Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks

To investigate the behavior of Li isotopes during magmatic differentiation and the petrogenesis of Cu-bearing ore deposits, a suite of post-collisional adakitic rocks from Qulong region, southern Tibet, was studied. Their lithologies range from diorite through granodiorite to granite porphyry with the latter containing giant Cu deposits. Detailed evidence of field observation and geochemical signature suggest that these three sets of rocks were most likely formed by various degrees of partial melting and fractional crystallization from the same source. The dioritic enclave, granodiorite and granite porphyry have d7Li values ranging from 0.2 to 8.2 parts per thousand , 3.1 to 6.8 parts per thousand , and 3.9 to 7.4 parts per thousand , respectively. Most of these samples are overlapping in Li isotope composition, comparable to other granitoids worldwide, indicating insignificant Li isotope fractionation during partial melt-ing and magma differentiation in adakite-like rocks. By contrast, their Li concentrations are mainly controlled by fractional crystallization as suggested from different modal mineralogy. This process does not lead to further enrichment of Cu although they have initial high concentrations (an average of similar to 104 ppm). In comparison, granite porphyry has extremely high Cu con-tents (up to 2000 ppm) and their d7Li values are positively correlated with Cu content, suggesting the involvement of mag-matic fluids that most likely exsolved from deep magma chamber. Such fluids not only modified the Li isotopic compositions of granite porphyries, but also extracted metal elements from the highly evolved magma, eventually resulting in the Cu mineralization. Our work here provides new insight into the formation and evolution of the porphyry Cu -bearing deposit.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

By studying rocks from the Qulong region in southern Tibet, it was found that these rocks were most likely formed by various degrees of partial melting and fractional crystallization from the same source. The Li isotope composition did not show significant fractionation during partial melting and magma differentiation. The Cu mineralization was related to magmatic fluids exsolved from a deep magma chamber.