Diagenetic influences on iron-bearing minerals in Devonian carbonate and siliciclastic rocks of Estonia

The Estonian Devonian sediments consist of carbonate, siliciclastic, and mixed carbonatesiliciclastic deposits that have undergone extensive dolomitization. Chemical analysis of 165 samples and XRD mineralogical analysis of 10 samples from 8 drill cores were carried out and combined with previous results, obtained particularly on the heavy and clay fraction residues of 131 samples. The carbonate residues include illite, illite-smectite, and chlorite, with accessory biotite, muscovite, pyrite, goethite, hematite, siderite, sphalerite, and magnetite. The residues of siliciclastic rocks consist of quartz, K-feldspar, illite, montmorrilonite-chlorite and chlorite, with accessory biotite, muscovite, leucoxene, ilmenite, pyrite, hematite, goethite, and siderite. The iron content of rocks is related to the detrital input during primary sedimentation or diagenetic products formed during cementation, dolomitization, and authigenic mineral growth. Iron minerals underwent chemical alteration during diagenesis and are partly corroded and dissolved. Fe(III) minerals, responsible for red coloration of rocks, became dominant due to oxidation, low water table, and arid climate, which prevailed during diagenesis. The red coloration of siliciclastic rocks is due to hematite coatings of quartz grains. It may be primary in origin, but also diagenetic due to alterations of magnetite to hematite and dispersed distribution of Fe-oxides in dolomite cement. The red coloration of carbonate rocks (dolostones and marlstones) may have an early diagenetic origin prior to dolomitization, with possible redistribution of colour at later diagenetic stages. The variable origin of iron minerals (primary, early, middle, and late diagenetic) is distinguishable by thin-section petrography, and can be also supported by chemical and mineralogical data. The average Fe2O3total/Al2O3 ratio in the studied carbonates is higher than in siliciclastics, being in accordance with the distribution of these elements in the Earth’s crust. In contrast to the Earth’s crust data, the average TiO2/Al2O3 and K2O/Al2O3 ratios are higher in Devonian siliciclastics. This could be explained by alteration of clay during late diagenesis.