Petrologic attributes and genesis of the hydrothermal-sedimentary dolostone in Baiyinchagan Sag, Erlian Basin, China

The hydrothermal-sedimentary dolostone has been discovered in the deep-lacustrine mudstone of the Lower Cretaceous Tenggeer Formation in the Baiyinchagan Sag, where is in the western Erlian Basin of northern China. 46 thin... [ view full abstract ]

The hydrothermal-sedimentary dolostone has been discovered in the deep-lacustrine mudstone of the Lower Cretaceous Tenggeer Formation in the Baiyinchagan Sag, where is in the western Erlian Basin of northern China. 46 thin sections and 20 polished blocks were made for petrographic study of dolostone, which was carried out using both optical and scanning electron microscopy, the latter in backscatter electron mode, electron probe microanalysis of minerals and QEMSCAN. Petrological characteristics show that the hydrothermal-sedimentary dolostone is mainly composed of dolomite, ankerite (Fe 0.5%-5%), natrolite and albite. In addition, minor minerals are pyrite, apatite, siderite, k-feldspar, illite, barite, sjogrenite and so on. Meanwhile, three principal types of dolomite are recognized: (1) Dolomite with crystal size ranges from 5μm to 10μm (D1), coeval with albite, K-feldspar and illite as matrix of hydrothermal sedimentary rock. It is difficult to distinguish crystal boundary shape due to micrite-size crystal. D1 dolomite is volumetrically more abundant (>60%) than other types and reflects lamellar structure. (2) Dolomite (D2) replaces natrolite grains from margins to cores of grains, with a relatively high Fe content (up to 5%). D2 dolomite forms fine planar-e to planar-s crystals (10-50μm). Then euhedral pyrite replaces assemblage of ankerite and natrolite. (3) The third type of dolomite is vug- or fracture-filling saddle dolomite (D3) and it postdates matrix dolomite (D1, D2). D3 forms coarse planar-e to non-planar crystals with curved faces and ranges in size from 300μm to 600μm. Volumetrically, this type of dolomite is less abundant (<1%) than the other types of dolomite. This sedimentation with high content of U (40~60ppm) and Th (60~100ppm) is distinguished from other dolostones. And the composition of trace element such as Cu、Ni、Co、Fe、Mn is consistent with that of modern submarine hot water deposition in the Red Sea. It is suggestion that D1 is primary dolomite directly precipitating from brine. The microbe was likely to play a formative role in the precipitation of D1.And D2(iron-rich) and D3 saddle dolomite were precipitated by the hydrothermal exhalative processes of the sublacustrine hot springs．