| Interlayer oxidation zone is the prospecting mark of sandstone type uranium deposits, and it usually controls the distribution of uranium ore. In this paper, the paleo-interlayer oxidation zone of the Daying uranium deposit in the northern Ordos basin is the research object. Zoning and distribution of the interlayer oxidation zone have been identified and figured out. Mineralogical and geochemical characteristics of the internal different kinds of rock in paleo-interlayer oxidation zone have been made clear through the study of field outcrops, core description, polarizing microscope, scanning electron microscope and electron microprobe analysis, X-ray powder diffraction (XRD), X-ray fluorescence spectrometry, ICP-MS, and other testing methods. The mineralogical and geochemical characteristics of the initial paleo-interlayer oxidation zone are rebuilt. The mineralogical and geochemical characteristics of sandstones in different zones in paleo-interlayer oxidation zone and are systematically compared and analyzed. The variation of fluid environment and material composition in the process of diagenesis (alteration) and mineralization is discussed. Finally, water-rock interaction in the forming of the paleo-interlayer oxidation zone of the Daying uranium deposit is revealed.The main understanding and conclusions have been obtained as follows.(1) The residual red altered sandstone in the rock and outcrop is the original record of the paleo oxidation zone in the mineralization period. Signs of altered minerals of initial paleo oxidation are kaolinization, limonitization and sericitization. And HREE and LREE elements are moved out significantly. Complex evolutionary process of the initial formation of the paleo oxidation zone, the transformation of the ancient oxidized zone by the calcium cementation and secondary reduction of the paleo oxidation zone has been revealed. The red altered sandstone and the green altered sandstone are the components of the paleo interlayer oxidation zone. In the mineralization period, there was a large-scale regional initial paleo interlayer oxidation in study area, and the fine classification of the paleo-interlayer oxidation zone has been carried out.The initial paleo-oxidation zone has been not existed because of the later trans- formation. Large scale red alteration sandstone was partially cemented by calcium, which formed the residual red sandstone with partial paleo-oxidation information. Large scale red sandstone without calcareous cementation was secondly reduced to green stone. In this paper, the mineralogical and geochemical characteristics of the initial paleo-oxidation zone are rebuilt according to the mineralogical and geochemical characteristics of the residual red sandstone and the effect of calcareous cementation on the elements geochemical characteristics.The typical mineral assemblages and alteration features of the initial paleo-oxidation zone are kaolinization, limonitization and sericitization. Geochemical characteristics of elements in the initial paleo-oxidation zone are as follows. Compared to the residual red calcareous cementitious sandstone, the value of Fe2O3 and Fe2O3/ FeO are a little lower, and pH value is significantly decreased and less than 7, and AEh value is slightly higher. Except the contents of TFe2O3, CaO and MnO decrease obviously, the other major elements increase significantly. Some trace elements increase significantly, such as V, Zn, Ga, Cs, Rb, Ba and Th; and some decrease obviously, such as Co, Ni and Mo. The geochemical characteristics of the rare earth elements in the initial paleo-oxidation zone are recovered by semi quantitative analysis. The content of LREE, HREE and ∑REE decrease significantly and are lower than the residual red calcareous cementitious sandstone and the primary gray sandstone. The rare earth elements of the initial paleo-oxidation zone were taken out obviously in the process of the interlayer oxidation.In the study area, the paleo-interlayer oxidation zone experienced complex evolution process of initial paleo oxidation zones formation stage, metallogenic belt formation stage, the stage of paleo oxidation zones transformed by calcareous cementation and the secondary reduction in paleo oxidation zone.