埃达克岩与热液成矿过程中钾镁同位素分馏及其指示意义:以驱龙斑岩铜矿床为例

Potassium and magnesium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks and its indicative significance:A case study of the Qulong porphyry copper deposit,southern Tibet

为研究后碰撞埃达克质岩岩浆分异和热液过程中的钾镁同位素分馏行为及其对斑岩铜矿床岩石成因的指示意义,本文对藏南驱龙斑岩铜矿床的一套埃达克质岩石开展了钾镁同位素分析。该套岩石包括闪长质包体、花岗闪长岩和花岗斑岩,后者是超大型斑岩铜矿床的成矿母岩。已有研究结果证实,这3套岩石很可能是由同一来源的不同程度的部分熔融和分离结晶作用形成的。研究结果表明,闪长质包体、花岗闪长岩和花岗斑岩的δ^(41)K组成分别为-0.38‰~-0.22‰、-0.43‰~-0.34‰和-0.59‰~-0.36‰。尽管3类岩石各自变化范围较小,但整体上δ^(41)K与K 2O和SiO_(2)含量呈现明显负相关趋势,表明全岩钾元素迁移和钾同位素发生了解耦,钾同位素产生了分馏。在闪长质包体花岗闪长岩演化期间,δ^(41)K与Sc和Y含量呈明显正相关关系,这指示了在该演化期间可能主要是角闪石分离结晶导致了钾同位素分馏。相比之下,花岗斑岩中具有明显的钾同位素组成变化,在花岗斑岩演化期间,δ^(41)K与Eu/Eu*和Sr含量呈正相关关系,表明在岩石演化晚期钾同位素的分馏可能主要跟斜长石分离结晶有关。然而,通过瑞利分馏模拟表明仅矿物分离结晶难以实现如此大的钾同位素分馏,花岗斑岩中的钾同位素分馏是岩浆热液共同作用的结果。值得注意的是,闪长质包体具有比地幔更重的钾同位素组成,导致这种现象的原因可能是复杂多样的:一方面,下地壳可能具有比地幔更重的钾同位素组成,岩浆混合机制下导致源区具有重钾同位素组成;另一方面,驱龙斑岩铜矿床的岩浆源区受到俯冲板片来源的富重钾流体/熔体所交代,导致其源区的钾同位素组成偏重。相比之下,这3类岩石的镁同位素组成比较类似(闪长质包体:-0.36‰~-0.19‰;花岗闪长岩:-0.28‰~-0.13‰;花岗斑岩:-0.44‰~-0.13‰),并且落在了地幔和下地壳范围内。这3类岩石的δ^(26)Mg跟MgO和SiO_(2)含量之间不存在相关性,表明埃达克质岩浆分异过程中(角闪石和黑云母)也不产生镁同位素分馏,与前人对花岗岩中镁同位素分馏行为的结论相一致。作为典型的热液型矿床,藏南驱龙斑岩铜矿床和江西德兴斑岩铜矿床在热液蚀变过程中均产生了钾镁同位素分馏。德兴斑岩铜矿床大部分蚀变岩的钾镁同位素值都高于新鲜岩浆岩,具有显著的钾镁同位素组成变化;相比之下,驱龙斑岩铜矿床埃达克质岩岩石的钾镁同位素组成大部分位于岩浆岩基线值范围内,受热液蚀变作用影响不显著。此外,驱龙斑岩铜矿床花岗斑岩中铜大量富集,岩浆热液流体出溶是导致铜大量富集的主要原因,可能至少有两种不同性质(温度、盐度)的热液流体导致花岗斑岩中钾镁同位素组成局部波动变化,这些流体不仅改变了花岗岩斑岩的钾镁同位素组成,并且从高度演化的岩浆中提取金属元素,最终导致铜成矿。

The study focused on investigating the behavior of potassium and magnesium isotopes during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks from the Qulong porphyry copper deposit in southern Tibet.The analyzed rocks included dioritic enclaves,granodiorites,and granite porphyries,with the latter containing a super-large copper deposit.The research confirmed that these rocks likely originated from different degrees of partial melting and fractional crystallization from the same source.Theδ^(41)K values for the dioritic enclaves,granodiorites,and granite porphyries ranged from-0.38‰to-0.22‰,-0.43‰to-0.34‰,and-0.59‰to-0.36‰,respectively.While the variation inδ^(41)K values among the rock types was small,there was a clear negative correlation betweenδ^(41)K and the concentrations of K 2O and SiO_(2),indicating potassium isotope fractionation during the migration of potassium in the rocks.The study suggested that the evolution of dioritic enclaves to granodiorites showed a positive correlation betweenδ^(41)K and Sc and Y,implying that the separation and crystallization of hornblende played a significant role in potassium isotope fractionation.In contrast,the composition of potassium isotopes varied significantly in granite porphyries.The evolution of granite porphyries displayed a positive correlation betweenδ^(41)K and Eu/Eu*and Sr,suggesting that potassium isotope fractionation in these rocks may be primarily related to the fractional crystallization of plagioclase during late-stage rock evolution.The study also indicated that the observed potassium isotope fractionation in granite porphyries likely resulted from the combined influence of magma and hydrothermal fluid,rather than solely from mineral separation and crystallization processes.The heavier potassium isotopic composition in dioritic enclaves compared to the mantle was attributed to potential factors such as the lower crust having a heavier K isotopic composition than the mantle,and metasomatism of the magma source area by heavy potassium-rich fluid/melt from the subduction plate.In terms of magnesium isotopes,theδ^(26)Mg values for the three types of rocks(dioritic enclaves,granodiorites,and granite porphyries)were similar and fell within the range of the mantle and lower crust.There was no correlation betweenδ^(26)Mg and MgO or SiO_(2)in these rocks,indicating that magnesium isotope fractionation did not occur during the differentiation of adakitic magma,consistent with previous findings on magnesium isotope behavior in granites.Comparing the Qulong porphyry copper deposit in southern Tibet to the Dexing porphyry copper deposit in Jiangxi,both as typical hydrothermal deposits,the study noted that the altered rocks in Dexing showed higher potassium and magnesium isotope values than fresh magmatic rocks,with significant changes in their isotopic composition.In contrast,the potassium and magnesium isotopic compositions of adakitic rocks in Qulong were mostly within the baseline range of magmatic rocks and were not significantly affected by hydrothermal alteration.The study highlighted that the large copper enrichment in granite porphyries at the Qulong deposit was mainly due to the exsolution of magmatic hydrothermal fluid.It suggested the presence of at least two types of hydrothermal fluids with different properties(temperature and salinity)causing local fluctuations in the isotopic composition of potassium and magnesium in the granite porphyries.These fluids not only altered the isotopic composition of potassium and magnesium but also facilitated the extraction of metallic elements from highly evolved magma,ultimately leading to copper mineralization.