| There are abundant bauxite resources in Wuchuan-Zheng’an-Daozhen Area, North Guizhou (Wuzhengdao for short). Two competing views on the forming environments of bauxites in Wuzhengdao are debatable in the long term, including "marine facies" and "continental facies ". The studies on redox conditions, pH etc. in the forming environment of bauxite during penecontemporaneous period are still far from certain. Paleogeographic environment and characters of penecontemporaneous metallogenic environment have been analysed here, based on sedimentary, mineralogy and geochemistry study, which illustrates the relationship between paleogeography and bauxite, effects of penecontemporaneous metallogenic environment on high grade bauxite and the beneficial subenvironment of bauxite formation.Wuzhengdao bauxite formed in a elliptic basin, extends to Wulong on the north, Nanchuan on the west, Yanhe on the east and Suiyang-Fenggang on the south, with the south side higher than of the north. The ore-bearing horizon overlies limestone beds of late Carboniferous Huanglong Formation or mud shales of early-middle Silurian Hanjiadian Formation, and underlies the shales of early Permian Liangshan Formation or limestone of Qixia Formation, the contact relationship is parallel unconformity. Source tracing of trace elements is simple and reliable. Cr-Ni, Zr, Hf, Nb, Ta have been used to analyse the sources of bauxite. Cr-Ni value has a wide distribution in Wuzhengdao bauxite, indicating parent rocks of aluminosilicate rocks and carbonatites and small part of basalt parent rocks. High Field-Strength Elements (HFSE) Zr-Hf and Nb-Ta chart indicate mud shale of Hanjiadian formation has a closer relationship with bauxite. Source tracing of trace elements indicates mud shale of Hanjiadian Formation and limestone of Huanglong Formation are important material sources of bauxite, and mud shale of Hanjiadian Formation even contribute more to bauxite. Besides, there is another small part of basalt provides metallogenic material to bauxite. Metallogenic materials transported form areas around the basin were deposited in the centre of the basin, forming bedded ore-bodies, which are typical sedimentary bauxite. Bauxite is mainly composed of diaspore and clay minerals like kaolinite, chlorite and illite etc., and contains a bit of heavy minerals, such as anatase, zircon and rutile. and minute quantity of quartz, feldspar and pyrite. Striped and elliptic spherical parasites (Ce2Ca(CO3)3F2) have been recognized in ZK14904, ZK3402and ZK9A04. Binary structure of bauxite is clear, generally, diaspore takes the content of40%-95%in the middle-upper part, the rest are mainly clay minerals like kaolinite, chlorite and illite etc. Bauxites achieving the industrial grade mainly distributes in this part. The lower part is dominated by clay minerals, diaspore takes the content of5%-40%, while clay minerals can reach up to95%. Constant compositions of bauxite are A12O3(26.13-75.16%), SiO2(4.52-44.46%), Fe2O3(0.77-27.67%)and TiO2(1.05-5.24%), these elements have wide distributions. Basic element content is low and rather changeable (0.01-4.77%). A12O3has negative correlations with SiO2and Fe2O3, while displays positive correlation with TiO2, indicating bauxite metallogenic process is a process enriching Al, Ti and losing Si, Fe. Trace elements of Zr, Cr, V and Li are enrichment in bauxite. In parallel, Zr, Hf, Nb, Ta and Cr with similar changing rules display an increasing trend in content from north to south, while in vertical, the content shows an opposite trend from the top to the bottom of the section. Contents of Zr, Cr and Nb are relatively high, Zr is up to1835ppm, Cr, V, Li are more than150m. Hf and Ta are relatively low. Ni is special with different changing rules from other elements both in parallel and vertical. The contents of Ni in the samples are significantly different. Ranges of LREE, HREE, ΣREE in bauxite are18.49×10-6-993.6×10-6,12.7×10-6-47.3×10-6,38×10-6-1040.9×10-6, respectively, average values are198.82×10-6,26.3×10-6,225.1×10-6. LREE, HREE, ΣREE show an increasing trends from top to the bottom of the drill section. LREE displays large variation, while HREE has the opposite trend positive anomaly of Ce in the drill is obvious.The iron weathering crust, less than1m has been discovered at the bottom of ore-bearing rock series of bauxites. The ore-bearing rock series is composed of clay rocks and bauxites. Clay rocks exhibit homophaneous structure. Bedding and fossils are absent in the ore-bearing rock series of bauxites. Black carbonaceous interbeds and inferior coal interlayer appear in some drills. All these characters of bauxite indicate the forming environment should be low energy lacustrine, bay or coastal lagoon.Sedimentary environment can be accurately determined by paleosalinity analysis. Terrestrial lake is fresh water environment, bay belongs to salt water environment, while coastal lagoon is fresh water-salt environment. Trace elements of B, Sr, Ba and REE are good indicators