锡的地球化学性质与华南晚白垩世锡矿成因

锡在地质过程中表现出亲氧、亲硫和亲铁三重特性。在地幔岩浆过程中,锡是一种中等不相容的金属元素。锡成矿主要与酸性岩浆活动有关,其地球化学性质决定了其成矿主要受源区性质、氧逸度以及挥发分含量、岩浆结晶分异等因素控制。高度结晶分异可使锡在岩浆中进一步富集,是锡成矿相关花岗岩的普遍特征;锡为变价元素,岩浆体系氧逸度影响源区中锡的迁移能力和分离结晶过程中锡的元素行为,还原性岩浆体系有利于锡富集成矿。富含F、Cl和B等挥发组分对锡元素的迁移和富集起积极的作用。全球锡矿床分布与俯冲带关系密切,特提斯和环太平洋构造域是主要蕴藏区。重要的锡成矿事件表现出区域性和阶段性的特征。结合锡的地球化学特性以及锡矿分布特征,我们认为最有利锡成矿的动力学机制是俯冲板片后撤机制。俯冲板片后撤引发深部软流圈地幔上涌,导致强烈的壳幔相互作用,形成低氧逸度、富F、Cl和B等花岗岩,有利于锡成矿。对于华南晚白垩世锡成矿事件,新特提斯洋俯冲板片发生后撤是其成矿地球动力学背景。

The geochemical behavior of tin and Late Cretaceous tin mineralization in South China

Tin shows triple characteristics of chalcophile, lithophile and siderophile affinities in geological processes. It is a moderately incompatible element during mantle magmatism. Tin mineralization is mainly related to felsic magmatic activities. The geochemical properties of tin determine that its mineralization is mainly controlled by magmatic differentiation, source region properties, oxygen fugacity and volatile components. The high degree of crystallization differentiation can further enrich tin in magma, which is a common feature of granites related to tin mineralization. Whether tin mineralized granites are derived from enriched source rocks or not, however, remains controversial, the presence of tin-rich protoliths may increases the scale of tin mineralization. The decomposition of tin minerals in the source region is one of the key factors to control whether the magmatic melt is enriched in tin, hence high temperature melting is the basic condition for tin migration from source region. Tin is sensitive to oxygen fugacity. A reducing magma system is beneficial to tin mineralization, whereas volatile components, such as F, Cl, and B, play an active role in the migration and enrichment of tin. The distribution of tin deposits in the world is closely related to subduction zones, e.g., the Tethys and the circum-Pacific tectonic domains have the main tin reserves of the world. Important tin mineralization events show regional and phased characteristics. Combined with the geochemical characteristics of tin and the distribution characteristics of tin deposits, we propose that the most favorable dynamic mechanism for tin mineralization is the roll-back of subducting slab, which triggers deep asthenosphere mantle upwelling, results in strong crust-mantle interaction, and forms granite rich in F, Cl, and B with low oxygen fugacity, which is favorable for tin mineralization. For example, the Late Cretaceous tin mineralization event in South China was controlled by the roll-back of the subduction plate of the New Tethys Ocean.