The Formation Process And Evolution Law Of Pyrite In Uranium Reservoir In Zhiluo Formation,Northern Ordos Basin

The sulfide mineral pyrite,one of the most common reducing media in sandstone-hosted uranium deposits,can effectively record the geochemical processes of uranium mineralization.Therefore,detailed analyses of the mineralogical and geochemical characteristics of pyrite play an essential role in elucidating the mechanism of uranium mineralization.At present,however,there is a lack of systematic investigation and research on the complex characteristics of pyrite in sandstones,and the different geologic contexts from different regions have led to the diversified genetic mechanisms.Pyrite is closely related with uranium minerals and widely distributed in sandstone-hosted uranium deposits occurring in the Zhiluo Formation in northern Ordos Basin.Therefore,these deposits provide favorable conditions for the study of its multi-stage formation processes and evolution laws.By drilled core and outcrop observations,and several analyses and determinations such as optical microscope(OM),scanning electron microscope(SEM),energy disperse spectroscopy(EDS),electron probe X-ray microanalysis(EPMA),laser ablation multi-collector inductively coupled plasma mass spectrometer(LA-MC-ICPMS),and secondary ion mass spectrometer(SIMS),a variety of characteristics of pyrite,such as the spatial distribution,macroscopic and microscopic morphologies,occurrence state and morphological and compositional evolution in uranium reservoir sandbodies have been clarified,the relationship between pyrite and uranium mineralization has been evaluated,and the genetic mechanism of pyrite has been revealed via sulfur isotopic compositions,finally the formation and evolution model of pyrite has been established under the multi-scale restricting factors.Macroscopically,the types of pyrite are divided into nodular,disseminated and banded,with nodules being the most abundant.Pyrite mainly distributed in gray sandstones is closely associated with organic reducing media(coal seam,carbonaceous debris and mud gravel),and the diameter and quantity of pyrite are negatively related to the distance from the media.Moreover,the stronger the reducibility is,the greater the abundance of pyrite will be.Microscopically,the morphologies of pyrite can be categorized as framboidal,euhedral and cement.Among them,framboids are the most widely distributed types and pyrite cement has the highest abundance.All micromorphological pyrite is closely associated with uranium minerals.The positive correlation between the diameter of ore-stage framboidal pyrite and the cross-sectional area of uranium minerals suggests that pyrite has adsorption and reduction effects on dissolved uranium with oxidation residue on the rim of pyrite,and the pyrite can be replaced by uranium minerals.The crystallization of pyrite is strongly dependent on carbonaceous debris,clay minerals,biotite,pre-existing pyrite and iron-titanium oxides.Those components either act as favorable physical or geochemical sites,or can supply the required material source.The distribution pattern of pyrite can be classified as periphery,infilling and the combined form.Taking the metastable framboidal pyrite as a carrier,the framboids can evolve into polyframboidal,euhedral and cement pyrite,respectively,which increases the stability of the structure.Pyrite has the highest content in mineralized,gray sandstones and the lowest content in red sandstones,while the contents in barren,gray sandstones are greater than those in yellow sandstones,indicating that the transformation of pyrite has been promoted by chemical oxidation,preferentially producing goethite as pseudocrystals,and then converting into hematite by dehydration.Goethite and hematite are the main color-causing minerals in yellow and red sandstones,respectively.The?³⁴S values of pre-ore framboidal pyrite associated with carbonaceous debris range from-19.1‰to-8.0‰,indicating the biogenic mechanism,while the values of euhedral pyrite and pyrite cement coexisting with other types of carriers range from+7.9‰to+20.3‰,suggesting the thermogenic sulfate reduction processes.Undoubtedly,the dissolution of pre-ore pyrite provides the main material sources for the precipitation of ore-stage pyrite.The ore-stage framboidal pyrite produced?³⁴S values from-31.2‰to-3.8‰,indicating the bacterial sulfate reduction processes.The?³⁴S values of ore-stage euhedral pyrite and pyrite cement range from-56.9‰to-34.3‰,and combining micromorphological evolution processes and sulfur isotopic fractionation mechanisms,it is believed that they were produced via Ostwald ripening from framboids oxidized to metastable sulfur species,producing ³⁴S-enriched sulfate and ³⁴S-depleted H₂S/HS^-through disproportionation,and this process can be controlled by the changes in Eh/p H conditions reflected by paragenetic sequence of sulfur-and selenium-bearing minerals.Twenty-three key elements affecting the sources of sulfur and iron,the storage spaces or the microenvironments,etc have been proposed from five level scales,namely kilometer-sized basin and orogens,meter-sized boreholes,millimeter-sized specimen,micrometer-sized slices and nanometer-sized crystals.On this basis,with regional redox events in uranium reservoirs as marker,the multi-stage formation and evolution model of pyrite,namely,“precipitation,dissolution,reprecipitation,preservation and redissolution”was established.The complex processes of mobilization,migration and fixation of uranium can be indicated by multi-stage developmental rules of pyrite,which also have certain guiding significance for the exploration of uranium resources.