GEOLOGIJA 49/1, 69–84, Ljubljana 2006
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije Manganese nodules in Jurassic limestone of the Southern Alps in Slovenia
Bojan OGORELEC1, Stanko BUSER2 & Miha MI[I^1
1Geolo{ki zavod Slovenije, Dimi~eva 14, SI-1000 Ljubljana, bojan.ogorelec@geo-zs.si,
miha.misic@geo-zs.si 2Univerza v Ljubljani, NTF, Oddelek za Geologijo, A{ker~eva 12, 1000 Ljubljana
Klju~ne besede: Mn-gomolji, jura, mikrofacies, Ju‘ne Alpe, Slovenija Key words: Mn-nodules, Jurassic, microfacies, Southern Alps, Slovenia
Kratka vsebina
Posebnost jurskih plasti v Julijskih Alpah so rde~i pelagi~ni apnenci, ki vsebujejo manganove gomolje. Zasledimo jih na ve~ lokacijah okrog Bovca in Krna ter na ve~ krajih v Trenti, koncentrirani pa so na nekajmetrski horizont zgornjeliasne starosti. Gomolji so diskaste oblike in merijo do 12 cm v premeru. V njihovi mineralni sestavi nastopajo piroluzit, psilomelan, manganit, todorokit in hausmannit. Dele‘ Mn v gomoljih je nizek, med 0,8 in 3,5 %, dele‘ Fe pa do 1,7 %. V nekaterih gomoljih Fe nad Mn prevladuje.
Na Mangartu in na Begunj{~ici nastopa manganovo orudenje v tanj{i plasti znotraj paketa skrilavega glinovca, radiolarita in krinoidnega apnenca, dele‘ MnO v njej zna{a do 60 %.
Abstract
One of geological peculiarities in the Julian Alps is red pelagic limestone containing manganese nodules. They are encountered at several localities in the Bovec, Trenta and Krn area, occurring in some metres thick horizon of the Upper Liassic age. The nodule forms are disc-like attaining up to 12 cm in diameter. Mineral composition is characterized by pyrolusite, psilomelane, manganite and hausmannite. The shear of Mn in the nodules is low ranging from 0,8 % to 3,5 %. The Fe amount attains up to 1,7 %, in some samples, however, Fe exceeds Mn.
At Mangart and at Begunj{~ica, Mn ore beds occur in a thinner horizon in a sequence of slate, radiolarite and crinoidal limestone. Herein, the amount of MnO reaches up to 60 %.
Uvod
Ena od stratigrafskih in sedimentolo{kih posebnosti v slovenskem delu Ju‘nih Alp so rde~kasti apnenci jurske starosti, v katerih se javljajo manganovi gomolji. Te plasti dobimo na ve~ lokalnostih v Julijskih Alpah, v Dolini triglavskih jezer (Salopek, 1933; Grim{i~ar, 1962; Buser, 1986; [muc , 2005), na ju‘nem pobo~ju Bav{kega Gam-sovca (Ramov{, 1985), na Krnu (Babi}, 1980/81), v grapi Slatenik pri ^ezso~i in na
ju‘nem pobo~ju Rombona pri Bovcu (Winkler, 1920; Buser, 1986), v dolini Koritnice (Ku{~er et al., 1974), na Vasi na skali (J u r -kov{ek et al., 1990), na Mangartu (Selli, 1963; Jurkov{ek, 1987; [muc , 2005) in na Begunj{~ici v Karavankah (Teller, 1899; Buser, 1980) pa so razvite tudi kot plastovito in ‘ilno manganovo orudenje (sl. 1).
V {ir{em  alpskem  prostoru  so plasti z manganovimi gomolji poznane kot »ammo-
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Bojan Ogorelec, Stanko Buser & Miha Mi{i~
Sl. 1. Lokacije jurskih plasti z manganovimi gomolji ali manganovim orudenjem v slovenskem delu
Julijskih Alp in Karavank.
Fig. 1. Position of Jurassic beds with manganese nodules or manganese mineralization in Julian Alps
and Karavanke Mts. in Slovenia.
nitico rosso« ali kot »Knollen-Kalke« (Jenkyns, 1974; Bernoulli & Jenkyns, 1974). V jurskem obdobju so te plasti dokaj raz{irjene v mediteranskem prostoru Tetide (sl. 2) in jih sledimo od Sicilije do Severnih Apneni{kih Alp, Dolomitov in Mad‘arske (Jenkyns, 1970; 1971; 1978; W endt, 1970; Jürgen, 1969; Germann, 1971; Mind-szenty et al., 1986, Cronan et al., 1991, Vörös , 1991; K r a i n e r et al., 1994; Haas, 2001). Posebno znani in dobro raziskani so spodnjejurski rde~i hemipelagi~ni apnenci z manganovimi gomolji iz Adneta pri Salz-burgu, ki jih {e danes masovno uporabljajo kot cenjen arhitektonski kamen (Jenkyns, 1974; Böhm, 1992; Böhm et al., 1999). Prvi, ki je primerjal liasne manganove gomolje iz Gosaua v Severnih Alpah z recentnimi iz globokih morij, je bil Heim (1924).
Pri na{ih raziskavah smo pozornost posvetili predvsem plastem z manganovimi gomolji iz Doline triglavskih jezer, Lu‘nice pri Krnu, grape Slatenik ter iz okolice Bovca (sl. 1). Omenjene plasti smo raziskovali v okviru izdelave Osnovne geolo{ke karte v merilu 1:100.000, listov Tolmin-Udine (B u -s e r , 1986) in Beljak (Jurkov{ek, 1987) v letih 1979-1982.
Namen pri~ujo~e raziskave je bil, da ugotovimo in podamo stratigrafski polo‘aj, osnovne petrografske in mineralo{ke zna-~ilnosti Mn-gomoljev ter facies kamnin, v katerih se ti pojavljajo, delno pa tudi njihovo kemi~no sestavo. Za primerjavo navajamo v kratkem tudi nekaj podatkov o sestavi in okolju nastanka recentnih manganovih gomoljev.
