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A link between metamorphism and the formation of ore deposits in the Mid-Bosnian Schist Mts., Central Dinarides

The Mid-Bosnian Schist Mountains (MBSM) are situated within the Central Dinarides. This is a Palaeozoic metamorphic complex, which covers an area of about 3000 km2. It is composed of pre-Variscan, Variscan and post-Variscan formations (Fig 1). Pre-Variscan unit, situated in the northernmost parts of MBSM (Busovača unit), is composed of phyllites and mica schists, interlayered with metavolcanics, amphibolites and gneisses. The lowermost portion of the Variscan unit is composed of fossiliferous Silurian metaclastics, with pelagic limestone, chert and metavolcanic interlayers. The metavolcanics correspond mainly to metarhyolites (Vranica unit), together with large masses of pyroclastic equivalents and subordinate mafic rocks, and are considered to be Silurian– Devonian in age. They occupy central part of the MBSM. The Devonian formation is conformably overlain Silurian metaclastics and is composed of marbles, fossiliferous platform limestones, dolomites, recrystallized limestones and dolomites (Hrvatović, 1996). The Variscan formations are disconformably overlain by the post-Variscan, mainly Permian non-fossiliferous breccias and conglomerates, which are composed of redeposited Silurian metaclastics and metarhyolites. The post-Variscan stage of volcanic activity produced less altered Permian rhyolites associated with subordinate metabasalts, and Lower Permian sediments. Existing K/Ar ages support multiple metamorphic events (Pamić et al., 2004). New 40Ar/39Ar ages on 13 mineral concentrates (muscovite, biotite, K-feldspar) from 10 samples are reported from pre-Variscan Busovača unit and the Variscan Vranica unit: 1) from the Busovača unit, mica-schists and phyllites were sampled, while from 2) the Vranica unit mainly metarhyolite were collected. (1) The lower, Busovača unit, is characterized by penetrative, mostly mono-stage foliation and a high ductile strain. 40Ar/39Ar ages reveal the existence of a low-grade metamorphic dome, which was formed during Eocene metamorphism erasing all record of older metamorphic events. This metamorphic dome was formed within upper greenschist facies conditions (P-T conditions between 350-450 °C and ca. 3-5 kbar, Majer et al., 1991), during a stage of ductile, Adria-directed deformation. The tectonothermal overprint allows distinction of two stages, a likely prograde stage at ca. 45– 42 Ma, and a second retrograde stage with peak temperature conditions at 39– 35 Ma. A plateau age of 31 Ma has been determined on hyalophane from open vein fillings, formed by retrogressive metamorphic fluids under brittle conditions (Balogh et al., 1999). We interpret this Eocene metamorphic unit to represent a metamorphic core complex, which juxtaposed to the upper, Vranica unit, which is characterized by Variscan and early Triassic tectonism and associated metamorphic overprint. (2) The Vranica unit represents the upper unit and is characterized by low ductile strain, domain microfabrics, and a weak metamorphic overprint. This unit is dominantly overprinted by a Variscan metamorphic event (305-285 Ma), which corresponds in age to the Austrolapine and Southalpine Quartzphyllite units in its northwestern extension. This Variscan metamorphism is overprinted by a Late-Permian to Early Triassic (245 Ma) thermal overprint, interpreted to record a crustal extension. The Eocene overprint is insignificant. Within the Palaeozoic of the Mid-Bosnian Schists Mountains (MBSM) more than 250 ore occurrences exist. The widespread hydrothermal vein deposits were formed by extrusion and intrusion of rhyolitic magma in Late Carboniferous-Early Permian time. Different depth of formation and distance from the parent magmatic body as well as erosion level gave rise to a variety of mineral parageneses (Jurković, 1957). The zonal distribution of mineral parageneses around the centre of volcanic activity can be recognized, although a volcanic edifice has not been reconstructed. Fluid inclusion were studied from several locations from the MBSM (Fig 1): Busovača (quartz, hyalophane, Berberuša (quartz), Raštelica (quartz, barite), Hrmza (quartz), Čemernica (quartz) and Kreševo (quartz, fluorite) region by petrographic, microthermometric, laser Raman and ion chromatographic techniques (Strmić et al, 2000). Variability in FIs composition points to presence of two end-members. 1. Within the Busovača unit, FI-s record a typical metamorphic fluid (Busovača quartz-hyalophane veins) characterized by aqueous-carbonic FI-s enriched in N2 (1-8 mol %). Inclusions show presence of low to moderate salinity fluids (6 to 14 wt. %.NaCl equ.) with Th recorded in interval from 270 to 410°C. The similar FI-s composition was found in the Berberuša locality from the south eastern part of the MBSM. Both deposits were probably formed during retrograde stage of Eocene metamorphism, as the Ar/Ar data suggests. 2. The southeasternmost deposits Raštelica and Hrmza show no evidence of metamorphic fluids. FI-s corresponds to highly saline (32 to 33 wt. %. NaCl equ.) CaCl2-NaCl-KCl± MgCl2 aqueous fluids. Composition of the FI-s, together with temperature of total homogenization (Th) into liquid phase between 150 and 345°C, suggests ore-forming processes, spatially related to rhyolite-metarhyolites volcanism. 3. Several ore-occurrences (Čemernica, Kreševo) are found to posses both FI-s types suggesting the involvement of primary non-metamorphic, rhyolite-metarhyolites related fluids, which are later overprinted by the retrograde metamorphic fluids.

metamorphism; ore deposits; variscan; alpine overprint; Mid-Bosnian Schists; Central Dinarides

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