(2) Mineralogical characteristics of three types of altered sandstone and non oxidized sandstone (reduction zone) in the paleo oxidation zone were compared fully. Calcareous cement reconstruction paleo oxidation zones inherited the mineralogical characteristics of initial paleo oxidation zones. Calcitization cementation is most prominent. The secondary reduction paleo oxidation zone remained a very small amount of ferritization, sericitization and other oxidizing acid alteration minerals. And the alteration of the chloritization and montmorillonitization is remarkable. The non oxidized reduction zone, which has great difference to initial paleo oxidation zones, is characterized by the mineralization of the pyritization and the clastic uranium mineralization.The differences of the mineralogical characteristics of different zones are obvious. Limonitization, kaolinization and sericitization tend to be accumulated in the initial paleo-oxidation zone. Limonitization, calcilization, and a small amount of residual sericitization and kaolinized tend to be accumulated in the calcareous cementitious paleo-oxidation zone. Clayzation (including chloritization, montmorillonite, little sericitization), residual limonitization in red sandstone in mineralization and a little later calcitization tend to be accumulated in the secondary reduction paleo-oxidation zone. Pyritization, clastic uranium mineralization and little calcitization tend to be accumulated in the reduction zone. The dissolution of the internal clastic grain becomes weaker from the paleo-oxidation zone to the reduction zone.(3) Geochemistry characteristics of three types of altered sandstone and non oxidized sandstone (reduction zone) in the paleo oxidation zone were compared completely. The values of △ Eh, U, organic carbon and S are almost equal between the calcareous cementitious paleo-oxidation zone and initial paleo-oxidation zone. But the values of other elements, especially pH, REE and CaO, are great different. The secondary reduction paleo oxidation zone with highest value of △ Eh has the most content of Fe, Mg, Al and REE. The reduction zone, has greatIn the initial paleo-oxidation zone and the calcareous cementitious paleo-oxidation zone, the value of Fe2O3 and Fe2O3 / FeO are the biggest, and the value of △Eh and the content of U, organic carbon and S are the lowest. Besides, the value of pH is lower than 7 in the initial paleo-oxidation zone. And the content of REE is lowest. The content of CaO and HREE is the most in the calcareous cementitious paleo-oxidation zone. The content of U, organic carbon and S in the secondary reduction paleo-oxidation zone are the similar to the initial paleo-oxidation zone. But the values of FeO, MgO, Al2O3, TFe2O3, REE and △Eh are the highest. The calue of Fe2O3 / FeO is the lowest. In the reduction zone, the value of AEh and U are relative high, and the contants of K2O, Na2O and SiO2 are the highest, but the contant of TFe2O3 is the lowest.(4) The mineralization happened in the gray sandstone which located in the front line of the paleo-interlayer oxidation zone and its edge part with non oxidized in the reduction zone. Mineralogical and geochemical characteristics are similar to the non oxidized reduction zones. An important difference is that U and other valence elements are gathered here. In this zone, the paleo oxidized sandstones and non oxidized sandstones are presented as the finger like interaction. So it Is also generally referred to as the "transition zone". But mineralization is only concentrated in the non oxidized grey sandstone.It is found that uranium mineralization, pyritization, montmorillonitization, early part of sericite and late calcitization are the main mineralogical characteristics in the gray ore bearing sandstone in the transition zone. The values of U, V, Mo, organic carbon and S are the highest. The values of Fe2O3, FeO, K2O, Na2O, SiO2, Fe2O3/FeO and △Eh in the gray ore bearing sandstone are between that of initial paleo oxidation zone and reduction zone. The value of Al2O3, MgO, TFe2O3 and REE are higher than that of reduction zone.(5) The diagenetic alteration and mineralization fluid environment has obvious changes during the paleo-interlayer oxidation zone development stage. The fluid changed from weak alkaline to acid in the early mineralization stage. But it changed from acid to alkaline in the late mineralization stage and became acid after mineralization. These issues reveal the complex evolution process of water rock interaction which includes the initial oxidation zone formation stage, large scale mineralization, paleo oxidation zone transformed by calcareous cementation and transformation of the ancient oxidation zone to be reduced. Uranium mineralization happened in the alkaline fluid environment at front line of the paleo-interlayer oxidation zone and its edge part with non oxidation.