Genesis And Growth Mechanism Of Pyrite Nano-micron Polycrystalline Aggregates In The Baiyunpu Pb-Zn Deposit

Mineral is a record of geological process,as well as important resources.The information of geological environment can be expressed by the characteristics of mineral?Pyrite is the most common sulfide mineral in the nature.Previous scholars have found that the morphology,particle size,elemental and isotopic compositions,structural parameters and physical properties of pyrite have huge significance in the research of geologic environment and ore-forming process.Colloform structures of pyrite are invariably mentioned in various geologic bodies which have received more attention in recent years.The characteristics and their evolution laws of mineral in zoned textures,which commonly develop in colloform structures,not only record instantaneous conditions during the primary ore minerals precipitation,but also provide sequential information about the developing physical and chemical conditions.This study focuses on the Baiyunpu Pb-Zn deposit,Hunan and analyses the mineralogical characteristics of three generations of pyrite(framboidal Pyl,colloform Py2,and granular Py3)from this deposit using FESEM,EPMA,?XRD,and EBSD.A detailed study has been carried out aiming at the colloform pyrite(Py2),and we found the existence of monocentric and multicentric aggregates having clear multilayer bands.And the inner core is composed of nano-submicron pyrite euhedral crystals with a small amount of fine grained galena.Pyrite grains distribute randomly and have no preferred orientation.Pyrite microcrystals in discrete layers have a larger grain size than in the core and elongate perpendicular to the layering direction,which show either<100>or<110>preferred orientation.Observation results show that both the core and layered zones of colloform pyrite in this study are consist of visibly nano-micron-sized grains.On the basis of mineralogical research,this study analysed in situ sulfur isotopic compositions of pyrite formed in three stages of the Baiyunpu deposit.Pyl has very negative?34S values(-22.7‰--14.9‰)that are interpreted in terms of bacterial reduction of marine sulfate during the sedimentation and diagenesis.Py2 formed in the early ore-stage has slightly negative ?34S values(-8.4‰--0.29‰),and it may indicate that the sulfur of Py2 is mainly derived from both thermochemical sulfate reduction and bacterial sulfate reduction.Py3 grains formed in the ore-stage are texturally and compositionally zoned due to the differences in arsenic contents.Arsenic-rich zones of Py3 have lighter ?34S values(-19.9‰--7.39‰)than arsenic-depleted zones(-11.1‰-+2.3‰),which is interpreted to result from the mixing of intermittent meteoric waters.The oxygen fugacity and arsenic contents of the mixing fluid were significantly increased,and it caused the reduce of ?34S values in arsenic-rich zones.According to the microstructures,compositions,growth orientations,and in situ sulfur isotopic results of pyrite nano-micron polycrystalline aggregates,combining with the geological background of this deposit,we proposed the aggregates are mainly formed by the rapid mixing of metal-bearing fluid and reduced sulfur,precipitation and crystallization.The sulfur source may be associated with microbial activity.The growth occurs from the core to the outside and contains:(1)formation and congregation of microcrystals in the inner core;(2)growth and geometrical selection of pyrite crystals in outer layers.According to the theories of crystal nucleation,growth and aggregation,the formation of the inner core indicates a rapid nucleation and crystallization under highly supersaturated conditions,and a agglomeration of microcrystals driven by the reduce of total surface free energy.The later pyrite crystals continue to grow with the polycrystalline aggregate formed at the early stage as a substrate,and a banding pattern with a preferred orientation will appear consisting of pyrite microcrystals running perpendicularly to the substrate surface through geometrical selection.The intermission and resumption of pyrite growth are inevitably involved in the environmental conditions,and the process is repeated,which will lead to the formation of multiple discrete layers.The formation of monocentric or multicentric aggregates mainly depends on the aggregation level of the inner cores before the start of crystal growth in outer layers.By discussing the effects of environmental factors,such as sulfur sources,trace elements,microbial activity,sulfur fugacity,temperature and degree of supersaturation,on the evolution of crystal characteristics in pyrite polycrystalline aggregates,we suggest that the changes of grain sizes,forms,and CPOs from the inner core to layered zones may be influenced by the degree of supersaturation and the sulfur fugacity comprehensively.This study reveals the agglomerate characteristics of nano-micron crystals within colloform pyrite,and gains insight into its genesis and growth mechanism based on microcrystalline mineralogical and geochemical characteristics.It deepens the relevance of single crystal growth and polycrystalline aggregation of nano-micron-sized minerals,and enriches the research contents of crystallography and mineralogy theories,which will provide an objective basis for understanding complex geological environments and processes.