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Geochemical anomalies on Permian/Triassic boundary in Lokve barite deposit, NW Outer Dinarides, Croatia

Permo/Triassic boundary (PTB) is the most intricate and intriguing geological feature marked with the most extensive mass extinction in the history of life. It is also marked with expressive geochemical and isotopic anomalies which still ask the proper explanation. There have been numerous possible explanations for this profound break in the evolution of life, such as Sibirian trap volcanism at the Tethys/Panthalassa margin which caused "volcanic winter" (dust, sulphate aerosols, acid rains, etc.), sea- level fluctuation, changes in sea water chemistry, extra-terrestrial impact event, or their combinations. The reconstruction of the scenario for the PTB biotic crisis requires further investigation combining paleoenvironmental studies and geochemical elemental and isotopic data. Lokve barite deposit in Gorski Kotar, NW Croatia, is a unique accumulation of only two minerals, barite and pyrite. It is placed at the very PTB and represent geochemical anomaly par excellence with a unique genesis, still puzzled in some details. Lokve mineralization is a stratabound ore deposit conformably situated between Paleozoic clastics and lowermost Triassic dolomites. It bears only two minerals barite and pyrite exclusively, separated into two distinctive juxtaposed horizons stretching for tens of kilometer (PALINKAS et al., 1993). The discovery of cryptalgal fabrics and other conspicuous sedimentary features in underlying silici- clastics with massive pyrite and surmounting barite-bearing dolomites supports their affiliation to a tidal flat facies or eventually to Sabkha environment (PALINKAS and SREMAC, 1989). Barite and pyrite were formed by early diagenetic, bacteriogenic sulfate reduction in a peritidal muddy environment, con- comitant to a widespread process of evaporative dolomitization. The early diagenetic model is supported by persistant increase of  34 S of pyrite, from negative to highly positive values down the pyrite horizon. Different  34 S in coegsisting barite and pyrite suggests absence of their cogenesis. 87Sr/86Sr ratio in barite, significantly higher than the contemporaneous Permian or Triassic sea, points to intensive ingress of fresh underground water from the continent and mixing with the sea water. REE element patterns, chondrite-normalized, confirm highly reducing environment by expressive Eu/Eu* anomalies, and con- cave upward distribution characteristic of terrestrial barites. Elemental anomalies of Cu, Pb, Zn, Hg and other heavy metals, although not expressive, are present at the very boundary as well. The genesis of deposit is still not completely defined, but it is exceptional in many respects. Equivalent deposits can be found at other PTB boundaries in the Zechstein basin (WEDEPOHL, 1980), Andian realm and elsewhere.

Lokve; barite deposit; pyrite

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