Palaeo-sedimentary Environment And Enrichment Mechanism Of Iodine For Late Sinian Phosphorite From Weng'an-Fuquan Of Guizhou,China

Doushantuoian was the first massive phosphorus forming peak period within China's Yangtze platform,and the sedimentary phosphorite distributes widely in Hubei,Guizhou,Hunan,Jiangxi,Zhejiang,Sichuan and Shaanxi Provinces.World's phosphorite which had been formed during the period equivalent to the Doushantuo Formation was generally situated above a tillite layer.Phosphorite deposited after the glacial stage is well acknowledged as a common feature of this period,the phosphorite sedimentation can thus be related to the environment change and reconstruction of paleo-ocean and paleo-ecology after the “Snowball Earth” event.In the initial stage of Doushantuoian seawater transgression,the sedimentary foundation was uneven,and the undersea highland or paleo-uplift became phosphorite-enriched areas.Study on lithofacies palaeogeography reveals that the secondary sag and the uplifted epicontinental marginal sea were the favorable phosphorus forming sites.Previous research has obtained many achievements on elements geochemistry,sedimentary characteristics for Weng'an-Fuquan Late Sinian phosphorite since the discovery of Wengfu phosphate deposit,but some deficiencies and gaps still exist,such as,study of sedimentary environment,panlinspastic reconstruction,phosphate mineralization model,and enrichment mechanisms for iodine concentration in phosphorite,etc.After Paleaozoic,Guizhou suffered from a number of strong crustal deformation,which was a serious impediment to the Late the two-stage phosphorus geological period in Sinian Doushantuoian,which specifically led to the formation of the under phosphate ore layer(referred as ore bed a)and the upper phosphate ore layer(referred as ore bed b).Therefore,the reconstruct and remodeling into phosphorus period sedimentary environment,the restoration of the palaeogeography of the pattern,the relation of bio-mineralization,enrichment distribution of the iodine and sedimentary environment are of great significance.This research was financially supported jointly by the Project of Public Welfare and Infrastructure form Guizhou Provincial Department of Land and Resources,China(no.2012294),and Disciplinary Construction Funds for Guizhou University,China.Based on sorting,analysis and previous studies,this research analyzes and investigates phosphorus containing rock series in the Weng'an-Fuquan area.A large number of regional geological drilling data section measurement,rock-mineral determination,geochemical test and analysis of phosphorus phosphate rocks were conducted for the purpose of studying the Late Sinian paleo-geographic characteristics,phosphorus environment and lithofacies period into the ore controlling factors,material origin and the mineralization model system.On the basis of above,the iodine enrichment distribution and controlling factors and enrichment mechanism have been investigated and the results are as follows:(a)Through the comparison between several sections in the study area,the ledge in study area has been divided into two phases(sedimentary cycle): the first cycle is the shore terrigenous sand-mud steep hill sedimentation stage,which then gradually transited to a steep hill formation of early carbonate sedimentary phosphorus(phosphate layer that is formed locally,that is ore bed a);the second depositional cycle is the late steep hill phase from erosion on the section deposited to the main industrial phosphate layer(ore bed b)of the deposition,until the phosphorous depletion and a large number of Dolomite Precipitation so far.The two cycles reflect the variation of water from shallow to deep and then shallow,the energy from low to high and to low.The sedimentary environment can be characterized by low energy,turbid blocking and reducing conditions,namely a transition to the open water and high energy environment which forms a transgressive depositional sequence in the sedimentary area.(b)As shown by rock-mineral determination,electron microscope and SEM/EDX analysis,the main mineral composition is francolite,which consists of amorphous collophaneite,cryptocrystalline apatite,microcrystalline apatite and apatite four needle columnar crystal types,mainly in the amorphous collophaneite,and contains algae debris a large number of relics and biological characteristics.The depositional feature,organic carbon and Mo,V,Ni,Cu and rare earth element analysis indicates that ore bed a was formed under a reducing condition during the early Doushantuoian phosphorite forming age,while the condition for ore bed b is relatively oxide.Sedimentary environment is the main controlling factor in the formation of different types of phosphorite.In addition,trace element analysis shows that hydrothermal deposition characteristics of phosphorites may be attributed to the deep water mass(raised by the ocean current)inheriting hydrothermal features of the bottom of the seawater.