Characteristics and Research Significance of Polymorphic Pyrite in Logatchev Hydrothermal Area, North Atlantic

The variety of morphology and compositions of pyrite in the Logatchev hydrothermal area of North Atlantic is particular and representative. In this paper, it analyzes the mineralogical characteristics of pyrite systematically by scanning electron microscopy (SEM) and electron probe microanalysis(EPMA). It is found that the size of granular pyrite is inhomogeneous with high contents of Fe and S and low contents of trace elements. There are three types of pyrite framboids in this area:loosely arranged framboids, spherical framboids and framboids with enhedral crystal structure. The content of Cu in framboids is large, and the different framboid crystals have different chemical compositions. Part of colloidal pyrite develops multilayer girdles, and it is formed by the polymerization of nano-micro pyrite crystals. From the core to the girdles, the contents of S/Fe and Zn show a decreasing trend, while the As content shows an increasing tendency. The analyses show that the earlier granular pyrite is formed directly in a hydrothermal system at higher temperatures, the formation of pyrite is mainly composed of nano and micro pyrite crystals with the attenuation of hydrothermal action. The loosely nano and micro pyrite crystals have a tendency to aggregate framboids. The size and accumulation of framboids vary with the change of the super saturation of solution. The growth of polycrystalline aggregation is from inside girdle to outside girdle, and the difference of aggregation of girdles indicates the alternation of metallogenic environment. Multi-morphologies of pyrite in the area have a complete evolution series from dispersed microcrystals to polycrystalline aggregate to euhedral crystal, which is significant to the understanding of the ways and characteristics of modern seafloor hydrothermal activity and the exploration of the evolution process of the nano crystalline crystal.