中国东南部荒芜的巫山岩浆-热液演化:洞察成矿潜力的控制因素

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2024-05-07 DOI:10.1093/petrology/egae047
Zhe Chi, Jun-Yi Pan, Pei Ni
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引用次数: 0

摘要

A 型花岗岩通常会富集和矿化钼和锡等对新兴技术至关重要的关键金属。然而,影响其成矿潜力的关键因素仍然难以捉摸。对贫瘠系统的研究很少,这阻碍了对这一问题的理解。在此,我们对贫瘠的巫山岩浆岩进行了详细的熔融和流体包裹体研究,以重建其岩浆演化和岩浆-热液转变过程,并探索控制A型花岗岩中钼和锡成矿潜力的因素。巫山岩浆岩表现出明显的岩性分带,由中粒绢云母至斑状碱长石花岗岩和细粒斑状花岗岩两大部分组成。每种岩性中都广泛发育有米鲕状空洞。两个花岗岩相的硅酸盐熔融包裹体为流纹岩,F含量适中(0.06-0.53 wt %),H2O含量较低(2.0-3.5 wt %)。熔融包裹体的不相容元素含量范围很广,如 Cs(9-1977 微克/克)和 Rb(268-2601 微克/克),这表明巫山经历了高度的岩浆演化。钼在岩浆演化过程中表现不一致,其含量随着分晶程度的增加而富集,但在铯含量超过 50 微克/克后保持不变。雷利分馏模型表明,大量的钼从流体外溶解中提取出来,从而抑制了钼的进一步富集。与此相反,在整个岩浆演化过程中,硒表现为轻度不相容元素。与钼元素的变化趋势相比,硒元素的含量增长缓慢,在高度演化的熔体中,硒元素的最大含量达到~30 μg/g。生铁、磁铁矿和榍石等含硒矿物的分离和结晶抑制了硒的富集。寄存在斜长石石英中的中间密度(ID 型)流体包裹体代表了从岩浆中流出的初始流体,显示出较高的钼浓度,但锑浓度较低。通过对世界范围内的钼矿化侵入体和贫瘠侵入体进行对比,可以发现熔体和流体中的金属含量并无本质区别。矿化侵入体的特点是熔体粘度较低和裂隙发育,这两种情况都有利于从大型岩浆室中提取金属和流体,然后将其浓缩到较小的岩石体积中。由此看来,在钼矿化过程中起关键作用的是物理和结构条件,而不是化学成分。熔体中锡的富集对锡的矿化是必要的,但不是决定性的,而初始解理流体中锡的富集则决定了特定花岗岩体系的锡矿化潜力。源熔融和分馏结晶通常会提高高演化熔体的最终锑富集度,与源熔融和分馏结晶中的锑富集度相比,熔体和流体之间的锑分配效率在将熔体富集度转化为锑富集流体,从而提高岩浆-热液系统的成矿潜力方面起着根本性的作用。我们的研究结果表明,在华南沿海A型花岗岩带,钼的勘探前景广阔,而硒的成矿潜力预计有限。
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Magmatic-Hydrothermal Evolution at the Barren Wushan Pluton, Southeast China: Insight into Controls on Mineralization Potential
A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic-hydrothermal transition and to explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potential of a given granitic system. Compared to Sn enrichment in source melting and fractional crystallization which commonly enhance final Sn fertility in the highly-evolved melts, the efficiency of Sn partitioning between melt and fluid plays a fundamental role in converting melt fertility into Sn-enriched fluids and thereby high mineralization potential of the magmatic-hydrothermal system. Our findings suggest a prospect for Mo exploration in the coastal A-type granite belt in South China, while the potential for Sn mineralization is expected to be limited.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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