| The Shandong province hosts the largest gold producing region in the southeastern margin of North China Craton. It is separated by the Tan-Lu fault into two distinct domains, namely, the Luxi to the west and the Jiaodong to the east. In addition, the big gold districts in Jiaodong and Luxi are all controlled by the Tan-Lu fault. Those gold deposits in Jiaodong and Luxi regions have been studied to a varying degree by many previous researches and many models have been developed to focus on their genesis, and the nature and extent of gold metallogeny between Luxi and Jiaodong regions display marked contrast. However, the gold deposits occurred in the the Yi-Shu metallogenic belt have not been well understood so far. In this study, we focus on the Tongjing gold deposit(Luxi), Longquanzhan gold deposit(the Yishu metallogenic belt) and Qibaoshan gold deposit(Ludong) in the middle segement of Tan-Lu fault zone, southeastern North China Craton. Based the detail field investigation of the Tongjing, Longquanzhan and Qibaoshan gold deposits, we present the zircon U-Pb age, major and trace element geochemistry, and Sr-Nd-Pb isotope compositions for the mafic dykes from the Longquanzhan gold district, the pyrite Rb-Sr chronology, S-Pb-He-Ar isotope data of ores from Tongjing, Longquanzhan and Qibaoshan gold deposits. These result are used to (1) document the Early and Late Cretaceous magmatisms and gold metallogenic events,(2) study the petrogenesis, source and geodynamic setting of the Early Cretaceous intrusives, and (3) discussed the magmatic sources for the ore fluids and metal concerning the Cretaceous gold ore system and analyzed the genetic links between gold metallogenesis and magmatism. The main results and conclusions are summarized as follows:(1) The mafic dikes are widely distributed in the Longquanzhan gold district, and they are mainly gabbro, diabase and diorite-porphyrite. LA-ICP-MS zircon U-Pb dating on the gabbro dike s in the Yishu fault zone reveals that they formed at131±2Ma with four major groups of inherited ages at2453±44-2552±47Ma,1852±44-1900±44Ma and730±8Ma. These gabbro dikes exhibit a wide range of SiO2and are characterized by Fe2O3, TiO2, CaO, Na2O/K2O and low abundance of MgO(Mg#=40-49), Na2O+K2O, A12O3, Cr, Ni. They are enriched in large ion lithophile elements (Rb, Ba, Th, U) and light rare earth elements without significant Eu anomalies, and depleted in high field strength elements, they are all belong to the tholeiitic series rocks. All samples show a wide range of Sr compositions with high87Sr/86Sr (0.71019-0.71676),d(t)(-8.66--4.30) and large T2DM ages (1272-1625Ma). Their Pb isotope compositions show a wide range ((206Pb/204Pb)i=17.7006-18.2049,(207Pb/204Pb)i=15.4781-15.5627,(208Pb/204Pb)i=38.2248-40.1191). In combination with the mineralogy, geochemistry and Sr-Nd-Pb isotope compositions, we contend that the gabbro dikes could have originated from partial melting of the upwelling asthenospheric mantle, with a small amount of crustal contamination. Dynamical background is probably related to the second extensional sinistral strike-slip movement of Tan-Lu fault system, which triggered the upwelling asenospheric mantle. Meanwhile, the chronology of mafic dikes also provide the time constraints for the transition from compressional to extensional mechanism.(2) Whole-rock geochemical data show that the Tongjing diorite-porphyrite can be classified as high-potassium calc-alkaline metaluminous rock, which are characterized by high Na2O content and high abundance of Mg#, and they are also enriched in LILE and LREE, and depleted in heavy earth elements (HREE) and HFSE, without significant Eu anomalies. In addition, the studied intrusion exhibit more radiogenic lead ((206Pb/204Pb)i=16.917-17.956), and are characterized by (87Sr/86Sr)i ranging from0.707105to0.70788, low εNd(t) values from-15.17to-16.41. In combination with the mineralogy, geochemistry and Sr-Nd-Pb isotopic compositions of Tongjing diorite-porphyrite, we contend that the Tongjing diorite-porphyrite could have originated from the enriched mantle, accompanying with some crastual contamination. The