Study Of The Relationship Between Early Cretaceous Magmatism And Copper Mineralization In Zouping Volcanic Basin,Shandong Province

The Zouping volcanic basin is located in the south of Zouping County,western Shandong Province.There are abundant Early Cretaceous intrusive and volcanic rocks cropped out in the basin,among which the Wangjiazhuang quartz monzonite,Beilou monzonite porphyry and Nandongzi monzonite are associated with Cu(-Mo-Au)mineralization.Based on comprehensive field investigation,this study presents zircon U-Pb dating,whole-rock geochemistry,Sr-Nd-Pb-Hf isotopic compositions and in situ zircon Hf-O isotopic compositions of the plutonic and volcanic rocks,as well as molybdenite Re-Os dating result,sulfide S-Pb isotopic compositions,ore-forming-related quartz H-O isotopic compositions,and fluid inclusionsmicrothermometry and component analysis of theWangjiazhuang Cu(-Mo)deposit.LA-ICP-MS zircon U-Pb analyses for plutonic rocks and SHRIMP zircon U-Pb analyses for volcanic rocks yield an Early Cretaceous emplacement age of 130-123 Ma.Seven molybdenite samples of Wangjiazhuang Cu(-Mo)deposit yield a Re-Os weighted mean age of 127.8±0.7 Ma(2σ),which is identical to the zircon U-Pb age of the intrusive and volcanic rocks,suggesting an Early Cretaceous magmatism and mineralization in Zouping area.The intermediate-basic to intermediate-acid intrusive rocks(SiO2=54.4-65.5%)and intermediate-basic volcanic rocks(SiO2=49.8-57.3%)from the Zouping volcanic basinare characterized by high concentrations of alkali,potash,large ion lithophile elements(LILEs,such as Rb,Ba and Sr)and light rare earth elements(LREEs),and low concentrations of high field strength elements(HFSEs,such as Nb,Ta and Ti)and heavy rare earch elements(HREEs)as well as no obvious Eu anomaly.The Wangjiazhuagn quartz monzonite and Beilou monzonite porphyry also havehigh Sr/Y and(La/Yb)N ratios,similar to adakitic rocks worldwide.According to geochemical and whole-rock Sr-Nd-Pb-Hf and in situ zircon Hf-O isotopic characteristics,three geochemical groups of these plutonic and volcanic rocks have been identified in the Zouping volcanic basin.Group A,including Wangjiazhuang quartz monzonite and Beilou monzonite porphyry,are characterized by relatively low(87Sr/86Sr)i ratios(0.705493-0.706484),high εNd(t)values(-1.31 to+3.08),high radiogenic Pb[(206Pb/204Pb)i=18.3474-18.5195,(207Pb/204Pb)i=15.5431-15.5949,(208Pb/204Pb)i=38.186-38.396],high zircon εHf(t)values(-2.1to 4.3)and δ18O values(6.3-7.9‰).It is indicated that these magmatic rocks were primary derived from partial melting of the subducted Paleo-Pacific slab with the overlain sedimentary componentsand subsequent interaction with mantle peridotite.Group B,including Nandongzi monzonite,Shifanlu and Huangshan volcanic rocks,are characterized by relatively low(87Sr/86Sr)i ratios(0.705360-0.706300),low zirconδ18O values(5.6-6.6‰),relatively high εNd(t)values(-2.74 to-1.12),relatively high whole-rock εHf(t)values(-1.13 to 1.55),relatively high zircon εHf(t)values(-2.8 to 4.7)and relatively high radiogenic Pb[(206Pb/204Pb)i=18.0871-18.4204,(207Pb/204Pb)i=15.4942-15.5850,(208Pb/204Pb)i=37.937-38.158)].It is suggested that the magmas werederived from the partial melting of the asthenosheric mantle which was previously metasomatised by the subducted Paleo-Pacific slaband sediments derived fluids.The ascent magma experienced crystallization differentiation of olivine,pyroxene and P-Ti-rich minerals.Group C,including the Chayeshangabbroic diorite,Xueshan monzonite,Xueshan gabbroic diorite and Hebanshan volcanic rocks,with low(87Sr/86Sr)i ratios(0.704387-0.705781),low εNd(t)values(-14.95 to-8.89),low whole-rock εHf(t)values(-17.89 to-17.12),low zirconεHf(t)values(-25.3.to-8.1),low radiogenic Pb[(206Pb/204Pb)i=16.5852-17.4732,(207Pb/204Pb)i=15.2353-15.4062,(208Pb/204Pb)i=36.424-37.374)],and high zirconδ18O values(6.7-8.4‰).It is inferred that the magmas were derived from the partial melting