Nahajali{~a plasti z manganovimi gomolji
in manganovim orudenjem ter njihova
stratigrafska lega
Koncem srednjega liasa je do takrat stabilna in enotna Julijska karbonatna platforma razpadla (Buser, 1987). Na tej se je v noriju in retiju odlagal dachsteinski apnenec, v spodnjem liasu pa plitvovodni biomikritni, biosparitni in oosparitni apnenec. Na dvignjenih in okopnelih karbonatnih blokih je pri~elo intenzivno zakrasevanje. Ob tem je bil na ve~jem delu spodnjeliasni apnenec izlu‘en oziroma korodiran. Ohranjen je le {e v okolici Bovca, Krna, na Mangartu ter na Bav{kem Gamsovcu, drugod pa zgornjeliasni apnenec z manganovimi gomolji nalega neposredno na dachsteinski apnenec.
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
71
	0/		(a l	^		
	*2i		J\  F		•   Localities of Jtirassis Mn-nodules	
i	?J		fe-		^ Julian Alps and Karavanke Mts. (present paper)	
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Sl. 2. Polo‘aj jurskih apnencev z manganovimi gomolji v mediteranskem delu Tetide (povzeto po J e n k y n s u 1970, dopolnjeno z lokacijo Julijskih Alp).
Fig. 2. Jurassic beds with manganese nodules in the Mediterranean part of Tethys (after J e n k y n s 1970, studied area added).
Na pogreznjenih karbonatnih blokih je pri{lo do kondenzirane sedimentacije rde~-kastih biomikritnih apnencev. Poglobitev karbonatne platforme je bil globalni dogodek, ki je verjetno nastopil povsod isto~asno. Na Begunj{~ici najdemo amonite, ki ta dogodek uvr{~ajo v zgornji lias oziroma v toarcij.
V nadaljevanju na kratko opisujemo posamezna nahajali{~a manganovih gomoljev:
Dolina triglavskih jezer. Tod le‘ijo zgor-njeliasni toarcijski rde~i apnenci na dach-steinskem apnencu v loferskem razvoju. Sti~na ploskev je le rahlo valovita, vendar je med obema apnencema konkordanca. Jurske plasti za~enjajo z rde~im, ve~inoma krino-idnim apnencem, vmes pa so mikritni apnenci s kamnotvornimi {koljkami oziroma filamenti. Okoli 40 cm nad stikom so v krinoidnem apnencu manganovi gomolji s koncentri~no teksturo. Lepo vidni so na planinski poti severno od ko~e pri Triglavskih
jezerih. Debelina horizonta z Mn-gomolji je do 3 metre. Navzgor sledi gomoljast apnenec tipa »ammonitico rosso« s {tevilnimi {e ne-dolo~enimi amoniti. Nad tem apnencem le‘i doggersko-spodnjemalmski radiolarijski horizont, nad tem pa paket tithonijsko-ber-riasijskega apnenca s kalpionelami (plasti »biancone«).
Jezero v Lu‘nici. Pri jezeru v Lu‘nici, jugovzhodno od Krna, so v spodnjeliasnem oolitnem apnencu do 10 metrov globoke in do 30 cm {iroke razpoke, ki jih zapolnjujejo v obliki sedimentnih dajkov krinoidni apnenci in pisane apnen~eve bre~e. Med njimi je tanka plast mikritnega apnenca z do 10 cm velikimi okroglimi manganovimi gomolji in nepravilnimi manganovimi skorjami. Navzgor sledi 4 m ro‘natega apnenca s {tevil-nimi filamenti, ki vsebuje poredke manganove gomolje. Vi{je je {e 2 m ro‘natega gomoljastega apnenca tipa »ammonitico rosso« z amoniti.
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Bojan Ogorelec, Stanko Buser & Miha Mi{i~
Sl. 3. Presek Fe-Mn gomolja s conarno rastjo.
Ozna~eni del gomolja je pove~an na tabli 2, sl.
3. Koritnica pri Bovcu.
Fig. 3. Ferromanganese nodule in cross section.
Marked part of the nodule is enlarged on tab.
2, fig 3. Koritnica at Bovec.
Ravni Laz. Na Ravnem Lazu, severovzhodno od Bovca, je nad spodnjeliasnim belim biosparitnim apnencem okoli 2 m rde-~ega krinoidno-mikritnega apnenca s {tevil-nimi filamenti ter z manganovo mineralizacijo. Vi{je sledi mikritno-krinoidni apnenec z do 5 cm velikimi manganovimi gomolji.
Koritnica. V dolini Koritnice pod Klu-‘ami, vzhodno od Bovca, opazujemo v skalah
ob vodi nad spodnjeliasnimi oolitnimi apnenci toarcijski mikritni apnenec s filamenti. V njem so redki, okrog 5 cm veliki gomolji sivkasto rjave barve (sl. 3). Edini kemi~no raziskani gomolj vsebuje 2,1 % Fe in le 0,3 % Mn, zato lahko v tem specifi~nem primeru govorimo o feromanganovih gomoljih. Zaradi sivkaste barve so bili ti gomolji opisani kot limonitni (Ogorelec, 1970; Ku{~er et al., 1974). V tem apnencu je bila dolo~ena pelagi~na foraminifera Globigerina helveto-jurassica Haeusler.
Slatenik. V grapi Slatenik pri ^ezso~i je na severnem pobo~ju Polovnika razvit le nekaj decimetrovdo 2 m debel horizont ro‘na-tega apnenca z izredno lepimi manganovimi gomolji, velikimi do 12 cm, ki se mestoma celo dotikajo drug drugega (sl. 4). Ta apnenec le‘i s stratigrafskim hiatusom konkordantno na svetlem spodnjeliasnem oolitnem apnencu. Lateralno ga pokrivajo rde~i zgornje-kredni lapornati apnenci z globotrunkanami (»scaglia«) ali fli{ne plasti.
Bav{ki Gamsovec. Na ju‘nem pobo~ju Bav-{kega Gamsovca, severno od prelaza Luknja, dobimo ob planinski poti na nekaj m2 veliki skali rde~kastega toarcijskega apnenca do 5 cm velike ~rnorjave manganove gomolje. Apnenec z gomolji le‘i na spodnjeliasnem gastropodnem svetlo sivem apnencu, ki ga sledimo navzgor do vrha Bav{kega Gamsovca.