(c)According to the data from regional geologic sections and exploratory boreholes,sedimentary characteristics of phosphorus-bearing system,geochemical analysis of samples,regional sedimentary paleogeography maps of Nantuo stage,Early and Late Duoshantuo stages were presented respectively.Further,various sedimentary facies and microfacies were described precisely in detail.Through the relationship between the paleogeographic frameworks and phosphorite distribution,it can be proposed that archipelago gulf paleogeographic framework is the main controlling factor for the grand phosphorite deposits in Wengfu area,gulf morphology and size control of phosphorite distribution and reserve scale.(d)The distribution of spatial mineralization is controlled by the phosphorus-bearing series,the paleo tectonic conditions and the paleogeographic patterns.Complete sedimentary sequence deposited in the edge of shallow-water,platform marginal in Central Guizhou Uplift,besides,incomplete sedimentary sequence deposited in the central platform near the edge of uplift,and the thickness of phosphorus-bearing series and different sedimentary sequences directly restrict the spatial distribution of ore deposits.The archipelago and gulf pattern in the Nantuo stage has evolved into a large-scale and embayed coast in the Duoshantuo stage;specifically the western part was a paleo uplift area,but the eastern part was the paleo landform feature of the paleo sag,and the phosphorus bearing rocks are distributed in the north and south direction,and the overall outline of the eastern and western changes.By the division of the secondary paleo uplift(e.g.Xinqiao Peninsula),the forming environment of the northern Gulf shoal and the vast beach in the south was controlled by the formation of the phosphorite and the spatial distribution.The aforementioned study of lithofacies palaeogeography shows that the beach,with strong hydrodynamic conditions conducive to phosphorous winnowing purification,and the barrier function of the Gulf and the peninsula is advantageous to growth and preservation of alga,which is beneficial to the accumulation of phosphorus.(e)Phosphorite-forming mechanism and mineralization model on the basis of comprehensive analysis of mineralization space,as well as the mineralization epoch and mineralization series of phosphorite deposits in the Wengfu area were established.The eastern Cathaysian plate keeps diving to the Yangtze plate,a large number of multi-period submarine volcano eruption brought massive phosphorous and led to accumulation of phosphorous at the deep sea bottom,long-term accumulation of reserves and high phosphorus in seawater containing driving rising currents,from North East to South West transgressive,which diffuses on the whole carbonate platform,in subtidal and intertidal environment.Through chemical precipitation and concentration,algae absorb and transfers coagulation in the sea is considered as the first completing phosphorus enrichment.The late steep hill in a shallow water environment was dominated by wave mechanical jigging sedimentary transformation under the action of the formation of phosphate granules and mechanical enrichment.At last,according to the Wengzhaoba,Dawan-Qianchang,Tuanpo and Yingping under the gulf environment formed in two phases grand phosphorite deposits during the early and Later Doushantuoian(f)Relationship between lithology,ore bed structure,maceral,TOC content,alga content etc.and iodine content shows that iodine concentration in phosphorite is related to the biological effect,however,in the neritic platform facies,the Yingping brown alga shoal is the iodine-enriched facies belts of the phosphorite,and iodine-enriched phosphorite distributed in this belt in Wengfu area.Under the shallow water environment,calm,phosphorus rich seawater supply abundant nutrients for algae species.Biological brown algae blooms,and iodine gets into the algal cells by biological adsorption and active transportation,completing biological preliminary step.When iodine rich brown alga dies,it is broken by the tidal process,and large algal subsides by phosphatization precipitation,and gradually gathers in diagenetic mineralization.Phosphorus is not directly involved in the preliminary iodine enrichment process,but in the process of iodine rich kelp debris phosphate fixation,preserved in phosphate rock in the formation of algae containing iodine rich phosphorite bond.Moreover,the isoline map of iodine content in Wengfu area was complied.