for certain facies. Systematic sampling for trace elements and REE analysis by ZK202, ZK5604, ZK3228, ZK3402, ZK.14904covering the whole research area. A large number of statistics indicate B has significant meaning in facies, B is less than60ppm in terrestrial fresh water, while in transient phase it is60-100ppm. It is more than100ppm in salt water. B in Wuzhengdao bauxites ranges between45ppm-148ppm. More than60%of the sample indicates sea-land transition environment. However, eluviation is the necessary condition for bauxite formation, in this condition B may enrich downward, causing some bias in judgement, so it is necessary to check the content of B first. Kaolinite, the main composition of bauxite, is also the key component of laterite, chlorite is mainly formed during sedimentation or diagenesis, hence the bottom of the section enriched kaolinite (content of diaspore less than10%), far away from the high grade bauxite, is the level experienced weak eluviation, which can indicate the original sedimentary environment. While in samples from the middle-lower part of the section, though contains high diaspore (>10%), the content of B cannot directly indicate the sedimentary environment. If the ore-bearing rock series experienced serious eluviation, B in the middle-upper part will be massive lose, B in the bottom will be largely increased, the vertical curve of B must be changeable, on the contrary, the vertical curve of B in the ore-bearing rock series experienced weak eluviation should be straightness. The formation of high grade bauxite should be experienced serious eluviation, so the drills with high content of industrial bauxite can be tested to vertically contrast the content of B. If the upper high grade bauxite contains high B, it means there is still high content of B after the eluviation, the environment should be marine facies. If the content of B is low, then other indicators should be considered. In conclusion, characters of industrial banxite (high grade bauxite) in thickness, ore-bearing rock series and variation of the curve of B are significant reflections of the degree of eluviation. It is the key point to choose the drill experienced weak eluviation to decide the sedimentary environment by B. It is suggested from the data that ZK202and ZK5604experienced relatively weak eluviation. The content of B in drill2indicates a marine-continental transitional environment. While the ZK14904and ZK3402experienced serious eluviation result in great loss of B, but the contents of B in the middle-upper part of the two drills are still very high, which indicates a transitional-marine environment. As a whole, the content of B in bauxite indicates the bauxite deposited in transitional facies. Sr/Ba is a common method to recovery the paleosalinity, Sr/Ba>1in marine environment, Sr/Ba<1in terrestrial environment and0.6<Sr/Ba<1in transitional facies. It is generally recognized that Sr/Ba in mudstone is the best indicator to reflect the paleosalinity of sedimentary medium. The characters of massive bauxite are similar to that of shale in Wuzhengdao bauxite, kaolinite or chlorite exceed50%in some samples. From the perspective of petrology they are kaolinite clay rock and chlorite clay rock, so the paleosalinity of sedimentary medium can be recognized through analyzing the massive bauxite samples.1<Sr/Ba<2in massive samples of Wuzhengdao bauxite indicates the sedimentary environment was affected by marine. V/Zr and LREE/HREE in Wuzhengdao bauxite also reflect the same condition.It is suggested that Wuzhengdao bauxite formed in a special dynamic environment, from evidences of the mineral assemblages, outcrop charaters, geochemistry of trace elements and REE of bauxite, subjective environment is partly closed bay. Although the sea level is fluctuated, the sedimentary environment switched between partly closed bay and terrestrial lake. When the sea level was high, metallogenic province connected with Yangtze sea, it appears to be salt-brackish water sedimentation. When the sea level was low, metallogenic province out of touch with sea, it appears to be terrestrial salt-brackish water sedimentation. The input of atmospheric precipitation and land fresh water, salt water was diluted constantly. After a period of desalination, some areas changed into fresh water environment. The degree and scale of desalination are determined by length of time of desalination. After lacustrine salt or brackish water totally desalted, sedimentary environment became a brackish water or salt water environment again over the next transgression. After the sea level reduced, lacus or partly closed bay experienced desalination again. In this cycle, metallogenic environment switched in terrestrial lacus, partly closed bay and bay, paleosalinity was in the transition between fresh-brackish water-salt water, so that the sedimentation of bauxite have both continental and transitional characters.Wuzhengdao bauxite is divided into4types, including massive, earthy, debris and ooid, different