Metallogenesis Of The Silver-lead-zinc Deposits Along The Southern Margin Of The North China Craton

Southern margin of the North China Craton is one of the most important metallogenic belts in China, that hosts numerous Au, Mo, W, and Ag-Pb-Zn deposits, including the Luanchuan Mo belt being the largest molybdenum district and the Xiaoqinling gold district being the second largest gold district in China. In the last two decades, a number of Ag-Pb-Zn deposits have been discovered in several districts, making this area to be an important Ag-Pb-Zn producer throughout the North China Craton and the Qinling Orogen. However, the mineralization type, fluid evolution, metals source, tectonic setting of these Ag-Pb-Zn deposits have been controversial. In this paper, mineralization characteristics, occurrences of precious metals, mineralization ages, origins of ore-forming fluids and metals of the Ag-Pb-Zn deposits from three key ore fields (Nannihu, Xiayu, Fudian) were systematically studied, to reveal the ore genesis and the relationships to other types of deposit in the same ore field, and finally to summarize regional metallogeny and to discuss the geodynamic background of the Mesozoic large scale mineralization at the southern North China Craton.In the Nannihu ore field, Luanchuan district, Fe-Pb-Zn-(Ag) skarn and hydrothermal Ag-Pb-Zn vein deposits are widespread. The former type is represented by the Luotuoshan deposit, and the latter type is represented by the Lengshuibeigou deposit. Both types of deposits are located at the periphery of the Nannihu granite porphyry and Nannihu Mo-(W) porphyry deposit, and the vein type of deposits are more far from the intrusion. The Luotuoshan deposit is located in the western periphery of the Nannihu porphyry deposits and hosted in the Proterozoic marble. The ore bodies are controlled by the interlayer fault zone of the skams. Three stages of mineralization have been recognized in the Luotuoshan deposit:(1) the skarn stage,(2) the polymetallic sulfide stage, and (3) the quartz-carbonate stage. Lengshuibeigou deposit is located in the NNE-trending faults in the Northwest of ore field, and the mineralization consists of three stages:(1) the quartz-pyrite stage,(2) the quartz-polymetallic sulfide stage, and (3) the quartz-carbonate stage.Trace elements of pyrite, analyzed by LA-ICP-MS, suggest that the porphyry and skarn deposits are enriched in high temperature elements (Co and Ni) whereas the vein Zn-Pb deposits are rich in low temperature elements (Pb、Zn、As and Sb). For example, pyrites from the Nannihu deposit are rich in Co and Ni and those from the Luotuoshan deposit are rich Mn, Ni and Bi, whereas pyrites from the Lengshuibeigou deposit are rich in Pb、Zn、Cu、Au、Ag、As and Sb. Most of the pyrites from the three types of deposits have the Co/Ni≥1, with Co and Ni contents decreasing from the porphyry deposit, skarn deposit, to hydrothermal vein deposit. The834S values of5sulfide samples from the Nannihu deposit are0.93～2.86‰(mean value=2.0‰),15sulfide samples from the Luotuoshan deposit are0.24～6.46‰(mean value=2.6‰), and11sulfide samples from the Lengshuibeigou deposit are0.70～8.07‰(mean value=4.0‰). The834S values have a wide range but are close to those of wall rocks (-13～-8‰and6～19‰) of the deposits. The H-O isotopes also show a tendency from magmatic fluids to meteoric waters following the porphyry deposit to the Pb-Zn-Ag veins. Therefore, the ore-forming fluids and materials of the three types of deposits are mainly derived from the magmatic-hydrothermal fluids, with gradually adding of meteoric waters and strata sulfur from the Nannihu, Luotuoshan, to the Lengshuibeigou deposits. Zricon U-Pb age of the Nannihu porphyry (144.9±1.6Ma) is in well consistent with molybdenite Re-Os date of the Nannihu deposit (144.9±0.7Ma and142.8±0.6Ma). Sericite of S031vein from the Lengshuibeigou deposit has a Ar-Ar plateau age of121.5±1.2Ma. Consequently, the Nannihu, Luotuoshan, and Lengshuibeigou deposits are a suite of magmatic-hydrothermal deposits which were genetically related to the intrusion of the Nannihu granite porphyry.Numerous Ag-Pb-Zn veins are located in the Xiayu ore field, Xiong’ershan district. These veins are distributed in the metamorphic rocks of the Taihua Group, and are controlled by the NE-to NNE-trending faults. The ore field has three key Ag-Pb-Zn deposits, including the Shagou, Haopinggou, and Tieluping deposits. The Shagou deposit is located in the northwest of the ore field, and consists of four stages of mineralization:(1) the quartz-siderite stage,(2) the quartz-sphalerite stage,(3) the quartz-silver minerals-galena stage, and (4) the quartz-carbonate stage. Haopinggou deposit is located in the north of the ore field, and consists of Au-bearing quartz-pyrite veins and Ag-bearing sphalerite and galena veins which were formed in the shearing and