www.geologia-croatica.hr Plate tectonic aspects of the Triassic carbonate-hosted stratiform-stratabound base-metal deposits in the Western Balkan, NW Bulgaria Irina Marinova* and Zhelyazko Damyanov Bulgarian Academy of Sciences, Institute of Mineralogy and Crystallography, Acad. G. Bonchev Str., Block 107, 1113 Sofia, Bulgaria (*corresponding author: [email protected]) 65-73 4 Figs. 2 Pls. doi: 10.4154/gc.2016.05 ABSTRACT Article history: The Triassic carbonate-hosted stratiform-stratabound base-metal deposits in the Western Received November 06, 2015 Balkan, NW Bulgaria, have well defined regional geological and tectonic settings, style of Revised and accepted December 09, 2015 mineralisation, mineralogical, geochemical and isotopic data. Their genesis, however, re- Avaliable online February 29, 2016 mains controversial in the framework of plate tectonic models, and is not supported by comparative analysis of possible counterpart candidates within the NW Tethyan domain. Besides, the crucial missing information is the geochronology of ore mineralisation relative to the Triassic carbonate host rocks. The article stresses the ambiguities due to the lack of mineralisation age dating, and emphasises the necessity for building a convincing plate tectonic model based on the relevant geodynamic setting. A prospective direction of re- Keywords: siderite-base metal deposits, search is the critical comparison with equivalent sediment-hosted base-metal deposits with- Kremikovtsi-Vratsa ore district, Triassic in the continental margin environments such as those in the Alps and Dinarides as well as metallogeny, Moesian carbonate platform others in the Alpine-Himalayan orogen and worldwide. 1. INTRODUCTION These deposits share many similarities with the sediment- The Triassic carbonate-hosted stratiform-stratabound base- hosted base-metal deposits in the Alps and Dinarides metal deposits in the Western Balkan, NW Bulgaria are me- (PALINKAŠ et al., 2014 and references herein), the Cracow- dium- to small-sized deposits of lead, zinc, copper and silver Silesian District in Poland (BANKS et al., 2003), Iran (RA- jABI et al., 2013), and Ireland (HnatySHIN et al., 2015), located to the south of Vratsa town. They also include the etc. large iron-manganese-barium (+subeconomic lead-zinc) Kremikovtsi deposit situated to the north of Sofia, together The Triassic sediment-hosted base-metal deposits in the comprising the Kremikovtsi-Vratsa Ore District (POPOV, Alps and Dinarides formed within the Adriatic-Dinaridic car- 1985) (Fig. 1). bonate platform – a passive continental margin of Gondwana (PALINKAŠ et al., 2014). The Triassic carbonate-hosted base-metal deposits in the Western Balkan (Bulgaria) formed within the Balkan terrain – a Precambrian- Cambrian ophiolite-island arc assem- blage, unconformably overlain by a Pal- aeozoic sedimentary sequence. This ter- rain was accreted to the Palaeozoic base- ment of the Moesian platform (located to the northward) in Upper Carboniferous time. Basements of the Balkan and the Moesian terrains are of Gondwana origin (HAydoutov & YaneV, 1997). Thus, the Hercynian orogonesis resulted in clo- sure of the Palaeo-Tethys and accretion of Gondwana and the eurasian plate (DABOVSKI & ZAGORCHeV, 2009). The location of ancient and old mine workings are known for almost all depos- its in the Western Balkan. It is believed Figure 1. Location of the Kremikovtsi-Vratsa Ore District in the Bulgarian early Mesozoic that the Thracians were the first to start tectonic setting (a) from DAMyANOV (1995) and distribution of the carbonate Triassic mining, followed by the Romans and then formation in the NW Bulgaria (b) from ZAGORCHeV & BUDUROV (2009). the Saxons. During the Ottoman rule, min- Vol. 69/1 ers hired by the Republik of Dubrovnik, after primitive melt- primary zone of tectonic deformation (DABOVSKI & ZA- ing, carried the metal to Dubrovnik proper. In 1902, the mo- GORCHeV, 2009). dern mining industry in the Vratsa Ore District started with the The deposits in the Kremikovtsi-Vratsa Ore District are opening of the Plakalnitsa mine. The mine had been operating mainly hosted by the Lower to Middle Triassic limestones, do- for around 90 years until 1998, when the last mines were lomitized limestones and dolostones in areas of their synclinal closed (PeTROV et al., 2011). Unlike the Kremikovtsi depo sit foldings. Base-metal mineralisations are also known in the th which has been known since the end of the 19 century. Small- Lower and Middle jurassic carbonate-terrigenous rocks (e.g. scale production began in 1913. The modern production start- the Breze iron deposit in the Western Balkan, KANOURKOV, st ed in 1963 and continued until the early 21 century. 