耿庄金矿含FeS_2晶须环带重晶石巨晶的特征及成因意义

The characteristics and genetic significance of giant barite crystal with FeS_2 whiskers girdle found in the Gengzhuang gold deposit,Shanxi Province,China

在山西耿庄金矿区含金隐爆角砾岩体的晶洞内,发现了含FeS2晶须环带重晶石巨晶,用扫描电镜、电子探针、X射线粉晶衍射等手段系统研究了重晶石及其所含FeS2晶须的矿物特征。结果表明,重晶石化学成分单一;晶体结构标准;发育完好;晶须为FeS2-Fe(Ni,Co)S2的类质同象系列。根据地质背景,结合包裹体研究成果,运用晶体生长和地球化学理论,讨论了重晶石巨晶及所含晶须的成因。认为重晶石巨晶是该区燕山期花岗岩浆活动的期后热液演化到后期阶段时,SO2-4与围岩中的钡反应而形成;所含的FeS2晶须是持续进行的火山隐爆作用所产生的大量H2S、SO2等与热液中分解出的Fe2+、Ni2+、Co2+反应而形成的;富碱、H2S溶解度增大,S2-增高有利于晶须的生长;重晶石和晶须生长的地质环境具有宏观上的一致性和具体条件的差异性。当pH值为1～8.5、lgfO2为-31～-38时,优先进入FeS2生长的优势场,形成FeS2晶须矿物。当pH值为5.5～10、lgfO2在-25～-36时形成重晶石;FeS2环带和重晶石的共生现象是两者共结晶交互生长的结果;重晶石的套叠生长和晶须环带特征,是耿庄地区隐爆作用脉动性和周期性作用的结果。耿庄含晶须重晶石巨晶,明确标示出所形成的地质条件和地质作用过程,为深入研究隐爆作用和理解该区成矿作用提供了矿物学证据。

The giant barite crystals with FeSz whiskers girdle were found in the geode in the concealed explo- sion breccia in Gengzhuang gold deposit, Shanxi Province, China. The mineral characteristics of the barite and the FeS2 whiskers contained were systematically analyzed by SEM, EPMA and XRD systematically. The re sults showed that the barite chemical composition is single, its crystal structure is standard and its crystal growth is perfect. The whiskers belong to the FeS2-Fe（Ni, Co）$2 isomorphism series. According to the geo- logical background and the inclusion research results, combined with the crystal growth and the geochemical theory, the genesis of the barite and the whiskers contained has been discussed. The barite was the product of the reaction between the barium in the surrounding rock and the SO42 when the hydrothermal evolution came to the last period after the Yanshanian granitic magmatic activity. The FeS2 whiskers contained were the prod- ucts of the H2 S and SO2 which were formed by continued concealed volcano explosion and the Fe2＋ , Ni2＋ and Co2＋ which came from the decomposition of the hydrothermal fluids. The rich alkali, the increasing H2 S solu- bility and the increasing S2- content were conducive to the whisker growth. The geological environments of the growths of barite and whisker had consistency in macro and differences in specific conditions. While the pH ranged from 1 to 8.5, and the lgJ^e ranged from --31 to --38, the superior field for the FeS2 growth was at- tained and the FeSz whiskers formed. While the pH ranged from 5.5 to 10 and the lgf% ranged from --25 to --36, the barite formed. The symbiosis of the FeSe whiskers girdle and the barite was the result of the interac tive growth. The barite intussusceptive growth and the presence of FeSe whiskers girdle are the signs of the pulsated and cyclical concealed explosions in the Gengzhuang area. The giant barite crystal with FeSz whiskers girdle clearly marked the geological conditions and processes of their formation, which provided a mineralogical evidence for further research of concealed explosion and understanding the mineralization in that area.