Superimposed Biotic,Hydrothermal And Seawter Mineralization And Its Variations In Early Cambrian Black Shales Of South China

The early Cambrian black shales of South China and their associated mineralization are one of the research highlights for decades in the field of Earth Sciences.However large disputes still exist.Aiming at the scientific issue of main controlling factors and variations of mineralization(i.e.,multiple,superimposed and differential factors of mineralization),we investigate two typical examples in this metallogenic belt,including the Ni-Mo polymetallic deposits in Sancha,Hunan province and the barite deposits in the Tianzhu Dahebian,Guizhou province.Systematic analyses were conducted,including geology,petrology,mineralogy,element geochemistry,organic geochemistry and sulfur isotope geochemistry.Based on the analytieal results,in combination with the geological characteristics of the ore deposits,we address their origin.Furthermore,integrated with previous studies on other types of deposit,such as phosphorus and vanadium deposits,we discuss the variations of mineralization,i.e.,differential mineralization.We propose a new mineralization mechanism as it is controlled by three factors(biotic,hydrothermal and seawater)with superimposition at different stages.This not only clearly reveals the biotic impacts on mineralization with direct evidence(better the understanding of previous studies),but also combines the hydrothermal,seawater and biotic mineralization as a whole.This is different from previous studies,in which hydrothermal and seawater are disputed as two counteracting factors and biotic mineralization has received relatively little research attention.In addition,we discuss the mineralization variations in the early Cambrian black shales of South China,and suggest that the variations are caused mainly by the activating differences of the three mineralization factors under variable tectonic,lithofacies and paleogeographic background.The ore mineral compositions of the Sancha Ni-Mo polymetallic deposits are complex.The metallic minerals contain the C-Mo-S mixed-layer mineral(MoSC),millerite,pyrite,and a small amount of gersdorffite,vaesite,sphalerite and chalcopyrite.Among them,the MoSC and millerite are the predominant accumulated minerals of Mo and Ni,respectively.Meanwhile,a large amount of metallic sulfide ellipsoid has been found,which was identified as the cystocarp of red alga.This type of ellipsoid is rich in Mo outside and Ni inside,displaying a differential mineralization of organism.Analytical results of trace elements and REEs show that the Ni-Mo ores are formed mainly in an anoxic and reducing seawater environment and influenced by hydrothermal water and others factors(e.g.,organisms).Organic geochemistry indicates that the ores have relatively high organic carbon contents(>1.0%),and the organic precursors are mainly from the lower plankton,algae and bacteria.High bitumen reflectance(>2.0%)and biomarker characteristics imply that the ores were formed in an anoxic seawater environment,and were highly influenced by hot water.The ?34S value of the ores ranges in-13.7‰?+3.9‰,with an average of-4.5‰,indicating that the sulfur were formed from a mixture of seawater and hot water.Based on the above results,we suggest that the deposit belongs to a superimposed and multi-stage formation,with biotic,hydrothermal and seawater impacts.Mo was formed mainly in the early stage,sourced from seawater with biotic impacts.In contrast,Ni was formed mainly in the late stage,sourced from hydrothermal influx,and the biotic impacts are relatively weaker in general.The ore mineral compositions of the Tianzhu Dahebian barite deposits are relatively simple,clearly different from those of the Sancha Ni-Mo ores.The primary mineral is barite,while small amounts of quartz,zoned hyalophane and pyrite were observed.Among them,the zoned hyalophane,which can indicate a multi-stage hydrothermal effect,is discovered for the first time in this area,and infers that the ore formation may experience a fault-controlled,multi-stage and progressive process.Trace element geochemistry shows that the deposits were formed in an anoxic and reducing seawater environment,and the negative ?Ce anomaly(<0.6)and positive ?Eu anomaly(>3.0)indicate that the deposits were affected by hot water.Organic geochemistry reveals that the organic carbon content in wall rocks(1-8%)is relatively higher than in the ore bed(<3%).The content of chloroform bitumen varies significantly and the organic matter is over mature.These reflect that the ore formation has experienced a strong alteration(e.g.,hydrothermal).The organic precursor is mainly from the lower plankton,algae and bacteria.The organic elots(i.e.,red algae)found in the Sancha Ni-Mo ores were not observed.The ?34S values of the ore are distributed like a tower in+36.7‰?+43.8‰,with an average of +40.2‰.These values are higher than that of the early Cambrian seawater(?+30‰),indicating a strong impact of sulfate reducing bacteria.The deposits were formed in a closed to semi-closed basin.Considering that the ore formation has a close relationship with hydrothermal and biotic effects,we propose a superimposed mineralization model between organism and hydrothermal exhalative sedimentation.Through systematic comparison of the multiple deposits hosted in early Cambrian black shales of South China,including Ni-Mo,barite,phosphate and vanadium deposits,we find some commons of the ore formation.For example,all the deposits have hydrothermal and seawater supplies with different ratios and the organism plays an important role in the ore formation.However,some variations still exist.In the Ni-Mo deposits,Ni is mainly from hydrothermal water while Mo is mainly from seawater and affected by organism.In the barite deposit,organism also impacts the ore formation,but such a large amount of barium cannot be predominantly provided by organism.It sourced mainly from the basal Ba-rich strata that were leached by hydrothermal brine.In the phosphate deposit,the phosphate accumulates mainly in the form of colloidal phosphorite or collophanite,and large amounts of biological remains were found,which can reflect a biotic effect.In the vanadium deposit,a combined impact of terrestrial,seawater and hydrothermal was indicated.Based on these understandings,we therefore propose a new genesis model of superimposed mineralization with tri-factors(biotic,hydrothermal and seawater),and this model may have a general significance to the black-shale-hosted deposits in South China.All the results in this study can be referred in the global research of black-shale-hosted or even sediment-hosted deposits,and have implications for regional ore exploration.