A Review of Research Progress in the Genesis of Colloform Pyrite and Its Environmental Indications

Colloform pyrite is a special form of nano-micro polycrystalline aggregation growth, for which a suitable term is ＂aggregates of nano-micro crystals＂. This kind of colloform texture is observed in various geological bodies, such as ancient sedimentary rocks, modern marine and lake sediments, various types of ore deposits, and modern seafloor hydrothermal vents. This paper summarizes the latest developments and research into the definition, formation mechanisms, and environmental indications of colloform pyrite. There appears to be three main formation mechanisms of colloform pyrite： pseudomorphic replacement; biogenic precipitation; and inorganic precipitation. The morphology, particle size, trace element content and preferential growth orientations of coUoform pyrite microcrystals can be important indicators for sedimentary environments, hydrothermal activity, and ore-forming processes. We suggest that the microscopic features of nano-micro crystals in colloform pyrite and their aggregation growth patterns need further investigation. The relationships between formation mechanisms of colioform pyrite, organic activity and depositional environments require further exploration. To reveal the nature of nano-micro grain aggregation growth in colloform pyrite and analyse its growth environment and evolutionary history, it is supposed to apply nanoscientific and nanotechnological methods, further integrate consideration of macroscopic geological backgrounds and microscopic mineral growth phenomena, combine high-resolution imaging systems and in situ quantitative microanalysis methods and constitute a mergence of earth science, thermodynamics and kinetics, life science, material science, and chemistry in the study.

Genesis of Strata-bound Sulfide Orebodies in the Tongling Polymetallic Mineralization Cluster, SE China: Evidence from Colloform Pyrite

Colloform pyrite(CPy)is widely distributed in the Tongling mineralization cluster of the Middle-Lower Yangtze River Mineralization Belt(MLYRMB),China.There have many debates as to whether such CPy is associated with Late Mesozoic igneous or Carboniferous sedimentation.CPy from the Xinqiao deposit,a representative of the stratabound sulfide deposits in the MLYRMB,was studied by powder X-ray diffraction(XRD),field-emission scanning electron microscopy(SEM),and high-resolution transmission electron microscopy(TEM).The results show that CPy mainly comprises pyrite,pyrrhotite,quartz,and illite.Pyrite in CPy shows cubic,globule,and xenomorphic morphologies.No octahedral or pyritohedron was observed.Most of the quartz crystals display xenomorphic morphology,where pyrite mold are popular on the surface.Organic matter(OM),which is usually bound to illite,is an important component in CPy.Morphological investigations which exhibit detrital features of quartz and clay minerals indicate that they were derived from continental weathering.Specially,some hexagonal pyrrhotite nanoparticles which show mackinawite morphology are coexisted with OM.The results indicate that the transformation process of sulfides possibly is mackinawite(the precursor)—hexagonal pyrrhotite-pyrite.Thus,compositional and micro-textural characteristics of CPy in Xinqiao deposit suggest it to be a sedimentary origin rather than a hydrothermal origin which is associated with Yanshanian magmatism.Moreover,the coexistence of CPy and stratabound sulfide orebodies in the MLYRMB suggests a causal link between the two.It is considered that CPy might have served as a Cu mineralization geochemical barrier for the Cu-bearing ore-forming fluids,which originated from the Mesozoic magma in the MLYRMB.