Study On Gold-polymetallic Mineralization In Jidong Middle And Late Proterozoic Carbonate Region,Hebei Province,China

The eastern Hebei(Jidong)region is located in the southeastern part of the Yanshanian tectonic belt,along the northern margin of the North China Craton(NCC),which lies in the junction of the Paleo Asian Ocean and Marginal-Pacific tectonic domains.Following its final cratonization in the late of the Paleo-Proterozoic,the NCC was tectonically reactivated in the Mesozoic after prolonged stabilization.During the Mesozoic,the region experienced extensive magmatic activity and various types of associated mineralization,which makes this region become an important metallogenic belt in the northern margin of the NCC.Geochemical data,zircon U-Pb,and Hf isotope analysis of the Shepantu syenogranite in the Xiaoyingzi batholith and coeval rhyolite porphyry from the Jidong region were used to investigate their origin,magma source and tectonic evolution during this period.Zircon U-Pb dating of the Shepantu syenogranite and Jianchangying rhyolite porphyry reveal the emplacement age of 199.6 ± 1.5 Ma and 192.5 ± 0.9 Ma respectively,indicating that the magmatism formed during the Early Jurassic.The geochemical characteristics of plagioclase in Shepantu syenogranite indicate that the plagioclase belongs to albite-feldspar.During the formation of the granite,it may have been affected by basic magma and magmatic mixing occurred.The geochemical characteristics of biotite indicate that biotite belongs to magnesian biotite with the characteristics of crust-mantle mixed source.The geochemical characteristics of biotite and zircon indicate that the rock mass has high oxygen fugacity.The geochemical characteristics of feldspar phenocrysts in Jianchangying rhyolite porphyry indicate that the plagioclase and alkaline feldspar belongs to albite and orthoclase,respectively.During the formation process,it may be affected by metabasic magma and magmatic mixing has occurred.The trace element characteristics of zircon indicate that the rock mass has high oxygen fugacity.Geochemical characteristics indicate the Shepantu syenogranite and Jianchangying rhyolite porphyry can be described as highly fractionated I-type granitoids.Geochemical and Hf isotopic compositions characteristics show that the primary magmas of them were derived from partial melting of the ancient lower crust and fractional crystallization during the magmatic evolution.The rhyolite porphyry experienced wall-rock material assimilation during the emplacement.The emplacement of the Shepantu syenogranite and Jianchangying rhyolite porphyry was during a post-collisional extensional environment related to the collision and amalgamation of the Siberian plate with North China plate along the northern margin of the NCC.Our results shows that the homogenization temperature of ore-forming fluid in stage I of Changcheng gold deposit is 205~259?,and the fluid is mainly magmatic water,which may be mixed with a small amount of interlayer water.Decarbonization,silicification and barite mineralization occur in the deposit.The fluid is acidic and has high oxygen fugacity.Sulfur may exist in the form of sulfate,which is not conducive to the crystallization of sulfide.In the late stage,with the decrease of fluid temperature and oxygen fugacity,a small amount of euhedral-stellate disseminated pyrite and gold began to precipitate.The homogenization temperature in stage ? is 140~190?,and this period is the main metallogenic stage of gold.Due to the addition of a large amount of interlayer water and atmospheric precipitation,the fluid temperature and salinity decreased,and the fluid gradually changed from acidic to neutral,reductive and weakly alkaline,which promoted the occurrence of strong carbonation(mainly dolomitization).At this time,disseminated-veinlet disseminated pyrite and other metal sulfides precipitated in large quantities,accompanied by gold precipitation.With the continuous addition of atmospheric water,the ore-forming fluid changes from magmatic water to interlayer water and atmospheric precipitation in stage III,and the homogenization temperature is122~135?.With the progress of metallogenic process,the fluid gradually changed into weak oxidation-weak alkaline fluid,carbonation(mainly calcitization)was developed,a small amount of star point pyrite was deposited,and the hydrothermal metallogenic period was basically over.The occurrence state of gold was micro-ultramicro wrapped gold and lattice gold,and had the symbiotic combination of Au,As,Sb and Tl.It is further revealed that the ore-forming fluid is the mixed water of magmatic fluid and interlayer water,and atmospheric precipitation was added in the late stage.The ore-forming materials mainly come from mantle derived magma.The mineralization is controlled by the carbonate strata,which is similar to Carlin type gold deposit.Matouniu gold polymetallic ore(occurrence)is the first skarn-type ore occurrence discovered in Middle-Late Proterozoic carbonate strata in the northern margin of the NCC.Phase,temperature and pressure of fluid inclusion,in-situ composition of quartz and and C-O-S-Pb isotopes from the deposit were used to investigate the origin,magmatic evolution,mineralization,and tectonic setting of the Matouniu gold polymetallic ore(occurrence).The occurrence state of gold was finely characterized by electron microprobe and scanning electron microscope.The study shows that the ore-forming fluid originated from the medium salinity(6~8 wt.