Thallium isotope variations in an ore-bearing continental igneous setting: Collahuasi Formation, northern Chile
Thallium isotope variations in an ore-bearing continental igneous setting: Collahuasi Formation, northern Chile
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DOI：
10.1016/j.gca.2010.04.068
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发表时间：
2010-08
影响因子：
5
通讯作者：
R. Baker;M. Rehkämper;C. Ihlenfeld;C. Oates;R. Coggon
中科院分区：
文献类型：
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作者：
R. Baker;M. Rehkämper;C. Ihlenfeld;C. Oates;R. Coggon
铊是一种高度不相容的元素，因此，预计大部分块状硅酸盐地球铊预算将驻留在大陆地壳中。然而，大陆岩石的铊同位素系统学目前基本上还没有被探索。在这里，我们提出了新的Tl同位素组成和浓度数据的一套36个侵入和喷出的火成岩附近的斑岩铜矿在Collahuasi形成的中安第斯山脉在北方智利。岩浆岩的岩性受到与铜矿床形成相伴的热液蚀变作用的双重影响。样本中的铊浓度变化超过一个数量级，从0.1到3.2μg/g，而ε 205铊的范围在-5.1和+0.1之间（ε 205铊是样本的205铊/203铊同位素比与标准的偏差，单位为每104份）。这些变化主要被认为是热液蚀变过程的结果，包括Tl的交代运输，和含Tl矿物的形成/分解，这与小但显著的Tl同位素效应有关。Tl丰度与K和Rb浓度具有良好的相关性，但与Cu无协变。这表明，Tl显示只有有限的亲铜亲Collahuasi组的大陆地壳，但作为一个亲石元素的分布，主要是由分割的Tl+到K+-轴承阶段。Collahuasi样品与propylitic蚀变特征，这是来自热液系统的边缘部分，平均而言，稍轻的Tl同位素组成比更中央的绢云母和粘土蚀变带的岩石。这个小的，但统计上显着的差异最有可能反映优先保留的同位素重铊蚀变相，如白色云母和粘土，绢云母和粘土蚀变过程中形成的。
Thallium is a highly incompatible element and a large fraction of the bulk silicate Earth Tl budget is, therefore, expected to reside in the continental crust. Nonetheless, the Tl isotope systematics of continental rocks are essentially unexplored at present. Here, we present new Tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry Cu deposits in the Collahuasi Formation of the Central Andes in northern Chile. The igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the Cu deposits. The samples display Tl concentrations that vary by more than an order of magnitude, from 0.1 to 3.2μg/g, whilst ε205Tl ranges between −5.1 and +0.1 (ε205Tl is the deviation of the205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). These variations are primarily thought to be a consequence of hydrothermal alteration processes, including metasomatic transport of Tl, and formation/breakdown of Tl-bearing minerals, which are associated with small but significant Tl isotope effects. The Tl abundances show excellent correlations with both K and Rb concentrations but no co-variation with Cu. This demonstrates that Tl displays only limited chalcophile affinity in the continental crust of the Collahuasi Formation, but behaves as a lithophile element with a distribution that is primarily governed by partitioning of Tl+into K+-bearing phases. Collahuasi samples with propylitic alteration features, which are derived from the marginal parts of the hydrothermal systems, have, on average, slightly lighter Tl isotope compositions than rocks from the more central sericitic and argillic alteration zones. This small but statistically significant difference most likely reflects preferential retention of isotopically heavy Tl in alteration phases, such as white micas and clays, which formed during sericitic and argillic alteration.