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Bauxites in croatia - mineral resources and beyond

For a long time now bauxites in Croatia have been treated only as an aluminum ore with all research efforts focused on their detection, exploitation and refining. More than 1000 bauxite deposits located in the 150 km long zone of the Dalmatian karst have been mined during the second half of the 20th century. In recent times, however, most of the ore is exhausted, leaving only small deposits which are not commercially cost effective. Despite a great number of mineralogical and chemical analyses from the bauxite deposits and occurrences in Croatia published in various studies and professional reports, the issues of bauxite genesis, its relation to the adjacent karst paleo-environment, and its tectonostratigraphic constraints have been largely underestimated. Also, they were treated earlier in the context of paleoclimate related geochemical and mineralogical processes and of local tectonics. Lately, they were brought again into the focus of interest when the newly constructed models of orogenic evolution of the Adriatic (Adriatic-Dinaric) Carbonate Platform (ADCP) region have underlined the importance of a number of hiatuses of variable duration marked by bauxite deposits. Recognizing bauxites as tectonic and climatic event markers at regional unconformities, new investigations call for additional correlative studies within and across different stratigraphic horizons, thus enabling the utilization of this immense potential indispensable in building the regional models of platform evolution. Croatian bauxites are hosted in carbonate rocks marking the regional unconformities between several stratigraphic levels. In the recent investigations, addressing the root causes of geochemical variations observed in bauxite deposits, Lower Paleogene bauxites were specifically targeted. It is because their variable time span of emplacement (Early Cretaceous to Early Eocene) within the ADCP as the extent of subaerial exposure during the over-all regression of the carbonate platform is of particular importance in understanding the origin of bauxites. Multiple discriminant function analysis, which proved very effective in these investigations, is a statistical technique that is particularly useful when applied to problems involving discrimination between several pre-defined groups characterized by the great number of observations. The purpose of the first investigation was assessing the geochemical contrast (aided by qualitative mineralogical analysis) among the several groups of Lower Paleogene bauxites previously defined in relation to their paleographical/paleotectonic settings and thus elucidating the paleoenvironmental conditions and intervening processes prevailing on the emergent carbonate platform. A total of 50 bauxite samples of the Early Paleogene age were collected from various sites of Istria, North Adriatic islands, North and Central Dalmatia (fig.1.) and chemical and mineralogical analysis was performed on bulk samples. Two discriminant function models were created accommodating the four groups of bauxites to be neatly separated into their characteristic compartments according to specific combinations of major and trace elements. A second investigation (fig.2.) focused on analogous issues related to a particular portion of the platform – the Istrian Peninsula – due to its specific geodynamic evolution through Cretaceous to Paleogene times. It relied on adoption of the recent perspectives on development of the collision-induced diachronous discontinuity surface in the Istrian part of the ADCP. This process, related to the forebulge uplift at the latest stages of Alpine orogenesis, is amply recorded in the geochemical signature of contemporaneous bauxites. A discriminant function model based on Compositional Data analysis (major and trace elements) was constructed allowing for complete distinction between the four a priori defined Istrian Lower Paleogene bauxite groups overlying successively the Pula through Gornji Humac formations.

Bauxite geochemistry, Compositional data, Discriminant function model, Adriatic–Dinaric Carbonate Platform, K–Pg unconformity, Istrian Peninsula, Croatia

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