① The initial paleo-oxidation zone formation stage during the mineralization period.In the mineralization period, the pH value decreased and was less than 7. The value of Fe2O3/FeO increased and the value of △Eh decreased obviously. The fluid environment changed from reductive and weak alkaline to oxidative and acidic. Large scale oxidation occurred in the sandstones, and they appeared to be red due to ferritization (Fe2+ was oxidized to Fe3+). Besides, organic matter and pyrite were oxidized and decomposed; the content of U decreased sharply due to U was oxidized and moved out. Feldspar was kaolinized, and Ca2+ and SiO44+ were released. REE was activated and moved out. The content of REE in the initial paleo-oxidation zone was the lowest. In this stage, the material composition of the paleo interlayer oxidation zone is dominated by the regional large-scale red alteration sandstone.② Large-scale mineralization stage.The values of AEh and Fe2O3/FeO in the gray ore bearing sandstone are between that of initial paleo oxidation zone and reduction zone. This shows that O2 in the fluid is constantly consumed. Oxidation ability decreased and reduction ability increased. The phenomenon that the content of montmorillonite increased obviously and quartz grains were dissolution into SiO44- showed that pH value of the fluid in this stage was greater than 7. U was transported as [UO2(CO3)3]4- and [UO2(CO3)2]2-,and RE3+ was transported as [REE(CO3)3]3- and [REE(CO3)4]5-. The content of Pyrite and organic matter were very rich at the front line of the paleo-interlayer oxidation zone and its edge part with non oxidized in the reduction zone. Uranyl ion was reduced into U4+ and CO32-by pyrite and organic matter. U4+ combined with SiO44- formed uranium ore and coffinite. CO32- combined with Ca2+ formed calcite cement. When the CO32- was separate out, REE was precipitated and the content of REE increased in transition zone. In this stage, the material composition of the paleo interlayer oxidation zone is mainly dominated by the regional large-scale red alteration sandstone and the gray ore bearing sandstone.③ The transformation stage of the paleo-oxidation zone with calcareous cementation after mineralization period.After mineralization period, large scale red alteration sandstone was partially cemented by calcium. The fluid environment was alkalinity (pH>7). Ce existed the negative anomalies and the value of Ceanom was less than -0.1. The highest content of Fe2O3 and the value of Fe2O3 / FeO showed that the paleo-oxidation information was reserved in the red calcareous cementitious sandstone. Compared to the initial paleo-oxidation zone, the content of CaO and REE in the paleo-oxidation zone with partial calcareous cementation increased obviously due to the influence of calcareous cementation.④ The formation stage of the large scale secondary reduction paleo- oxidation zone after mineralization period.After mineralization period, the fluid with a lot of oxygen and uranium was blocked by the Hetao downfaulted movement. The reducing gas from underlying strata ascended into the uranium reservoir along faults. The fluid environment was changed into reductive alkaline (pH>7). Large scale red sandstone without calcareous cementation was secondly reduced to green stone. The contents of U, S and organic carbon in the secondary reduction paleo-oxidation zone decreased. The content of FeO and the value of △Eh were the highest. As suffered the strongest transformation, it had the highest content of clay minerals mainly chlorite and montmorillonite. The contents of TFe2O3, MgO and Al2O3 were also the highest in this zone. Clay minerals had strong adsorption capacity for the rare earth elements. So the content of the rare earth elements in this zone was the highest too.The innovative achievements of this paper are as follows.(1) The original record of the paleo interlayer oxidation zone is identified. And its mineralogical and geochemical characteristics are summarized. The typical alteration mineral features are kaolinization, ferritization and sericitization; and LREE and HREE elements were moved out obviously in this period.(2) This article reveals the process of the paleo-interlayer oxidation zone forming and the two diagenetic alteration transformations. The change process of the diagenetic alteration and mineralization fluid environment is restored form the water -rock interaction perspective. |
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