Mangart. Feromanganovi gomolji na Mangartu so bili raziskani v 30 cm debeli plasti manganove rude, ki se javlja znotraj 15 metrov debelega karbonatno-klasti~nega zaporedja na Mangartskem sedlu (sl. 5), (J u r k o v { e k et al., 1990; [ m u c , 2005). Ta
Sl. 4. Manganovi gomolji v
rde~em biomikritnem apnencu.
Grapa Slatenik pri ^ezso~i.
Fig. 4. Manganese nodules in red
biomicritic limestone. Slatenik
gorge at ^ezso~a.
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
73
Sl. 5. Drobni silicizirani manganovi gomolji v orudeni plasti na Mangartskem sedlu.
Fig. 5. Small manganese nodules, influenced by silicization in the Mn-ore bed at Mangartsko sedlo.
paket se javlja neposredno nad plitvovodnim biomikritnim in oolitnim liasnim apnencem. Med krinoidnim apnencem in lapornatim apnencem z radiolariji se javljajo manganski skrilavec, radiolarit in glinovec. Te plasti so zaradi manganovih oksidov obarvane ~rno. Okside sestavljajo piroluzit, kriptomelan in todorokit. Dele‘ MnO v raziskanem vzorcu je 45 %, dele‘ FeO pa 8 %, ostalo pripada kremenu (ca 15 %) in mineralom glin. Gomolji na Mangartu merijo v premeru 2-3 cm in so mo~no silicizirani.
Begunj{~ica. Toarcijske jurske plasti na ju‘nem pobo~ju Begunj{~ice v globjemor-skem razvoju le‘ijo v stratigrafskem hiatusu na zgornjetriasnem koralno-stromatoporid-nem apnencu. Hiatus nakazujejo neptunski dajki, ki jih dobimo v zgornjetriasnem apnencu. Zapolnjujejo jih pisana jurska bre~a ali krinoidni apnenci. Jurske plasti sestavljajo rde~i do sivkasti gomoljasti apnenci tipa »ammonitico rosso«. Nad njimi sledijo sivi plo{~asti apnenci z ro‘enci. V spodnjem delu rde~ih apnencev je plast skrilavega gli-novca orodena z manganovo rudo, ki so jo neko~ kopali in iz nje na Jesenicah izdelovali manganovo jeklo. Ruda je vsebovala 30 do 36 % in celo do 60 % mangana (Teller, 1899). Na poredkih mestih najdemo v apnencu tudi okrogle manganove gomolje. V apnencu pod in nad orudenitvijo so najdeni zna~ilni toarcijski amoniti. Med pomembnimi so Hildoceras heterophyllum, H. bifrons, H. commense, H. cf. opalinoide in Phylloce-ras nilssoni (T e l l e r , 1899; M i h a j l o v i } & Ramov{, 1965). Amoniti ka‘ejo, da je emerzija in nastopanje gomoljastih apnencev
z manganovimi gomolji za~ela v zgornjem liasu oziroma toarciju. Omenjenemu apnencu navzgor sledi doggersko-spodnjemalmski radiolarit in apnenec z gomolji ro‘enca.
Litologija in mikrofacies
Talnino zgornjeliasnega rde~kastega apnenca z manganovimi gomolji v Julijskih Alpah in Ju‘nih Karavankah sestavlja ve~ sto metrov debela skladovnica norijsko-retijskega apnenca v dachsteinskem razvoju ali spodnjeliasni apnenec. Dachsteinski apnenec se je odlagal na plitvem odprtem {elfu z ob~asnimi litoralnimi pogoji in ka‘e vse znake loferskega razvoja (Buser, 1986, 1987; Ogorelec & Buser, 1996). Apnenec v neposredni talnini rde~kastih plasti z gomolji je po strukturi intrapelmikrit s {tevilnimi drobnimi izsu{itvenimi porami in kopu~ami neskeletnih alg ter oosparit.
Plasti z manganovimi gomolji imajo zna~ilno rde~kasto barvo, ki se menja od svetle oran‘no do temno rjavorde~e. Po strukturi uvr{~amo ta apnenec v biomikrit tipa wackestone-packstone. Osnovo kamnine sestavlja gost mikrit, od alokemov so prisotni le fosili, izjemoma tudi drobni intra-klasti. Najbolj pogostne so plo{~ice ehi-nodermov, manj je tankih {kolj~nih lupin oziroma filamentov, mikrogastropodov in foraminifer, mestoma pa so {e juvenilni amoniti in kalcitizirani radiolariji (tab. 1, sl. 1, 4-6, tab. 2, sl. 1,2). Krinoidne plo{~ice so ponekod tako pogostne, da apnenec lahko poimenujemo kar »krinoidni« (tab. 1, sl. 4,5).
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Bojan Ogorelec, Stanko Buser & Miha Mi{i~
Energijski indeks apnenca je nizek do zelo nizek in po njem lahko sklepamo na se-dimentacijo v relativno mirnem okolju, kjer je pri{lo do spiranja karbonatnega blata. Stopnja sedimentacije je bila zelo nizka, kar je omogo~ilo kopi~enje organskih skeletov. Rde~kasta barva apnenca je pogojena s primesjo ‘elezovih in manganovih oksidov, predvsem goethita. Ti oksidi se javljajo skupaj z minerali glin kot pigment v osnovi, bolj koncentrirani pa so v {tevilnih stilolitnih {ivih. Dele‘ karbonata v raziskanih vzorcih apnenca se giblje med 86 in 97 %. V nekaterih vzorcih opa‘amo nadome{~anje kalcita z avtigenim kremenom, predvsem v plo{~icah ehinodermov in {kolj~nih lupinicah.
Raziskani manganovi gomolji
Manganovi gomolji v raziskanih plasteh so koncentri~ne diskaste oblike ter merijo v premeru od nekaj mm do najve~ 12 cm, ve~ji del pa med 2 in 5 cm. Makroskopsko se od prikamnine odli~no lo~ijo po temnej{i rjavkasti barvi in po tem, da reliefno izstopajo na izlu‘enih povr{inah (sl. 4 in naslovnica revije).
