Contrasting geochemistry of apatite from the Jurassic W- and Sn-mineralized granites in the Qitianling field, Nanling Range, South China

Abstract

Tungsten and tin are vital to human society, however, their spatio-temporal decoupling mechanisms remain debated. The Qitianling batholith in the Nanling Range hosts the Xintianling W deposit in the northeast and the Furong Sn deposit in the southwest, making it an ideal place to study the spatial separation of Jurassic W-Sn deposits. This study analyzes apatite geochemistry and Nd isotopes in the Xintianling W-mineralized granites, comparing them with published data of the Qitianling Sn-mineralized granites. Apatite in the Qitianling Sn-mineralized granites exhibits ε_Nd(t) values of −8.19 to −5.88 with T_DM2 ages ranging from 1615 to 1428 Ma, while apatite in the Xintianling W-mineralized granites displays clustered ε_Nd(t) values of −9.27 to −8.06 with T_DM2 ages ranging from 1706 to 1608 Ma. The overlapping T_DM2 ages suggest that both were derived from the partial melting of Paleo- to Mesoproterozoic crustal materials, with the addition of mantle-derived melts in the Qitianling granites. The Sr and ΣREE contents, along with (La/Sm)_N, (La/Yb)_N, and Eu/Eu* ratios of apatite indicate that the Qitianling granites underwent significant feldspar and hornblende fractional crystallization, while the Xintianling granites experienced significant feldspar, monazite, and allanite fractional crystallization. Apatite in the Qitianling granites has higher FeO contents and lower MnO, SO₃ contents and Eu/Eu* ratios than apatite in the Xintianling granites, suggesting more reducing conditions for the former. The Qitianling Sn-mineralized granites have lower F and SO₃ contents and higher Cl contents than the Xintianling W-mineralized granites. Therefore, source magma chemistry and its subsequent magmatic evolution, along with intensive physical-chemical variables together play a significant role in controlling the spatial segregation of W-Sn deposits. Supervised orthogonal partial least squares discriminant analysis (orthoPLS-DA) was used to evaluate the suitability of apatite as a discriminator for the decoupling of W-Sn deposits. Results reveal that LREE, Mn, Hf, Nd, and Eu are the key elements contributing to the discrimination of apatites between the Qitianling Sn and Xintianling W granites, which can be a discriminator for decoupling of W-Sn deposits in the Nanling Range and beyond in South China.