extensional zones, respectively. Two stages mineralization and alteration have been recognized in the quartz veins, including (1) the quartz-siderite stage and (2) the quartz-pyrite stage; while the Ag-bearing sphalerite and galena veins are composed of three stages:(1) the quartz-ankerite stage,(2) the quartz-polymetallic sulfide stage, and (3) the quartz-calcite stage.The carbonate minerals from Shagou and Haopingou deposits have δ13CPDB and δ18OSMOW values of-5.82～1.39‰and9.62～17.61‰, respectively, and the calculated δ13C and δ18O values for ore-forming fluids are-7.47～-0.57‰and2.35～11.50‰, respectively. It seems that the ore-forming fluids have higher C-O isotopes than those of magmatic fluids.18sulfide samples from the Shagou deposit have834S values of1.1～6.3‰, and6sulfide samples from the Au-bearing quartz-pyrite veins and8sulfide samples from the Ag-bearing sphalerite and galena veins from the Haopinggou deposit have834S values of-0.65～3.72‰and1.09～6.12‰, respectively. In addition with sulfur isotopes of the Tieluping deposit,δ34S values of the Ag-Pb-Zn veins in the Xiayu ore field, range from-8.8to6.3‰, showing a much wider range than deep-sourced sulfur (such as magmatic sulfur or mantle-derived sulfur). The Pb isotopic compositions of galena from the Shagou and Haopinggou deposits are quite similar, and11samples have206Pb/204Pb,207Pb/204Pb and2Q8Pb/204Pb values of17.472～18.153,15.411～15.572, and38.178～38.630, respectively. They plot in the range of the Proterozoic Guandaokou and Luanchuan Groups in the Pb-Pb diagram, but far away from the ranges of the Taihua and Xiong’er Groups. Pyrites from the Au-bearing quartz-pyrite veins and Ag-bearing sphalerite and galena veins have various Co and Ni contents, and their Co/Ni ratio scattered in the ranges of volcanogenic deposits, skarn deposits, and sedimentary environments in the Co versus Ni diagram, indicating the ore-forming materials may be of several origins. C-O-S-Pb isotopes and Co/Ni ratios of pyrite demonstrate the ore-forming fluids and materials of the Ag-Pb-Zn veins were derived from the metamorphic devolatilization of the marine sedimentary rocks previously subducted beneath the Xiong’ershan district, but with the involvement of some magmatic fluids. Ar-Ar plateau age of the sericites from S8vein in the Shagou deposit and H16vein from the Haopinggou deposit are139.8±1.2Ma and144.4±1.6Ma, respectively, indicating the Ag-Pb-Zn veins in the Xiayu ore field are formed during late Jurassic to early cretaceous. These ages are earlier than the zircon U-Pb ages of the Haopinggou granite porphyry, demonstrating there is no direct relationship between porphyry emplacement and Ag-Pb-Zn mineralization, but the mineralization may be related to the regional deep-seated intrusions. The emplacement of these intrusions induced the devolatilization of the Meso-Neoproterozoic marine sedimentary rocks and generated a lot of fluids and materials. Magmatic fluids may mix with the metamorphic fluid during this process. Moreover, based on the trace elements and sulfur isotope study, the Au-bearing quartz-pyrite veins and Ag-bearing sphalerite and galena veins maybe formed in two independent ore-forming events. The former may be the same type of deposit located in the Xiong’er Group in the south of Xiayu ore field; whereas the later veins may be the same type of those Ag-Pb-Zn veins in the Shagou deposit.Lots of Ag-Pb-Zn veins are distributed surrounding the Donggou granite porphyry and Donggou Mo deposit in the Fudian ore field, Waifangshan district, represented by proximal Sanyuangou Pb-Zn-Ag deposit and distal Laodaizhanggou Ag-Pb-Zn deposit. The Sanyuangou deposit is located in south of the Donggou granite porphyry, and consists of three stages of mineralization:(1) the quartz-pyrite stage,(2) the polymetallic sulfide stage, and (3) the quartz-carbonate stage. Laodaizhanggou deposit is located in distal northwest of the Donggou granite porphyry, and the ore veins are controlled by the EW-trending faults zones. Four stages of mineralization have been recognized in the Laodaizhanggou deposit:(1) the quartz-pyrite stage,(2) the dark brown sphalerite-galena-pyrite stage,(3) the light brown sphalerite-galena stage, and (4) the quartz-carbonate stage.The carbonate minerals from the Sanyuangou and Laodaizhanggou deposits have513CPDB and δ18Osmow of-9.05～-3.94‰and12.10～16.61‰, respectively, and the calculatedδ13C and δ18O values of ore-forming fluids for the Sanyuangou deposit are-6.35～-3.92‰and5.18～10.50‰, respectively, and for the Laodaizhanggou deposit are-8.60～-3.49‰and4.15～8.33 ‰, respectively. Both of the δ13C and δ18O values from Sanyuangou and Laodaizhanggou deposits are consistent with magmatic fluids, indicating the ore fluids may mainly be of magmatic origin. The Pb isotopic compositions of galena from the Donggou, Sangyuangou, and Laodaizhanggou deposits