engleski

Organic geochemistry of Lokve sabkha early diagenetic barite deposit, Gorski Kotar, Croatia

1. Introduction Barite mineralization in Lokve is a stratabound ore deposit conformably situated at the Permian-Triassic boundary. It is composed exclusively of pyrite and barite, separated into two distinct, juxtaposed horizons, stretching for tens of kilometers (PALINKAŠ et al., 1993). Stable isotopic composition of organic matter and carbonates are widely used as an indicator of depositional environment, and for the assessment of paleoclimates and paleoceanography of ancient marine environments. The distributions of hydrocarbons (HC) from ancient sediments have been used to infer biological origin, maturity, and proportions of marine and terrestrial organic matter, as well as sedimentary facies and paleosalinity. The isotopic compositions of the individual HC serve to identify the biological precursors of HC in ancient sediments (SCHWAB & SPANGENBERG, 2004). The aim of this study is to estimate the type of organic matter associated with barite-pyrite mineralization. 2. Geological setting Middle and Upper Permian sediments siliciclasitc sediments occurring near Lokve village originated from newly exposed dry land areas, uplifted during an early continental rifting phase in the Dinarides. The final members of the Upper Permian clastic rocks are green-grey and grey-violet fine-grained siltstones and pelites. Extensive low-lying coastal areas in the Dinarides became sites of evaporate deposition in lagoons and coastal sabhkas. Smoothing of the relief halted clastic deposition, which together with a concurrent change in climate from humid-arid to moderate-warm, triggered carbonate deposition and formation of the basal dolomite. All the Lower Triassic carbonate lithotypes with stratabound mineralization in the Gorski Kotar area, adjacent to the conformable contact with the Permian clastics, originated in a sedimentary environment near mean sea level (BABIĆ, 1968 ; ŠĆAVNIČAR, 1973). The Lokve barite deposits are situated along the edges of the Upper Permian terrain, adjacent to the contact with the Lower Triassic dolomites. The largest deposits occur in the vicinity of the villages of Homer and Mrzle vodice (PALINKAŠ et al., 1993). 3. Samples and methods Four samples collected from Lokve deposit were selected for organic geochemical analyses. Two samples from Homer locality are sandstones with pyrite. Two samples of barite-pyrite ore are collected at Školski brijeg locality. To remove any superficial contamination from handling and weathered material, the rock samples were cut in slabs with a water cooled saw, washed with analytical grade and glass distilled acetone, ethanol, and water, and dried at 50°C for 48 h. The cleaned samples were powdered in an agate ball-mill, and analyzed for distribution and isotopic composition of HC according to the procedure described by SPANGENBERG & MACKO, 1998. The extracts were fractionated by silica-alumina liquid chromatography into aliphatic, aromatic and NSO compounds. Chemical characterization of the aliphatic and aromatic HC were performed with an Agilent Technologies 6890 GC coupled to an Agilent Technologies 5973 quadrupole mass selective detector (GC/MSD). The compound specific C isotope analyses of the aliphatic HC were obtained by the use of an Agilent Technologies 6890 GC coupled to a Finnigan MAT Delta S IRMS by a combustion (C) interface III (GC/C/IRMS) under a continuous He flow. 4. Results The n-alkanes in the C11-C28 range and the acyclic isoprenoids pristane (Pr) and phytane (Ph) are the dominant resolvable compounds in the m/z 71 ion chromatograms of all samples. The low unresolved complex mixture (UCM) eluting between n-C11 and n-C30 is presented in sandstone samples. The n-alkanes have distribution with no odd-even predominance. Isoprenoids in the range C15 (farnesane) to C20 (phytane) are generally present in all samples. Non-aromatic cyclic HC, such as terpanes and steranes, are present in the saturated HC fraction extracted from the sandstones samples. Terpanes are found in one sample of barite-pyrite ore. Terpanes in the range from C23 to C32 were detected in the m/z 191 ion chromatograms. A series of low molecular-weight steranes were detected in the m/z 217 ion chromatograms. The gas chromatograms traces of the aromatics extracted from all samples from the Lokve deposit are dominated by methylated-naphthalenes, methylated-fluorenes and methylated-phenanthrenes. The &#61540; 13C values of the measured alkanes range from -24.2 to -35.4 &#8240; V-PDB. The low molecular weight n-alkanes (C<14) are depleted on 13C. The &#61540; 13C value of pristane was recorded only in one sample of barite-pyrite ore (-36.9 &#8240; ). The &#61540; 13C values of phytane were recorded in the both barite-pyrite ore at -27.5 &#8240; . 5. Discussion and conclusion Low UCM, higher concentration of n-C17 and n-C18 vs. isoprenoids and relatively high concentration of short chain n-alkanes in the studied samples indicate that biodegradation and water-washing have not significantly affected the HC distribution (SCWAB et al., 2005). The dominance of n-alkanes with carbon number lower then C20 points to marine and/or bacterial input of organic matter. The Pr/Ph ratios between 1 and 3 point to oxidizing conditions in depositional environment and to organic matter derived from marine algal source (LARGE & GIŻE, 1996). The absent of hopane TS and the presence of TM suggest immature organic matter (PETERS & MOLDOWAN, 1993). Maturity ratios involving methylated naphthalenes confirm immature organic matter. Most of these parameters are based on the notion that naphthalenes with &#61538; -substituted methyl groups are more stable than those with &#61537; -substituents. Differences in &#61540; 13C values of the short chain n-alkanes may indicate the different biological source of these compounds. References: BABIĆ, LJ. (1968): O trijasu Gorskog Kotara i susjednih područja. &#8211; Geol. Vjesnik, 21, 11-18. LARGE, D.J. & GIŻE, A.P. (1996): Pristane/phytane ratios in the mineralized Kupferschiefer of the Fore-Sudetic Monocline, SW Poland. -Ore Geology Reviews, 11, 89-103. PALINKAŠ, L., PEZDIČ, J. & ŠINKOVEC, B. (1993): The Lokve barite deposit, Croatia: an example of the early diagenetic sedimentary ore deposit. &#8211; Geol.Croat., 46/1, 97-106. PETERS, K.E. & MOLDOWAN, J.M. (1993): The Biomarker Guide. Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, p. 363. ŠĆAVNIČAR, B. (1973) Clastic sediments of Triassic in the Gorski Kotar region. &#8211; Acta geologica, VII/3, 105-160. SCHWAB, V. & SPANGENBERG, J.E. (2004): Organic geochemistry across the Permian-Triassic transition at the Idrijca Valley, W. Slovenia. &#8211; Applied Geochem., 19, 55-72. SCHWAB, V., SPANGENBERG, J.E. & GRIMALT, J.O. (2005): Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps. &#8211; Geochem. Cosmochem. Acta, 69, 1825-1840. SPANGENBERG, J.E. & MACKO, S.A.(1998): Organic geochemistry of the San Vicente zinc-lead district, eastern Pucará Basin, Peru. -Chem. Geol., 146, 1&#8211; 23.

sabkha; barite mineralization; Permian-Triassic boundary; organic matter; carbon isotopes; biomarkers

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