THE ORIGIN OF THE ZHANGJIALONG TUNGSTEN DEPOSIT, SOUTH CHINA: IMPLICATIONS FOR W-Sn MINERALIZATION IN LARGE GRANITE BATHOLITHS

摘要

The Nanling region is the largest W-Sn metallogenic district on Earth and hosts several giant W-Sn deposits, all except one of which are spatially and genetically associated with highly evolved Mesozoic granitic stocks. Volumetrically, however, Caledonian granites (Paleozoic), mainly batholiths, approach their Mesozoic equivalents in importance and have been the target of recent exploration. This has resulted in the discovery of a number of economic W-Sn deposits in or near the Caledonian batholiths, including the giant Zhangjialong deposit, which is located on the southern margin of the Penggongmiao granite batholith. The unresolved question is whether this is evidence for an important Caledonian epoch of W-Sn mineralization. In this contribution, we report the results of high-precision secondary ion mass spectrometry (SIMS) zircon U-Pb, muscovite Ar-Ar, and molybdenite Re-Os age determinations that constrain the timing relationships among granitic magmatism, greisenization, and W mineralization related to the Zhangjialong deposit. The molybdenite Re-Os age of the W mineralization is 160.2 ± 2.2 Ma, which is similar to, albeit slightly older than, the muscovite Ar-Ar age of the greisen (153.5 ± 1.0 Ma). These ages, however, are considerably younger than the zircon SIMS U-Pb age of 441.3 ± 2.4 Ma for the spatially associated granite. These data demonstrate that the W mineralization and greisenization of the Zhangjialong W deposit is of Late Jurassic rather than Silurian age, which precludes a temporal and genetic link between the hydrothermal W mineralization and the regional Caledonian magmatism. Instead, the W mineralization is interpreted to be genetically related to a hidden Late Jurassic granitic pluton. A compilation of published whole-rock geochemical data indicates that the large granite batholiths are less differentiated and poorer in W and Sn than the W-Sn bearing granite stocks, irrespective of whether they are Paleozoic or Mesozoic in age. This suggests that the metallogenic potential of the large granitic batholiths is limited, and that W-Sn deposits hosted within granite batholiths are likely to be genetically related to highly evolved granitic stocks that in some cases have not been exposed.