Pod mikroskopom opazujemo, da imajo gomolji {tevilne, 30 do 100 µm debele kon-centri~ne ovoje z nagubano, stromatolitom podobno teksturo (tab. 2, sl. 3-5), kakr{na je zna~ilna tudi za recentne manganove gomolje (»cauliflower structure«, T h i e l & S c h n e i -d e r 1988) (tab. 2, sl. 6). Rentgenske analize ka‘ejo, da gomolje sestavljajo nizko magne-
zijev kalcit in goethit, manganovi minerali pa so psilomelan - Mn5O10(Ba, H2O)2, manganit -MnO(OH) in hausmannit - Mn2+Mn3+2O4. V sledovih so prisotni {e minerali glin in kremen.
Vsebnost mangana v 12 preiskanih gomoljih je relativno nizka in niha med 0,8 in 3,5 % Mn ter 0,03 do 1,7 % Fe. Apnenec iz prikamnine vsebuje 0,04 do 0,07 % Mn, 0,01 do 0,08 % Fe in okrog 380 ppm Sr (analize B. Ogorelec na In{titutu za sedimentologijo Univerze v Heidelbergu , metoda atomske absorpcije). Zaradi dokaj{nega dele‘a ‘eleza v nekaterih gomoljih glede na mangan, je zato pravilno, da govorimo v teh primerih o feromanganovih in ne samo o manganovih gomoljih. Izraz »feromanganovi« se uporablja tudi za recentne Fe-Mn gomolje (Halbach et al., 1988).
V primerjavo navajamo, da vsebujejo gomolji iz jurskega apnenca v Severnih Alpah med 0,05 in 24 % Mn ter 0,1 do 17 % Fe (Wendt , 1970; Böhm, 1992), gomolji iz enakih plasti na Siciliji pa do 40 % Mn in do 50 % Fe (Jenkyns, 1978).
Nekateri gomolji in mikrogomolji iz raziskanih plasti so brez opaznega jedra. Pri teh opazujemo koncentri~no rast skorij od same sredine gomolja navzven. Pri drugih pa manganove skorje obra{~ajo drobce mi-kritnega in biomikritnega apnenca ali skelete fosilov (tab. 1, sl. 2, 3). Zgrban~eno teksturo skorij so avtorji prvotno tolma~ili z dehidratacijo goethita in njegovim prehodom v hematit v ~asu diagenetskih procesov, kar povzro~a redukcijo volumna gomoljev (Jürgen , 1969; Mangini, 1988). Novej{e
Tabla 1 - Table 1
1         Manj{i manganov gomolj v biomikritnem apnencu. Smaller manganese nodule in biomicritic limestone {t./Coll. No.: GeoZS 63432.
2         Manganove skorje obra{~ajo plo{~ico ehinoderma. Manganese encrustation of echinoid plate. Ravni Laz pri Bovcu. Merilo/scale bar 1 mm, zbrusek {t./Coll. No.: GeoZS 57782.
3         Manganovi mikrogomolji v biomikritnem apnencu. Mn-micronodules in biomicritic packstone. Ravni Laz pri Bovcu. Merilo/scale bar 1 mm, zbrusek {t./Coll. No.: GeoZS 57782.
4         Krinoidni apnenec. Plo{~ice ehinodermov so obdane z manganovimi ovoji. Crinoidal packstone. Echinoid plates with Mn incrustations. Dolina triglavskih jezer. Merilo/scale bar 2 mm, zbrusek {t./Coll. No.: GeoZS 42537.
5         Biomikritni apnenec s {tevilnimi plo{~icami ehinodermov, drobci {kolj~nih lupin, foraminifer in juvenilnih amonitov. Biomicritic packstone with echinoid plates, shell debris, foraminifers and juvenile ammonites. Dolina triglavskih jezer. Merilo/scale bar 2 mm, zbrusek {t./Coll. No.: GeoZS 42540.
6         Manganovi mikrogomolji v biomikritnem apnencu. Mn-micronodules in biomicritic wackestone. Jezero v Lu‘nici. Merilo/scale bar 1 mm, zbrusek {t./Coll. No.: GeoZS 42543.
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
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Bojan Ogorelec, Stanko Buser & Miha Mi{i~
Sl. 6. Paleogeografske razmere v zahodni Sloveniji v ~asu zgornjega triasa in spodnje jure z nakazanim polo‘ajem nastanka manganovih gomoljev (a) in manganove rude (b).
Fig. 6. Paleogeographic situation in the western part of Slovenia in the time-span of Upper Triassic to Lower Jurassic. Superimposed origin of Mn-nodules (a) and Mn ore-mineralisation (b) is
indicated.
raziskave pa govore, da je zgubana struktura ‘e primarna. To pripisujejo raziskovalci aktivnosti bakterij in drugih kemolitotrofnih organizmov v procesu nastajanja gomoljev (Monty, 1973; W endt, 1974; Thiel & Schneider, 1988). Monty imenuje zato te gomolje kar »oceanski manganovi stroma-toliti«.
Recentni manganovi gomolji
Pojav manganovih oziroma feromanganovih gomoljev so prvi~ zasledili ob priliki raziskovalnega kri‘arjenja z ladjo H.M.S. Challenger v letih 1872-76, ki je obplula vse oceane razen Arktike, in sicer v Tihem oceanu. Kasneje, ob raziskavah morskega
Tabla 2 – Table 2
1         Biomikritni apnenec z juvenilnimi amoniti in ehinodermi. Biomicritic packstone with juvenile ammonites and echinoids. Dolina triglavskih jezer. Merilo/scale bar 2 mm, zbrusek {t./Coll. No.: GeoZS 42542.
2         Biomikritni packstone z manganovimi mikrogomolji. Biomicritic packstone with Mn-micronodules. Dolina triglavskih jezer. Merilo/scale bar 1 mm, zbrusek {t./Coll. No.: GeoZS 42536.
3         Del Fe-Mn gomolja z drobno »stromatolitno« laminacijo. ^rno so polja limonitiziranega pirita. Detajl s slike 3. Part of ferromanganese nodule with »stromatolithic« structure. Black spots belong to liminitised pyrite. Detail from fig. 3. Koritnica pri Bovcu. Merilo/scale bar 2 mm, zbrusek {t./ Coll. No.: GeoZS 51332.
4-5 Detajl Fe-Mn gomolja s conarno »stromatolitno« teksturo. Jedro gomolja je mikritni/belongs to micritic limestone. Jezero v Lu‘nici. Merilo/scale bar 1 mm, zbrusek {t./Coll. No.: GeoZS 42531.
