摘要
丹池成矿带是我国华南重要的锡多金属集中区域,大厂矿田锡多金属成矿与晚白垩世黑云母花岗岩(笼箱盖岩体)密切相关。然而,岩浆热液演化过程中熔/流体化学性质、挥发分、体系氧逸度变化等尚不明确,从而限制了对大厂矿田锡多金属矿床锡富集机制和矿床成因的理解。笔者首次在大厂矿田东段大幅楼Sn-Zn矿床22-1号矿体中发现了与锡矿化密切相关的自形粗粒热液磷灰石,结合已有黑云母花岗岩、花岗斑岩脉中未蚀变岩浆磷灰石和蚀变磷灰石资料,利用岩浆热液演化过程中熔/流体化学性质变化,系统探讨锡富集机制和矿床成因的关键科学问题,同时,建立利用磷灰石地球化学数据区分锡矿与其他成因类型矿床的新型综合图解,为找矿勘查提供指导。为此,笔者对大幅楼矿床热液磷灰石开展了精细矿相学研究、激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)U-Pb定年、X射线荧光光谱(XPS)和电子探针(EPMA)主量成分分析工作。磷灰石矿相学结果表明,自形粗粒磷灰石(粒径为50~200μm)主要与铁氧化物、锡石、钛铁矿和石英共生。电子背散射(BSE)图像和XPS面扫描分析下,磷灰石表面均一,不发育环带,Ca、P和F元素分布均匀。电子探针主量元素分析表明,磷灰石明显富F(含量为3.63%~4.47%),贫Cl(含量<0.036%),属于氟磷灰石。磷灰石原位U-Pb定年结果显示,大幅楼矿床热液磷灰石形成于(96.2±5.7)Ma。未蚀变岩浆磷灰石高F特征表明,大厂矿田初始岩浆相对富F,可有效降低岩浆固相线温度和黏度,延长岩浆演化过程,有利于Sn在残余熔体中富集。岩浆磷灰石低的As含量(含量为(1.15~7.18)×10^(-6)),以及大幅楼矿床流体包裹体中CH_(4)的大量出现,表明岩浆演化和大幅楼矿床热液成矿阶段,体系氧逸度(f_(O_(2)))始终维持在较低水平。结合前人流体包裹体成分分析和测温结果,认为流体降温是导致大幅楼矿床锡石沉淀的主要原因,锡石与毒砂的氧化还原缓冲在锡石硫化物阶段锡石沉淀过程中发挥了重要作用。华南典型锡矿和其他不同成因类型矿床磷灰石微量元素的主成分分析(PCA)和核密度估算(KDE)结果显示,锡矿相关磷灰石富集La、Ce、Pr、Sm、Gd、Dy、Yb和Y元素,可作为区分锡矿与其他成因类型矿床的判别标志。
The Danchi fold-and-thrust belt is one of the most important tin polymetallic regions in South China.Tin polymetallic mineralization is closely linked to the Late Cretaceous biotite granite(the Longxianggai pluton)in the Dachang district,middle of the Danchi fold-and-thrust belt.However,the chemical compositions of melts and ore-forming fluids,volatiles,and oxygen fugacity(f O_(2))during the evolution of magmatic-hydrothermal process remain unclear.Fortunately,euhedral coarse-grained hydrothermal apatite associated with tin mineralization was identified in the No.22-1 orebody of the Dafulou Sn-Zn deposit,eastern segment of the Dachang district.Combined with previous geochemical data of unaltered magmatic apatite,hydrothermally altered apatite in biotite granite and granite porphyry dike.These materials provide an opportunity to investigate the geochemical properties of melts and ore-forming fluids,as well as their constraint on tin mineralization and ore genesis.At the same time,we attempt to establish a new comprehensive diagram to distinguish tin deposits from other deposit types,providing guidance for exploration.Herein,detailed mineralogical studies,laser ablation-inductively coupled plasma-mass(LA-ICP-MS)U-Pb dating,X-ray photoelectron spectroscopy(XPS)analyses,and electronic probe analyses(EPMA)were conducted on hydrothermal apatite of the Dafulou deposit.The mineralogical features of apatite from oxide stage show that euhedral coarse-grained apatite(50200μm)is intergrowth with Fe-oxides,cassiterite,ilmenite,and quartz.The apatite does not show any zoning in the electron backscatter images(BSE).Calcium,P,and F elements show the relative homogeneous distributions in apatite grains by X-ray mapping.EPMA suggest apatite is significantly enriched in F(3.63%4.47%),but depleted in Cl(<0.036%).In situ U-Pb dating implies apatite formed at(96.2±5.7)Ma.The high F characteristics of unaltered magmatic apatite indicate the initial magma of the Longxianggai pluton is relatively F-rich,which effectively reduces the magmatic solidus temperature and viscosity,prolongs the process of magmatic evolution,and facilitates Sn enrichment in residual melts.The low As contents(1.157.18 ppm)of magmatic apatite,as well as the occurrence of CH_(4)in fluid inclusions of the Dafulou deposit,indicate that the oxygen fugacity(f_(O_(2)))were relatively low during the evolution of magma and the hydrothermal mineralization stage of the Dafulou deposit.Combined with previous fluid inclusion composition analysis and microthermometric results,we conclude that fluid cooling is the main trigger for cassiterite deposition and the redox buffering between cassiterite and arsenopyrite also plays an important role in cassiterite precipitation of the cassiterite-sulfides stage in the Dafulou Sn-Zn deposit.The results of principal component analysis(PCA)and kernel density estimation(KDE)of trace elements in apatite from typical tin deposits in South China and other deposit types show that the apatite associated with tin mineralizations is relatively enriched in La,Ce,Pr,Sm,Gd,Dy,Yb,and Y elements,which can be used to distinguish tin deposits from other deposit types.
作者
张宇
肖昌浩
于萍萍
张闰峰
胡利娟
ZHANG Yu;XIAO Changhao;YU Pingping;ZHANG Runfeng;HU Lijuan(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;Laboratory of Dynamic Diagenesis and Metallogenesis,Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China;Key Laboratory of Paleomagnetism and Tectonic Reconstruction,Ministry of Natural Resources,Beijing 100081,China;State Key Laboratory of Deep Earth and Mineral Exploration,Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100094,China;Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources by the Province and Ministry,Guilin University of Technology,Guilin 541006,China)
出处
《地学前缘》
北大核心
2026年第2期88-104,共17页
Earth Science Frontiers
基金
国家科技重大科技专项项目(2024ZD1001701)
中国地质科学院基本科研业务费专项经费项目(No.DZLXJK202206,No.DZLXJK202203)
自然资源部古地磁与古构造重建重点实验室开放课题(KLPTR-08)
中国地质调查局项目(DD20240127,DD20230344)
广西重点研发计划项目(2024AB21027)。
