Geology And Geochemistry Study On The Lala IOCG Deposit,Southwestern Margin Of Yangtze Block

The Lala iron oxide-copper-gold(IOCG)deposit is located at the central of the Kangdian region,southwestern margin of Yangtze Block.The orebodies are hosted in the palaeoproterozoic Hekou Group.This volcanic-sedimentary sequnce has undergone upper greenschist-lower amphibolite facies metamorphism.The orebodies are beded,lentoid and vein shaped,and roughly strata bound.The ores are mainly net vein-breccia and veined ores,followed by disseminated-massive and stripped-layered ores.This deposit contains more than 200 Mt of ores with an average grade of 15.28%Fe,0.83%Cu,0.03%Mo,0.02%Co,0.16g/tAu,1.87 g/tAg and 0.14%REE.In this paper,through the field geological survey and indoor comprehensive analysis,the microscopic petrography and mineragraphy observation,rare earth elements geochemistry,stable isotope geochemistry,radioisotope geochemistry and fluid inclusions geochemistry are investigated.The geological and geochemical characteristics of the Lala IOCG deposit have been systematically and comprehensively studied,and the results and achievements are as fellow:(1)The mineral composition and paragenesis of the LaLa IOCG deposit were identified,and the ore-forming period was re-divided.It is believed that the main metallogenic periods of the deposit include volcanic eruption-sedimentation,metamorphism,gas-hydrothermal and hydrothermal periods,among them,the gas-hydrothermal and hydrothermal period are the main periods.Apatite,monazite and molybdenite are discovered in the hydrothermal period.(2)Based on the study of rare earth elements,it is confirmed that the Hekou Group was formed through the metamorphism of the submarine hydrothermal sedimentary rocks and the felsic magmatic rocks.The REE in the ore-forming fluid of the volcanic eruption-sedimentation period derived from the evolution of alkaline-calc-alkaline magma in rift environment.The REE in the ore-forming fluid of the metamorphism period derived from host rocks,inheriting the geochemical characteristics of the volcanic eruption-sedimentation period.The REE in the ore-forming fluid of the gas-hydrothermal period derived from the evolution of medium-acid magma in the same period.The REE in the ore-forming fluid of the hydrothermal period derived from medium-high temperature hydrothermal fluid formed by the evolution of basic magma and(or)the host rock of the Hekou Group.(3)Based on the study of H-O,C and S isotope,the characteristics of the ore-forming fluid and the source of mineralizers(C,S)of the LaLa IOCG deposit are revealed.It is believed that the metamorphism period was dominated by metamorphic water,the gas-hydrothermal was dominated by magmatic water,and the hydrothermal period was dominated by magmatic water with some meteoric water.The mineralizaers derived from mantle.(4)The source of ore-forming material are confirmed by stable isotopes,such as Pb,Sr,Nd and Os.The composition of ore-forming materials of LaLa IOCG deposit is complex,with mixed source characteristics of crust and mantle.What's more,the source of ore-forming material are different in different ore-forming period.The source of ore-forming metals(Cu and Mo)in the same period is different.(5)Using various dating method,such as monazite U-Pb,biotite Ar-Ar,sulfide Re-Os,sulfide Pb-Pb,the time of the four mineralization period of the Lala IOCG deposit was accurately determined.(1)The volcanic eruption-sedimentation period occurred in the late of the paleoproterozoic era(ca.1725Ma-1647Ma).Fe-Cu-(L)REE mineralization occurred and Fe-Cu source bed formed.(2)During the metamorphic hydrothermal period,occurred at the middle of the Mesproterozoic(ca.1235Ma-1218Ma),the ore-forming elements in the source layer redistributed,transformed and accumulated.Fe-Cu-REE mineralization occurred with strip-shaped,foliated ores.(3)During the large scale gas-hydrothermal mineralization at the end of the Middle Proterozoic(ca.1097Ma-907Ma),the Fe-Cu-Mo-REE mineralization occurred with breccia-shaped,net-vein shape,vein-shape,disseminated and massive ore.(4)Fe-Cu-Mo-U-REE mineralization occurred at the early-middle Neoproterozoic hydrothermal period(ca.860Ma-816Ma),and the alkali metasomatic rock and veined ore formed through alkali metasomatism.It is believed that the Lala IOCG deposit has multi-period and long-term continuous mineralization characteristics.(6)Based on the study of fluid inclusions,it is confirmed that the ore-forming fluid in the gas-hydrothermal period is mixture,mixed with high-temperature,high-salt,medium-acid magmatic effluent fluid and low-temperature and low-salt basin brine/deteriorated water.Phase separation-fluid and overpressure is the main mechanism of mineral precipitation in this period.The ore-forming fluid in the hydrothermal period is mixture,mixed with magma discharge fluid and meteoric water.Fluid mixing is the main mechanism leading to the mineral precipitation.(7)It is believed that the fourth-stage ore-forming events of LaLa IOCG deposit coincides with the tectonic-magmatic-thermal event time of the Proterozoic in the Kangdian area.The volcanic eruption-sedimentation period corresponds to the Paleoproterozoic Kangdian continental rift.The metamorphism and gas-hydrothermal period are consistent with the subduction tectonic-magmatic events of the late Mesoproterozoic.The hydrothermal period is consistent with the Neoproterozoic Kangdian continental rift.It is proposed that the mineralization of LaLa IOCG deposit is the response of the crust-mantle interaction during the Proterozoic in the southwestern margin of the Yangtze block and the LaLa IOCG deposit is a sensu stricto IOCG deposit.