% Na Cl eqv)supercritical fluid exsolved in the magmatic chamber.In the process of upward migration,the surrounding rock of carbonate rock reacted.Early skarn stage: the homogenization temperature of some fluid inclusions in garnet is greater than 550?,and the salinity is 15.7~19.3% wt.% Na Cl eqv.The homogenization temperature of fluid inclusions in pyroxene is 314~439?,and the salinity is 7.3~16.1 wt.% Na Cl eqv.The ore-forming fluid is characterized by high temperature,medium and high salinity,and have characteristics of magmatic hydrothermal.At this stage,only WL-type fluid inclusions are developed,indicating that there is no obvious decompression of fluid and no boiling phenomenon.The oxygen fugacity of ore-forming fluid is high,and sulfur may exist in the form of sulfate,which is not conducive to sulfide precipitation.Late skarn stage: the homogenization temperature of inclusions in epidote is 337~354?,and the salinity is 11.7~13.0 wt.% Na Cl eqv.Precipitate a large number of oxides represented by magnetite and metasomatize early skarn minerals.The crystallization of magnetite leads to the decrease of oxygen fugacity of fluid,the fluid gradually changes from partial oxidation environment to partial reduction environment,and S6+ in sulfate is gradually reduced to S2-.In the oxidation stage,the temperature and salinity decline,and the oxygen fugacity of the fluid also decreased further.In the early sulfide stage,WL,WV,S,V and L-type fluid inclusions were developed in quartz.S-type inclusions coexisted with WL and WV-type inclusions at the same stage.In addition,the fluid has boiling,and the volatiles overflow from the ore-forming fluid,resulting in the reduction of the stability of the gold complex,the destruction of the stability of the [Au(HS)2]-complex,and the Au unloading precipitates with the crystallization of pyrite.At this stage,the ore-forming fluid is still dominated by magmatic fluid,but the continuous addition of atmospheric precipitation leads to the continuous reduction of temperature and salinity.In the late sulfide stage,the ore-forming materials have basically precipitated,the mineralization is gradually weakened,and the temperature and salinity of the ore-forming fluid are reduced to 174~274 and 2.6? ~7.2wt.% Na Cl eqv.The ore-forming fluid is dominated by atmospheric water at this stage.Fluid boiling and temperature reduction are the main mechanisms of mineral enrichment and precipitation in Matouniu gold polymetallic ore(occurrence).And fluid mixing and water-rock reaction also have a certain impact on mineral enrichment and precipitation.Pyrite is the most important gold-bearing mineral.Gold mainly exists in metal sulfide in the form of nano fissure gold and inclusion gold.The ore-forming material mainly comes from deep-seated magma.The mineral and rock geochemistry,zircon U-Pb and Hf-Sr-Nd-Pb isotopes were were carried out on the intrusive rocks related to Matouniu gold polymetallic ore(occurrence)and Changcheng gold deposit to investigate the metallogenic characteristics.For the Matouniu gold polymetallic ore(occurrence),the zircon U-Pb age of diorite porphyrite related to mineralization is determined to be 146.7 ± 1.1 Ma,and the magma has high humidity(H2Omelt between 5.4% and 6.0%)and oxygen fugacity(?NNO+1.0~?NNO +1.3,logf O2=-12.2~-12.0).when amphibole began to crystallize,the mother magma of diorite porphyrite was emplaced at a depth of about 3.8 km.The rock came from the partial melting of mafic lower crust,and a small amount of mantle derived material was added in the formation process.Diagenesis and mineralization occurred in the period from compression to extension of the tectonic system on the northern margin of the NCC caused by high angle subduction and rotation of the paleo Pacific plate.For the Changcheng gold deposit,the zircon U-Pb age of lamprophyre before mineralization is138.5 ± 5.8 Ma,and that of pyroxene diorite porphyrite related to mineralization is 124.1± 3.5 Ma.The magma is derived from the partial melting of enriched mantle wedge metasomatized by subduction fluid.During the process of magma ascending and emplacement,there was basically no obvious mixing of crustal materials.Since lamprophyre emplacement,the region has entered a period of strong extension caused by subduction plate rotation.The continuous extension led to the emplacement of pyroxene diorite porphyrite related to mineralization.In addition,mantle derived magma provided heating power,fluid and main metallogenic materials for gold mineralization.Finally,the regional metallogenic regularity was summarized and metallogenic prediction was carried out.Skarn type gold deposits in the middle-late Proterozoic carbonate rock distributed in eastern Hebei(Jidong)region formed in the late Jurassic.The contact zone between small diorite-type neutral intrusive rock mass(including vein rock)and carbonate rock stratum is the main target geological body of this kind of gold deposit.Carlin gold deposits in this region were formed in the early Cretaceous.The favorable parts for mineralization are near deep and large faults such as Lengkou fault zone +distribution area of impure carbonate rocks in middle and late Proterozoic + development area of interlayer fracture zone and steep fault structure + development area of pyroxene diorite porphyrite vein and lamprophyre vein.Through the comprehensive prospecting information,many prospecting target areas were determined.