Origin Of Tungsten-bearing Granites In The Eastern Jiangnan Orogen Belt

In recent years, a series of Tungsten deposits have been discovered in eastern Jiangnan orogen belt. We refer this belt to be "Jiangnan type" Tungsten ore belt, due to its special tectonic setting, diagenetic and metallogenic age, multi-metal assemblages. Especially in the last two years, dahutang tungsten mine and zhuxi tungsten deposits in northeastern Jiangxi Province, become the world’s largest tungsten deposit, respectively, aroused more and more concern. Three granites were selected for this study:Dongyuan as a representative tungsten-bearing granite and Jingde, Taoling as barren granites. Contrast study of tungsten-bearing and barren granites by geology and geochemistry, in order to clarify the differences in diagenetic process and composition of magma, to discuss the mechanism of the magmatism and mineralization of "Jiangnan type", and to propose the criteria of tungsten exploration.Dongyuan pluton is composed of granodiorite, monzogranite, syenogranite, showing obvious of sericitization and carbonatation for the feldspar, muscovitization and weak chloritization for biotite, of which accessory mineral association is zircon+ apatite+ monazite+ allanite+ ilmeite+ tungsten+magnetite+rutile+pyrite+ molybdenite+ titanite±tourmaline. Zircon LA-ICPMS U-Pb dating shows that the pluton intruded at 146.7Ma. In addition, abundant inherited zircons are identified in the granite, the average age of Neoproterozoic is 770.2±9.7Ma. Biotite in Dongyuan pluton is classified into high-Mg type, and is plotted in the crust-mantle zone on the material source discrimination diagram. The Dongyuan pluton is characterized by high potassium calc alkaline and shoshonitic series, metaluminous-peraluminous, high Mg#, relatively high Sr and Ba, low Y and Yb, weak negative Eu anomalies (δEu=0.74～0.83), enriched in light rare earth elements (LREE) and lighe ion lithophile elements (LILE) depleted in high field strength elements (HFSE), with strongly fractionated REE pattern. Rocks exhibit high initial 87Sr/86Sr ratios (0.7124), low εNd(t) (-5.53), and variable zircon εHf(t) (-13.0～-7.0) values. Zircon saturation temperatures reveal the magma temperatures are 772℃. Interpretation of elemental and isotopic data suggests that the Dongyuan granite has some affinities with the adakite, and was most likely derived from partial melting of thickened lower crust with little contribution of mantle components. The mafic magmas ascended from the metasomatized and heated the lower crust, triggering partial melting to form the Dongyuan pluton, with input of mantle-derived materials.Barren granites:Jingde pluton is composed of granodiorite, monzogranite, and Taoling pluton is composed of monzogranite, syenogranite. Zircon LA-1CPMS U-Pb dating show the plutons intruded at-140Ma, slightly later (5Ma) than ore-bearing pluton. The barren plutons underwent weak alternation and have relatively few accessory minerals. Biotite in barren pluton is classified into high-Mg type, and is plotted in the crust-mantle zone on the material source discrimination diagram. The pluton is characterized by high potassium calc alkaline series, metaluminous-weakly peraluminous, high Mg#. The Jingde pluton has relatively high Sr and Ba, low Y and Yb, slightly negligible to positive Eu anomalies (δEu=0.81～1.18), enriched in LREE and LILE, depleted in HFSE, with strongly fractionated REE pattern. The initial 87Sr/86Sr ratios of the pluton is 0.7096～0.7101, variable eNd(t) (-12.92～5.53), and variable zircon εHf(t) (-5.7～-1.1) values. Zircon saturation temperatures reveal the magma temperatures are 792℃. Taoling pluton enrich in LREE and LREE, depleted in HFSE, Sr and Ba. Medium-weak negligible Eu anomalies (δEu=0.62～0.81) (except sample TL-1 with δEu=0.47). The initial 87Sr/86Sr ratios and εNd(t) of the pluton are (0.7131～0.7141), and (-7.43～-6.71), respectively. Taoling pluton has variable zircon εHf(t) (-10.3～-3.7) values. Zircon saturation temperatures reveal the magma temperatures are 783℃. Elemental and isotopic data suggest that the barren granites also have some affinities with the adakite. The mafic magmas ascended from the metasomatized and heated the lower crust, triggering partial melting to form the barren pluton, with relatively more mantle-derived materials input, due to reltively high εHf(t) value and zircon saturation temperatures.In ore-bearing pluton, biotite has high AlⅣ value, low AlⅥ value, low Fe3+/Fe2+ ratio, suggesting relatively lower fO2 Plagioclase in the ore-bearing pluton belong to albite, has high SiO2, TiO2, low Al2O3, low TFe2O3. Plagioclase in the barren pluton mainly is andesine and oligoclase. The ore-beraing pluton has little chlorite, having low Fe, high Mg.Geochemical discrimination includes the following:elevated metallogenic element (W: 30～214×10-6), higher F(>700×10-6), high K (3.4-5.74), for bulk rock of ore-bearing pluton and high F, low Fe, low Fe3+/Fe2+(0.122) for biotite in ore-bearing pluton. Plagioclase mainly is albite, and chlorite has low Fe in ore-bearing pluton.The earliest mafic magmas derived by partial melting of metasomatized, having higher contents of volatiles (such as F), and metallogenic matter, ascend and heated the lower crust, triggering partial melting to form the tungsten-bearing pluton. The barren plutons intruded slightly later (-5Ma) than ore-bearing pluton and show a higher magma temperature and more input of mantle-derived magma, lower contents of volatiles and metallogenic substances. The ore-forming fluids in the metallogenic process experienced immiscibility or boiling and mixing with heated meteoric water, and in weak acids-weak bases, and weak reduction conditions, causing unstableness of W and Mo complexes in the system and forming scheelite and molybdenite, respectively, resulting in enrichment of large quantities of ore-forming materials to form the ore deposit. This model is different from traditional tungsten-tin deposit related to S-type granite, which has important significance for further understand the genesis of W-Cu, and W-Mo deposits related to granite.