6        Tangencialni presek gomolja s slik 4 in 5 z »ohrovtovo« teksturo. Tangential cut of the nodule from
figs. 4 and 5, »cauliflower structure«. Merilo/scale bar 2 mm, zbrusek {t./Coll. No.: GeoZS 42533.
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
77
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dna z ladjo Valdivia v letih 1898-99, so jih na{li tudi v Atlantskem oceanu (Halbach et al., 1988).
Dolga desetletja so ti gomolji veljali bolj kot mineralo{ka posebnost, dokler ni leta 1965 Mero zbral {tevilne, do tedaj znane podatke o regionalni razprostranjenosti recent-nih ‘elezovomanganovih gomoljev v oceanih. Zaradi vsebnosti nekaterih kovin, kot Cu, Ni in Co so ti gomolji v nekaterih predelih oceanov postali zanimivi kot potencialni vir na{tetih metalov v bodo~nosti. Zaradi mangana samega gomolji seveda niso zanimivi, saj so svetovne zaloge tega na kopnem tako velike (Nova Kaledonija, Kuba, Kanada, Rusija), da morsko »rudarjenje« s stali{~a mangana sploh ni perspektivno.
V zadnjih desetletjih so bila izvedena {te-vilna sistemati~na raziskovalna kri‘arjenja ter {tudije nastanka ‘elezovomanganovih gomoljev, ki naj bi posredovala pomembne podatke, predvsem o koli~ini gomoljev na oceanskem dnu ter o njihovi kemi~ni in mi-neralo{ki sestavi (Bonatti & Nayudu, 1965; A h r e n s et al., 1967; C r o n a n & T o -o m s , 1969; Bonatti et al., 1971; Sorem & Fewkes , 1979; Bender, 1972; Stoffers et al., 1984; I n g r i , 1985; M ü l l e r et al., 1988; H a l b a c h et al., 1988; B o l t o n et al., 1988). Dodelane so bile tudi raznovrstne tehnologije, kako ~imbolj enostavno in najceneje spraviti manganove gomolje z morskega dna na ladje (»deep ocean mining«), od sistemov »grabljenja« do »podvodnega sesanja«, vendar pa so vsi ti projekti trenutno zamrli in zaradi visoke cene zbiranja gomoljev ~akajo na ugodnej{e ekonomske razmere v svetu.
Polimetalni ‘elezovomanganovi gomolji se na dnu oceanov pojavljajo ve~ji del v globinah med 4 in 6 km, pod tako imenovano CCD mejo (carbonate compensation depth), predvsem v ekvatorialnem pasu in v severnem delu Tihega oceana, ju‘no od Havajev, v manj{em obsegu pa tudi v Atlantskem in Indijskem oceanu. Ve~ji del se gomolji javljajo kot samostojne, nekaj do 15 cm velike tvorbe, ve~krat pa tudi kot mikro-gomolji mm dimenzij (Halbach & Putea-n u s , 1988). [tevilna obmo~ja v oceanih vsebujejo tudi do 10 kg gomoljev na m2 oziroma 103ton/km2 (Bernhard & Blissenbach, 1988). Redkeje se mangan pojavlja tudi v tankih skorjah na morskem dnu, to je predvsem tam kjer je sedimentacija odsotna (podvodni platoji).
Bojan Ogorelec, Stanko Buser & Miha Mi{i~
Kemi~no razmerje ekonomsko zanimivih prvin v popre~nih gomoljih je slede~e -Mn:Ni:Cu:Co je 125:7:6:1, pri ~emer niha dele‘ Mn med 25 in 45 %, Fe med 5-8 %, Ni+Cu 1-2 % ter Co do 0,3 %. Razmerje Mn/ Fe je v povpre~ju 3-5 (Halbach & Puteanus, 1988; Friedrich et al., 1988).
Recentni ‘elezovomanganovi gomolji nastajajo na morskem dnu kot produkt diage-netskih in hidrogenetskih procesov. Zaradi menjavanja kristalnega in amorfnega materiala imajo gomolji zapleteno sestavo in ka‘ejo skorjasto kolumnarno, stromatolitom podobno teksturo (»struktura ohrovta«, Thiel & Schneider, 1988). Gomolje sestavljajo avtigen, terigen, vulkanski in biogen material. Pri samem nastajanju oziroma rasti gomoljev igrajo pomembno vlogo Mn/ Fe razmerje v pornih raztopinah (morju) ter dele‘ organske snovi. Slednja vpliva na dele‘ bakterij v sedimentu, ki sodelujejo pri iz-lo~anju manganovih mineralov, 4 do10 Å manganatov in todorokita (Buser & Grüt-te r , 1957; Giovanoli & Arrhenius, 1988). Ve~ji del imajo manganovi gomolji diskasto ali »krompirjasto obliko« in so zelo porozni. Poroznost lahko dose‘e tudi do 80 % gomolja.
Sama rast gomoljev ponavadi pri~enja z ob-ra{~anjem nekega nukleusa. To je lahko mikrozrno kremena, vulkansko steklo ali de-triti~no zrno. Ve~krat manganove skorje ob-ra{~ajo tudi drobce ali cele fosile, predvsem plo{~ice ehinodermov, {kolj~ne lupine, amonite in foraminifere ali pa tudi ve~je kose ‘e li-tificiranega apnenca iz okolja, kjer prihaja do manganove mineralizacije (obmo~ja »hard-grounda«).
Sediment, na povr{ini katerega »rastejo« gomolji, je radiolarijska glina ali karbonatno blato. Rast gomoljev je zelo po~asna. Raziskovalci jo ocenjujejo na 1-4 mm/milijon let v glinasti osnovi in na 3.6 mm/106y v karbonatnem blatu (Müller & Mangini, 1980; Mangini, 1988), izjemoma v nekaterih ekvatorialnih predelih pa tudi do 25 mm/milijon let. Ko mati~ni sediment izgubi polteko~o konsistenco, tako da je pretok por-nih raztopin mo~no zmanj{an in upo~asnjen, gomolji prenehajo rasti. Ko so prekriti z nekaj do 20 cm novega sedimenta, oziroma ko preide gomolj iz oksidacijskega v redukcijsko okolje, se pri~no raztapljati in Mn se seli nazaj proti povr{ini sedimenta, medtem, ko ‘elezo in ostali metali ostanejo v sedi-
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
79
mentu. Tako pride do cikli~nega kro‘enja Mn- ionov, izlo~anja manganovih mineralov na povr{ini sedimenta in do njihovega raztapljanja pod povr{jem (Lynn & Bonatti, 1965; Mangini, 1988).
