Application of in situ titanite U-Pb geochronology to volcanic-hosted magnetite deposit: New constraints on the timing and genesis of the Zhibo deposit, Western Tianshan, NW China
Application of in situ titanite U-Pb geochronology to volcanic-hosted magnetite deposit: New constraints on the timing and genesis of the Zhibo deposit, Western Tianshan, NW China
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原位钛矿 U-Pb 年代学在火山岩磁铁矿矿床中的应用：对中国西北天山智博矿床时代和成因的新约束
DOI：
10.1016/j.oregeorev.2018.03.001
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发表时间：
2018
影响因子：
3.3
通讯作者：
Li Fengming
中科院分区：
文献类型：
--
作者：
Jiang Zongsheng;Wang Dachuan;Zhang Zuoheng;Duan Shigang;Kang Yongjian;Li Fengming
阿吾拉勒成矿带位于中国西北部的西天山造山带内，包括四个大型氧化铁矿床，总资源量约10亿吨。其中，淄博存款是一个大型（337吨，26-68重量%铁）火山容矿磁铁矿存款，其中大量贫钛磁铁矿矿石被托管在石炭纪火山和火山岩序列。本文利用激光烧蚀ICP-MS对淄博地区的钛铁矿和锆石进行了原位U-Pb分析，对铁矿化的时间和成因进行了严格的限定。磁铁矿矿石中的钛铁矿与磁铁矿和由阳起石、绿帘石和方解石组成的钙蚀变组合密切相关。测年的钛铁矿显示出强烈的稀土元素分馏模式，重稀土富集，中性至负Eu异常，并具有低Th和U浓度，低Th/U比值。结构特征和地球化学特征表明，钛铁矿为热液成因，与磁铁矿共生。3个磁铁矿中的钛铁矿207 Pb校正206 Pb/238 U年龄加权平均值分别为310.3 ± 1.8Ma（MSWD = 0.17）、310.1 ± 1.8Ma（MSWD = 0.30）和315.3 ± 2.5Ma（MSWD = 0.26），将淄博铁矿成矿时代限定在315 ~ 310 Ma之间。安山岩中的岩浆锆石U-Pb年龄为316.3 ± 3.4Ma（MSWD = 0.079）。磁铁矿矿石和寄主火山岩的年龄重叠证实了它们之间的成因关系，并与岩浆对成矿系统的贡献相一致，这也表明磁铁矿的Fe和O同位素数据在以前的工作。这些新的U-Pb结果也与阿吾拉勒铁成矿带其他主要磁铁矿矿床的矿化和岩浆活动的年龄估计一致，表明与ca. 315-300 Ma火山作用。结合前人的地质、地球化学证据，我们认为淄博磁铁矿存款主要是由火山-深成岩构造中的基性-中质岩浆的富铁流体形成的。
The Awulale metallogenic belt within the Western Tianshan orogenic belt of northwestern China includes four large iron oxide deposits with a total resource of ∼1000 million metric tons (Mt) Fe. Among these, the Zhibo deposit is a large (337 Mt at 26–68 wt% Fe) volcanic-hosted magnetite deposit, where massive Ti-poor magnetite ores are hosted in the Carboniferous volcanic and volcaniclastic sequences. Here we use in situ U–Pb analyses of titanite and zircon by laser ablation ICP-MS to place tight constraints on the timing and genesis of iron mineralization at Zhibo. Titanite in the magnetite ore are closely associated with magnetite and Ca alteration assemblages consisting of actinolite, epidote, and calcite. The dated titanite exhibit strongly fractionated REE patterns with heavy REE enrichment, neutral to negative Eu anomalies, and have low Th and U concentrations, and low Th/U ratios. The textural and geochemical characteristics indicate that the titanite are hydrothermal in origin and coeval with magnetite in the paragenetic sequence. Titanite from three magnetite ores yield weighted mean207Pb-corrected206Pb/238U ages of 310.3 ± 1.8 Ma (MSWD = 0.17), 310.1 ± 1.8 Ma (MSWD = 0.30), and 315.3 ± 2.5 Ma (MSWD = 0.26), constraining the iron mineralization at Zhibo to a time interval between 315 Ma and 310 Ma. Magmatic zircon from a host andesite sample yield U–Pb age of 316.3 ± 3.4 Ma (MSWD = 0.079). The overlapping ages for magnetite ores and the host volcanic rocks confirm a genetic relationship between them, and are consistent with a magmatic contribution to the mineralization system, as also indicated by Fe and O isotope data of magnetite in previous work. These new U–Pb results are also consistent with age estimates for mineralization and igneous activity in other major magnetite deposits in the Awulale iron metallogenic belt, indicating a significant iron mineralization event related to the ca. 315–300 Ma volcanism. Combined with previous geological and geochemical evidence, we conclude that the Zhibo magnetite deposit was formed mainly by iron-rich fluids derived from a mafic to intermediate magma in a volcano-plutonic structure.