type of bauxite has different environmental significance. Massive bauxite with characters of muddy texture, hard and smooth and small porosity, indicates it didn’t experience serious reform. Ore grade of massive bauxite is low, in most cases, the content of clay minerals exceeds50%, from the perspective of petrology it can be treated as clay rock, representing low energy sedimentary environment. Clastic bauxite is dominated by grained clastic and sand cutting, reflecting it has experienced water transportation with certain intensity, which indicates the relatively high energy position of sedimentary environment. Ooids in Oolitic bauxite are mostly coarse fragment-core. Coexistence of broken ooid, complete ooid and new ooid with broken ooid-core is very common, which indicates the formation of ooids also experienced certain intensity of water power. Besides bauxitic ooids, hematite ooids have been discovered in some drills. Hematite ooids reflect nearshore oxidation environment. Earthy bauxite is the final state of evolution, massive appearance of half of soil bauxite indicates sediments have experienced serious eluviation after exposure. Different nature type of bauxite reflects different sedimentary environment, the division of sedimentary facies of bauxite according to the spatial distribution pattern and combination feature of different types of bauxite. Clastic and oolitic bauxites are common at Xinmo and Zhangjiayuan synclines on the south, clastic bauxite also appears in the east limb of Liyuan syncline. Earthy bauxite is mainly distributed in Liyuan syncline. Anchang syncline is a transitional belt of all kinds of ores. Zhangjiayuan syncline and Xinmo syncline yield oolitic hematite and maroon clay rock. It is suggested that Anchang-Zhangjiayuan-Xinmo-Liyuan syncline is located as nearshore environment, southwest-southeast of it close to land is relatively higher, water power is relatively strong. It is far away from continent in northwest direction. The convergence zone of Datang syncline and Daozhen syncline is the center of Huishui basin, so that the sedimentary palaeogeographic environment of bauxite is divided into three facies belts:nearshore plain, coastal wetland and partly closed bay. Coastal wetland is high and easier to exposure to accept leaching, it is the favorable zone of bauxite mineralization, ore quality is better. Partly closed bay is relatively low, it is close to sea and difficult to exposure, ore quality is worse. The ore-bearing rock series thickness is controlled by sedimentary paleogeography of bauxite, and appears positive correlation with the thickness of industrial bauxite beds. The formation of high quality bauxite is controlled by many factors, although sedimentary paleogeography controls the thickness of ore-bearing rock series, do not directly control the formation of high quality of bauxite. Only under the condition of other conditions are suitable, sedimentary paleogeography plays the decision significance on high quality bauxite formation.Grade of bauxite is closely related with texture and color. Quality of ores in decreasing order, they are semi-earthy bauxite, clastic bauxite, oolitic bauxite and massive bauxite. Lighten of the color and loosen of the structure, the ore quality is better. Color of bauxite is mainly reduced color of light grey-grayish green, which indicates bauxite formed in reducing environment, but periodic changes of color in inner ore-bearing rock series from light to deep is the reflection of eluviation. Bauxite of penecontemporaneous period exposed to the surface many times and experienced leaching. Pore water with oxygen enrichment, organic matter from plant debris and pyrite react to organic acid and sulfuric acid, metallogenic environment became acidic. Si, Fe dissolved in the acidic solution and migrated to the bottom of ore series. Al is relative enrichment in the middle-upper part of ore series to form high grade bauxite. The middle-upper part of the section belongs to oxidation environment in penecontemporaneous period, oxygen content gradually reduced, to the bottom of the reductive environment. Alkali elements in bauxite greatly lost indicates a significant acidic environment during the mineralization process. Because of the formation of sulfuric acid and organic acid in penecontemporaneous period, it is acidic environment in the middle-upper part of metallogenic environment. Acidity gradually reduced to the bottom of the section, it is alkaline to the bottom by the influence of transgression. According to the vertical changing rules of color, texture and mineral assemblages, metallogenic environment can be vertically divided into vadose zone and phreatic zone. Vadose zone is oxidation and acidic environment. Desilication and eisenaustrag of penecontemporaneous period mainly occur in the vadose zone. Phreatic zone is not favorable to mineralization. The lower the phreatic surface is more conducive to mineralization. |
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