Petrogenesis And Metallogeny Of The W-and Sn-bearing Granites In Southern Hunan Province

The Nanling Range in South China is characterized by well-developed non-ferrous and rare metal deposits as well as related various cycles of polygenetic granitic rocks.The Wangxianling area is located in the middle part of the Nanling Range.In situ zircon and cassiterite U-Pb,molybdenite Re-Os and muscovite 400Ar-39Ar dating techniques are used to refine the age spectra of granitic magmatism and W-Sn mineralization in the Wangxianling-Hehuaping area.Zircon U-Pb dating results of the central-phase medium-grained tourmaline two-mica granite and marginal-phase medium-grained tourmaline muscovite granite from the Wangxianling pluton defines granite emplacement age at-224 Ma.Tungsten-bearing quartz veins from both the Shuiyuanshan and Yejiwo tungsten mines have molybdenite Re-Os age of-224 Ma,consistent with the zircon U-Pb age of the Wangxianling granite and indicative of close association of the W mineralization with the Late Triassic granite.Muscovite of scheelite greisen in the Shuiyuanshan tungsten mine has an 40Ar/39Ar plateau age of 214 Ma suggesting the cooling history of about 10 Ma for the Wangxianling pluton.The Hehuaping biotite granite contains zircon with U-Pb age of?156 Ma.The granite porphyry dykes have zircon U-Pb age of?156 Ma.In situ LA-MC-ICP-MS U-Pb dating for cassiterite of both the Daguling tin skarn and Yejiwo tin greisen yields an average age of?156 Ma,indicative of their relation with the Late Jurassic granitic magmatism.Two mineralization events can be distinguished in the Wangxianling area.The W-dominant mineralization event is related to the Late Triassic Wangxianling granite,whereas the Sn-dominant mineralization is associated with the Late Jurassic Hehuaping biotite granite and granite porphyry dykesThe Late Triassic Wangxianling W-bearing granites have relatively high P2O5(0.22 wt%)and Al2O3(14.37 wt%),low Zr+Nb+Ce+Y(118.3 ppm)and REE(65.6 ppm)contents.Their initial 87Sr/86Sr ratios,?Nd(t)and?Hf(t)values range from 0.7172 to 0.7305,-7.1 to-9.5,-10.2 to-11.5,respectively,corresponding two-staged Nd and Hf model ages in the range from 1.83?1.95 Ga and 1.5?2.0 Ga.The Late Jurassic Hehuaping Sn-bearing biotite granite and granite porphyry have low P2O5(0.04 wt%)and Al2O3(12.91 and 12.25 wt%),high Zr+Nb+Ce+Y(334 and 385 ppm)and REE(287 and 303 ppm)contents.They have lower initial 87Sr/86Sr ratios(<0.720),higher sNd(t)and ?Hf(t)values(>-8)and younger Nd-Hf model ages(?1.5 Ga)than those of the Wangxianling granite.The Wangxianling granites,and Hehuaping granite and granite porphyry have affinities to the S-type and A-type granites,which were derived from partially melting of Paleoproterozoic metamorphic basement and granulitized Mesoproterozoic metamorphic basement,respectively.The Xintianling granitic complex in the central Nanling Range hosts one(80.9 Mt@0.36%WO3)of the largest scheelite skarn deposits in the world.The complex is composed of a central phase of medium-grained biotite granite and a marginal phase of fine-grained biotite granite,both of which have been crosscut by later granite porphyry dykes.The two granite phases and granite porphyry have zircon U-Pb ages of 165.0 Ma,164.0 Ma and 147.5 Ma,respectively.The biotite granite phases are metaluminous to slightly peraluminous,and of high-K calc-alkaline composition.These biotite granites have high initial 87Sr/86Sr ratios of 0.7116 to 0.7198,lower ?Nd(t)values of-7.1 to-9.5 and zircon ?Hf(t)values of-4.1 to-11.4 and two-stage Nd and Hf model ages of 1.6 to 1.7 Ga,indicative of S-type granite affinity and partial melting of Paleoproterozoic immature graphite-absent metagreywacke.They were formed by fractional crystallization of apatite,monazite,K-feldspar and plagioclase from one magma system.Granite porphyry dykes cut through the biotite granites and the skarn ore body,and have affinity to A-type granite.Compared with biotite granite,the dykes have similar initial 87Sr/86Sr ratios(0.7123 to 0.7153),slightly high ?Nd(t)(-6.2)and in situ zircon ?Hf(t)values(-5.2),and young two-stage Nd and Hf model ages(1.45 Ga and 1.53 Ga,respectively),suggesting a generation by dehydration melting of Mesoproterozoic granulitic restite in the lower crust.The Xintianling biotite granites are characterized by accessory mineral association of magnetite+pyrophanite+scheelite+wolframite+rutile+ilmenite.They have high magnetite/ilmenite,whole rock Fe3+/Fe2+ ratios and magnetic susceptibility.These features indicate a high oxidation state of the Xintianling granite magma(magnetite-series),distinctly different from other W-bearing granites in the Nanling Range,which are of ilmenite-series affinity and preclude absence of metallogenic potential of tin.Skarn in Xintianling deposit formed in four stages:Stage ?,grandite-diopside;Stage ?,scheelite-homblende-actinolite-andradite;Stage ?,scheelite-tourmaline-epidote-flourite-quartz vein;Stage IV,including molybdenite-quartz vein substage,and calcite-spessartine-sulfide vein substage.Scheelite mineralization occurred mainly in stage ?.Molybdenite Re-Os dating yields an age of 161.8 Ma for Xintianling scheelite skarn mineralization,consistent with the zircon U-Pb age of the host biotite granites,indicating a direct genetic relationship.C-O isotopic compositions of calcite veins indicate that the carbon of skam was derived by mixing of magmatic carbon and sedimentary carbon.H-O isotopic compositions of scheelite quartz veins demonstrate that the ore-forming fluid was derived by magmatic water and lesser meteoric water.Sulfur of the ore is originated from magmatic and sedimentary sulfur.Pb isotopes indicate that the skarn orebodies were derived from crustal materials.