Diskusija in zaklju~ki
^e strnemo ugotovitve terenskih in laboratorijskih raziskav ‘elezovomanganovih gomoljev in manganovih plasti iz slovenskega dela Julijskih Alp in Karavank, ugotavljamo, da so zgornjeliasni apnenci pele-gi~nega razvoja, v katerih se ti pojavljajo, na ve~ lokalnostih.
Gomolji se javljajo v razli~nih oblikah, najve~krat kot veliki, nekaj do 12 cm veliki primerki diskaste oblike, mestoma pa tudi kot manj kot 2 mm veliki mikrogomolji ter in-krustacije fosilnih drobcev, predvsem plo{~ic ehinodermov. V njihovi mineralni sestavi nastopajo piroluzit, psilomelan, manganit, to-dorokit in hausmannit. Dele‘ Mn v raziskanih vzorcih niha med 0,8 in 3,5 %, dele‘ Fe pa dose‘e do 1,7 % Fe. V nekaterih gomoljih ‘elezo nad manganom tudi prevladuje.
Spodnjejurski apnenci, v katerih se javljajo ‘elezovomanganovi gomolji, ka‘ejo sli~en mikrofacies kot enaki apnenci z Mn-mineralizacijo v Severnih Alpah.
Na Mangartu in na Begunj{~ici nastopa manganovo orudenje v tanj{i plasti znotraj paketa skrilavih glinovcev in lapornatih apnencev. Dele‘ MnO v rudi zna{a 45 %.
Zanimivo vpra{anje, ki si ga danes zastavljajo raziskovalci, je okolje nastanka ‘elezovomanganovih gomoljev in zakaj se ti pojavljajo na prostoru Tetide le v juri? Sicer so podobni nodularni rde~i pelagi~ni apnenci znani iz kambrijskega obdobja v [paniji, devonski nodularni in cefalopodni apnenci (»griotte«) iz Francije ter srednjetriasni amonitni in kondenzirani apnenci iz Alp in Notranjih Dinaridov, vendar so ‘elezovo-manganovi gomolji razviti le v jurskih apnencih (B e r n o u i l l i & J e n k y n s , 1974; Jenkyns, 1978). Verjetno so bile na prostoru zahodne Tetide v tem obdobju posebne razmere, v katerih so bili zdru‘eni tako pa-leogeografski, geokemi~ni in diagenetski parametri, da so se gomolji ohranili.
Prvotno so menili, da je bilo okolje nastanka jurskih apnencev, v katerih se javljajo manganovi gomolji, globje morje. Novej{e
raziskave (Jenkyns, 1970, 1974; W endt, 1974) pa zagovarjajo tudi mnogo manj{e globine morja, tudi le do samo nekaj deset ali sto metrov. To povezujejo z ugotovitvami, sa se med plastmi tipa »amonitico rosso« pojavljajo ob~asno stromatolitne plasti, pa tudi dejstvo, da le‘e liasni rde~i apnenci obi-~ajno tik nad svetlimi dachsteinskimi apnenci.
Manganovi gomolji so na prostoru Tetide redno vezani na stratigrafsko kondenzirane plasti, katere ka‘ejo znake izlu‘evanja in »hardgrounda«. Taka obmo~ja z brez ali z zelo pi~lo sedimentacijo so bili podvodni platoji, ki so bili ob prelomih potopljeni razli~no globoko, v~asih le nekoliko.
Na sliki 6 je shematsko prikazan paleo-geografski razvoj Julijske in Dinarske karbonatne platforme z vmesnim Slovenskim bazenom in predviden polo‘aj nastanka manganovih gomoljev ter Mn-orudenja. Medtem, ko naj bi na podvodnih platojih nastajali manganovi gomolji (a), ki so predmet tega ~lanka, pa je v vmesnih globjih jarkih pri{lo do manganskega plastovitega orudenja (b), kakr{nega poznamo z Mangarta in z Begunj{~ice.
Zakaj so gomolji danes sploh ohranjeni? Znano je, da se recentni ‘elezovomanganovi gomolji pojavljajo le na oceanskem dnu. Ko so prekriti s tanj{o plastjo sedimenta, pride do njihovega raztapljanja in do cikli~ne migracije manganovih ionov, kateri se ponovno vgrajujejo v nove gomolje na meji oksidirani sediment/voda (Lynn & Bonatti, 1965). Ohranjeni gomolji v apnencu zato z verjetnostjo ka‘ejo, da je pri{lo po njihovem nastanku do relativno hitrih fizikalno-ke-mi~nih sprememb v sedimentu, ki so pre-pre~ile njihovo raztapljanje ‘e v ~asu zgodnje diageneze.
Zahvala
Avtorji se lepo zahvaljujemo prof. Simonu Pircu za koristne sugestije pri nastanku ~lanka in njegov prevod v angle{ki jezik. Zahvala velja tudi Andreju Stoparju za izdelavo mikroskopskih preparatov, Mladenu [tumergarju za pripravo rentgenskih preparatov, Stanetu Zakraj{ku in Bernardi Bole pa za ra~unalni{ko obdelavo slik in tehni~no pomo~ pri kon~ni verziji ~lanka. Raziskave so del programa P1-0025/215 (Sedimento-
80
logija in mineralne surovine), ki ga financira Agencija za raziskovalno dejavnost Republike Slovenije.
Manganese nodules in Jurassic
limestone of the Southern Alps
in Slovenia
Introduction
One of the stratigraphic and sedimento-logic peculiarities of the Slovenian part of the Southern Alps are reddish limestones of Jurassic age in which manganese nodules occur. These beds outcrop at several localities (fig. l), largely with Mn-nodules (Buser, 1986; Jurkov{ek , 1987), and in places with layered and vein type manganese mineralization, as on Mt. Mangart (J u r k o v { e k et al., 1990; [muc, 2005) and Begunj{~ica (Teller , 1989; Buser, 1980). In Julian Alps beds with Mn-nodules were found in the Valley of Triglav lakes, on southern slopes of Bavski Gamsovec, at the lake in Lu‘nica, in the Slatenik brook by ^ezso~a, and in the surroundings of Bovec on southern slopes of Mt. Rombon and in the Koritnica valley.
