Cu and Fe skarns are two economically important types of skarn deposit worldwide, but the critical factors controlling the difference in metal associations remain enigmatic. The Edong ore district, China, presents an excellent opportunity to study the differences between Cu–Fe and Fe skarn deposits. We have measured He–Ar isotopes trapped in fluid released by crushing pyrite and chalcopyrite from four well known Cu–Fe and Fe deposits in the Edong district, Eastern China, with the aim of constraining their different fluid source and then discussing the factors controlling their variations between Cu–Fe and Fe skarns. He–Ar isotopic compositions are markedly different between the Cu–Fe and Fe skarn deposits in the Edong district. 3He/4He ratios in the Cu–Fe deposits are 0.75–1.87 Ra and 40Ar/36Ar ratios are 300–472. By contrast, He–Ar isotopic compositions in minerals from the Fe deposits have lower 3He/4He and 40Ar/36Ar ratios of 0.08–0.93 Ra and 299–361, respectively. These results suggest that noble gas of the Cu–Fe and Fe skarn deposits in the Edong district formed by variable degrees of mixing between a magmatic fluid containing a mantle component, and modified air–saturated water (MASW). Importantly, He–Ar isotope data provide compelling evidence that contrasting fluid sources were involved in the formation of the Cu–Fe and Fe deposits, i.e., mineralizing fluids of the Cu–Fe deposits could have a greater contribution from mantle component, and little involvement of MASW than those of the Fe deposits in the Edong district. This conclusion is consistent with obvious differences in the nature of the intrusions related to mineralization, as well as sulfur isotopic compositions of sulfides in the Cu–Fe and Fe deposits. It is most likely that different proportion of mantle-derived noble gases play an essential role in controlling differences between the Cu–Fe and Fe skarn deposits.
The Caojiaba tungsten deposit (19.03 [email protected] 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The deposit is characterized by an early prograde skarn overprinted by a retrograde assemblage, and then by quartz–scheelite–sulfide veins. The Caojiaba skarn is characterized by grossular garnet (Grs65–85Adr7.1–31) and hedenbergitic pyroxene (Hd63–86Di11–33), similar to the well–studied reduced tungsten skarns worldwide. In this paper, we present in-situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analysis of U–Pb isotopes and trace elements of wolframite grains coexisting with scheelite in the quartz–scheelite–sulfide veins. Uranium is correlated positively with Nb5+, tetravalent (Ti4+, Sn4+, Zr4+, Hf4+), and trivalent (Sc3+, V3+, Y3+, REE3+) cations in all wolframite samples, suggesting that the incorporation of U into wolframite is controlled by the coupled substitution mechanisms. Two wolframite samples (CJB–43 and CJB–44) yielded 206Pb/238U ages of 212.4 ± 1.0 Ma (2σ, n = 50, MSWD = 0.92) and 211.8 ± 1.1 Ma (2σ, n = 43, MSWD = 1.4), overlapping with the late Triassic granitic rocks (225.6–204.5 Ma) in the XZMP. Mineralogical and geochronological evidence collectively indicate that Caojiaba is a typical example of the late Triassic reduced tungsten skarn deposits in the XZMP. This study demonstrates the potential of U–Pb dating of wolframite and highlights its importance for directly dating hydrothermal ore–forming processes.
The Caojiaba tungsten deposit (19.03 [email protected] 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The genesis of the Caojiaba tungsten deposit remains to be determined, and the role of trace element enrichment and precipitation in scheelite has received little study to date. The deposit is characterized by early prograde skarn overprinted by retrograde skarn assemblage, and then by quartz–scheelite–sulfide veins. Scheelite (CaWO4) across the mineralization stages can be divided into four generations, i.e., scheelite type–1 (Sch1) in the prograde skarn, early scheelite type–2 (Sch2) and late scheelite type–3 (Sch3) grains in the retrograde skarn, and scheelite type–4 (Sch4) in the quartz–scheelite–sulfide vein. In the cathodoluminescence (CL) images, Sch1, Sch2, and Sch3 have core–to–rim textures displaying CL–dark core and CL–bright rims, whereas Sch4 is homogeneous in the CL response across the grains. Sch1 contains high REE + Y and Ta concentrations, and has negative Eu anomalies relative to the other three types of scheelite grains, consistent with the contribution from magmatic fluids related to the tungsten granites. The increasing fluid–rock interaction intensity, precipitation of REE–rich minerals, and pH change in ore–forming fluids increase positive Eu anomalies and LREE/HREE fractionation of scheelite from Sch2 to Sch4. The in-situ initial 87Sr/86Sr ratios of Sch2 range from 0.71727 to 0.72142, which are within or slightly higher than those of the exposed Late Triassic tungsten granites in the XZMP. The broadly increasing initial 87Sr/86Sr ratios of Sch3 (0.72070 to 0.72764) suggest a source derived from magmatic fluids and contribution of the Neoproterozoic Banxi Group slate. This study highlights that the variations in trace element and Sr isotope compositions of scheelite can be used to reveal the source of ore–forming fluids and metal, and constrain the ore–forming processes.
