Thermal processes in Triassic SEDEX ore deposit Vareš and adjacent pile of pillow lavas, Central Bosnia and Herzegovina

The abstract deals with the two prominent representatives of Triassic advanced rifting magmatism, SEDEX ore deposit Vareš and its principal driving mechanism for fluid convection, a huge pile of pillow lavas. Their position, at the head of the obducted ophiolite front, accomplishes a general pattern which can be followed from the Zagorje-Mid- Transdanubian megaunit, Dinarides, Albanides and Hellenides. The products of Triassic advanced rifting magmatism, are situated within the basement of the growing carbonate platform in Mesozoic time, invaded by bulldozing, obducting slab of the oceanic crust, during collision and closure of the Neotethyan ocean in Jurassic-Lower Cretaceous time. Their present position is in the mélange, chaotic mixture of complex lithology, developed during ophiolite emplacement, or as para-autochtonous, at the base of or even within the carbonate platform itself. The both objects are situated at the head of the obducted ophiolite Krivaja-Konjuh massif.
   The Vareš Fe-Pb-Zn-Ba SEDEX deposit is located 40 km northern from Sarajevo, Bosnia and Herzegovina. Mineralization is derived from hydrothermal sea floor exhalations within a Triassic rift depression of the Tethys.
   The Vareš basalt quarry, situated in the vicinity of the deposit, exposes Middle Triassic pillow lavas, peperites and hyaloclastite breccias formed by extrusions of basaltic lava into unconsolidated seawater sediments. Interactions of basaltic lava with the seawater produced mostly chloritic alteration of basalt and precipitation of hydrothermal calcite in feeding channels of lava lobes and hyaloclastite breccia matrix. Calcite is associated with siderite, chlorite, zeolites and minor epidote.
   Primary fluid inclusions (FIs) hosted by hydrothermal calcite reflect conditions of fluid/rock interaction. The coexistence of liquid-rich and vapour-rich FIs points to precipitation from boiling fluid and allows the estimation of trapping conditions. Assuming recent seawater salinity, homogenization temperatures in an interval between 275 and 290°C correspond to the formation depth between 650 and 900 m. Secondary two-phase (L+V) aqueous FIs display salinity close to the salinity of recent seawater (3.5-5.7 wt.% NaCl equ.) and their homogenization temperatures (TH) are in the range of 60–110°C.
   The Vareš Fe-Pb-Zn-Ba SEDEX deposit comprises stratiform siderite–hematite–chert beds formed by exhalation of hydrotherms onto the bottom of rifting basin. The mineralization displays a distinct vertical zoning, reflecting a gradual change of redox conditions in the depositional environment. The sequence starts with bituminous, thinly bedded shales with pyrite and base metal sulfides, overlain by barite and siderite, deposited under reducing conditions. Overlying clastics and oolithic limestone are succeeded by hematite shale, hematite ± chert beds, deposited in oxidizing environment. The principal
minerals are siderite, manganese-rich hematite, barite, pyrite, marcasite, chalcopyrite, galena, sphalerite, tetrahedrite and Pb-sulphosalts.
   Microthermometric investigations on siderite and barite samples distinguished several FI types including: 1) Two-phase (L+V) NaCl-CaCl₂-H₂O FIs (2-6 wt.% NaCl equ.; TH ≈+110°C), 2) H₂O-CO₂-NaCl FIs (~4.5 wt.% NaCl equ.; TH ≈+100°C), 3) Aqueous FIs with liquid hydrocarbons, 4) Pure hydrocarbon FIs, and 5) Mono-phase aqueous FIs.
   The FIs data indicate that modified seawater represents the major constituent of the mineralizing fluids and that magmatic activity in the region had served as principal driving mechanism for fluid convection. The presence of hydrocarbons reflects interaction of mineralizing fluids with organic rich sediments.
   The total surface of the exposed pillow lava on both banks of the Stavnja rivulet exceeds several square km. The pillow lava pile is made of m-dm sized pillows rounded, semirounde, squeezed, contorted, green, red and gray in colour.
   The foundation of the pile are lobs, partly disintegrated, turning into hyaloclastite breccias and pink peperites, within the inter-pillow space. The carbonate part of peperite contains Triassic index conodont fauna Paragondollela excelsa, Paragondollela foliata foliate, Paragondollela foliate inclinata, Nurella sp., CAI 6½, what appropriates to the Langobardian. The basalt was extruded into soft, unconsolidated sediments, of the Triassic rifting basin, and there are still no evidences of the oceanic crust which developed afterwards in Jurassic time.