北大西洋Logatchev热液区多形貌黄铁矿特征及其意义

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

北大西洋Logatchev热液区产出的黄铁矿形貌丰富、成分各异,极具特殊性和代表性.运用扫描电镜和电子探针系统观测黄铁矿矿物学特征,发现粒状黄铁矿粒径不一,具高Fe、S,低微量元素的特点;草莓状黄铁矿见有松散莓体、球形-次球形莓体和自形结构莓体,其Cu含量总体较高,不同晶体特征的莓体成分存在差异;胶状黄铁矿见有多层环带,内核到环带由纳微米晶体聚合生成,由内到外成分具有S/Fe比、Zn含量减小,As含量增大的趋势.分析认为早期粒状黄铁矿是在较高温度下的热液体系中直接形成,随热液作用衰减,生成的粒状黄铁矿多呈纳-微米晶产出;草莓状黄铁矿是由松散纳-微米晶逐步聚集形成,受溶液过饱和度影响,莓体粒度和聚集程度存在差异;胶状黄铁矿多晶聚集环带生长方向是由内向外的,聚集程度差异指示沉积环境的反复交替.研究区多形貌黄铁矿具有由分散的微晶→多晶聚合体→自形单晶的完整演化系列特征,对于解读现代海底热液活动的方式和特点,揭示纳米晶体-宏观晶体的演化过程有积极意义.

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.Thegrowth 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.