Mineral precipitation sequence and formation of the lacustrine hydrothermal sediments in the Lower Cretaceous Tenggeer Formation in the Baiyinchagan Sag, China

Abstract

Cores from the Lower Cretaceous Tenggeer Formation in the Baiyinchagan Sag of the Erlian Basin present dolomitic mudstones and argillaceous dolostone with white grains and laminae. Thin section, X-ray diffraction (XRD), electron microprobe (EMPA), and scanning electron microscopy (SEM) analyses show that in addition to terrigenous clastics, dolomite, analcime, natrolite, pyrite, pistomesite, fluorite, talc, barite, etc. are abundant. Compared with the conventional lacustrine sediments, these rocks contain highly abundant exhalative textures and structures, such as laminae and banded textures, disseminated textures, mesh-vein structures, breccia structures, and contemporaneous deformations. These types of mineral associations, rock textures and structures are specific and uncommon. Previous works indicate that these rocks are of hydrothermal origin. In this study, we provide supplementary evidence and recognition to support the lacustrine hydrothermal sediment origin and further study the mineral precipitation sequence and formation conditions and processes. With the use of 3D seismic data, the magmatic intrusions, hydrothermal ducts, and hydrothermal vents are first identified on seismic sections. The Ni-Co-Zn and Fe-Mn-(Cu + Ni + Co) × 10 ternary diagrams confirm the hydrothermal origin of these sediments. The Sr/Ba ratio, V / (V + Ni) ratio, oxygen and carbon isotope values of carbonate, pH and EH values suggest anoxic, reducing and alkaline sedimentary conditions. Based on the analysis of fluid inclusions, rock textures and structures, we find that the precipitation of minerals occurred in a specific sequence: pyrite-pistomesite-analcime-natrolite-dolomite. The nature of the magmatic intrusions, the changes in environmental variables and fluid energy, and the flow distance may be the controlling factors of this specific sequence. A mode of lacustrine “white smoker”-type hydrothermal sedimentation is presented in this study: as the hydrothermal fluid sourced from the intermediate-basic magmatic basement enriched in Fe2+, Mg2+, Al3+, Si4+, Ca2+, Na+, and CO32– rose and mixed with cool lake water, ferromagnesian, aluminosilicate, and carbonate minerals formed successively from the ducts to the vicinity of the vents. Pulsing exhalation and overflow formed alternating deposits of hydrothermal sediments and lacustrine mudstones.