Geology, Geochemistry And Metallogenesis Of The Shangfang Tungsten Deposit In Jianou County,Northern Fujian Province, SE China

The newly-discovered Shangfang large-scale scheelite deposit is located in Jian’ou County, Northwest Wuyishan metallogenic belt. Based on detailed field investigation and drilling-core logging, the author carried out a comprehensive study in order to reveal the ore-forming controls, composition and characteristics of of the ores, age and evolution of mineralization and ore-related magmatism, and source of mineralizing fluids and components in the fluids. The results provide, for the first time, significant insights into the genesis of the Shangfang tungsten deposit, enabling to build a metallogenic model for the deposit. When combined with existing geological and geochronological data, this study demonstrates a genetic association between the late Jurassic (160～150Ma) igneous magmatism and tungsten mineralization in the Shangfang mine and within the Wuyishan metallogenic belt, both formed under a continental-margin setting related to the westward subduction of the Paleo-pacific plate. The ultimate objective of this study is to build a predictive model as a guide for future prospecting and exploration of tungsten deposits in northern Fujian Province.Orebodies of the Shangfang scheelite deposit are localized along the contact zone between the biotite syenitic granite and the amphibolites of the Paleoproterozoic Dajinshan Formation. The Dajinshan Formation consists mainly of amphibolites and biotite-plagioclase granulite. Petrographic and geochemical analysis suggests that protoliths of the granulite are likely monzonite derived from reworking of Paleoproteroozic crustal components. Rocks of the monzonite are high K-calc-alkaline, and peraluminous with A/CNK ranging from1.08to1.95. These rocks are depleted in Mk Ta、P、Ti、Sr, but enriched in Rb、K、Pb、Th, and reselmbe S-type granites. The amphibolites have ISr=0.706452～0.708575and εNd(t)=0.15～1.09; and resulted likely from metamorphism of precursor gabbroic intrusions derived from an enriched mantle source. An amphibolite sample yielded a LA-ICP-MS zircon U-Pb age of388±10Ma (MSWD=1.7), which likely record the regional metamorphic event of the Late Caledonian.Rocks of the Shangfang biotite syenitic granite have71.11%to75.39%SiO2,7.78%to8.94%K20+Na20with K20/Na2O ratios from1.39to2.07and A/CNK of1.01and1.11, but low MgO、TiO2、P2O5contents. They belong to weak peraluminous to peraluminous and high-potassium calc-alkaline granite. They are depleted in LILE (e.g Sr、Ba) and HFSE (e.g. Ti、 P), and have (La/Yb)N=2.20～13.47and δEu=0.11～0.43. The bulk samples have ISr=0.712804～0.711713, εNd(t)=-10.10～-9.35,208Pb/204Pb=38.9450～39.0650,207Pb/204Pb=15.6544～15.6705, and206Pb/204Pb=18.5630～18.5789, which are consistent with zircon εHf(t) of-19.2～-14.7. It is suggested that the granites were formed by remelting of the Paleoproterozoic crustal material of Cathaysian Block.The orebodies are mostly layered, stratiform or lenticular and mostly confined within amphibolites that is intruded by the Shangfang syenitie granite. Metallic minerals are dominated by scheelite, molybdenite, pyrrhotite and pyrite with minor chalcopyrite, galena and sphalerite. Nonmetallic minerals are mainly composed of garnet, diopside, albite, actinolite, tremolite, epidote, quartz, chlorite, sericite, fluorite, and calcite. According to mineral assemblages, two types of ores can be recognized:actonolite-pyrrhotite-scheelite and diopside-quartz-scheelite veins.The paragenetic sequences of the ore and alteration minerals indicate four hydrothermal stages in the formation of the deposit, including garnet-diopside (skarn) stage, oxide (actinolite-scheelite stage) stage, quartz-sulfide stage and carbonate stage. Scheelite was mainly precipitated during the oxide stage and, less siginificantly, in the quartz-sulfide stage.The