In the alpine region the beds and formations with manganese mineralizations are known as “ammonitico rosso” and “KnollenKalke” whiche were during Jurassic of considerable extension in the Mediterranean region of Tethys, from Sicily to Northern Limestone Alps and Hungary (Jenkyns, 1970, 1971, 1978; W endt, 1970; Jürgen, 1969; German, 1971; Vörös, 1991; Haas, 2001). On figure 2 are shown localities of outcropping Jurassic beds that contain Mn nodules in the Mediterranean region (J e n -k y n s , 1970) with localities in the Slovene Southern Alps added. Their genesis is explained as a product of reduced sedimentation of the “hard ground” type.
In our investigations special attention was paid to Mn nodules in the Valley of the Triglav lakes, in Lu‘nica near Mt. Krn, in the Slatenik brook on the northern slope of Mt. Polovnik, and in the surroundings of Bovec (fig. 1).
The aim of this investigation was to establish, along with the basic petrographical and minralogical characteristics of Mn-nod-ules, and the facies of surrounding rocks, especially   their   diagenetic   characteristics
Bojan Ogorelec, Stanko Buser & Miha Mi{i~
and geochemistry with respect to the recent Mn concretions in oceans.
Stratigraphic position of beds containing manganese nodules
At the end of Middle Lias in the wider region of Southern Alps the formerly stable and unique Julian carbonate platform disintegrated (Buser, 1987). On this platform during the Upper Triassic shallow limestones of the Lofer development were deposited, and during Lower and Middle Lias-sic biomicritic, biosparitic and oosparitic limestones. On carbonate blocks which were uplifted above the sea level intensive kar-stification of limestone began. During this event the large part of the Lower and Middle Lias limestone was chemically corroded by solution cavities. Therefore this limestone is preserved only in the surroundings of Bovec, Krn and Mangart. Elsewhere the considered Upper Lias limestone which contains Mn nodules immediately overlies the Upper Tri-assic Dachstein limestone.
On the subsided carbonate blocks condensed sedimentation of reddish biomicritic limestones took place. Deepening of the carbonate platform was a global event that most probably occurred everywhere at the same time. On Mt. Begunj{~ica ammonites were found that place this event in Upper Lias, in Toarcian.
In the following important localities of manganese nodules are briefly desceribed:
The Triglav lakes valley. The red Upper Lias Toarcian limestones overlie Dachstein limestone in the Lofer development. Jurassic beds begin with red, largely crinoidal limestone with intercalated micritic limestone containing numerous bivalves and filaments. Approximately 40 cm above contact occur in crinoidal limestone the manganese nodules that display concentric structure. They can be observed along the mountain path north of the alpine hut at the Triglav lakes. Thickness of the Mn-nodules horizon attains 3 metres. Upward follows nodular limestone of the »ammonitico rosso« type containing numerous not yet determined ammonites. Above this limestone lies the Dogger-Lower Malm radiolarian horizont, and above it a package of Tithonian-Berriasian limestone with calpionellas (the »biancone« beds).
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
81
The Lake in Lu‘nica. At the lake in Lu‘nica southeast of Krn occur in Lower Lias oolitic limestone up to 10 metres deep and 30 cm wide fissures that are filled in the form of sedimentary dikes by crinoidal limestone and variegated limestone breccias. Within them appears a thin bed of micritic limestone with manganese nodules and irregular manganese crusts.
Ravni Laz. At Ravni Laz northeast of Bovec occurs above the white Lower Liassic biosparitic limestone around 2 m of red crinoidal-micritic limestone with numerous filaments and with manganese mineralization.
Koritnica. In the Koritnica valley below Klu‘e, east of Bovec, in rocks along the water above the Lower Liassic oolitic limestones the Toarcian micritic limestone with filaments can be observed. In it occur rare, up to 5 cm thick nodules of greyish brown colour (fig. 3). A single chemically examined nodule contains 2.1 % Fe and only 0.3 % Mn, so in this specific case we have matter with ferromanganese nodules.
Slatenik. In the Slatenik ravine at ^ezso~a in northern slope of Mt. Polovnik occurs a several decimetres to 2 m thick horizon of rosy limestone with extremely beautiful manganese nodules, up to 12 cm across, in places touching each other (fig. 4). This limestone overlies in stratigraphic hiatus con-formingly the light Lower Liassic oolitic limestone. Laterally it is overlain by reddish Upper Cretacous marly limestones with globotruncanas (»scaglia«) or by flysch beds.
Mt. Bav{ki Gamsovec. On southern slope of Bav{ki Gamsovec, north of the Luknja pass, are found at the mountainous path in a several m2 sized rock of reddish Toarcian limestone up to 5 cm large black-brown manganese nodules.
Mt. Mangart. Ferromanganese nodules on Mangart were examined in a 30 cm thick bed of manganese ore that occurs within a 15 metres thick carbonate-clastic succession on the Mangart pass (fig. 5). This package appears directly on the shallow-water biomicritic and oolitic Lias limestone. Between crinoid limestone and marly limestone with radiolarians occur manganese shale, radiolarite and mudstone. These beds are colored black by manganese oxydes pyro-lusite, cryptomelane and todorokite. The MnO grade of the examined sample is 45 %, of FeO 8 %, the rest accounted by quartz (ca
15 %) and clay minerals. The nodules on Mt. Mangart measure 2-3 cm across, and they are intensely silicified.
Mt. Begunj{~ica. Toarcian Jurassic beds in southern slopes of Begunj{~ica in deep marine facies occur above stratigraphic hiatus on Upper Triassic coral-stromatoporid limestone. The gap is indicated by neptunian dikes in the Upper Triassic limestone. The dikes are filled by variegated Jurassic breccia or crinoidal limestones. Jurassic beds consist of red to greyish nodular limestone of the »ammonitico rosso« type. Above these follow grey platy limestones with chert. In the lower part of red limestones occurs a bed of shaly mudstone mineralized with manganese ore. The ore contained even to 60 % manganese. In rare places in the limestone also spheric manganese nodules are found. In limestones below and above the ore characteristic Toarcian ammonites were found. Ammonites indicate the emersion, and occurence of nodular limestones with manganese nodules was started in Upper Lias, in Toarcian.
