FORMATION MECHANISMS OF FE-MN CONCRETIONS IN THE VIJENAC QUARRY, DINARIC OPHIOLITE ZONE

Rudnik Vijenac, smješten u blizini Tuzle, Federacija Bosne i Hercegovine, sastoji se od karbonatnih stijena, koje su lokalno proslojene tektonski poremećenim naslagama siltita i pješčenjaka s Fe-Mn konkrecijama. Sam kamenolom nalazi se u zoni dinaridskoga navlačnog pojasa (Pogari serija naslaga) koje diskontinuirano leže na ofiolitnome melanžu i ofiolitnim navlačnim naslagama. Petrografske, kemijske i XRD analize pokazale su da se konkrecije mogu podijeliti u dva tipa: (i) Mn-obogaćene konkrecije s ≈ 17 wt.% udjela Mn s kompaktnom strukturom te (ii) Mn-osiromašene s ≈ 8 wt.% udjela Mn te poroznijom strukturom. Udjeli Ca, Mg, Fe i organske tvari obaju tipova konkrecija podjednaki su. Udio Ni pozitivno korelira s Mn, udio Cr pozitivno korelira s Fe. Prema petrografskim i XRD analizama konkrecije se sastoje od kalcita, dolomita, hematita, todorokita i takanelita. Todorokit je karakterističan mineral za konkrecije formirane dijagenetskim rastom. Petrografska istraživanja potvrdila su rast konkrecija u trima fazama s ukupno dvjema generacijama kalcita. Promatrane konkrecije nalaze se unutar konsolidiranih pješčenjaka te nasljeđuju njihove sedimentne strukture – stoga se pretpostavlja kasnodijagenetski proces rasta.Vijenac quarry, near Tuzla in Bosnia-Herzegovina, is composed of carbonate rocks locally embedding tectonically disturbed siltite and sandstone with Mn-Fe concretions. The quarry itself represent a part of Dinaric overstep sequences (Pogari series) unconformably overlying ophiolite mélange and ophiolite trust-scheets. Petrographic, chemical and XRD analyses had shown that the concretions may be divided into two types: (i) Mn-rich concretions with ≈17% of Mn and compact texture and (ii) Mn-poor (≈8% of Mn) and porous texture. The amount of Ca, Mg, Fe and organic matter, as well as analysed microelements (Ni, Cr) in both concretion types are similar. According to petrographic and XRD analyses, concretions are composed of calcite, dolomite, hematite, todorkite and takenelite. Todorokite is characteristic for concretions formed by diagenetic growth. Petrographic study confirmed the development of concretions within three stages including two generations of calcite. The studied concretions are formed within consolidated sandstones inheriting their sedimentary textures - therefore a late diagenetic process is assumed