The Genesis Of Exhalative Rocks Of The Lower Permian Fengcheng Formation In Urho Area,Northwestern Junggar Basin

The exhalative rocks, also named the hydrothermal sedimentary rocks, attract increasing attention in the geological field. Current researches are mainly focused on marine exhalative rocks. Less continental exhalative rocks are reported, particularly there are the least reported lacustrine exhalative rocks. The hydrocarbon exploration and development has gained exciting achievements in lacustrine “white smoke type” exhalative rocks recently, and it lack of the knowledge about lacustrine “white smoke type” exhalative rocks and their geological significance. Based on the petrology, mineralogy and geochemistry, the genesis of lacustrine “white smoke type” exhalative rocks of Lower Permian Fengcheng Formation in Urho area, Junggar Basin were discussed. The main results were as follows:(1) Exhalative rocks of Fengcheng Formation were formed in the semi-deep to deep lacustrine environment at the late stage of plate subduction. The fault related to magmatic and hydrothermal activities were well developed. The paleogeothermal gradient was obviously higher. All the factors above laid a good foundation for hydrothermal activities.(2) Abnormal sedimentary textures and structures were identified in Fengcheng Formation, such as lamellars or bands composed of tuff, dolomites, cryptocrystalline quartz, and complex saline minerals, contemporaneous deformation structures, contemporaneous step faults, convolute beddings, veined-networks, eruptive breccias and intraclastic textures. These structures or textures and their combination were significant evidences of hydrothermal genesis.(3) The saline minerals such as dolomite, ferrodolomite, shortite, eitelite, halite, reedmergnerite and anhydrite were recognized in lacustrine “white smoke type” exhalative rocks in Fengcheng Formation. Microlitic dolomites reflected rapid chemical precipitation. We found a negative correlation between the order degree and the content of dolomites, differing from the secondary origin such as burial dolomitization. The order degree of dolomites showed different crystallization velocity. The distribution of high temperature minerals such as reedmergnerites and shortites was controlled by fault and volcanic edifices obviously. All the features above indicated hydrothermal genesis and the main passage was faults.(4) The content of microcrystalline dolomites in reedmergnerite and cryptocrystalline quartz bands increased or decreased from one side to the other, and big dolomite crystals had always ferrodolomite girdle. Then it was considered that early microcrystalline dolomites were rolled up and parceled by the hydrothermal fluid which was rich of iron. It was consistent with the result of EPMA in eitelites and magnesites, and the results of the high content of pyrite in saline rocks. Therefore, the hydrothermal fluid of saline minerals had higher iron content than that of dolomites.(5) The exhalative rocks of Fengcheng Formation were rich of hydrothermal trace elements such as Li, Mo, W and U. REE were similar to the andesites in same horizon and peripheral Late Carboniferous volcanic rocks, reflecting the magma chamber in the deep crust was not only the driving force of hydrothermal activity but also participated in the hydrothermal activity. Meanwhile the hydrothermal fluid was also affected by the continental crust. Different rock types always had different accumulation regulations of trace elements. Combining with the diversity of REE pattern, it was believed that the hydrothermal fluids were multi-sourced. The regular lamellars and the REE geochemistry indicated the alternation between hydrothermal flow and deposition. Significantly, the extremely weak Ce and moderate negative Eu anomaly of dolomitic exhalative rocks were apparently different from marine exhalative rocks, but were similar to the reported lacustrine “white smoke type” exhalative rocks. Combining with the fluid inclusions in saline minerals, it was considered that the hydrothermal fluid had high density and salinity, and the hydrothermal deposition occurred at the bottom of deep lake brine with alkaline and reducting, mid-low temperature and high pressure.(6) The results of carbon and oxygen isotopes showed that the sedimentary environment was relatively closed with high temperature and salinity. The hydrothermal fluid before eruption to the surface was influenced by heavy carbon of surrounding rocks. Strontium isotopes were slightly lower than the Permian global carbonates, and this feature was much more apparent in dolomites, which indicated that the hydrothermal fluid was mixed with mantle substance. Boron isotopes from dolomites and reedmergnerites were obviously different, which indicated the former was influenced by mantle fluid and the latter by altered oceanic crust. These are all significant evidences of deep fluids.(7) The hydrothermal fluid of Fengcheng Formation was multi-source brine from the deep crust. The deep circulation, migration, surface eruption and deposition were drived by high pressure, magma heat, gravity and compaction in the subduction zone. The high pressure and magma heat were main driving forces for fluid transportation.(8) The lacustrine “white smoke type” exhalative rocks of Fengcheng Formation had special materials and differed from the reported evaporate deposition and other lacustrine “white smoke type” exhalative rocks home and aboard.