Using elemental and boron isotopic compositions of tourmaline to trace fluid evolutions of IOCG systems: The worldclass Dahongshan Fe-Cu deposit in SW China
Using elemental and boron isotopic compositions of tourmaline to trace fluid evolutions of IOCG systems: The worldclass Dahongshan Fe-Cu deposit in SW China
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利用电气石的元素和硼同位素组成追踪 IOCG 系统的流体演化：中国西南部的世界级大红山铁铜矿床
DOI：
10.1016/j.chemgeo.2016.08.030
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发表时间：
2016
期刊：
影响因子：
3.9
通讯作者：
Zhou Mei-Fu
中科院分区：
文献类型：
--
作者：
Su Zhi-Kun;Zhao Xin-Fu;Li Xiao-Chun;Zhou Mei-Fu
世界级大红山铁氧化物铜金矿床产于中国西南康甸地区晚古元古代大红山群中。矿体在空间上与次火山侵入体有关，并受构造控制。新元古代广泛的变质叠加使确定成矿流体的初始成分变得困难。然而，电气石是一个有用的相，因为它的耐火性和化学多样性。它广泛存在于大红山矿床成矿蚀变的四个阶段，包括矿化前钠盐蚀变(I期)、铁氧化物矿化(II期)、硫化物矿化(III期)和晚期石英-方解石脉(IV期)。电气石与热液蚀变的时间组合为研究流体演化提供了很好的机会。四个阶段的电气石都属于碱性群，成分范围从闪锌矿到闪锌矿。用LA-MC-ICPMS对电气石进行了原位B同位素分析，得到了δ11B值在−为14.7‰到+55.9‰之间的总范围，并且在不同的阶段显示出明显的差异。第一阶段白长石化的电气石具有δ11B值，范围从−-14.7‰到−-5.7‰。铁氧化物矿石中电气石的δ11B值为−11.6‰至−为6.1‰，与第一阶段相似，而铜硫化矿石中电气石的δ11B值为−为4.4‰至−为0.6‰，显示出明显的重同位素。第四阶段的电气石以强烈蚀变围岩中贫瘠的石英-方解石-电气石脉体为代表，其δ11B值为正，范围为+22.9‰~+55.9‰。观测到的B同位素变化不能仅由成矿流体的分馏作用产生。因此，在地质约束和模式计算的基础上，提出了至少两个不同来源的B的混合，即一个岩浆源和具有高度正的δ11B值的外部源。与电气石共生的热液矿物平衡流体的氧同位素组成分别为7 4‰~10 2‰、8 4‰~8 6‰和3 8‰~8 0‰。II期至IV期流体δ18O的下降趋势与流体混合侵入盆外卤水的趋势一致。我们的数据表明，大红山IOCG系统的成矿流体最初是岩浆成因的(第I期和第II期)，后来经与外部卤水混合而逐渐变质(第III期和第IV期)。
The worldclass Dahongshan iron-oxide copper gold (IOCG) deposit is hosted within the late Paleoproterozoic Dahongshan Group in the Kangdian region, SW China. The orebodies are spatially associated with sub-volcanic intrusions and are structurally controlled. Extensive metamorphic overprinting during the Neoproterozoic makes it difficult to determine the initial compositions of the ore fluids. Tourmaline, however, is a useful phase for this purpose because of its refractory nature and chemical diversity. It occurs widely in the four stages of mineralization and alteration of the Dahongshan deposit, including pre-mineralization sodic alteration (stage I), iron-oxide mineralization (stage II), sulfide mineralization (stage III), and late quartz-calcite veining (stage IV). The temporal association between tourmaline and hydrothermal alteration provides an excellent opportunity to investigate the fluid evolution.Tourmaline from all four stages belongs to the alkali group and ranges from dravite to schorl in composition. In-situ B-isotope analyses of tourmaline conducted by LA-MC-ICPMS yielded a total range of δ11B values from − 14.7‰ to + 5.9‰ and the values show significant variations among the different stages. Tourmaline of stage I albitization has δ11B values from − 14.7‰ to − 5.7‰. Stage II tourmaline from Fe-oxide ores shows δ11B values of − 11.6‰ to − 6.1‰, similar to those of stage I, whereas stage III tourmaline from Cu-sulfide ores displays significantly heavier isotopes with δ11B values from − 4.4‰ to − 0.6‰. Tourmaline of stage IV, represented by barren quartz-calcite-tourmaline veins in intensely-altered wall rocks, has positive δ11B values from + 2.9‰ to + 5.9‰. The observed variations of B isotopes cannot be produced by fractionation of the ore fluids alone. Mixing of B from at least two distinct sources, a magmatic source and external sources with highly positive δ11B values, are hence proposed on the basis of geological constraints and modal calculations. The calculated oxygen isotope compositions of fluids in equilibrium with hydrothermal minerals that associated with tourmaline from stage II, stage III, and stage IV are 7.4‰ to 10.2‰, 8.4‰ to 8.6‰, and 3.8‰ to 8.0‰. The decreasing trend of δ18O of fluids from stage II to stage IV is consistent with the incursion of external basinal brines by fluid mixing. Our dataset thus demonstrates that ore fluids of Dahongshan IOCG system were initially of magmatic origin (stages I and II) and were later progressively modified by mixing with external brines (stages III and IV).