early Paleogene emersion phase.

The assumption that the primary bauxite might have been formed during an emersion phase between the Liassic and the Upper Tithonian is not very probable because there are no sediments of Jurassic and Cretaceous age in Mt. Ravna gora.

In the Republic of Slovenia, west of Celje, 70 km west of Mt. Ravna gora there are a few bauxite deposits, which were attributed to be of Oligocene age. They are underlain by Triassic limestones and dolomites and overlain by Middle Oligocene limestones which represent the oldest Ceinozoic sediments in the area, and can be found only as erosional remnants. The bauxites are of diasporic-boehmitic composition (B u s e r & L u - k a c s, 1970; B u s e r, 1973). According to their geological position and other characteristics they are very similar to the bauxites of Mt. Ravna gora. Their resemblance is also indicated by their contents of microelements (Table IV). On the basis of these facts we consider them to represent the same bauxite horizon.

The problem is how to call this bauxite horizon. It is generally accepted that the bauxites are dated the age of oldest sediments which overlain it. According to this, the bauxite of Mt. Ravna gora and the bauxites of the Celje area (if they represent the same horizon, which is highly probable) should be called Middle Eocene bauxites. However, the name of a bauxite horizon is also based on the recognised bauxitogenic processes which took place immediately before the transgression and the sedimentation of the cover. In the case of the bauxite of Mt. Ravna gora, the assumption was put forward that they may have been resedimented from bauxites of Triassic age. Nevertheless, we would like to stress that the facts which are in favour of this assumption do not exclude other ideas. So far, it is not possible to find out whether the bauxites of Mt. Ravna gora and the surroundings of Celje originate from resedimentation of Triassic bauxites only. It is quite possible that some other materials (terra rossa) which covered the ' Triassic carbonate rocks and were exposed to bauxitogenic processes in the Eocene also contributed as parent materials. We propose that, until new facts are discovered, the bauxite of Mt. Ravna gora should be considered as Middle Eocene bauxite. Received 1996.03.27. Accepted 1996.09.1 7.

REFERENCES A v a n i C , R., S i m u n i C , An., S i r n u n i t , Al. and S i k i ~ , ~ .

(1990): Klastiti egera ("Socka" slojevi) u Hwatskorn zagorju. (Egerian clastic rocks C'Socka beds") in Hwatsko zagoje). Zbornik radova XIII. kongresa geol., 202-219, Ohrid.

B u s e r, S. (1973): Oligocenski boksiti u kamniaih Alpah. (Bawit

Rud.-geol.-naft. zb., Vol. 8, Zagreb, 1996.

-Vorkomrnen des OligozAn in den siidlichen Steiner Alpen). 2. Jugosl. simp. istraf. ekspl. boksita, A-I, 9 st. Tuzla.

B u s e r, S. & L u k a c s, E. (1970): Bauxite in Slowenien. Anna- les Inrt. GeoL PubL Hungarici 1/: Lw F m . 3, 209-220, Budapest.

G o r j a n o v i C - K r a m b e r g e r, D. (1904): Geologijska prije- gledna karta Kraljevine Hrvatske i Slavonije. TumaE geologijske karte Rogatec-Kozje. (Geologische Ubersichsts Karte des Gnigreiches KroatienSlavonien, Erlautemngen zur geologischen Karte von Rohitisch-Drachenburg). Zone 21, Col. XIII. Naklada Kralj. zemalj. vlade, Odjela za unutarnje poslove, 1-24, Zagreb.

K u i E e r, D. (1967): Zagorski tercijar, Geologjq 10,s-58, Ljub- ljana.

MaksimoviC,Z,Mindszenty,kandPantoGy.(1991): Contribution to the geochemistry of Hungarian karst bauxites and the allochtonylautochtony problem. Acta GeoL Hung., V: 34 No 4,317-334, Budapest.

S i k i C, L. (1985): The Rupelian in the area of Ravna gora (north- western Croatia) based on foraminiferous fauna. CeoL vjmW38,73-80, Zagreb.

S i k i C , ~ . , S imun iC , An., P ik i j a ,M. and S i m u n i 6 , ~ l . (1976): Gornji eocen na podmEju Ravne gore (sjevero- zapadna Hrvatska). (The Upper Eocene on Mt. Ravna Gora, NW Croatia). GeoL v j w 29, 191-197, Zagreb.

S i k i ~ , ~ . , S i m u n i C , ~ n . a n d S i k i C , ~ ( 1 9 8 5 ) : 0 l i ~ o c e n u s j e - verozapadnoj Hrvatskoj. (The Oligocene in northwestern Croatia). Ge?lokiglamik 28 (11), 119-129, Sarajevo.

3 i m u n i 6, Al. & S i m u n i C, An. (1980): Petrografski sastav i ge- neza trijaskih naslaga Iva&ice, Kalnika i Ravne gore. (Petrographic composition and genesis of Triassic deposits of

IvanSBca, Kalnik and the Ravna gora Mountain). GeoL vim&, 32,243-253, Zagreb.

S i m u n i C, An. (1992): GeoloSki odnosi srediinjeg dijela Hwat- skog Zago rja. (Geological relationships of the middle part of Hwatsko Zagoqe Area). Disertacija, 1-189, SveuZilSte u Zagrebu.

S i m u n i C, An. & H e C i m o v i 6, I. (1979): Tektonski odnosi sje- verozapadne Hrvatske (Ivanfica, Kalnik Ravna gora). (Tectonic relationships of northwestern Croatia - Ivanifica, Kalnik and Ravna gora Mountain). Zbornik rad. 4 god. man. skupa, Sekcije za primj. geol., geof., Znan. savjeta za naftu. JAZU, 187-198, Zagreb.

Simuni~,~n.,SimuniC,AI.andMilanoviC,M.(1979): Geoldka grada Ivanitice i Ravne gore. (Geology of Mt. IvadEica and Mt. Ravna Gora (northwestern Croatia). GeoL vjcm& 31,157-174, Zagreb.

3 i m u n i 4 An. & P a m i 6, J. (1993): Geology and petrology of Egerian-Eggenburgian andesites from the easternmost parts of the Periadriatic zone in Hrvatsko Zagorje (Croatia), Acta GeoL Hung., 3613,301-316, Budapest.

3 i n k o v e c, B. (1970): Geology of the Triassic bauxites of Lika, Yugoslavia. Acta GeoL V71 (Prir. h. 39), 5-70, Zagreb.

S i n k o v e c, B. (1976): SadrZaj nekih mikroelemenata u boksitima SR Hrvatske. (Contents of some microelements in bauxites of Croatia). TV Jug. simp. istraZ eksp. boksita, 6 st. Hercegnovi.

S i n k o v e c , ~ . , SakaE ,K , Pa l i nkaS ,L . and Miko,S. (1989): Geology of bauxite deposits in the Litica (Hene- govina) and Imotski (Dalmatia) regions, Yugoslavia. Travaux ICSOBA, 19, NO 22,459-477, Zagreb.