| Abstrakt: |
The Lower Cretaceous Tenggeer Formation in Baiyinchagan Sag is a lacustrine argillaceous deposit. Thin section, X-ray diffraction (XRD), electron microprobe (EMPA), and scanning electron microscopy (SEM) analyses show that large amounts of ferromagnesian carbonates occur extensively. The ferromagnesian carbonate has similar content of FeO and MgO, and it is defined as pistomesite. Associated minerals, e.g., dolomite, analcime, natrolite, barite, and talc are recognized. The crystals of pistomesite are mostly euhedral hexagonal in laminae, bands, grains, and fractures. Few rhombohedral crystals appeared at the rim of the grains. This specific occurrence of pistomesite and mineral associations is uncommon and rare in lacustrine deposits. EMPA and EDS analysis results show that within a single crystal, the Mg/Fe ratio decreases from 1.37 to 0.73 from the core to the edge, which indicates that the formation temperature of the crystal core is the highest and that it gradually decreases to the rim. The crystallization pattern requires a high temperature and sufficient growth space. The pistomesite in laminae, bands, grains, and fractures has similar values of δ18OVPDB ranging from − 16.3 to − 12.9‰ and δ13CVPDB ranging from 2.6 to 5.4‰. Several samples from grains are isotopically distinct, withδ18OVPDB ranging from − 6.9 to − 5.1‰ and δ13CVPDB ranging from − 1.3 to 1.2‰. The measurement data of fluid inclusions indicate that the pistomesite is formed at high temperature, with homogenization temperature from 241 to 295 °C and salinities from 16.24 to 18.72 eq. Wt % NaCl. Some fluid inclusions in grains tend to have distinctly low values, with homogenization temperature from 128 to 156 °C and salinities from 7.42 to 16.39 eq. Wt % NaCl. These results suggest that the pistomesite deposits of the Tenggeer Formation might have a hydrothermal precipitation origin. It is considered that the hydrothermal fluids might be from the underlying intermediate–basic magmatic intrusions, leaching metals from these rocks and forming the ferromagnesian carbonate deposits. But in post-diagenetic process, some grains that disseminated in the lacustrine mudstone are metasomatized. The pistomesite recrystallized and replaced the host minerals via porewater solutions. [ABSTRACT FROM AUTHOR] |
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