The Mesozoic Tectonic-magmatic Evolution Process And Gold Metallogenesis In Chifeng-chaoyang Region, Northern North China Craton

The North China Craton (NCC) is bounded to the north by the Hingan-Mongolian Orogenic Belt (HMOB) and to the southeast by the Yangtze Craton, and it is an Archean craton as indicated by the presence of≥3.8Ga crustal remnants. However, the lithospheric mantle beneath the Eastern Block of the NCC experienced widespread reactivation and thinning peaking in the Early Cretaceous, contemporaneous with extensional basin formation, the development of metamorphic core complexes (MCC), intense magmatism and large-scale gold metallogenesis. Until now, there are still many controversies concerning the timing of suturing between the NCC and HMOB, the thinning mechanism of NCC, and the genesis of the large-scale gold metallogenesis. Based on the detailed field investigation about the Chifeng-Chaoyang gold district, we present zircon U-Pb ages, mineralogy, major and trace element geochemistry, and Sr-Nd-Pb-Hf isotope compositions for the dykes from Jinchanggouliang (JCGL), the Duimiangou (DMG) and Jiguanzi (JGZ) intrusions, and the enclaves from JGZ to (1) document the Triassic and Cretaceous periodic magmatisms and mineralizing events,(2) investigate their magma sources and petrogenesis, and (3) provide potential information about crust-mantle interaction beneath the northern NCC. Moreover, we discussed the magmatic sources for the ore fluids and metal concerning the cretaceous gold ore systems according to the H-O-S-Pb isotopes, and analyzed the genetic links between gold metallogenesis, adakitic magmatism and formation of MCC. The main results are summarized herein:1. The Chifeng-Chaoyang region is underlain by the Archean Jianping Group gneiss with a metamorphic age of about2.5Ga, and the Jurassic-Cretaceous volcanic-sedimentary rocks. The regional structures are dominated by the Late Paleozoic-Early Mesozoic EW-striking faults and the Late Mesozoic NE-striking MCC. Nearly all intrusions and gold deposits are located in the lower-plate of the MCC, and these intrusions and gold deposits always cluster together. Gold veins are hosted by the NE-trending faults, which are developed in the metamorphic basement or intrusions, and the maximum principal compressive stress is SN-NW oriented. Pyrite, chalcopyrite, galena and sphalerite are common sulfides in different gold deposit, and the gold often occurs as native gold or electrum in pyrite and chalcopyrite. Moreover, the quartz, pyrite, chlorite, sericite and carbonate are prevalent ore-related hydrothermal alteration minerals. Three stages of hydrothermal alteration and hypogene mineralization are recognized, largely on the basis of megascopic and microscopic textural relationships, and mineral assemblages:coarse grained quartz-pyrite stage, Polymetallic sulfide stage and the low-temperature quartz-pyrite-carbonate stage.2. LA-ICP-MS zircon U-Pb dating reveals three major age groups of2500Ma (n=2),253±6Ma (n=5) and227±1Ma(n=9) of dykes. The inherited ages of2500Ma, contemporary with the Archean NCC continental growth, imply that crustal material was involved in the magma source. The igneous zircons with concordia age of227±1Ma may record the emplacement age of the JCGL dykes. Both diorite and diorite porphyry exhibit a wide range of SiO2and MgO contents and are characterized by high concentrations of Na2O+K2O and Al2O3and low abundances of P2O5and TiO2. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) without significant Eu anomalies, depleted in high field strength elements (HFSE), and all are categorized as shoshonitic rocks. All samples show a narrow range of Sr isotope compositions with initial87Sr/86Sr ratios from0.70394to0.70592, variable εNd(t) values (1.1to-9.8) and TDM2ages (913～1972Ma). Their Pb isotope compositions form continuous variation trends and plot in the fields between EMI and LCC. The above results suggest that the JCGL dykes studied could have been derived from mixing of lower crust, lithospheric mantle of the NCC and ascending asthenospheric melt, in a post-orogenic extensional geodynamic setting. These shoshonitic dykes, together with the geochronologic data of regional ENE-trending retrograded eclogites, ophiolites, continental arc magmatic belt, A-type granite, alkaline intrusions and metamorphic core complex from the northern NCC and Central Asian Orogenic Belt (CAOB) suggest that closure of the Paleo-Asian Ocean (i.e. stage of pre-collision to collision) had completed during the latest Permian to earliest Triassic, and that the CAOB was subsequently tectonically dominated by post-orogenic extensional regimes. The involvement of asthenospheric melt in the magma source implies that the sub-continental lithospheric mantle (SCLM) of the NCC had been modified and the onset of lithospheric destruction and thinning beneath the northern NCC may have occurred in the Middle-Late Triassic as a result of post-orogenic subducting slab detachment and lithospheric delamination.3. LA-ICP-MS zircon U-Pb dating reveals that the DMG quartz monzonite from JCGL (128±1Ma), the JGZ quartz monzonite from the Anjiayingzi (133±1Ma), and enclaves from the host JGZ intrusion (133±1Ma) all formed in the Early Cretaceous. All amphiboles from these quartz monzonite and enclaves belong