1988) and in the basement rocks (Vratsa Ore district, The deposits in the Kremikovtsi-Vratsa Ore District were KANOURKOV, 1988; Kremikovtsi deposit, DAMyANOV, intensively explored and studied over a long time with the on- 1996, 1998) but of smaller extent. th set in the 1960-1990s of the 20 century. Comprehensive re- The base-metal mineralisations in the Western Balkan crop searchers of the Vratsa ore region and the biggest Sedmoch- out largely along the Iskar River where deeply incised valley islenitsi deposit are ATANASOV (1972) and MINCHeVA- display the thick Triassic sequence. The Iskar riverbed is at STeFANOVA (1988) with their comprehensive reviews con- 300-400 m above sea level; the peaks in the Vratsa Balkan are cerning the deposit mineralogy and geochemistry. Tectonics of high, at about 1300-1600 m a.s.l., and the relief is strongly in- the ore deposits were studied by KalaYdzieV (1977, 1982), cised. In the Western Balkan the Triassic carbonate rocks do KalaYdzieV & POPOVA (1981) and by CHeRNeV not occupy a large area because denudation after the Triassic (1993). The Kremikovtsi deposit was studied by many re- eroded most of the formation exposed in positive elevation searchers (review in KANOURKOV, 1988) among whom the structures (Fig. 1). The Triassic section in boreholes in the studies of RATIeV (1977), KalaYdzieV et al. (1982), Moesian platform exceeds 1 km (VAPTSAROVA et al., 1979). KANOURKOV (1988), POPOV (1989), BAKARDZIeV & POPOV (1995), and DAMyANOV (1993) are more compre- The area in the Western Balkan which hosts stratiform- hensive. stratabound base-metal mineralisations is a part of the large Berkovitsa anticline which includes wide and flat second-order Past research constrained the general features of the depo- synclines and anticlines. It developed between the Triassic- sits including the geological setting, tectonics, style of mine- jurassic and the Upper Cretaceous (yOVCHeV, 1961). Its Pa- ralisation, ore and gang mineralisation and geochemistry. laeozoic core crops out along the Iskar River and comprises Some issues, however, have remained unresolved. The dating Hercynian intrusive rocks (granodiorites and granites). The of ores is based only on Pb isotope data, which remains contro- latter are overlain discordantly by Upper Carboniferous quartz- versial. The sources of metals and fluids are poorly defined, ites, sandstones and coal-bearing argillites. Passing upwards, and the data on the geodynamic setting of the deposits needs Permian terrigenous-volcanogenic rocks crop out. The Triassic updating. Hence, the genetic models created are very different sequence begins with a regional discordance – the Lower Tri- and often contradictory. assic monogenic conglomerates and monomictic (quartz) to The paper reviews the published data on the Triassic car- oligomictic sandstones overlie a variegated basement of Pre- bonate-hosted stratiform-stratabound base-metal deposits in Cambrian rocks, Palaeozoic low-grade metamorphic and mag- the Western Balkan, NW Bulgaria concerning their geology matic rocks or Permian red beds (Fig. 2). The exact age of the and mineralogy. It illustrates the principal ore textures, brief- Triassic transgression is not known. The Triassic system in ing the main ideas on their origin, and outlines directions for Bulgaria lacks discordances or discontinuities. A regional un- future research. The primary goal is to clarify and confirm the conformity at the end of the Triassic and the beginning of the relevant geodynamic setting in relation to the other Triassic jurassic is very well recorded as most tectonic units are related sediment-hosted base-metal deposits within the continental to the early Cimmerian orogenic phase (BONCHeV, 1940). margin environment, likewise equivalent deposits in Alps and The Triassic formations developed after the Hercynian oroge- Dinarides as well as the other similar deposits in the Alpine- ny, following denudation of the expressive topography, depo- Himalayan orogen and worldwide. sition of the Lower Permian molasse type sediments and the Late Permian. Differential subsidence, accompanied by em- 2. GEOLOGICAL FRAMEWORK bryonic rifting (the Chiren rift in the West Balkan terrain) led The Western Balkan is part of the Balkan Orogenic Zone. To to a marine transgression at the end of the early Triassic and a the east the Balkan Zone comprises the Central Balkan and marine regime in the Middle Triassic time with the foundation eastern Balkan units. The Western Balkan is composed of of the carbonate platform. At the end of the Triassic, activation Neoproterozoic-Cambrian basement, poorly preserved Pa- of the Late Cimmerian orogenic phase began, followed by laeozoic mantle (mainly Permian variegated sediments) and compression and weak folding which resulted in a gradual re- Mesozoic cover of Triassic clastic