1.Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring(Central South University),Ministry of Education of the People’s Republic of China,Changsha 410083,Hunan,China
2.School of Geosciences and Info-physics,Central South University,Changsha 410083,Hunan,China
The Dayingezhuang gold deposit, located in the northwest of the Jiaodong Peninsula, is characterized as an altered rock-controlled gold deposit. Gold mineralization predominantly occurs within the beresitization alteration zone situated in the footwall of the Zhaoping fault zone. The reddenization alteration is observed in the Mesozoic granitoid, whereas chloritization alteration is present in the metamorphic wall rocks of the Jiaodong Group. Despite these observations, the impact of various alteration conditions on gold mineralization, the migration pathways of ore-forming fluids, and the spatial dynamics of water-rock interactions that lead to gold precipitation remain inadequately understood. This study employs the TOUGHREACT software to simulate the chemical interactions between ore-forming fluids and wall rocks at different stages of the Dayingezhuang deposit. The simulation aims to quantitatively analyze the redox and acid-base properties associated with distinct alteration processes and to examine the chemical equilibrium concentrations of gold (Au) along with the volume fractions of key altered minerals at various stages. The simulation results indicate that during the primary mineralization stage, the pH value of the fault zone is below 7, and the lgf(O2) decreases from -27 to -44, suggesting acidic conditions with low oxygen fugacity. In contrast,the pH value in the Jiaodong Group and Linglong granites exceed 7,with lgf(O2) values ranging from -30 to -40, indicating alkaline conditions with higher oxygen fugacity. The volume fraction of pyrite and sericite near the fault zone varies from 0 to +0.025%, while the volume fraction of chlorite in the Jiaodong Group metamorphic rocks ranges from 0 to +0.01%. The volume fraction of potassium feldspar in the Linglong granites varies from 0 to +0.05%. These findings suggest that the alteration system surrounding the Dayingezhuang gold deposit functions as a conjugate reaction system, characterized by contrasting redox and acid-base properties. Chloritization and reddenization alterations occur under oxidative and meta-alkaline conditions. The spatial distribution of gold precipitation is predominantly located near the fault zone. As the ore-forming fluids migrate, beresitization alteration occurs under reductive and acidic conditions, leading to the formation of gold orebodies.
大尹格庄金矿床位于胶西北矿集区,是典型的破碎蚀变岩型金矿,金资源量超过150 t。大尹格庄金矿化主要发育在招平断裂带下盘的黄铁绢英岩化蚀变带中,黄铁绢英岩化蚀变矿物组合为黄铁矿、绢云母和石英(Yang et al.,2009,2014;Liu et al.,2021)。矿体上盘主要围岩为胶东群变质岩,在胶东群变质岩中存在绿泥石化蚀变(凌洪飞等,2002)。矿体下盘的中生代花岗岩围岩内发育红色蚀变,蚀变矿物主要为钾长石、钠长石、赤铁矿、金红石和碳酸盐矿物等(袁月蕾等,2023),众多学者将其称为红化蚀变(陈光远等,1997;袁月蕾等,2023)。已有研究表明,大尹格庄金矿床中典型的绿泥石化蚀变、红化蚀变与黄铁绢英岩化蚀变具有共生关系(孙伟清等,2019)。然而,不同类型蚀变的形成条件及其对金矿化过程的影响尚不清晰,矿化元素迁移形式和矿化沉淀的位置尚不明确。针对上述问题,热液蚀变过程化学反应数值模拟能够对多相流体流动、质量运移、热传导和热液对流以及离子、化合物、气体和矿物的化学反应进行计算,通过将热液流体的物理运移和演化过程与热液—围岩相互作用的化学反应进行耦合,实现对热液蚀变矿化过程的动态模拟(Zhao et al.,2018;Zou et al.,2017,2019),定量分析蚀变成矿环境和矿物的溶解沉淀,对探索蚀变分带与矿化富集之间的关系具有重要意义。
已有研究表明,大尹格庄金矿床的含金热液呈微酸性,其中Au元素主要与S形成化合物,以[Au(HS)2]-的形式赋存在成矿流体中(Yang et al.,2016)。为了定量分析成矿物质的迁移形式和蚀变矿化过程环境的pH值,使用TOUGHREACT软件计算主要流体组分(如 [Au(HS)2]-和H+等)的化学平衡浓度值,计算公式可表示为
式中:为矿物n的反应比表面积;为矿物的饱和指数;和为由实验测出的参数;为发生溶解反应的矿物相数量;为与温度相关的反应速率常数。由于大部分已公开的反应速率常数是在温度为25 ℃条件下测得的,所以不同温度条件下的反应速率常数(Lasaga et al.,1994)可近似表示为
式中:为表面活化能;为温度为25 ℃条件下的反应速率常数;R为气体常数;T为绝对温度。
4 实例模拟与讨论
4.1 模型构建与参数设置
本文收集的数据包括13张地质剖面图和81个钻孔数据,建立了大尹格庄金矿三维地质模型[图4(a)]。考虑到大尹格庄金矿体主要集中在招平断裂下盘,并沿着该断裂带展布[图 4(b)],基于建模数据,模型区域范围在南北和东西水平方向上的尺寸分别定义为7 km,纵深为3 km,根据区域范围勘探程度,将模型剖分为200 m×200 m×100 m的块体单元,共有36 500个网格单元。模型的主要地质单元包括断裂带、胶东群(上盘)和玲珑花岗岩(下盘)。为了更好地了解流体在断层内部和周围的流动模式及其发生的化学反应,本研究在主要地质单元设置观测点[图4(c)]。前人对胶东地区动力学演化过程和岩浆活动构造背景的研究表明,由于成矿后经历了复杂的碰撞造山和洋陆俯冲作用,构造体制不断发生变化,使得矿床遭受明显的剥蚀抬升作用(Yang et al.,2009;Zhang et al.,2022)。根据推算大尹格庄金矿床的成矿深度为6.8~9.5 km(丁正江等,2015;Wang et al.,2018;Zhang et al.,2022),钻探结果显示大尹格庄金矿在超过1 000 m深度金矿化程度较好。因此,本研究在现有地层的基础上添加了厚度约为7 km的覆盖层(Xie et al.,2022;单文法等,2023)[图4(d)],并将建立好的块体模型进行格式转换,导入到TOUGHREACT软件中进行数值模拟。
模型中各地质单元的岩石物理性质是TOUG-HREACT模拟的重要参数之一,通过参考已有的文献资料和实验室测试数据(Yang et al.,2009;单文法等,2023),设定岩石样本物理性质数据如表1所示。根据前人研究(戴雪灵,2012;刘育等,2014),大尹格庄矿床Au成矿温度为230~340 ℃,成矿主阶段压力范围为78~300 MPa,考虑到流体初始温度应略高于矿物形成温度(Liu et al.,2014),因此本文将成矿流体初始温度设为350 ℃,成矿压力设为中间值128 MPa。同时,结合前人对大尹格庄金矿床成矿阶段的研究(戴雪灵,2012;张炳林等,2017),设置各成矿阶段的温度范围如表2所示,并对每个阶段设置的模拟时间在数量级上保持一致。
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