| Liaodong peninsula is one of the most important gold-uranium metallogenic prospect areas in the North China Craton(NCC).Liaodong Peninsula,located in the northeastern part of the NCC,is also one of the important areas where the Mesozoic granitoid rocks are widely distributed.The Mesozoic granitic magmatism is closely related to mineralization,including large-scale Au,U,Pb,Zn,Cu,and Mo metallic ore deposits in Liaodong Peninsula.Previous studies of the Liaodong Peninsula were mainly conducted by a single deposit or two deposits.However,the comprehensive and comparative studies are still insufficient for the different periods of intrusions and deposits from the Mesozoic in Dandong district.The relationship between granitoids and gold-uranium deposits and the sources of the ore-forming materials and fluids are important to understand the genesis of the gold-uranium mineralization in this region.We discussed the magmatic sources for the ore fluids and metal concerning the Mesozoic gold-uranium ore systems,and analyzed the genetic links between gold-uranium metallogenesis and magmatism.The main conclusions are summarized herein:The zircon U-Pb dating studies show that The Saima-Bailinchuan alkaline complex(SBAC)is composed of alkaline syenite and aegirine-nepheline syenite,with zircon U-Pb age of 235-227 Ma.Aegirine,alkaline-augite,biotite,orthoclase and nepheline are the major minerals.The occurrence of the SBAC,together with many other alkaline complexes of similar ages(235-209 Ma)on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids.Triassic magmatism mainly comprises nepheline syenites,including the SBAC,indicating an intraplate anorogenic extensional setting in the northern part of the NCC during Middle to Late Triassic.Mineralogy,petrology and geochemical studies show that the parental magma of the Saima-Bailinchuan alkaline syenite is SiO2-undersaturated potassic,and is characterized by high contents of CaO,Fe2O3,K2O,Na2O.The Saima-Bailinchuan alkaline syenite was originated from a phlogopite-rich,enriched lithospheric mantle source in garnet-stable area.The studied samples(SM-6,SM-7,BLC-1 and BLC-2)have the calculated magmatic crystallization temperature average values of 686?,723?,766? and 816?,respectively.The Sanguliu granodiorite is defined as a granodiorite,according to TAS diagram.The Sanguliu granodiorite have high SiO2 contents,and low Mg#,Ni,and V concentrations,indicating that they were derived from crustal source.We propose that the Sanguliu granodiorite is I-type granite.Geochemical features such as enrichment of Rb and Ba relative to Sr,high K2O,and low CaO and MgO indicate a crustal source rock.During the Early Cretaceous,Sanguliu intrusion probably reflects the subduction of Pacific plate beneath the NCC plate and the resulting transpressive regime along the edge of the continent.Ore deposit geology studies show,Saima uranium deposit is closely related to the Indosinian alkaline complex.Ore body occurs in the contact zone between aegirine-nepheline syenite and wall rock.The Wulong gold deposit is controlled by a set of NNE extending faults on the western side of the Yalujiang fault.The sulfide minerals are mainly pyrite and minor pyrrhotite,bismuthinite,chalcopyrite;the gangue minerals include quartz,calcite and chlorite.Silicification,sericitization,chloritization and carbonatization are the dominant near-ore alteration types.Five stages of mineralization can be distinguished:(?)milky white quartz stage;(?)milky white quartz-pyrite stage;(?)gray quartz-pyrite-bismuthinite stage;(?)dark grey quartz-polymetallic sulfide stage;and(?)carbonate stage.Sidaogou is a typical large gold deposit in Liaodong peninsula.Four stages of mineralization can be distinguished:(?)milky white quartz stage;(?)quartz-pyrite stage;(?)quartz-polymetallic sulfide stage;and(?)chalcopyrite-quartz-carbonate stage.The sulfide minerals are mainly pyrite and chalcopyrite;the gangue minerals include quartz and calcite,sericitization,chloritization,chalcopyritization and carbonatization are the dominant alteration types.The fluid inclusions study show,the main type is two-phase fluid inclusions in SBAC area.They may occur in clusters or isolated within mineral growth plane.The homogenization temperature ranged from 116 to 380? with two peaks 116?168?,180?320?,respectively.Furthermore,temperature of final ice melting ranged from-3.5? to-0.8?,corresponding to a salinity of 1.4?5.7 wt.%NaCl eqv.The density varies from 0.68 to 0.92 g/cm3.For the Wulong gold deposit,microthermometric studies show that Tm,CO2 of CO2-H2O inclusions are concentrated in the range of-61.0?-56.8?,Tm,clath are in the range of 7.5?9.8?and Th,CO2 vary from 10.1?28.0?,with the salinities being 0.43%?4.80%NaCl eqv.and the densities being 0.64?0.86g/cm3,and most of inclusions show thatTh,total(206?385?).For the Sidaogou gold deposit,microthermometric studies show that Tm,NaCl of LH2O-VH2O-S inclusions are concentrated in the range of 245?340?,with the salinities being 34.67%-41.23%,and the densities being 1.02?1.13g/cm3.Tm,CO2 of CO2-H2O inclusions vary from-56.7?-57.3?,Tm,clath vary from 7.5-8.6?,with the salinities being 2.1-4.3wt%NaCl,Th,CO2 vary from 25.2?27.50C,Th,total vary from 276?320?.Tm,ice vary from-15.2?-19.4?,with the salinities being 18.8%?21.96%,Th,total vary from 150?280?,and the densities being 0.93?1.07 g/cm3.SRXRF suggests that Au element exists in CO2-rich inclusions.The carbon isotope compositions at Wulong gold deposit suggest that the ore fluids are derived from mixing of fluids from the mantle source.The ?D and the?18OH2O values suggest that magmatic fluids may have played an important role during the formation of the ore-forming fluids.The ?34SCDT values of primary sulfide ore minerals distribute narrowly in Wulong and Sidaogou deposit.The characteristics of sulfur isotopic composition show that the sulfur in these two deposits is derived from the mantle mixed seawater.Meanwhile,the Pb isotopic composition of ore sulfides is stable and the ore lead is ordinary common lead with little U and Th radiogenic lead.According to the tracer analysis regarding the characteristic parameters,lead composition model and ??-?? diagram,the conclusion is drawn that the ore lead is typically crustmantle mixed subduction lead.By aggregated investigations,this study proposes there are two periods of mineralization during Mesozoic in Liaodong peninsula:the first period is Indonian magmatic hydrothermal mineralization.The characteristics of U deposit fit more with the replacement type of mineralization in magmatic hydrothermal system.The hydrothermal replacement formed around Late Triassic intrusions,medium-low temperature and medium-low salinity ore-forming fluid migrate through faults and fractures and formed U mineralization.The second metallogenetic period is Yanshanian magmatic hydrothermal system.The Au mineralization formed at contact zone between intrusion and granite,the hydrothermal fluid migrated through the favorable deep thrust,mixed with meteoric water and formed Au mineralization. |
PDF Full Text Request