The Relationship Between Evolution Of The Mesozoic Magmatic Rocks And Gold Metallogenesis In Eastern Hebei-western Liaoning District

There are hundreds of lode gold deposits that were formed during Mesozoic time and clustered along the eastern,southern and northern margins of the North China Craton.Simultaneously,the whole eastern part of the craton is maked by numerous extensional structures,intense episodic magmatisms and significant lithospheric thinning.Previous studies have proposed the large-scale gold mineralization mainly occurred at the Early Createous and ascribed its metallogenesis to the processes of lithospheric thinning.However,gold mineralization along the northern margin of the craton did not just occur during the Early Createous,but also Jurassic.Its relations with episodic magmatisms and geodynamic setting still remain debated.Data on petrology,major and trace elements,Sr-Nd isotopic compositions of whole rocks,zircon U-Pb ages and is-situ Hf isotopes are reported for Triassic-Jurassic granitic rocks and lamprophyre dykes in the Eastern Hebei to Western Liaoning gold district,in order to investigate their sources,petrogenesis and implications for the Mesozoic evolution of lithosphere beneath the northern part of the North China Craton.Additionally,we discussed the characters and sources for the ore fluids and metals based on the H-O-S isotopes and electron probe data on sulfides,and further documented the genetic link between gold metallogenesis and magmatisms according to their geochronology,spatial relations and the lead isotopic evolution models proposed in this paper.The Dashizhuzi complex mainly consists of granodiorites and granites.The granites obtain an emplacement age of 226 Ma,and are metaluminous and can be categorized to high-K calc-alkaline series rock.All samples have distinctive features of high Na2O?4.5–4.9 wt.%?,Sr?? 458 ppm?,and low Y?? 8.3 ppm?with high Sr/Y?65–126?and?La/Yb?N ratios?35–48?.They are also enriched in large-ion lithophile elements and light rare earth elements,and depleted in high-field-strength elements and heavy rare earth elements.The initial 87Sr/86 Sr ratios and ?Nd?t?values vary from 0.70416 to 0.70449 and-11.0 to-9.1,respectively.The ?Hf?t?values of zircons vary from-12.2 to-1.4.These characteristics show that the Dashizhuzi granites originated directly from melting of mafic lower crust composed of pre-existing ancient crustal and enriched mantle-derived juvenile crustal materials at normal continental crustal depth of 33–40 km.The Baizhangzi granites obtain an emplacement age of 233 Ma,and are metaluminous and can be categorized to high-K calc-alkaline series rock.All samples are featured with low MgO?0.58–0.77 wt.%?,Mg#,Co?2.9–3.7 ppm?,Ni?0.9–2.9 ppm?,Cr?12.5–15.8 ppm?contents and high Sr/Y ratios?40–51?.They are also enriched in large-ion lithophile elements and light rare earth elements,and depleted in high-field-strength elements.The initial 87Sr/86 Sr ratios and ?Nd?t?values vary from 0.70445 to 0.70524 and-7.3 to-1.7,respectively.The ?Hf?t?values of zircons vary from-13.4 to-5.9.These characteristics suggest that the Baizhangzi granites were derived from melting of lower continental crustal materials,followed by crystallization of plagioclase to some extent.Petrogenesis of the Dashizhuzi and Baizhangzi granites may be related to post-orogenic extension caused by the assembly of the Mongolian composite terranes with the North China Craton.Basaltic underplating during the Early Mesozoic could modified the compositions of lower continental crust and provided heat for melting of lower crustal materials.The high Sr/Y raios and low Y,and depletion in Nb–Ta and Ti features indicated by these Triassic granitic rocks are mainly related to the intrinsic characteristics of the lower crust of the North China Craton.All rock types of lamprophyres are mainly minette and kersantite,and are dated at 167 Ma.The lamprophyre dykes display a wide range of SiO₂?48.5–56.4 wt.%?,moderate to low Na2O?1.3–3.9 wt.%?and high total alkalis?K2O+Na2O = 5.2–8.4 wt.%?.Two groups of dykes can be recognized based on their MgO and Al2O3 contents.The Group 1 dykes have high Al2O3?12.4–14.1 wt.%?,and low Fe2O3?5.3–8.0 wt.%?and MgO?5.7–9.7 wt.%?contents with Mg# values of 62 to 71.They also have low compatible elements and high rare earth elements contents,and MORB-like Th/U?3.3–3.8?,Ba/Th?53–180?and Ce/Pb?11.7–27.3?ratios,and variable Sr-Nd isotopic compositions?initial 87Sr/86 Sr = 0.70569–0.70629;?Nd?t?=-8.5 ^-0.1?.The Group 2 dykes have low Al2O3?11.7–12.4 wt.%?,and relatively high Fe2O3?7.4–8.0 wt.%?and MgO?11.7–13.7 wt.%?contents with Mg# values of 76 to 77.They also have high compatible elements and low rare earth elements contents,and enriched Sr-Nd isotopic compositions?initial 87Sr/86 Sr = 0.70618–0.70800;?Nd?t?