Genetic relationship between granitic magmatism and W mineralization recorded in the Nanling Scientific Drilling (SP-NLSD-2) in the Pangushan W mining district, South China

Abstract

The Nanling Region in South China hosts numerous W-dominated polymetallic deposits that are related to Mesozoic granites. Although considerable amounts of age data have been obtained, the genetic relationship between granitic magmatism and W mineralization remains unclear. The Nanling Scientific Drilling (SP-NLSD-2) in the Pangushan W deposit has penetrated Upper Devonian sedimentary rocks (4.1–721.9 m), Sinian sedimentary rocks (721.9–1287.9 m) and Mesozoic granite (1287.9–2012.1 m). The drill hole clearly reveals a zonation including exocontact W-bearing quartz veins, alkali autometasomatic zone and primary K-feldspar granite. The K-feldspar granite at the depth of 1880 m, and rocks of potassium autometasomatic zone at 1414 m and greisenized zone at 1288 m have magmatic zircon grains with U–Pb ages of 164.8 ± 1.4 Ma, 161.7 ± 1.6 Ma, and 153.8 ± 1.2 Ma, respectively. Molybdenite Re–Os ages of endocontact mineralization at the depths of 1302 m and 1291 m are 155.9 ± 3.1 Ma and 155.0 ± 2.3 Ma, respectively. Muscovite Ar–Ar ages of exocontact mineralization at the depths of 1050 and 885 m are 154.9 ± 1.5 Ma and 154.0 ± 3.0 Ma, respectively, and molybdenite Re–Os age of exocontact quartz-vein type mineralization at 860 m is 152.0 ± 2.6 Ma. These new ages indicate that the granite emplacement, potassium autometasomatism, greisenization, and W mineralization occurred at ∼164.8 Ma, ∼161.7 Ma, ∼153.8 Ma, and 155.9–152 Ma, respectively. The W mineralization was approximately 9–13 Myr younger than the hosting granite. The whole-rock and feldspar chemical compositions of K-feldspar granite, potassium autometasomatic zone, greisenized zone, and feldspar-quartz veins indicate the sequence of metal transportation and concentration to form the zonation. The granite crystallized from high-K calcalkaline, metaluminous to peraluminous, highly differentiated magmas that were enriched in W and Bi. During crystallization, the magmas underwent potassium autometasomatism caused by post-magmatic fluids which were rich in K, Fe, W, and rare-earth elements (REE) but poor in Na and Ca, and formed the potassium autometasomatic zone. The evolved post-magmatic fluids became rich in K, Si, Ca, REE, and W but poor in Na and Fe and initiated greisenization to generate the greisenized zone, which almost exhausted Ca, REE, Na, and Fe of the fluids. The later crystallization of feldspar quartz veins exhausted K and caused the residual post-magmatic fluids to be rich in Si and W. The fluids formed W-bearing quartz veins with decreasing temperature and pressure. The main reason for W enrichment in the hydrothermal fluids was due to the successive crystallization of minerals rather than autometasomatism.