叶蜡石化蚀变过程中的元素活动性与流体性质:以山西五台地区白云叶蜡石矿为例

发布时间：2018-05-30 16:41

本文选题：热液蚀变 + 元素迁移　；参考：《岩石学报》2017年06期

【摘要】：热液蚀变过程中的元素活动性与流体性质对深入理解矿物稳定性和成矿作用具有重要的意义。本文以华北克拉通中北部山西五台地区的白云叶蜡石矿为例,研究了蚀变过程中元素迁移特征和流体性质。该矿体围岩以绿片岩相酸性火山岩为主,岩性为绢云钠长石英片岩并夹有少量的绿泥钠长片岩。矿区内蚀变分带明显,可分为早期的黄铁绢英岩化(绢云母-石英-黄铁矿)和晚期叠加的叶蜡石化(叶蜡石-伊利石-高岭石-石英),而金矿化则主要发育于黄铁绢英岩化带内。Log fo2-pH相图模拟结果显示,早期黄铁绢云岩化蚀变热液具有弱酸性至偏中性(pH=5.24~5.87)和较低氧逸度(位于黄铁矿+黄铜矿稳定相区内)特征;而引起叶蜡石化蚀变的热液具有强酸性(pH=2.07~2.20)和高氧逸度(位于HM缓冲线以上)特征。质量平衡迁移分析结果显示,随着叶蜡石化蚀变作用的增强,叶蜡石矿石中的Al2O3行为较稳定,SiO2、Na2O和K2O含量相对于围岩绢云钠长石英片岩呈不同程度的迁入,而其余氧化物大量活化迁出。微量元素Nb、Ta、Th、U、Rb和Ga含量相对升高,Th/U比值略有升高;Sr、Ba、Zr、Hf明显亏损,Zr/Hf比值从34~41下降到17~22。稀土元素均发生一定程度的活化迁移,且轻稀土迁出程度更高。Y/Ho比值(28~32)高于球粒陨石的Y/Ho(26~28),表明Y-Ho在叶蜡石化蚀变过程中表现出不同的地球化学行为。Eu负异常明显增大,这可能与长石的分解关系密切。围岩绢云钠长石英片岩中金属元素含量较高且Au与As含量之间呈明显正相关性,但在叶蜡石矿石中大部分金属元素含量均低于检出限,说明金属元素在叶蜡石化蚀变作用过程中发生了强烈的活化迁移,这与岩相学上叶蜡石矿石中可见港湾状细粒赤铁矿而缺乏黄铁矿的特征吻合。本文研究结果表明叶蜡石化过程中,大量的所谓不活动元素(如P、Ti、Zr、Hf、Y和Ho等)发生了显著迁移并导致Zr/Hf和Y/Ho比值的解耦,并伴随着大量金属元素的迁出,说明叶蜡石化不利于金矿化的形成。
[Abstract]:Element activity and fluid properties in hydrothermal alteration process are of great significance for further understanding of mineral stability and mineralization. Taking dolomitic pyrophyllite deposits in Wutai area of Shanxi Province in the central and northern part of North China Craton as an example, the characteristics of element migration and fluid properties during alteration are studied. The host rock of this ore body is mainly greenschist facies acid volcanic rock, and its lithology is sericite albite schist with a small amount of green mud sodium schist. The alteration zones in the mining area are obvious, It can be divided into early pyrite (sericite-quartz pyrite) and pyrophyllization (pyrophyllite, Illite, kaolinite and quartz), while gold mineralization is mainly developed in the zonal. Log fo2-pH. The phase diagram simulation results show that, The early pyrite altered hydrothermal solution is characterized by weak acid to neutral pH = 5.24 ~ 5.87) and low oxygen fugacity (located in the stable phase region of pyrite chalcopyrite). The hydrothermal solution which caused pyrophyllite alteration was characterized by strong acidic pH = 2.07 ~ 2.20) and high oxygen fugacity (located above the HM buffer line). The results of mass equilibrium migration analysis show that with the enhancement of pyrophyllite alteration, the Al2O3 behavior of pyrophyllite ore is relatively stable and the contents of Na _ 2O and K _ 2O in pyrophyllite ore are different from those in the surrounding rock sericite albite schist. The rest of the oxides are activated and migrated out in large quantities. The contents of trace elements NbTa-Ta-Th, URb and Ga are relatively higher. The ratio of Th- / U is slightly increased. All rare earth elements were activated and migrated to some extent, and the migration degree of light rare earth elements was higher than that of chondrites. The ratio of Y / H / Ho = 2832) was higher than that of chondrites, which indicated that Y-Ho showed different geochemical behaviors and negative EU anomalies during pyrophyllite alteration. This may be closely related to the decomposition of feldspar. The content of metallic elements in albite schist of surrounding rock is high, and there is a positive correlation between au and as content, but most metal elements in pyrophyllite ore are lower than the detection limit. It is concluded that the strong activation and migration of metal elements occurred during the pyrophyllite alteration process, which coincides with the characteristics of harbour fine hematite and lack of pyrite in petrographic pyrophyllite ores. The results show that during pyrophyllization, a large number of so called inactive elements (such as PttiTiZR, HfCY and Ho, etc.) migrate significantly and lead to the decoupling of the ratio of Zr/Hf and Y/Ho, which is accompanied by the migration of a large number of metal elements. It shows that pyrophyllite is not conducive to the formation of gold mineralization.
【作者单位】：中国地质大学地球科学与资源学院;
【基金】：国家重点研发计划(2016YFC0600106) 国家自然科学基金重点项目(41230311、41530208)联合资助
【分类号】：P619.2

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