scheelite have rare earth elements (REE) ranging from581.22to3440.6μg/g with LREE/HREE ratios from1.09to8.75. They have two distinct chondrite-normalized REE pattern: low MREE with positive Eu anomalies and high MREE with negative Eu anomalies. Low MREE scheelite have LREE/HREE ratios higher than3.9and δEu from1.19to3.60, whereas the high MREE varieties have LREE/HREE lower than3.6and δEu from0.14to0.80. The author proposes that the Eu anomalies in scheelite were formed by metasomatism of ore-forming fluid with Eu enriched or depleted minerals in the country rocks. Scheelite with positive Eu anomalies were formed by fluids equilibrated with plagioclase, whereas the equivalents with negative Eu anomalies were precipitated from amphibole metasomatized fluids. REE3-and Na+can replaces Ca2+in scheelite lattice indicating ore-forming fluids are rich in Na-.Scheelite, quartz, and calcite contain abundant fluid inclusions. Primary fluid inclusions mostly have liquid phase with small vapor phase (L+V type)(more than90%). From the main metallogenic stage Ⅱ to the late metallogenic stage Ⅲ, homogenization temperatures decrease from190～240℃to150～180℃; the salinity of ore-forming fluid decrease from4～6wt.%NaCl equiv. to1.2～3.2wt.%NaCl equiv.; but fluid densities increase from0.80～0.90g/cm3to0.88～0.94g/cm3. Fluid inclusion extracted from quartz of stageⅢ have H isotope of-57.1‰to-76.3‰, and the quartz have δ18O from11.3%o to12.3%o, which are equilibrated with ore-forming fluid with δ18O of4.9%o to5.9‰. Sulfides have δ34S values between0.78%o and5.29‰; and the δ34S value of molybdenite, pyrrhotite and pyrite are decreased (δ34SMo>δ34SCpy>δ34Spy), indicating that sulfur isotopic equilibrium between the sulfides and ore-forming fluid. Results of the stable isotopes suggest that sulfur was dominantly derived from magmas, whereas ore-forming fluids have a mixing source of magmatic and meteoric fluids. Two samples collected from the Shangfang granites have identical LA-ICPMS zircon U-Pb ages, of157.6±1.4Ma (MSWD=1.3) and158.8±1.4Ma (MSWD=3.4), which represent the emplacement age of granite. The scheelite has a Sm-Nd isochron age of162±7Ma, which is within errors of molybdenite Re-Os isochorn age of158.1±5.4Ma. Both ages are in good agreement with zircon U-Pb ages suggesting that formation of the Shangfang scheelite deposit was synchronous with emplacement of granite intrusions and ore-forming fluids are derived from the intrusions. The ages are also contemporaneous with regional large-scale W mineralization in South China. Our newly obtained geochronological data have shown that the160～150Ma W mineralization in South China have extended from southern Hunan, northern Guangdong and southern Jiangxi (namly the Nanling metallogenic belt) to western and northern Fujian Province of the Wuyi Mountain. The distribution of the W deposits are nearly NE or NEE treading, rather than EW trending as previously thought. This indicates that the Late Jurassic (160～150Ma) large scale W deposites in South China were not limited in the well-known Nanling region, but in a much wider area.A combination of regional geology, ore-hosting rocks, mineralization styles, hydrothermal alteration, paragenetic sequences, fluid inclusions, stable isotopes, and geochronology support that the Shangfang scheelite deposit is of reduction skarn in origin (pyrrhotite type). Ore-forming-fluids and metals were derived from late Jurassic granites. A genetic model for the Shangfang W deposit is proposed, which is useful for future mineral exploration in the northwest Fujian Province.Using this model, combined with integrated geological information prediction technique (MgeoM technique), seven prospective zones and ten prospecting target areas have been delineated, among which two areas are level A prospecting targets, four areas are level B prospecting targets, and the predicted WO3resources are up to530000tons.