Qibaoshan subvolcanic intrusions can be divided into two stages. The early stage subvocanic intrusion belong to the alkaline magma series in terms of K2O+Na2O contents, and to shoshonitic series based on their K2O content. They are further characterized by an wide range of SiO2content with relatively low Mg#, positive anomalies in Rb, Ba, and Th, and are depleted in high field strength elements (Nb, Zr, Ti), without significant Eu anomalies. They show uniformly high (87Sr/86Sr)i (0.70900-0.70920), low εNd(t) values (-15.17to-16.41). However, the late stage subvocanic intrusions can be classified as high-potassium calc-alkaline and weaky peraluminous rocks. They show relatively high contents of SiO2and K2O and high abundance of Mg#, with lower degree of fractionation between LREE and HREE compared to those of the early stage subvolcanic intrusion, with positive Eu anomalies. In combination with the mineralogy, geochemistry and Sr-Nd isotopic compositions of Qibaoshan subvolcanic intrusions, we contend that the early stage subvolcanic intrusion could have originated from the partial melting of the enriched mantle, and lower mafic crust for the late stage subvolcanic intrusions.(3) The Longquanzhan gold deposit, as an important part of gold mineralization belt of the Yi-Shu fault zone, is the only one altered-fracture type gold deposit identified up to present. The Rb-Sr isotopic dating of10pyrites separated from the Longquanzhan gold deposit yielded an isochron age of96±2Ma (MSWD=1.2). It is the youngest high-precise age of gold deposits so far identified in Shandong Province. It may represent the Late cretaceous gold mineralization in Shandong Province. The Re-Os isochron age of molybdenite from Yinan Jinchang gold deposit was128±1Ma, and single grain and low background Rb-Sr isochrone dating on biotite from prograde and retrograde skarn gives133±6Ma and128±2Ma, respectively. They are contemporary with the monzonite porphyry (LA-ICP-MS zircon U-Pb age of126-128Ma) and Granite porphyry(LA-ICP-MS zircon U-Pb age of127Ma,133Ma). The zircon SHRIMP U-Pb dating of diorite from the Tongjing gold district in Yinan Country has yield concordant ages of128±3Ma, with which we reach the conclusion that the Tongjing gold deposit was formed at around128Ma. The single grain pyrite Rb-Sr dating of the Qibaoshan gold deposit give an average isochron age of117Ma±, which are later about7Ma than the Quartz diorite porphyry (K-Ar age of124.1Ma). These ages of ore-forming and magmatism provide the time constraints for the genetic relationship between the gold metallogeny and magmatic evolution in the Yishu fault zone and adjacent area.(4) The calculated δD and δ18O using measurements of fluid in inclusion waters extracted from auriferous quartz of Longquanzhan gold deposit is-77%o and3.97%o to4.07%o. the fluid inclusions from the Au and Cu mineralization stage of Qibaoshan gold deposit have δDH2O of-78.6‰-48.14%o and δ18Omo of+3.05‰-+5.77%o, and calculated δDH2O and δ18O values of fluid inclusion from quartz range from the quartz-sulfide stage of Tongjing gold deposit is-87%o and-0.4%o. The fluids of quartz from the above gold deposits are nearly similar to magmatic water and consistent with a predominance of magmatic fluids in the ore system. In addition, the δD and818O values of the carbonation stage fluid, extracted from fluid inclusions in calcite, plot towards the meteoric water line. This indicates the latter fluids likely had a high meteoric component.The3He/4He ratios in inclusion fluids of the Longquanzhan gold deposit are0.14Ra to0.78Ra with an average of0.39Ra. The40Ar/36Ar ratios are482-1811, averaging957, obviously higher than that of air (295.5), and the40Ar*/4He ratios of0.078-0.272. The3He/4He ratios of Yinan gold deposit vary from0.11Ra to1.02Ra. The40Ar/36Ar ratios are in the range of364-458with40Ar*/He ratios of0.154-0.735. The3He/4He ratios of Qibaoshan gold deposit vary from0.187Ra to0.194Ra, and40Ar/36Ar of291-842with40Ar*/4He of0-0.04. The elevated3He/4He ratios indicate a mantle-derived component in the ore fluids, and the percentage of mantle-derived He can be calculated