to magnesiohornblende, and all micas can be classified as magnesian biotite. The 'An' of plagioclase from DMG and JGZ intrusion ranges from15to26, thus they are defined as oligoclase. The reversely-zoned plagioclases are common in enclaves from JGZ intrusion. Calculated results from aluminum-in-hornblende geobarometry show that the DMG intrusion, the JGZ intrusion and enclaves from the JGZ host intrusion emplaced at0.77kbar,2.19kbar and1.59kbar respectively, and corresponding depth at2.7km,7.7km,5.6km. Temperatures of emplacement calculated with the hornblende-plagioclase thermometer are742C,647℃and647℃respectively. The log fo2estimated from biotite ranges from-14.8～-16.3,-17.8～-18.7and-18.0～-18.7, located between quartz-fayalite-magnetite and the Ni-NiO buffer assemblages.4. Bulk-rock analyses show that the DMG and JGZ intrusion are characterized by high contents of SiO2, Al2O3, Na2O, K2O and low abundance of MgO and Mg#, defining their subalkaline and metaluminous characteristics. The enclaves from JGZ intrusion has relatively low concentrations of SiO2and total alkaline, and higher MgO, CaO and FeO, and can be classified as subalkaline-alkaline metaluminous monzodiorite-diorite. The DMG intrusion, JGZ intrusion and the enclaves from it are all enriched in LILE and LREE, and depleted in heavy rare earth elements (HREE) and HFSE, without significant Eu anomalies (0.89～1.10,0.61～0.83,0.79～0.93). They have high (La/Yb)N ratios (20.5～31.0,12.6～40.2,14.1～37.8), and low and constant (Dy/Yb)N ratios (0.87～1.60,1.12～1.35,1.31～1.63). The DMG and JGZ adakitic intrusions formed in an intracontinental extensional setting contemporaneous with the formation of pull-apart basins, metamorphic core complexes and intense magmatism, rather than in a convergent margin. The DMG intrusion, JGZ intrusion and the enclaves from it have homogeneous (87Sr/86Sr)i ratios (0.7059～0.7066,0.7055～0.7060,0.7059) and εNd(t)(-6.2～-7.2,-8.2,-11.0). The Pb isotope compositions of DMG intrusion are also homogeneous ((206Pb/204Pb)i=17.289～17.375,(207Pb/204Pb)i=15.359～15.463,(208Pb/204Pb)i=37.130～37.472). Moreover, the εHf(t) of JGZ intrusion and enclaves from it ranges from-9.2～-12.0and-4.5～-10.0respectively. In combination with the mineralogy, geochemistry and Sr-Nd-Pb-Hf isotope compositions of these rocks, we contend that the DMG and JGZ intrusions could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle, accompanied by fractional crystallization of K-feldspar, biotite, apatite, Fe-Ti oxides and minor hornblende and plagioclase, and the enclaves from the JGZ intrusion could denote the feeder system during the late stage of host magma evolution. Thus, the DMG and JGZ adakitic intrusions may record the magmatic event associated with underplating of asthenospheric amgma in an intracontinental extensional environment, and both asthenospheric magma underplating and lithospheric delamination may have played important roles in the lithospheric thinning of the northern NCC.5. The fluid inclusions from Anjiayingzi gold deposit have final homogenization temperatures (Th) of180～402℃, ice-melting temperatures (Tm, ice) of-11.6～-0.8℃, salinities of1.3～15.6wt%NaCl and fluid pressures of319×105Pa with δD of-80.3‰～-96.5‰and δi8OH2O of3.7‰-5.5‰. The fluid inclusions from JCGL gold deposit have Th of205～390℃, Tm, ice of-19.1～0.8℃, salinities of1.3～21.7wt%NaCl and fluid pressures of422×105Pa with δD of-54.0‰～-110.9‰and δ18OH2O of1.9‰～8.9‰. The fluid inclusions from Paishanlou gold deposit have Th of187～337℃and salinities of4.6～12.2wt%NaCl with5D of-87.3‰～-116.2‰and518OH2O of0.7‰～6.9‰. The mineralization depths of gold deposits from the Chifeng-Chaoyang region calculated fluid inclusions parameters range from2-2.5km. The ore fluids derived from the Early Cretaceous intrusions dominantly, and the fluid unmixing and temperature drop controlled the gold deposition jointly.The Anjiayingzi, JCGL-Erdaogou and Paishanlou gold deposits have δ34S of-0.5～7.6‰,-2.8～2.3‰and0.3～6.5‰respectively. The corrected Pb isotope composition of metamorphic basement from Chifeng-Chaoyang region is highly variable, with (206Pb/204Pb)i=15.040～18.090,(207Pb/204Pb)i=15.065～15.655and (208Pb/204Pb)i=34.897～38.571. The corrected Pb isotope composition of the Early Cretaceous adakitic intrusions is relatively homogeneous with (206Pb/204Pb);=15.742～17.375,(207Pb/204Pb)i=15.237～15.665and (208Pb/204Pb)i=36.908～38.547. The Pb isotope of ore sulfides from regional gold deposits is nearly the same as that of the Early Cretaceous intrusions with (206Pb/204Pb)i=16.400～17.591,(207Pb/204Pb)i=15.210～15.578and (208Pb/204Pb)i=36.690～38.091, indicating their magmatic origin.6. The Triassic Mo (gold?) mineralizing event is confirmed by the Re-Os age of243.5±1.3from the JCGL Mo-bearing quartz veins, and it was developed in the post-orogenic extensional regime as the result of Paleo-Asian Ocean closure. The zircon U-Pb, biotite and K-feldspar40Ar-39Ar ages reveal that gold deposits from Chifeng-Chaoyang region mainly formed during the Early Cretaceous (133～117Ma), contemporaneous with the development of MCC (130～100Ma) and intense magmatism (133Ma～124Ma). Most importantly, the ore fluids and metals also derived from the Early Cretaceous intrusions. In combination with these results described above, this paper presents a two-stage mineralizing model.