星宿海

星宿海位于黄河上游的源头地区,在青海省曲莱县东北部,东与扎陵湖相通,为黄河散流地面而成浅湖分布的沙丘、沮茹地带,是一个长二、三十公里,宽十几公里开阔的积水盆地。这就是通常所说的星宿海。黄河上源之一的马曲,东流注入于星宿海。星宿海的由来是由于这里是一片广阔的草滩和沼泽,滩上密布着大小不一、状各异的水泊,

Depositional chemistry of chert during late Paleozoic from western Guangxi and its implication for the tectonic evolution of the Youjiang Basin

Continual deep-water sediments from the late Early Devonian to the Late Permian extended in wide areas of western Guangxi. We analyzed the major, trace, and rare earth elements of the Upper Paleozoic cherts in Badu, western Guangxi. High non-terrigenous SiO2 contents （Sinon_ter/Sibulk（%）〉 80%） and pure chert components （〉 70%） indicate a large extent of silicifi- cation in the Upper Paleozoic cherts, except for the Upper Devonian-Lower Carboniferous Luzhai Formation cherts, which have lower non-terrigenous SiO2 contents （avg. 71.8%） and pure chert components （40%-70%）. The Al/（AI＋Fe＋Mn） ratios and Feter/Febulk（%） values of samples from the lowest horizon of the Pingen Formation are 0.05-0.26, 13.1%-14.5%, respec- tively, indicating hydrothermal origins. All other samples show high Al/（Al＋Fe＋Mn） ratios （0.39±0.81） and high Feter/Febulk（%） values （23.1%-186.8%）, indicating non-hydrothermal origins. The Pingen Formation and Liujiang Formation cherts show slightly-moderately negative Ce anomalies （0.71±0.07, 0.81±0.08, respectively） and higher Y/Ho ratios （33.49±1.27, 36.10±2.05, respectively） than PAAS. This suggests that these cherts were deposited in the open marine basin, rather than in the intracontinental rift basin as previously assumed. The Luzhai Formation cherts may be deposited near the seamount or sea- floor plateaus with no negative Ce anomalies （1.09±0.07） and no significant Y-Ho fractionation （Y/Ho=28.60±1.25）. The Nandan Formation and Sidazhai Formation cherts were deposited in the open-ocean basin with moderately negative Ce anom- alies （0.67±0.08, 0.73±0.11, respectively） and high Y/Ho ratios （36.01±1.00, 32.00±2.25, respectively）. On the basis of our studies about cherts, we conclude that the Youjiang Basin originated as part of the Paleo-Tethys that controlled the deposition- al environments of cherts during late Paleozoic. The rift of the Youjiang Basin had occurred at least since the Early-Middle Devonian. The basin had a trend of evolving into an open-ocean basin during the Early-Middle Permian.

Primary dolostone related to the Cretaceous lacustrine hydrothermal sedimentation in Qingxi sag, Jiuquan Basin on the northern Tibetan Plateau

Based on comprehensive studies in petrography, petrofabric analysis and geochemistry, this paper describes a unique and rare laminated micritic ferruginous primary dolostone crystallized and precipitated from the alkaline hot brine under the conditions of the Mesozoic faulted lake basin. The main rock-forming mineral of this dolostone is ferruginous dolomite with a micritic structure. This dolomite mostly exhibits laminae of 0.1-1 mm thick and is often discovered with other minerals, such as albite, analcite, barite and dickite, which have at least two types of interbedded laminae. Petrogeochemistry reveals that this dolostone contains a large number of typomorphic elements of hydrothermal sedimentation, including Sb, Ba, Sr, Mn, and V. In addition, the LREE is in relatively high concentrations and possesses the typical REE distribution pattern with negative Eu anomaly. Oxygen isotope values （C^ISOpDB） range from 5.89%~ to 14.15%o with an average of 9.69%0. The ratio of 87Sr/86Sr is between 0.711648 and 0.719546, with an average of 0.714718. These data indicate that the depositional environment is a stable, blocked, anoxic low-lying hot brine pool in the bottom of deep lake controlled by basement faults. The hydrothermal fluid is the alkaline hot brine formed by the combination of the infiltration lake water and mantle-derived magmatic water, consisting of many ions, including Ca2＋, Mg2＋ and Fe2＋. Under the driving flow power of magmatic heat, gravity and compaction, the hy- drothermal fluid overcame the overburden pressure and hydrostatic pressure of the lake water body, and boiled to explosion, and then the explosion shattered the original laminated micritic ferruginous primary dolostone near the vent and then formed a new type of dolostone called shattered ＂hydroexplosion breccias＂. In the low-lying, unperturbed hot brine pool, far from the vent, the laminated micritic ferruginous primary dolostone was quickly crystallized and chemicals precipitated from the hy- drotherm. This study of special rocks contributes to research into the causes of the formation of lacustrine carbonate rocks and dolostone. In particular, it provides new examples and research insights for future studies of the lacustrine dolomite from the similar Mesozoic and Cenozoic basins in China.

Hydrodynamic regime as a major control on localization of uranium mineralization in sedimentary basins

Uranium deposits in sedimentary basins can be formed at various depths,from near surface to the basement.While many factors may have played a role in controlling the location of mineralization,examination of various examples in the world,coupled with numerical modeling of fluid flow,indicates that the hydrodynamic regime of a basin may have exerted a major control on the localization of uranium deposits.If a basin is strongly overpressured,due to rapid sedimentation,abundance of low-permeability sediments or generation of hydrocarbons,fluid flow is dominantly upward and uranium mineralization is likely limited at shallow depths.If a basin is moderately overpressured,upward moving fluids carrying reducing agents may meet downward moving,oxidizing,uranium-bearing fluids in the middle of the basin,forming uranium deposits at moderate depths.If a basin is weakly or not overpressured,either due to slow sedimentation or dominance of high-permeability lithologies,minor topographic disturbance or density variation may drive oxidizing fluids to the bottom of the basin,leaching uranium either from the basin or the basement,forming unconformity-type uranium deposits.It is therefore important to analyze the hydrodynamic regime of a basin in order to predict the most likely type and location of uranium deposits in the basin.