according to the crust-mantle mixing model. The percentage of mantle helium involved in the ore-forming fluids trapped within the pyrite grains from gold deposits of the six metallogenic belts were calculated with the equation mentioned above. The results show1.63%to9.66%(average4.72%) mantle helium involved in the Longquanzhan gold mineralization. They are similar to Yinan gold deposit (1.29-12.60%, average5.09%), and higher than those of Qibaoshan gold deposit (2.21-2.30%, average2.26%). However, they are obviously lower than those of Zhaoyuan-Laizhou-Pingdu metallogenic belt and Mouping-Rushan metallogenic belt. The radiogenic Ar isotope compositions is present in ore-forming fluids and the caculated proportion of Longquanzhan gold deposit is38.7%-83.7%, Tongjing gold deposit of18.8%-35.5%and Qibaoshan gold deposit of0-64.9%. These results indicate that the ore-forming fluid was a mixture of mantle fluid, crustal magmatic fluid and MSAW. The occurrence of a mantle component in ore-forming fluid indicates the gold mineralization, including Tongjing, Longquanzhan and Qibaoshan was the result of crust and mantle interaction. However, the proportion of the mantle He of the Longquanzhan and Yinan gold deposit is a little higher than the Qibaoshan gold deposit, and more air-saturated water participate in the Yinan and Qibaoshan gold deposit. δ34S values of the Tongjing, Longquanzhan and Qibaoshan gold deposits range from0.9%o to4.4%o,1.53%o to5.6%o and0.3%o to5.89%o, respectively, similar to magmatic sulphur.3sulfide samples from the Longquanzhan gold deposit have206Pb/204Pb=15.779-19.037,207Pb/204Pb=15.076-15.643,208Pb/204Pb=36.744-40.488. three wallrock samples have206Pb/204Pb,207Pb/204Pb and208Pb/204Pb calculated using the time (96Ma) of hydrothermal deposition of pyrite, were15.40-17.754,15.053-15.472and35.734-37.801, Ores from the Longquanzhan gold deposit have distinct Pb isotopic compositions to age-corrected data of wallrock and probably indicate that the ore-forming metals were sourced from a crustally-derived magma. This view is also consistent with interpretation of the He-Ar isotope data.10sulfide from the Tongjing gold deposit have206Pb/204Pb=19.04-23.08,207Pb/204Pb=15.69-16.30and208Pb/204Pb=38.40-41.46, they are rich in U-radiogenic Pb and Th-radiogenic Pb. The calculated206Pb/204Pb,207Pb/204Pb and208Pb/204Pb using the time (130Ma) of hydrothermal deposition of molybdenite of2diorite porphyry samples are17.855-17.956,15.505-15.528and37.826-37.883, which are significantly lower than the Pb compositions of sulfide in Tongjing gold deposit, but they share the same linear relationship in the207Pb/204Pb and208Pb/204Pb versus206Pb/204Pb diagram. These isotopic compositions confirm a genetic relationship between the gold mineralization and magmatism and indicate the Pb isotopic compositions of sulfide are probably derived from the lower crust, mantle and sediment strata source.7sulfide samples from Qibaoshan gold deposit have206Pb/204Pb=16.624-17.725,207Pb/204Pb=15.238-15.512,208Pb/204Pb=36.414-37.721, Ores from Qibaoshan gold deposit have similar Pb isotopic compositions to the Pb isotopic data of volcano-subvolcanic rocks(206Pb/204Pb=16.485-17.056,207Pb/204Pb=15.149-15.388,208Pb/204Pb=36.445-37.484), combing with H-O, He-Ar isotopic data, we conclude that these provide multiple constraints for the genetic relationship between the gold mineralization and magmatism.(5) we integrate high-quality radiometric ages of gold mineralization in the study area which are reported in recent years. These results show that two gold metallogeny episodes can be identified, the Early Cretaceous gold metallogeny are related with those of enriched mantle-derived high-K calc-alkaline series, Most important, the ore fluid and metals also derived from the Early Cretaceous and Late Cretaceous (?) magmatism, in combination with these results described above, this paper presents a genetic model of Early Cretaceous skarn gold deposit+cryptoexplosion breccia pipe Au-Cu polymetallic deposit and Late Cretaceous altered rock type gold deposit. |
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