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高校地质学报 ›› 2019, Vol. 25 ›› Issue (6): 888-.DOI: 10.16108/j.issn1006-7493.2019028

• 其他学科 • 上一篇    下一篇

黄铁矿LA-ICP-MS微量元素特征:对青海锡铁山矿床成因的启示

姚希柱,杨涛,朱志勇,党奔,丰志杰,吴昌志   

  1. 1. 内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院,南京 210023;
    2. 中国地质科学院地质研究所,北京 100037
  • 收稿日期:2019-03-29 修回日期:2019-05-06 出版日期:2019-12-20 发布日期:2020-01-06

Characteristics of LA-ICP-MS Trace Elements in Pyrite: Implications for the Genesis of Xitieshan Deposit in Qinghai Province

YAO Xizhu,YANG Tao,ZHU Zhiyong,DANG Ben,FENG Zhijie,WU Changzhi   

  1. 1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;
    2. InstituteofGeology,ChineseAcademyofGeologicalScience,Beijing100037,China
  • Received:2019-03-29 Revised:2019-05-06 Online:2019-12-20 Published:2020-01-06

摘要: 锡铁山铅锌矿床位于柴达木盆地北缘,主要呈层状、脉状及不规则状三类矿体产于绿片岩相的片岩、大理岩及其过渡带中。赋矿围岩的原岩主要为晚奥陶世滩间山群的浅海相基性—酸性火山-沉积岩。前人对该矿床的成因类型存在SEDEX、VMS 和变质叠加的喷流沉积矿床等诸多争议。根据矿石宏观特征和显微组构,文章将锡铁山矿床中层状矿体中的黄铁矿分为胶黄铁矿(Py-I)、他形黄铁矿(Py-II) 和半自形—自形黄铁矿(Py-III) 3 种类型,分别代表了原始沉积型黄铁矿、低程度重结晶黄铁矿和高度重结晶的黄铁矿。使用激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS) 分别对这三种类型黄铁矿进行了微量元素的测试,结果表明三者间具有明显的差异。Py-I 中富含Cu、Pb、Zn、Co、Ni 等成矿元素,Co/Ni比值大于1,并具有相对平滑的时间-空间信号曲线。这表明Ni、Co、As 可能以固溶体形式赋存于Py-I 晶格之中,Cu、Pb、Zn 等元素可能以纳米级矿物微粒存在于Py-I 晶格缺陷中。Py-II 中微量元素含量变化较大,明显低于Py-I,LA-ICP-MS 时间-空间信号曲线表现为多个“尖峰”,暗示着这些元素以微米级矿物包裹体形成存在。Py-III 中微量元素含量极低,甚至低于检测限,除了个别测试点以外。根据上述元素的赋存状态和组构特征,笔者认为Py-I 是火山活动海底喷流沉积过程中形成的,Py-II 和Py-III 则是由Py-I 经后期不同程度的成岩和变质重结晶作用而形成的产物。结合近年来锡铁山矿床赋矿围岩岩相学、锆石U-Pb 年代学、地球化学和构造背景的研究结果,此次研究认为锡铁山矿床应属于VMS 型块状硫化物矿床,在成矿后的造山运动过程中经历了显著的矿体变质和成矿元素再活化作用。

关键词: 铅锌矿床, 黄铁矿, 微量元素, LA-ICP-MS, 锡铁山

Abstract: The Xitieshan Pb-Zn deposit is located in the northern margin of Qaidam basin. Three type of ore bodies, that is, stratiform, veined and irregular were identified from the deposit in schist, marble and their transition zone. The Tangjianshan group, host rock of the deposit, is greenschist metamorphosed mafic to felsic volcano-sedimentary formation which deposited from late Ordovician shallow marine. There are many disputes about genetic types of the deposit, such as sediment-hosted Zn-Pb deposit (SEDEX), volcanic-associated massive sulphide deposit (VMS) and metamorphic overprinting exhalative sedimentary deposit. According to macro-and micro-observation on ore fabric, three main types of pyrite from stratiform ore bodies had been identified, that is, colloidal pyrite (Py-I), anhedral pyrite (Py-II) and subhedral-euhedral pyrite (Py-III), which could represent primary depositional pyrite, low level of recrystallized pyrite and high level of recrystallized pyrite, respectively. In this contribution, laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) was used to measure minor and trace element concentration for the above three types of pyrite, and obvious differences had been recognized. Metallogenic elements, such as Cu, Pb, Zn, Co, Ni and so on, are enriched in Py-I, Co/Ni> 1, and yield smooth time-space resolved signal of LA-ICP-MS analysis. Among them, Ni, Co and As might be situated in lattice of pyrite as solid solution, while Cu, Pb, Zn and other elements might exist in lattice(defect)of pyrite in form of nanometer mineral inclusions. Metallogenic elements in Py-II are variable and obviously lower than which in Py-I, and yield lots of pumps in time-space resolved signal of LA-ICP-MS analysis, indicating their existence of micrometer mineral inclusions in pyrite lattice. Metallogenic elements in Py-III are extremely low and generally below detection limit, except for some spot with minor amount of trace elements. Based on their occurrence and fabric characteristics, we assume that the Py-I was formed in the process of the submarine-exhalative sedimentation with volcanic activities, while Py-II and Py-III were produced from Py-I by later diagenetic and metamorphic
recrystallization in different degrees. Combined with recent studies of petrographic, zircon U-Pb geochronology, geochemistry and tectonic background of their host rocks, it is strongly suggested that the Xitieshan deposit should be a VMS, which had be undergone significant metamorphism of orebodies and reactivation of metallogenic elements during regional orogenic process after mineralization.

Key words: lead and zinc deposit, pyrite, trace elements, LA-ICP-MS, Xitieshan