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

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。

Element mobility and fluid characteristics during pyrophyllite alteration: A case study from the Baiyun pyrophyllite deposit, Wutai County, Shanxi Province

Investigation on the element mobility during hydrothermal alteration processes can provide challenging insights into fluid properties and metallogeny. The Baiyun pyrophyllite deposit of the Wutai County, Shanxi Province, provides a natural case for study on fluid properties and elemental mobility, where the wall rocks mainly consist of albite-sericite quartz schist, associated with minor albite-chlorite schist. Hydrothermal alteration in the mine is dominated by pyrite-phyllic (sericite-quartz-pyrite) and pyrophyllite (pyrophyllite-illite-kaolinite-quartz). The gold mineralization is mainly developed in the pyrite-phyllic alteration zone. Log f_O2-pH diagram is constructed to depict the alteration process, and the phase diagram exhibites that the early pyrite-phyllic alteration fluid was weak acid to partial neutral (pH=5.24~5.87) with relatively low oxygen fugacity (which was located in the stable area of pyrite and chalcopyrite). The alteration fluid leading to the pyrophyllite alteration, in contrast, was acidic (pH=2.07~2.20) with relatively high oxygen fugacity (which was above the HM buffer line). According to the immobility of Al₂O₃ from the pyrophyllite ores, the mass-balance calculation shows that the contents of SiO₂, Na₂O and K₂O from the pyrophyllite ores are dramatically increased by comparison with the wall rocks (albite-sericite quartz schist), whereas, the other major oxides were prominently moved out. The abundances of Nb, Ta, Th, U, Rb, and Ga were relatively moved in, with Th/U ratios increased slightly. The abundances of Sr, Ba, Zr and Hf were relatively decreased with Zr/Hf ratios declined from 34~41 (wall rocks) to 17~22 (pyrophyllite ores). The rare earth elements were moved out as a whole, and the emigration amounts from LREE to HREE were gradually decreased. The Y/Ho (28~32) ratios are distinct with those of C1 chondrite (26~28), indicating that Y and Ho exhibit different behavior during pyrophyllite alteration. The negative Eu anomalies of the pyrophyllite ores are obvious, being attributed to decomposition of feldspars. The contents of Au and As within the albite-sericite quartz schist exhibit positive correlation ships with each other. By contrast, most metal ore-forming elements of the pyrophyllite ores are below the detected limit, suggesting the elements have been moved away during the pyrophyllite alteration. This phenomenon is consistent with the lithology features of pyrophyllite ores (visible harbor-shaped hematite and lacking of relevant pyrite). Our data indicates that the acidic and oxidized fluids can migrate amounts of so-called inactive elements, such as P, Ti, Zr, Hf, REE and other elements, simultaneously decoupled the ratios of Zr/Hf and Y/Ho during pyrophyllite alteration. Meanwhile, the fluids for pyrophyllite alteration leading to a superimposed destroy on the early formed gold mineralization, are not conducive for Au precipitation.