Original scientific paper

https://doi.org/10.4154/gc.2024.12

Tracing the evolution of the world's first mined bauxite from palaeotopography to pyritization: insights from Minjera deposits, Istria, Croatia

Ivor Perković ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia
Maja Martinuš ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia
Blanka Cvetko Tešović ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia
Igor Vlahović ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia
Darko Matešić ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia
Robert J. Newton ; School of Earth and Environment, University of Leeds, United Kingdom
Tianchen He ; College of Oceanography, Hohai University, Nanjing, China
Marin Šoufek ; Croatian Natural History Museum, Zagreb, Croatia
Ivan Razum ; Croatian Natural History Museum, Zagreb, Croatia
Goran Durn ; Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, Croatia

Abstract

The Minjera bauxites are the first analysed and mined bauxites in the world. They are a group of pyritised bauxites situated in northern Istria, developed during the subaerial exposure phase which marked a major part of the Late Cretaceous and Palaeocene in northern Istria. In this study, the morphology, petrography, mineralogy, geochemistry as well as stable sulphur isotopes of the D-1 and D-15 deposits from Minjera were studied, as well as the evolution of their bedrock and cover. This study found that those two deposits differ in morphology, mineralogy and geochemistry as a consequence of their different palaeotopographical positions, with the D-1 deposit located at a higher position at the time of its formation compared to D-15, which led to the higher degree of leaching and desilicification in the D-1 deposit. The pyritisation in the studied deposits was a multi-phase process, which began with the deposition of framboidal pyrite and micrometre-sized anhedral pyrite, over which colloform pyrite was precipitated. This indicates that the solutions were initially supersaturated with iron sulphide, saturation of which subsequently changed, as finally euhedral, dendritic and acicular pyrite were deposited, indicating undersaturated conditions. The final stage was marked by deposition of pyrite veins. This formational sequence of pyrites is also supported by stable sulphur isotopes, as the δ34S values exhibit a wide range from -40.86 to 2.32 ‰, where lower values indicate an open system with an unrestricted sulphate supply in which supersaturated conditions could have been achieved, while the higher values indicate a change towards a closed system with limited sulphate supply. The organic matter necessary for microbial sulphate reduction was derived from the marshy environment established atop of the bauxite. The initial flooding started in the Palaeocene, with the first part of the sequence being deposited under lacustrine conditions, which changed towards fully marine with the deposition of Foraminiferal limestones.

Keywords

Minjera; Istrian Palaeogene bauxites; Pyritisation in karst bauxites; Iron sulphide morphology in karst bauxites; Stable sulphur isotopes in karst bauxites

Hrčak ID:

318267

URI

https://hrcak.srce.hr/318267

Publication date:

21.6.2024.

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