MINERALOGICAL AND GEOCHEMICAL FEATURES OF UPPER CRETACEOUS MARINE IRONSTONES OF THE AYAT FORMATION (TURGAY DEPRESSION)

Abstract

The relevance. The formation of ooidal ironstones has been the subject of many scientific discussions for a long time. The purpose of this work is to study the mineralogical and geochemical features of ooidal ironstone to access the sedimentation and geochemical conditions of their formations. The methods: petrographic analysis, scanning electron microscopy, transmission electron microscopy, X-ray fluorescence analysis, mass spectrometry with inductively coupled plasma. The paper introduces the mineralogical and geochemical characteristic of the Ayat Formation indicating their diagenetic origin in a coastal marine environment. The presence of various authigenic minerals, such as glauconite, apatite, pyrite, wurtzite, barite, siderite, indicates the fluctuations of the geochemical conditions during the diagenesis of marine sediments. The formation of pyrite and wurtzite was controlled by oxygen-depleted conditions with the bacterial sulfate reduction at the water-sediment interface. The association of goethite, siderite, berthierine, wurtzite, pyrite and barite among iron-rich ooids indicates a single process of their input. According to mass spectrometry, the average total content of rare earth elements was 83,1 ppm, while there is a general deficit of La, Ce, Pr, Nd. The behavior of rare earth elements and microelements has similar features for marine ironstones of Ayat Formation and channel ironstones of Lisakovsk deposit. This indicates the same diagenetic process of ironstone formation, while the enclosing sediments have distinctive facies conditions.

Keywords

• ooidal ironstone
• diagenesis
• mineralogy
• geochemistry
• ayat formation
• turgay depression