| The shallow marine dolomite of Jixian System in Qishan area of Weibei Uplift is of great significance for the
deep exploration in Ordos Basin. Based on the field survey and sampling of Laochuan outcrop, by means of geochemical
analysis and testing such as carbon and oxygen isotope analysis, combined with the sedimentary evolution background and
stratigraphic distribution characteristics, the geochemical characteristics of the dolomite of Jixian System in Qishan area
are clarified, and the characteristics of the paleoenvironment and the genesis of the dolomite are deeply discussed. It is
shown that: (1) The lithology of Jixian System in Qishan area is mainly dolomite, siliceous dolomite and flint-bearing
banded dolomite, which develop stromatolites. According to their structure types, they can be divided into calcareous
dolomite, crystalline dolomite, residual structure dolomite, and dissolved dolomite. The sedimentary environment is tidal
flat of carbonate platform, and the sedimentary facies are intertidal zone and subtidal zone. (2) The mean value of δCe is
0.64, and Ce is negative anomaly, which indicates that the dolomite of Jixian System is in an oxidized environment during
the sedimentary period. Discrimination parameters of redox environment，such as V/Cr, Ni/Co, V/(V + Ni) and Ce/La,
indicate that the Jixian System is deposited in shallow water sedimentary environment with weak oxidation. (3) Paleowater-
temperature is in good agreement with values of Sr/Cu, which suggest that regional temperatures were much higher
during deposition period of Jixian System, furtherly indicating an arid and scorching climate background. (4) The contents
of Sc≤1.53 μg/g, Th≤1.47 μg/g, Hf≤0.47 μg/g, Zr≤17.6 μg/g，which are lower than those of the corresponding elements
in the upper crust (Sc=14.9 μg/g, Th=2.3 μg/g, Hf=5.8 μg/g, Zr=240.0 μg/g); the mean value of ΣREE+Y is 12.50 μg/g,
which is much smaller than that of terrigenous detritus (100 μg/g). These indicate that the influence of terrigenous detritus
is very weak. (5) The mean value of δEu is 0.75, and the negative anomaly is obvious, indicating that the dolomite in the
study area has hardly been modified by hydrothermal fluid; the average value of CaO/(CaO + MgO) is 0.58, and the
average value of Sr / Ba is 5.43, indicating that the salinity is high, and it is a marine environment. (6) The carbon and
oxygen isotope analysis shows that the dolomites of the study area are all formed in marine fluids; environment
discriminant graphs of Sr / Ba and CaO/(CaO + MgO) show that the dolomites of Jixian System are formed in marine
saltwater. (7) The genetic identification diagram of dolomite shows that the dolomite of Jixian System in Qishan area of
Ordos Basin is mainly of burial origin, and there are mixed water origin dolomite and reflux permeability origin dolomite
only in some areas. At last, the genetic model of dolomite in Jixian System in Ordos Basin is preliminarily established: the
calcareous dolomite and crystalline dolomite are mainly of burial origin, the residual structure dolomite is mainly of mixed
water origin, and the dissolution dolomite is mainly of reflux seepage origin. |