Lithology and microfacies
Below the Upper Liassic reddish limestone with Mn-nodules a several hundred meters thick package of Upper Triassic Dachstein limestone or Lower to Middle Li-assic limestone. The beds were deposited in a shallow open shelf with temporary inter-tidal conditions, and display all characteristics of the Lofer development (Buser, 1986,1987; Ogorelec & Buser, 1991). According to its sructure, the limestone from the immediate base is as intrapelmicrite with numerous thin dessication pores and agglomerations of non-skeletal algae.
Typical for Jurassic beds of the condensed facies is the reddish color of rock which varies from light orange red to dark brown red. According to structure the limestone is attributed to biomicrite of the wackstone-packstone type. The matrix of the rock consists of dense micrite of allochems only fossils are present. Most frequent are fragments of echinoderms, less abundant are pelecypods (thin walled shells), micro-gastropods and foraminifers, and in traces juvenile ammonites and calciticized radi-olarians (tab. 1, figs. 1, 4-6; tab. 2, figs. 1, 2).
82
Crinoid plates are locally so abundant that the limestone can be named “crionoi-dal«. The energy index of limestone is low to very low. The degree of deposition was low which permitted accumulation of the described organic skeletons.
The reddish color of limestone is due to the admixture of iron and manganese oxides. They occur with clay minerals as pigment in the matrix, more concentrated are in numerous stylolithic sutures. The carbonate content in the investigated limestone varies between 86 and 97%. In certain samples substitution of calcite by authigenic quartz can be observed, mostly calcite from echinoderm plates and pelecypod valves.
Manganese nodules
Manganese nodules are of concentric shape, somewhat flattened, measuring several mm to 12 cm in diameter, and mostly between 3 and 5 cm. They can be distiguished macroscopically from the encasing rock by their darker brownish color, and by their positive relief on weathered surfaces (fig. 4).
Under microscope the nodules display numerous, 10 to 30 µm thick concentric envelopes with stromatolithic structure (tab. 2, figs. 3-5) which is typical also for recent manganese nodules (“cauliflower structure”, Thiel & Schneider, 1988). X-ray and chemical analyses indicate among the components in nodules low magnesium calcite and goethite, and manganese minerals psilomelane, manganite and hausmannite. In traces occur clay minerals and quartz.
Manganese content in the 12 examined nodules is relatively low, and varies between 0,82 and 3,45% Mn and 0,03 to 1,7 % Fe, whereas the limestone of the enclosing rock contains between 0,04 and 0,07 % Mn and 0,01 to 0,08 % Fe. In cases, where Fe is prevailing over manganese, we can use the terminus ferromanganese nodules.
For comparison, the nodules from different studied localities Jurassic limestone in Northern Alps contain between 0.05 and 24% Mn and 0.1 to 17% Fe (Wendt, 1970, 1973; Böhm, 1992) and manganese nodules from equivalent beds in Sicily up to 40 % Mn and 50 % Fe (Jenkyns, 1978).
Certain nodules from investigated beds have no core. In these, concentric growth
Bojan Ogorelec, Stanko Buser & Miha Mi{i~
and structure can be observed up to the center. In other nodules, however, the manganese crusts overgrow fragments of micritic and biomicritic limestone up to 1 cm in diameter (tab. 1, figs. 2, 3). Wrinkled structure of crusts was earlier explained by dehydration of goethite and its transition to hematite during diagenetic processes, which resulted into a reduction of volume (e.g. Jürgen, 1969; Mangini, 1988). Recent studies indicate the primary character of the wrinkled structure. It is attributed by researchers to the activity of bacteria and other chemolithotrophic organisms during the process of genesis of nodules (Monty, 1973; Wendt, 1974; T h i e l & S c h n e i d e r , 1988). Monty refers these nodules as “oceanic manganese stromatolites”.
Discussion and Conclusions
The manganese nodules from the study area of the Slovenian part of Julian Alps occur in Upper Lias biomicritic reddish limestones of pelagic development. They are often associated with condensed beds overlying Dachstein limestone. They occur as up to 12 cm large specimens of discoid shape, and in places also as up to 2 mm large micronodules and incrustations of fossil fragments, especially plates of echinodids. Their mineral composition comprises pyrolusite, psilo-melane, manganite, todorokite and hausman-nite. The Mn contents in examined samples varies between 0.8 and 3.5 %, while that of Fe attains at most 1.7 %. In certain nodules iron predominates above manganese.
On Mts. Mangart and Begunj{~ica manganese ore occurs in a thinner layer within a package of shales and marly limestones. MnO contents in ore from Mangartu averages 45 %, and on Bergunj{~ica up to 60 %.
Middle Jurassic limestones with ferro-manganese nodules are of a similar micro-facies as analogous limestones with Mn-mineralisation in Northern Alps.
The interesting question posed at present by researchers concerns the environment of forming of the ferromanganese nodules, and farther, why they appear in the Tethyan realm only in the Jurassic? Otherwise are similar nodular red pelagic limestones known also from the Cambrian times of Spain, Devonian nodular and cephalopod limestones (»griot-
Manganovi gomolji v jurskem apnencu Ju‘nih Alp Slovenije
83
te«) from France, and Middle Triassic ammo-nitic and condensed limestones from the Alps and Internal Dinarides, but the ferroman-ganese nodules are developed in Jurassic limestones only (B e r n o u i l l i & J e n k y n s , 1974; Jenkyns, 1978). Most probably in the realm of the western Tethys at this time special conditions existed in which specific paleogeographic, geochemical and diage-netic parametres made possible preservation of the nodules.
Manganese nodules are in the Tethyan region regularly associated with strati-graphically condensed beds with indications of leaching and »hard ground«. Such areas wihout or with very modest sedimentation were submarine plateaus that were submerged along faults to various depths, at times not very deep. Figure 6 shows schematically the paleogeographic evolution of Julian and Dinaric carbonate platforms with the intermediate Slovenian basin, and supposed position of formation of manganese nodules and Mn ore. While on submarine plateaus manganese nodules (a) formed, the subject of this paper, whereas in intermediate deeper ditches the layered manganese ores formed (b), as known from Mangart and Begunj{~ica.
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