MINERALOGICAL AND GEOCHEMICAL FEATURES OF OOIDAL IRONSTONES OF HYDROTHERMAL ORIGIN

Abstract

The relevance. Hypotheses of ooidal iron ore formation remain debatable to this day. The study of ooidal ironstones in volcanic and hydrothermal activity areas will allow better understanding of the origin of ooidal iron ores. This work aims to study mineral and chemical composition of ooidal iron ores from the Vani hydrothermal deposit (Milos island, Cyclades archipelago) to assess the effect of hydrothermal solutions on the features of their material composition. The research technique included optical microscopy (petrographic analysis), X-ray fluorescence analysis, scanning electron microscopy with local X-ray spectral energy dispersive analysis, X-ray diffraction analysis, transmission electron microscopy and inductively coupled plasma mass spectrometry. As a result of the research, the textural and structural features were determined, and the mineralogical and geochemical characteristics of the ooidal ironstones from the Vani hydrothermal manganese deposit (Greece) were obtained. Conclusions. Three lithotypes for the studied sequence with ooidal ironstones were distinguished based on differing structural and mineralogical characteristics: chemogenic (manganese rocks, ooidal ironstones) and clastic (glauconite sandstone). It is assumed that the primary source of metals for ironstones was hydrothermal solutions according to the geological and structural position of the deposit. The multi-stage events of hydrothermal activity contributed to the formation of iron-rich minerals in two stages, which differ in structures and impurity elements. Rare earth elements enrich the ooidal ironstones compared to other lithotypes due to the high sorption capacity of iron hydroxides in conditions of metal precipitation. The main in situ minerals of rare earth elements of studied ironstones are arsenates represented by inclusions in the ooid cortex. The distribution of rare earth elements in ooidal ironstones indicates their hydrothermal sources.

Keywords

• ooidal ironstones
• iron-rich ooids
• iron-rich pisoids
• hydrothermal deposit
• origin
• metal sources
• geochemistry
• minerals
• lithotypes