Abstract

Abstract Over 170,000 metric tonnes of high-grade clinoptilolite tuff were extracted from the open-pit mine in Nižný Hrabovec (eastern Slovak Republic) in 2018, making it one of the world’s major natural clinoptilolite producers. The mine is hosted in a Miocene volcanogenic-sedimentary deposit in the East Slovak basin, with estimated 150 million tonnes of clinoptilolite tuff—the economically most important reserve in the European Union. The 100-m-thick tuff horizon is under- and overlain by SW-dipping shallow-marine Badenian sediments (16.30–15.03 Ma ≅ Langhian stage). The tuff is rhyolitic (74.3–77.6 wt % SiO2, 4.09–5.49 wt % Na2O + K2O) with a generally high-K calc-alkaline affinity (3.07–4.28 wt % K2O), comparable to volcanic rocks in the central segment of the Carpathian-Pannonian region of similar age. Although some parts of the tuff underwent slight geochemical changes during formation, this did not significantly change the whole-rock composition of the strikingly homogeneous raw material present in the mine. Rare earth elements (ΣREE = 117–141 ppm) show a uniform pattern, with enriched light REEs and negative Eu anomalies of 0.42 to 0.6 (when normalized to chondrite), comparable to the upper continental crust composition. Mean 87Sr/86Sr = 0.70880 and 143Nd/144Nd = 0.512463 indicate an enriched magmatic source, with a dominant crustal contribution. X-ray diffraction data and electron probe microanalyses show that clinoptilolite-Ca is the only zeolite phase present in the deposit. High-resolution electron probe microanalytical imaging and measurement techniques reveal that clinoptilolite is present as (1) coarse patches that either form pseudomorphs of volcanic glass shards or grew in voids and (2) ultrafine material making up the matrix of the tuff. Both textural types have Si/Al >4, and their evolution is connected to dissolution-(transport)-precipitation reactions from acidic volcanic ash under alkaline fluid influence and slightly elevated pressure-temperature (44°–84°C) conditions. Authigenic cristobalite, detected as ultrafine-grained matrix crystallites, formed during the zeolitization process from excess Si. The large-scale, remarkably homogeneous and monomineralic natural zeolite deposit formed from a well-equilibrated magmatic source, with no syn- to postsedimentary reworking and with essentially isochemical conditions during the zeolitization process.

Highlights

  • Zeolites occur in several geologic settings of differing ages, but most mineable deposits are of Cenozoic age, in the United States

  • One of the economically most important natural zeolite occurrences in the world is the deposit around Nižný Hrabovec in the Slovak Republic (Fig. 1A), with reserves of ~150 metric tonnes (Mt) of clinoptilolite tuff (Chmielewská, 2014)

  • This study presents a detailed and integrated petrological and geochemical investigation of one of the world’s largest actively mined clinoptilolite tuff deposits

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Summary

Introduction

Zeolites occur in several geologic settings of differing ages, but most mineable deposits are of Cenozoic age, in the United States. Along the northwest margin of the East Slovak basin, a lower Badenian (16.3–15.03 Ma) shallow-marine sequence crops out (Nižný Hrabovec Formation; 500–600 m thick), comprising fine-grained sandstones and mudstones grading into coarse-grained sandstones overlain by volcanogenic sediments of the Hrabovec tuff (Šamajová, 1979; Varga et al, 1985; Kovácet al., 1995). This tuff, which has a thickness of ~100 m in a 7-km-long belt running from the villages of Pusté Cemerné in the southeast, through Nižný Hrabovec, Kucín, Majerovce to Vranov nad Topl’ou in the northwest (Fig. 1A; Šamajová, 1979; Kovácet al., 1995), was strongly affected by 100 m (C)

E Burdigalian
Results
Potassic Granite
Ba U Ta La Sr Sm Ti Y
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