The 500 km-long NE-trending Jiangnan tungsten belt formed at 140–150 Ma at the northern margin of the Yangtze craton and has a total resource of approximately 6.0 Mt WO3. Most deposits are of granite-related porphyry/vein-style and skarn type. The three large W systems at Zhuxi, Shimensi and Shiweidong in the southwestern part of the belt are the most important. These deposits have reduced affinity and are related to ilmenite-series S-type granites. The many smaller W-Mo deposits in the northeastern part of the belt have oxidized affinity and are related to magnetite-series I-type granites. Based on geological features, magma formation temperatures, bulk-rock and biotite compositions, Sr-Nd-Hf isotope data and trace element modeling, we propose a model in which large, reduced W-related granitic magmas formed by crustal partial melting induced by intrusion of mantle-derived mafic magmas, followed by internal magmatic evolution. In contrast, the more oxidized W-Mo-related granitic magmas formed by hybridization of crustal melts and sills of mantle-derived mafic magma in hot zones, with minor magmatic evolution. This model explains the metallogenic features of the coeval large W and small W-Mo deposits in the Jiangnan belt and reveals that the magma source and corresponding oxidation state are the main controls on the significant size difference between the reduced and oxidized tungsten systems, although the general geodynamic situation is the same.
The Xiangzhong district is the largest low-temperature W-Au-Sb metallogenic area in the world. The Darongxi skarn W deposit in the north of the Xiangzhong district is closely related to biotite monzonite granite, muscovite monzonite granite, and felsophyre, but the nature of granitic magma and its relationship with mineralization is relatively weak. In this paper, U-Pb dating, Lu-Hf isotope, the in situ composition of zircon, and the apatite of biotite monzonite granite, muscovite monzonite granite, and felsophyre in the Darongxi mining area are systematically studied, and the formation age, magma property and source, and their relationship with mineralization are discussed. The values of zircon U-Pb age and the εHf(t) of biotite monzonite granite are 222.2 ± 0.54 Ma and −2.9~−6.4, respectively. The values of zircon U-Pb age and the εHf(t) of muscovite monzonite granite are 220.8 ± 0.58 Ma and −2.7 to −8.1, respectively. The values of zircon U-Pb age and the εHf(t) of felsophyre are 222.3 ± 2.20 Ma and −2.2~−5.4, respectively. Magmatic apatite grains from biotite monzonite granite and muscovite monzonite granite show distinctive core–rim and oscillatory zoning textures in CL images, and demonstrate a bright yellow in colorful CL images. The magmatic apatite has a total rare earth concentration (3766~4627 ppm), exhibiting right-inclined nomorlized rare earth element patterns and obvious negative Eu anomalies. The geochemical data of magmatic zircon and apatite indicate that magma sources are responsible for these intrusions in the Darongxi mining area, mainly derived from the partial melting of the Mesoproterozoic crust, which is rich in W; the magma is rich in F and poor in Cl (F = 2.4~3.3 wt%, Cl = 0.0024~0.0502 wt%). The oxygen fugacity of magmatic zircon (ΔFMQAVG = −4.02~−0.26), the high negative Eu anomaly (δEu = 0.06~0.12) and the low positive Ce anomaly (δCe = 1.09~1.13) of magmatic apatite, and the occurrence of ilmenite all indicate that the redox condition of magma from the Darongxi mining area is reduced. The reduced F-rich crust-source granitic rock and W-rich source provide favorable conditions for the mineralization of the Darongxi reduced skarn W deposit.
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