=-10.7 ^-7.4?.These distinctive features suggest that the Group 1 dykes were derived from a relatively fertile lithospheric mantle sources?garnet-facies amphibole-bearing lherolite?,which has experienced variable degrees of asthenospheric mantle-derived melt-peridotite interaction prior to melting.However,the Group 2 dykes were derived from an ancient garnet-facies phlogopite and/or amphibole-bearing lherzolite lithospheric mantle.According to the petrogenesis of the Jurassic lamprophyre dykes and Trassic granitic rocks,we proposed that thinning of the Early Mesozoic lithospheric mantle beneath the northern part of the North China Craton is dominantly through thermo-mechanical erosion prior to the Middle Jurassic.At the same time,the chemical compositions have been modified at the bottom of the lithospheric mantle through melt-peridotite interaction processes.There are numerous famous gold deposits,like Jinchangyu,Yu'erya,Baizhangzi and Niuxinshan,located at the Eatern Hebei to Western Liaoning gold district.They are all located to the south of the regional EW-trending Miyun-Xifengkou fault and in the interior of the NE-NNE-trending Xifengkou-Lingyuan fault zone,and generally cluster around the Dushan-Dashizhuzi complex.The orebodies commonly comprise auriferous quartz-sulfide veins or disseminated gold-bearing ores,and are controlled by different fault systems and geometry of granitic magmas.Pyrite,chalcopyrite,galena,shpalerite and tellurobismuthite are common sulfides in different gold deposit,and the gold oftern occurs as native gold or electrum.The alteration minerals are dominant by quartz,pyrite,sericite,chlorite and carbonate.Three hydrothermal metallogenesis stages can be recognaized,including coarse granied quartz-pyrite,quartz-polymetallic sulfides and quartz-carbonate.The quartz-polymetallic sulfides is the most important gold deposit stage.The fluid inclusions from the Baizhangzi gold deposit have homogenization temperatures of 300? to 380? and salinities of ⁵.3 wt.% NaCl.?¹⁸O and ?D values of ore fluids vary from 0.3 to 4.1‰ and-94.0 to-90.7‰,respectively.The fluid inclusions from the Niuxinshan gold deposit have homogenization temperatures of 260? to 360? and salinities of 3 to 11 wt.% NaCl.?¹⁸O and ?D values of ore fluids vary from-2.6 to 6.2‰ and-80.7 to-63.1‰,respectively.The fluid inclusions from the Yu'erya gold deposit have homogenization temperatures of 259? to 390? and salinities of 2.1 to 4.5 wt.% NaCl.?¹⁸O,?D and ?13C values of ore fluids vary from 4.6 to 7.6‰,-90.4 to-72.1‰ and-7.9 to-3.8‰,respectively.The fluid inclusions from the Jinchangyu gold deposit have homogenization temperatures of 256? to 370? and salinities of 3.0 to 12.5 wt.% NaCl.?¹⁸O and ?D values of ore fluids vary from-0.9 to 8.3‰ and-97.0 to-50.0‰,respectively.?13C values of ore fluids and ankerites vary from-8.4 to-2.3‰ and-7.6 to-5.4‰.These chacracteristics suggest that ore fluids were predominance magmatic fluids with contribution of a few paleometeoric water.Sulfide separates from the Baizhangzi,Niuxinshan,Yu'erya and Jinchangyu gold deposits have ?34S values of-1.5 6.3‰,3.1 6.3‰,1.0 3.7‰ and-4.9 2.5‰,respectively.Co contents of pyrite measured by electron probe from the Baizhangzi,Niuxinshan and Yu'erya gold deposits are dominantly 0.02–0.09%,0.08–0.25% and 0.04–0.31%,and the corresponding Co/Ni ratios are dominantly 0.9–16.7,0.5–3.2 and 0.6–8.3,respectively.These characteristics suggest that sulfides were derived from a magmatic sulfur source,and pyrites were predominance deposited from magmatic fluids.The lead isotopic evolution models in the Eastern Hebei to Western Liaoning district can be divided into two long-lasting evolution stages bounded by three significant Precambrian events,including 2.9–2.7 Ga major continental crust growth and crust-mantle differentiation,¹.85 Ga assembly of the Eastern and Western Blocks,and Mesozoic large-scale gold metallogenesis and intense magmatisms.According to the lead isotopic evolution models,we suggest that the Trassic magmas have dominant enriched lithospheric mantle sources,and the Jurassic magmas were mainly derived from lower continental crust,and crust-mantle interaction played an important role in the petrogenesis of the Early Createous magmas.Additionally,ore-forming metals of Jurassic gold deposits in the Eastern Hebei to Western Liaoning district have a dominant lower crust origin.However,metals of Early Createous gold deposits in Chifeng-Chaoyang district were dominantly derived from lithospheric mantle or processes of crust-mantle interaction.The formation of Trassic magmas and molybdenum mineralization along the northern margin of the North China Craton were mainly induced by post-orogenic extension caused by the assembly of the Mongolian composite terranes with the North China Craton.Subduction of the Izanagi plate may played critical role in the formation of the EW-trending Jurassic magmatic and gold,molybdenum mineralization belt.The NE-trending Early Createous magmatic and gold,molybdenum mineralization belt were also formed in a subduction setting,but related to the westwarding subduction of the Pacific plate to the eastern Eurasian continent.