of the EMI-type enriched lithospheric mantle underneath the North China Craton and experienced crystallization differentiation of olivine,pyroxene and P-Ti-rich mineral.The inclusion observed in the quartzs sampled from ore-bearing veins in both the breccia and porphyry ores of the Wangjiazhuang Cu(-Mo)deposit are mainly liquid-rich andgaseous-rich two-phase inclusions,and daughter minerals-bearing three-phase inclusions.The peak value of homogenization temperatures of primary fluid inclusions in the breccia ores ranges from 290 to 320℃,while the peak value of salinity changes from 5-10 wt.%NaCl eq and 40-45wt.%NaCl eq respectively.The peakvalue of homogenization temperature of the primary fluid inclusions in the porphyry ores ranges from 230 to 320℃while the peak salinity ranges from 5-10 wt.%NaCl eq and 55-60wt.%NaCl eq respectively.Raman spectra of the fluid inclusions from both types of ores indicate that water is dominant with small amount of CO2 in the gaseous phase.Based on the calculation according to the measured fluids density and pressure of fluid inclusions,the Wangjiazhuang Cu(-Mo)deposit was deduced to be formed about 1 km subsurface.The δ18OH2Oand δD values of quartzs in ore-bearing veins from both the breccia(δ18OH2O=6.26-9.45‰,δD=-64 to-57‰)and porphyry ores(δ18OH2O=1.26-7.24‰,δD=-89 to-69‰)are between the magmatic water and meteoric water,implying that the ore-forming fluidsfor both types of ores were mainly derived from magma and later mixed with meteoric water.These results confirmed that the Wangjiazhuang Cu(-Mo)deposit is a shallow porphyry-brecciaCu(-Mo)deposit and boiling and mixing of ore-forming fluids dominantly resulted in mineralization.Boththe breccia and porphyry oreshave a narrow δ34S range(breccia ores,-4.8to+0.4‰;porphyry ores,-2.8 to+2.1‰)and similar Pb isotopic compositions(breccia ores,206Pb/204Pb=18.3145-8.3528,207Pb/204Pb=15.5513-15.5650,208Pb/204Pb=38.221-38.294;porphyry ores,206Pb/204Pb=18.295-18.402,207Pb/204Pb=15.555-15.573,208Pb/204Pb=38.215-38.331),which are similar to the ore-bearing quartz monzonite.The molybdenitehave high concentrations of Re(155.7-244.2 ppm),which are consistent to the typical subducted oceanic slab-derived magma related porphyry Cu(-Mo)deposit(such as central Andes and Cordillera).The S,Pb isotopic compositions of sulfides and Re concentrations of molybdenite indicatethat themetalsin both type ores wereprimary extracted from the quartz monzonite or similar source.Inthe Early Cretaceous(～130 Ma),the roll-back of the subducted Paleo-Pacific slab underneath the North China Craton,and resulted in extension and delamination of lithosphere aswell as upwellingof asthenosphere in Zouping area.The delaminated EM1-type enriched lithospheric mantle,at the depth of 75-85 km,were heated by the asthenosphere and underwent partial melting to generate the Group C plutonic and volcanic rocks(Chayeshan gabbroic diorite,Xueshan monzonite,Xueshan gabbroic diorite and Hebanshan volcanic rocks).The asthenosphere mantle which was previously metasomatized by the subducted Paleo-Pacific slab and sediments-derived fluids,ascent to 75-85 km to underwent decompression partial melting to generate the Group B plutonic and volcanic rocks(Nandongzi monzonite,Shifanlu and Huangshan volcanic rocks).The subducted oceanic crust and sediments with high Cu,Mo contents and oxygen fugacity were heated by the asthenosphere and partially melted to primary magma,which upwelled and interacted with overlain peridotite and eventually emplaced into shallow crust to form the Group A adakitic plutonic rocks(Wangjiazhuang quartz monzonite and Beilou monzonite porphyry).At the late stage of the magmatic evolution,the magma-derived ore-bearing fluids ascended to the upper of the quartz monzonite.The decrease of temperature,mixing with meteoric water accompanied with fluid boiling causing copper and molybdenum deposition to form the Wangjiazhuang porphyry-breccia Cu(-Mo)deposit.