Petrogenesis and oxidation state of biotite granite, and cassiterite u-pb age from the Debao Cu-Sn skarn deposit, southwestern China: Implications for coupled Cu-Sn mineralization

Abstract

Copper and Sn mineralization are seldom formed from the same magmatism due to the distinct geochemistry of Cu and Sn (i.e., Cu and Sn mineralization are usually associated with oxidized mantle-derived intrusions and reduced crust-derived granites, respectively). Previous studies proposed that the formation of Cu-Sn deposits was due to injection of the oxidized mafic magma and/or associated fluids to the reduced magma chambers. The Debao skarn deposit in Guangxi (SW China) has uncommon coupled Cu-Sn mineralization, and it is unclear whether the Cu-Sn mineralization is associated with two different types of magmatism or not. This study presents zircon U-Pb dating, element and Hf-O isotopic data of mineralization-related granites, as well as cassiterite U-Pb ages for the Debao deposit, to constrain the genesis of the coupled Cu-Sn mineralization. The Cu-Sn mineralization is spatially associated with two types of biotite granites: the porphyritic and equigranular biotite granites which have zircon U-Pb ages of 442.5 ± 3.1 Ma to 439.0 ± 2.2 Ma and 439.1 ± 2.6 Ma, respectively. Cassiterite U-Pb dating yielded a mineralization age of 445.6 ± 3.2 Ma, coeval with the emplacement of both types of granites. The two granites have similar zircon δ18O values of 6.9–8.4 ‰, and variable εHf(t) values of − 9.9 to 6.1 (mostly − 3 to 3) with Hf model ages of 2042–1036 Ma, indicating that they were cogenetic and mainly derived from Mesoproterozoic metasedimentary rocks in the Sibao Group with some input of juvenile materials. The equigranular biotite granites have higher SiO2 contents (74.97–76.41 wt%) and Rb/Sr ratios (10.9–12.6), as well as lower Zr/Hf (23.4–24.6) and Nb/Ta ratios (4.4–5.3) than those of the porphyritic biotite granites (SiO2 = 70.70–73.27 wt%, Rb/Sr = 1.0–2.8, Zr/Hf = 23.4–36.7, Nb/Ta = 7.2–9.4), indicating that the former underwent a much higher degree of magmatic differentiation than the latter. Moreover, the two granites are relatively reduced, as indicated by low zircon Ce4+/Ce3+ ratios (5.1–113.9, mean 41.4) and calculated logfO2 values (−29.3 to − 12.7, mean − 20.9), which plot below the FMQ buffer. The Sn and Cu metals likely came from the Sn-rich metasedimentary rocks and the Cu-rich juvenile materials in the magma source region, respectively. Under the reduced and probably S-poor magma system, both the Sn and Cu are relatively incompatible and would be enriched in the residual melts. Therefore, we suggest that the Cu-Sn mineralization at the Debao deposit is genetically associated with the highly fractionated equigranular biotite granites, without the addition of oxidized mafic magma and/or associated fluids. Nevertheless, Cu endowment associated with such reduced magmas is expected to be small.