Assessing the W-Sn ore formation potential of large granite batholiths: Insights from zircon U-Pb and muscovite Ar-Ar geochronology of the Tianmuchong W-Sn deposit in the Nanling region, South China

Abstract

The Nanling region in South China represents one of the giant tungsten-tin (W-Sn) metallogenic belts on Earth, reserving a variety of W-Sn deposits associated with highly differentiated small granitic stocks. In contrast, large granite batholiths, characterized by a low degree of fractionation and a high degree of erosion/exhumation, are considered less favorable for forming W-Sn deposits. However, some W-Sn deposits are found to be related to large granite batholiths spatially and genetically, and the reasons remain a mystery. In this research, we take the Tianmuchong polymetallic W-Sn deposit as an example to assess the W-Sn ore formation potential of a large granite batholith. This deposit forms at the contact boundary between the Tashan granite batholith and wall rock. It is spatially related to the fourth intrusive unit (tourmaline granite) of the Tashan batholith, which contains abundant tourmaline and muscovite. Zircon U-Pb analysis on the tourmaline granite shows an age of 209.9 ± 2.2 Ma, in agreement with muscovite Ar-Ar age of 208.5 ± 2.07 Ma associated with W-Sn ores. The similar ages indicate that the emplacement of tourmaline granite and the W-Sn mineralization are coeval. The dated zircons display elevated U, Th, and Hf contents and negative Eu anomalies, implying an origin of highly differentiated magma. The occurrence of tourmaline and muscovite indicates that the Tashan tourmaline granite is volatile-rich (e.g., B and F), which facilitates the W-Sn enrichment under prolonged differentiation of the granitic magma. Additionally, the W-Sn ore mineralizations occur along the contact zones controlled by fault structures, which provide a favorable pathway for the upward intrusion of highly differentiated melt and the migration of W-Sn-bearing hydrothermal fluids. Therefore, the late phase of the large granite batholith that exhibits highly evolved characteristics also demonstrates a considerable potential to form W-Sn deposits, especially along the margins of the batholith with country rocks where faults are extensively developed.