In situ U-Pb and trace element analysis of accessory minerals in the Kiruna District, Norrbotten, Sweden: new constraints on the timing and origin of mineralization
In situ U-Pb and trace element analysis of accessory minerals in the Kiruna District, Norrbotten, Sweden: new constraints on the timing and origin of mineralization
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DOI：
10.1093/petrology/egp069
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发表时间：
2009-11
影响因子：
3.9
通讯作者：
Martin P. Smith;C. Storey;T. Jeffries;C. Ryan
中科院分区：
文献类型：
--
作者：
Martin P. Smith;C. Storey;T. Jeffries;C. Ryan
瑞典北方Norrbotten是波罗的海地盾的重要组成部分，提供了Svecokarelian造山运动期间岩浆、构造和相关叠加铁氧化物-磷灰石和铁氧化物-铜-金（IOCG）矿化的记录。采用激光烧蚀四极电感耦合等离子体质谱法（LA-ICP-MS）对该地区一系列矿床中的钛铁矿和褐帘石进行了U-Pb同位素系统学和微量元素化学分析。对区域方柱岩-钠长石蚀变岩的单个样品的分析表明，其年龄为1903 ± 8 Ma（2σ），可能与Fe成矿的早期阶段（1890-1870 Ma）同期。对未变形IOCG矿床中的钛铁矿和褐帘石的分析表明，初始蚀变发生在1862 ± 16 Ma。在许多矿床中，随后的变质作用重置了1790 ~ 1800 Ma的钛铁矿同位素系统，导致U-Pb同位素分析沿着协和面扩展。在某些情况下，核心区域可能记录了2050 Ma左右早期热事件的证据。主要剪切带上的变形IOCG矿床中的钛铁矿和褐帘石的年龄为1785 ± 21 ~ 1777 ± 20 Ma，与造山运动晚期的变形变质作用和次生热液蚀变作用相对应。钛铁矿和褐帘石微量元素化学没有明显的晶内变化，表明热液流体化学和金属来源是矿物微量元素化学的主要控制因素。未变形氧化铁-磷灰石和IOCG矿床中的钛铁矿是典型的轻稀土元素（LREE）富集，并显示低U/Th比值和低Ni的中酸性和基性火山岩矿床。这与金属的花岗岩来源是一致的。微量元素模式的微小变化是一致的水相络合物形成的相对稀土溶解度的影响。与Nautanen变形带有关的矿床具有相对较重的稀土元素（HREE）富集的钛铁矿和轻稀土元素亏损的褐帘石，具有高U/Th比值和高Ni含量，与当地基性火山岩中的金属浸出相一致。所有热液钛铁矿都是高场强元素富集的（Nb，Ta，Zr），表明它们的运输是由于高盐度或高F含量，或两者兼而有之。数据总体上支持IOCG型矿化模型，作为与主要地壳结构相关的盐水热液区域循环的结果，至少有一些金属成分来自该地区的花岗岩类岩石。这里的所有矿床都显示出后续变质作用的证据，尽管渗透性组构仅限于区域规模的变形带。
Northern Norrbotten, Sweden is a key part of Baltic Shield and provides a record of magmatic, tectonic and related, superimposed, Fe oxide–apatite and iron oxide–copper–gold (IOCG) mineralization, during the Svecokarelian orogeny. Titanite and allanite from a range of mineral deposits in the area have been analysed for U–Pb isotope systematics and trace element chemistry using laser ablation quadrupole inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses of a single sample from the regional scapolite–albite alteration give an age of 1903 ± 8 Ma (2σ) and may be contemporaneous with the early stages of Fe mineralization (1890–1870 Ma). Analyses of titanite and allanite from undeformed IOCG deposits indicate initial alteration at 1862 ± 16 Ma. In many deposits subsequent metamorphic effects reset titanite isotope systematics from 1790 to 1800 Ma, resulting in a spread of U–Pb isotope analyses along concordia. In some instances core regions may record evidence of early thermal events at around 2050 Ma. Titanite and allanite from deformed IOCG deposits on major shear zones record ages from 1785 ± 21 Ma to 1777 ± 20 Ma, corresponding to deformation, metamorphism and secondary hydrothermal alteration as a result of late orogenic movements. The lack of intracrystalline variations in titanite and allanite trace element chemistry suggests that hydrothermal fluid chemistry and metal source were the main controls on mineral trace element chemistry. Titanite from undeformed Fe oxide–apatite and IOCG deposits is typically light rare earth element (LREE) enriched, and shows low U/Th ratios and low Ni in both intermediate to acid and basic volcanic-hosted deposits. This is consistent with a granitic source for metals. Minor variations in trace element patterns are consistent with the influence of aqueous complex formation on relative REE solubility. Deposits related to the Nautanen Deformation Zone have relatively heavy REE (HREE)-enriched titanite, and LREE-depleted allanite, with high U/Th ratios and elevated Ni contents, consistent with leaching of metals from the local basic volcanic rocks. All hydrothermal titanites are high field strength element enriched (Nb, Ta, Zr) indicating their transport as a result of either high salinities or high F contents, or both. The data overall support models of IOCG-type mineralization as a result of regional circulation of saline hydrothermal fluids in association with major crustal structures, with at least some metallic components derived from the granitoid rocks of the area. All the deposits here show evidence of subsequent metamorphism, although penetrative fabrics are restricted to regional-scale deformation zones.