The Diorite From Yu'erya Gold Deposit Area In Eastern Hebei Province,China:Geochronology And Petrogenesis

The Jidong area is located in the northern margin of the North China Craton,it has successively influenced by the subduction of the Paleo-Asian Ocean in Paleozoic and the Paleo-Pacific oceanic tectonic domain in Mesozoic.The Jidong area is one of the important source of gold in China,nearly 200 gold deposits have been discovered,the gold deposits are closely related to the widespread Mesozoic intermediate-acidic intrusive rocks in the area.Therefore,the Mesozoic intermediate-acidic intrusive rocks have provided a platform for the formation age,origin study of gold deposits and geological prospecting.Located in Kuancheng County,eastern Hebei Province,the Yu'erya gold deposit is a typical representative of a great deal of porphyry gold deposits in Jidong area.The main rock types of the Yu'erya Jurassic granite pluton are the granites and diorites,the granites intruded into the Early Mesoproterozoic Gaoyuzhuang Formation strata,and the diorites intruded into granite and Gaoyuzhuang Formation strata.Previous studies have mainly focused on the formation age and genesis of the gold deposits and granites,considering that the Yu'erya gold deposits were mainly occurred in the granites,although some were located in the surrounding rocks adjacent to the granites,and were associated with genesis of the granites.We have recently discovered that a few diorites also host gold deposits,indicating a certain relationship with gold deposits.In the past work,the diorites in the area have been less studied,the scarce of precise chronological and geochemical datas limit the understanding to their petrogenesis.Moreover,it is not clear whether the diorites exist the relationship with gold mineralization and the granites.In order to solve these problems,this paper has undertaken detailed research on petrology,geochronology,and geochemistry of the Yu'erya diorites,identified the rocks characteristics and the formation age through studying the petrology and geochronology of the diorites,determined the Yuerya gold metallogenic epoch through the spatial relationship between the gold orebodies and the diorites,debated the petrogenesis through the analysis of geochemical characteristics of the diorites.Besides,based on the research of existing data,this paper revealed the relationship between the diorites and the gold deposits,discussd the geodynamic setting of diagenesis and mineralization from Yu'erya gold deposit area.Undoubtedly,the above research findings provide a unique help to the study of regional tectonic evolution and mineral exploration.1.Combination and formation age of the dioriteThe diorites of Yu'erya gold deposit area are composed of the biotite quartz diorite,biotite hornblende diorite and hornblende diorite.The result of LA-ICP MS U-Pb isotopic dating shows that the ages of the biotite quartz diorite is 163 Ma,the biotite hornblende diorite is 160 Ma and the hornblende diorite is 155 Ma,indicating that the diorite has been emplaced in Late Jurassic.2.Geochemical characteristics of the dioriteElements geochemical characteristics of all the diorite samples from the Yu'erya gold deposit area show that they have SiO₂=51.99%-61.88%,Al2O3=12.83%-18.25% MgO=2.21%-6.70%,Ti O2=0.68%-1.11%,CaO=3.28%-8.24%,TFe2O3=6.55%-9.79%,Na2O= 3.06%-5.52%,K2O=1.95%-3.38%,Mg#=37-64.The total alkali?K2O+Na2O?accounts vary from 5.08% to-8.90%,the Na2O/K2 O ratios range from 1.31 to 2.18,displaying that the diorites are Na-rich,mainly belong to the high-K calc-alkaline series,except for two samples belong to alkaline series.The Yu'erya diorites are metaluminous with A/NK values of 1.37 to 2.26 and an A/CNK values of 0.73-0.96.Other samples have similar characteristics of REE patterns which are characterized by relatively low total REE contents??REE=126.95×10-6-175.23×10-6?,except for one sample with high total REE contents??REE=311.61×10-6?,LREE/HREE=8.69-22.10,?La/Yb?N=9.17-39.35,relatively significant fractionation of LREE and HREE,no obvious negative Eu anomaly,showing that there is no fractional crystallization of plagioclase.These rocks are enriched in light rare earth elements?LREEs?and large ion lithophile elements?LILEs?,such as Ba,Rb,K and Sr,relatively enriched in Th and U,and individual strongly enriched in Th and U,depleted in heavy rare earth elements?HREEs?and high field strength elements?HFSEs?,such as Nb,Ta and Ti.Ba is mainly hosted in biotite and K-feldspar,the enrichment of Ba shows little obvious fractional crystallization of biotite and K-feldspar.3.Petrogenesis of the dioriteGeochemical analysis of the diorites of Yu'erya gold deposit area show that the diorites are characterized by low SiO₂ compositions,and comparatively high concentrations of V?97×10-6-258×10-6??Cr?10×10-6-360×10-6?,displaying that the diorites are characterized by the mantle-derived magma.All the diorite samples are enriched in LREEs and LILEs?Rb,Ba,K,Th,U,Sr?,depleted in HREEs and HFSEs?Nb,Ta,Ti?,La/Nb=4.05-5.40?>1.5?,La/Ta=63.80-89.50?>30?.Zircon Hf isotopic analysis of the diorites show homogeneous ?Hf?t?values?-16.6 to-6.5?,the individual ?Hf?t?values are slightly lower than ?Hf?t?average value of the enriched subcontinental lithospheric mantle?SCLM??-9.0?,it is suggested that the parental magma of the diorites were likely derived from partial melting of the enriched SCLM and contaminated slightly by ancient lower continental crustal materials.In addition,the Archean and Paleoproterozoic captured zircons are found in the samples of diorites from the gold deposit area indicate there are involvement of ancient lower continental crustal materials of the NCC in formation of rocks.Based on the geochemical characteristics of the major elements,trace elements and Hf isotope,it is considered that the diorites in Yu'erya gold deposit area should be derived from the partial melting of the enriched subcontinental lithospheric mantle,which are caused by subducted slab fluid metasomatism,the crustal contamination is little obvious.4.Relationship between diorite and gold mineralization and tectonic settingIn space,a few of diorites host gold deposits,the trend of diorites are the same as the trend of gold orebodies,which are dominated by NE direction,and control the distribution of gold orebodies;In time,the result of the zircon U-Pb dating indicates the concordant age of gold-bearing biotite quartz diorite is 163 Ma,the later hornblende diorite cutes the gold orebody and displays the concordant age of 155 Ma,speculating that the gold deposits have formed at Late Jurassic from 163 to 155 Ma.In formation,on the basis of the results of H,O,Pb and S isotope from the gold deposits,it is found that the ore-forming fluids are mainly derived from granitic magma.However,the influence of the later mantle-derived magma on gold mineralization can not be ignored,the mantle-derived magma can provide a few of mineral materials for gold mineralization.Therefore,this paper argues that the diorite has a certain contribution to gold mineralization.Moreover,combined with the regional tectonic evolution history,it is inferred that the diagenetic and metallogenic events of the Yu'erya gold deposit area were formed in the compressional orogenic environment,which are related to the subduction of the Paleo-Pacific plate to Eurasia.