Research Of Precambrian Sedimentary Exhalative Metallogenesis In Eastern Jilin Province

The scope of this study is one of the three Paleoproterozoic orogenic belts in the North China Craton(NCC)——the northeastern margin of Jiao–Liao–Ji active tectonic belt.The geodynamic evolution history of study area can be divided into four stages:Archean Archi continent formation stage;Paleoproterozoic"Liaoji ocean"tectonic evolution stage;Paleozoic–Early Mesozoic Paleo–Asian Ocean tectonic evolution stage;Mesozoic–Cenozoic Paleo–Pacific Ocean tectonic evolution stage.Based on the study of the metasedimentary rocks of the Dalizi Formation in the Laoling Group,the maximum age of deposition was ca.1.97 Ga,and the sediments were metamorphosed at ca.1.91 and1.84 Ga.The age of ca.1.91 Ga was regard as the peak metamorphic during the crustal thickening stage,and the age of ca.1.84 Ga formed during the post–orogenic extension that triggered retrograde metamorphism(i.e.,the exhumation and cooling).This suggests that middle Paleoproterozoic subduction within the northern Jiao–Liao–Ji Belt might have ended before ca.1.91 Ga,when the Longgang and Nangrim blocks collided and amalgamated,which are coeval with the global–scale collisional events from 2.1 to 1.8 Ga that led to the assembly of the Columbia supercontinent,into which the NCC was incorporated.Sedimentary exhalative activity usually occurs in the place of the earth's energy exchange and is one of the carriers for the release of energy from the earth's interior to the surface.Due to the simultaneous occurrence of internal sedimentary exhalative activity and external normal sedimentation,and the pulsating characteristics of sedimentary exhalative activity,exhalite is often interacted with terrigenous sediments and volcanic eruption sediments in the field,forming typical sedimentary exhalative structures,such as banded structure and laminated structure.Under normal sedimentation,sediments show regular changes as physical and chemical conditions change,sedimentary exhalative activity can form a set of specific mineralization zones.The combination of normal sedimentation and sedimentary exhalative makes the reality more complicated.Take the Dalizi iron polymetallic deposit as an example,the sedimentary hematite was formed in a high temperature and strong oxidation environment,and the oxidation environment of the vent forced the conduit facies products to be decomposed or could not be precipitated,so there were no conduit facies in the"double-layer"structure.Chalcopyrite is a product of high temperature and strong reduction environment.Field investigation shows that the copper body is closely related to hematite and magnetite bodies,and is generally in the upper part of the iron body.This vertical mineral succession records the change of hydrothermal sedimentary environment.Fe isotope studies show that the zonation of iron mineral phases is due to the different physical and chemical environments corresponding to the vent center and the far end,but the source of ore-forming materials and metallogenic mechanism are consistent.The hematite at the center is transitive to siderite at the far end,and magnetite as a late metamorphic product occurs in both hematite and siderite zones.The formation of copper sulfide in the center of the vent and the formation of lead and zinc sulfide at the far end are typical sedex mineralization zonation,which is a horizontal phase transition relationship formed by simultaneous deposition.Exhalite is one of the important markers to identify sedimentary exhalative deposits.A variety of exhalite has been identified in the study area,including siliceous rocks,siliceous albite rocks and marble in the Zhenzhumen and Dalizi formations in the Laoling Group,and tourmalite(tourmaline–quartzite),tourmaline rocks and marble in the Mayihe Formation in the Ji'an Group.It is also found that there is a kind of carbonaceous rock series which represents the seabed reduction environment and marks the end of sedimentary exhalative activity.The carbonaceous rock series has been identified in the bottom of Baifangzi Formation,the middle and upper section of the Dalizi,Huashan,Zhenzhumen and,Dataishan formations,and the upper section of the Mayihe Formation.Some of the organic matter has been transformed into graphite by later metamorphism.In the course of a sedimentary exhalative activity,the exhalative fluids extracted the beneficial elements in the ocean crust and deep strata through vertical convection circulation and ejected to the ocean bottom water-rock interface to form orebodies,and also formed a series of exhalite.When the end of the sedimentary exhalative activity,the vent surrounding biota died and formed carbonaceous rock series.The exhalite and carbonaceous rock series are the outcomes of different stages of sedimentary exhalative activity,which have indicative significance for the spatial location of orebodies.The exhalite and carbonaceous rock series together constitute the"sedimentary exhalative dual formations"model,which can be used as an important iconic rock assemblage to distinguish this kind of deposit.Based on the study of typical deposits,it is found that the exhalative fluids in this area belong to the homogeneous Na Cl–H₂O system with low salinity and low density,which is similar to the typical sedimentary exhalative deposits in the world.The higher temperature was similar to the highest temperatures recorded in modern submarine vents,and may have come from the deep circulation of the seawater.Based on traditional stable isotopes,non–traditional stable isotopes and in situ isotopic analysis,it is found that sea water plays an important role in the Precambrian sedimentary exhalative metallogenesis in eastern Jilin Province.With the convergence and final integration of the Longgang and Nangrim blocks,the subduction of oceanic crust triggered the formation of regional large–scale magmatic event temperature gradient,which provided the power source for regional sedimentary exhalative activities.The sedimentary exhalative deposits in the eastern Jilin Province are concentrated in the Paleoproterozoic evolution stage of"Liaoji ocean"tectonic domain and in the jiao-Liao-Ji belt of the eastern NCC.Four periods of sedimentary exhalative mineralization have been identified in this study,and developed episodic in time and distributed unevenly in space.The time is controlled by the stages of exhalative deposition and mineralization,and