are quite similar,206Pb/204Pb,207Pb/204Pb and208Pb/204Pb values of9samples are17.314-17.458,15.452-15.485and38.138-38.370, respectively. The values plot between the range of Orogen reserve and Mantle reserve in the Pb-Pb diagram, indicating a deep-seated lead source. However, from the porphyry to outside, sulfide samples from the Donggou deposit and Sanyuangou deposit both have a wide range of834S (0.36～9.91‰and5.52～16.24‰, respectively). In contrast, sulfides from the Laodaizhanggou deposit have concentrated δ34S with the mean value between the mean values of the Donggou deposit and the Laodaizhanggou deposit. Pyrites from the Sanyuangou deposit are rich in Cu, Au, and As, while the Laodaizhanggou deposit are rich in Sb and Pb. But both of them are rich in Ag. Co/Ni ratios of pyrites from the Donggou and Sanyuangou deposits are both>1, whereas those from the Laodaizhanggou deposit are scattered. Zricon U-Pb data of the Donggou porphyry (117.8±0.9Ma) and Molybdenite Re-Os data of the Donggou deposit (117.5±0.8Ma and116.4±0.6Ma) are in good agreement, and both are consistent with the Ar-Ar plateau age of the sericite (115.9±0.9Ma) from the Sanguangou deposit. However, the Ar-Ar plateau age of the sericite from the Laodaizhanggou deposit is124.7±1.2Ma, which is about7Ma older than the ages of the Donggou porphyry and the deposit. Consequently, the Donggou porphyry Mo deposit and the Sanyuangou hydrothermal vein Pb-Zn-Ag deposit surrounding the Donggou porphyry were formed by the same magmatic-hydrothermal event in the Fudian ore field, whereas the Laodaizhanggou deposit has no relationship with the Donggou granite porphyry. The Laodaizhanggou deposit is quite similar with the Shagou deposit on occurrence of vein, mineralizing characteristics, and mineral assemblage. The two deposits may belong to the same type.Based on study of the typical deposits in the Nannihu, Xiayu, and Fudian ore fields in the southern North China Craton, three types of Ag-Pb-Zn deposit have been recognized in the area, including skarn Fe-Pb-Zn-(Ag) deposit (Luotuoshan), porphyry-proximal hydrothermal vein Pb-Zn-Ag deposit (Lengshuibeigou, Sanyuangou), and crustal-extension vein Ag-Pb-Zn deposit (Shagou, Haopinggou, Laodaizhanggou). Among these types, skarn Fe-Pb-Zn-(Ag) deposit and porphyry-proximal hydrothermal vein Pb-Zn-Ag deposit have a genetic relationship with the porphyry intrusion in the ore field, and have similar origins of the ore-forming fluids and metals. Ore-forming metals in skarn Fe-Pb-Zn-(Ag) deposit are mainly composed of Fe and Zn with minor Pb, while the porphyry-proximal hydrothermal vein Pb-Zn-Ag deposit consists mainly of Pb and Zn with minor Ag. The crustal-extension vein Ag-Pb-Zn deposit generally has no relationship with the porphyry in the ore field, but is controlled by the regional fault and its secondary fault zones. The ore-forming fluids and materials of this type of deposit are mainly derived from the deep-seated magmas and regional metamorphism, and the ore-forming metals are mainly composed of Ag and Pb with minor Zn. In combination with research data from other areas in North China Craton, we proposed that widespread Mesozoic Ag-Pb-Zn mineralization in the southern margin of North China Craton is one of consequences of the destruction of North China Craton, which might be stimulated by the subduction and transition of the Paleo-pacific plate beneath the North China Craton. The subduction of Paleo-pacific plate not only led to the disturbance and upwelling of the asthenosphere, but also resulted in strong metasomatism of the overlying lithosphere, giving rise to the important transformation of the mechanical and chemical properties of the North China Craton lithosphere. The upwelling magma emplaced into the shallow crust and differentiated the magmatic fluids carrying material. The ore-bearing magmatic-hydrothermal fluids precipitated the sulfides at different sites to form different types of deposits, such as those in the Nannihu and Fudian ore fields. The emplacement of upwelling magma could also result in the metamorphic devolatilization of the marine sedimentary rocks and mixed with the metamorphic fluids. These fluids penetrated along the shallow fault zones and formed the Ag-Pb-Zn veins (such as Xiayu ore field). Based on our comprehensive dataset, we proposed several prospecting districts potentials for Mo, Ag-Pb-Zn, and Au deposits in the southern North China Craton:the Lushi-Luonan and the Waifangshan districts are potential for looking for porphyry Mo deposits and porphyry-related Pb-Zn-Ag depsoits, the Xiong’ershan district and the Waifangshan district hosting lots of fault zones are potential for looking for the crustal-extension vein Ag-Pb-Zn deposit, while the Taihua Group in the cores of Xiong’ershan district and Xiaoshan district are potential for looking for vein Au deposit.