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Zeolite deposits in Croatia

In spite of the fact that zeolites have been found at several locations in Croatia, so far only two regions with economically interesting concentrations are known. Both are situated in northern part of the country i.e. in the part that belongs to the Pannonian Basin. The first one is located in the Macelj area in north-western part of Hrvatsko Zagorje (NW Croatia) in which Lower Miocene (Egerian-Ottnangian) volcaniclastic rocks, up to 100 meters in thickness, have been described on several localities. They occur in several horizons interbedded with shallow-sea clastic sediments within the elongated syncline which gradually sinks towards the east. They have variable primary composition, and contain various alteration products of volcanic glass. The alteration products comprise zeolites, clay minerals (smectite, authigenic mica), SiO2 phases and authigenic feldspars. Clinoptilolite is the most abundant zeolite, however analcime and mordenite are also present. The type of exchangeable cations in clinoptilolite is variable ; therefore clinoptilolites were divided into three subgroups: Ca-rich, Ca-K, and Na-rich. Clinoptilolite content in the volcaniclastic rocks discovered in Macelj area vary significantly, in some rocks it is the dominant component, in others it is present only in traces. In Donje Jesenje quarry, in which due to exploitation the best outcrops of these rocks can be found, the variations, although present, are not so conspicuous, and the average content of clinoptilolite is approximately 50 wt. %. Alteration of volcanic glass is the result of burial diagenesis. Different alteration products are most probably due to different temperatures to which sediments were exposed, with increasing depth of burial clinoptilolite transformed to mordenite and analcime, while opal-CT changed to opal-C and recrystallized to quartz. However different immobile element content of rocks containing different alteration products indicates that alteration was probably also dependent on chemical composition of the rocks. Some types of alteration product were also dependent on chemical composition of primary material and its granulometric characteristics. Huge reserves of volcaniclastic rocks make them economically interesting, nevertheless it will be necessary to take care about raw material properties i.e. zeolite content and its chemical composition, when planning its particular application. The other deposit is located in Slavonian Mountains, on the southern slopes of Mt. Papuk in the vicinity of village Poljanska in Požega Valley . In the deposit four horizons that are results of changes in climate and related variations in sedimentaring basin characteristics can be differentiated ; two with analcime-bearing rocks (used in cement industry) which are underlain by sandy calcareous shales and separated by marly horizon. The analcime-bearing rocks are of hybrid composition. They contain, in layered or laminar alternation or mixed together, in various proportions, authigenic dolomite, volcaniclastic and pyroclastic particles from neighbouring ancient volcanoes as well as terrigenous psamitic and pelitic material. The analcime, that is present as dispersed grains or in monomineral laminae and thin layers, was formed by alteration of pyroclastic material within closed hydrological system i.e. shallow saline alkaline lake which existed during early Miocene (Ottnangian) time due to arid and semiarid climates. The conditions in such lake were also favourable for formation of dolomicrites. On the contrary calcareous footwall and marly interhorizon were deposited during more humid climates in a lake characterized by fresh water.

zeolite deposits; clinoptilolite; analcime; Croatia

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