the space is controlled by the sedimentary facies of different geological units,but all of them are concentrated in the transitional stage of carbonate to fine clastic filling sequence in the extensional basin,which is closely related to the evolution stage of tectonic.The ore formed in the first period is boron,which is occurs in the Mayihe Formation of the Ji'an Group.The formation of boron ore bodies is closely related to volcanic activities.The volcanic activities heated seawater to infiltrate and mixed with deep magma to form high-temperature hot water fluid,which leached the deep strata and basaltic ocean crust.The hot water fluid with rich boron was ejected from the ocean floor and mixed with cold seawater,resulting in the discharge of boron,magnesium,iron and silicon,and formed exhalite represented by tourmaline-quartzite,hyalotourmalite and silicified marble.At the end of the sedimentary exhalative activity,a large number of organisms died and formed carbonaceous rock series,which became crystalline graphite deposits through deep metamorphism.The sedimentary exhalative mineralization of this period is characterized by huge scale,close relationship with volcanic activities,high heat flow abnormal background,closed sedimentary water environment and less terrigenous supply to avoid boron dispersion,and the magnesium rich sedimentary strata promoted boron precipitation.The ore formed in the second period is lead and zinc,which is occurs in the Zhenzhumen Formation of the Laoling Group.During this period,the tectonic setting of sedimentary exhalative activity was relatively stable,and the deep magma chamber under the orebody drove the infiltration of heated seawater and formed a vertical convection cycle.The uncompensated starvation basin of the high-water level depositional system tract during the continuous extension of the back arc basin,the basin floor sediment is mainly carbonate,which is conducive to the development of ocean floor fractures and the migration of ore-bearing hot water fluid.In addition to the specific temperature,salinity,p H range and migration channel,the migration of Pb and Zn elements also needs to balance the concentration of H₂S in the ocean floor.The pyrite developed near the lead-zinc deposits in the Zhenzhumen Formation is probably the product of H₂S equilibrium mechanism.The black carbonaceous striped silicified dolomite marble and carbonaceous slate developed in the upper strata of the orebody and the graphite in the interstitial spaces around the dolomite marble fragments constitute"carboniferous rock series",indicating the end of the sedimentary exhalative activity in this period.The ore formed in the third period is copper,nickel,and cobalt,which is occurs in the Huashan Formation of the Laoling Group.The back-arc basin entered the extreme period of extension,and the deep materials(including the materials of upper mantle)of basement rupture were upwelled,resulting in the formation of deep source elements mineralization represented by cobalt and nickel,and the material and energy exchange between the surface layer and the deep layer reached the strongest.Partial basic magma activity is very important to the mineralization,and the magma chamber space location close to the high temperature water mixed with basic magma formation and carry more mantle source composition of hot fluid(Shansonggang copper nickel and cobalt deposits),and the magma chamber space position of seawater is far from the hot fluid temperature is not too high(Dahenglu copper and cobalt deposit).In the process of convective circulation,hot water extracts Co,Ni,Cu,As,Bi,Sb,Ag,Pb and other elements in the ocean crust and deep strata,and then spits out along the tectonic fractures,mixes with the cold seawater,offloads the elements for mineralization,and forms exhalite dominated by silicalite.The ore formed in the fourth period is iron polymetallic,which is occurs in the Dalizi Formation of the Laoling Group.The back-arc basin enters the extension-easing period,when the local extension-stretching may still be very strong,inducing lithospheric rupture and then developing into a small ocean basin.As a thermal driving force,the volcanic activities associated with the subduction collision heated the seawater to form a convection cycle,extracted the beneficial elements in the ocean crust and deep geological bodies and carried them to the ocean bottom water-rock interface,and deposited ore-bodies and silicalite in the form of central hot water jet.The constantly changing water environment and the specific mineralization zoning of the hot water deposition make the hot water jet activity in this period show the complex characteristics of horizontal and vertical mineral zoning.On the whole,the evolution of Precambrian sedimentary exhalative mineralization in eastern Jilin presents multi-period,directional and epoch specificity.The"sedimentary exhalative dual formations"was recognized in the four periods of sedimentary exhalative mineralization activity.The mineral types formed are gradually complex and the overall scale of the deposit is gradually small.Epoch specificity refers to the corresponding mineral types formed under certain sedimentary stratum conditions and tectonic background.With the final collision and fusion of the Longgang and Nangrim blocks as well as the passive collage of sedimentary formations on both sides,strong regional deformation and metamorphism were triggered,and the previously formed orebodies were forced to be reformed to some extent.On the micro level,it is mainly reflected in the migration and metamorphic redistribution of some elements.Macroscopic,the stability and continuity of the initial orebody are destroyed,and the occurrence,distribution and grade of the orebody are changed.Different minerals are transformed by difference due to their different properties.Such as the Dalizi deposit of iron ore body by transposition of structures transformation into"seems not layer,seems vein pulse"form,if pursue further on a foliation plane other than the original sedimentary bedding is futile.In order to find the"lost"ore body group,it is necessary to restore the original sedimentary exhalative mineralization zoning by restoring the structural deformation S₀,with iron ore body as the marker body.Therefore,the alteration of ore bodies by deformation and metamorphism must be fully considered in the exploration and prediction of sedimentary exhalative deposits in the study area.