Magmatism And Mineralization Of Ag-Pb-Zn Polymetallic Deposits In The Lindong District,Inner Mongolia

The Southern Great Xing'an Range(SGXR)is one of the most important metallogenic belts in China,that hosts numerous Ag,Pb,Zn,Fe,Sn,C u,and Mo deposits,the major deposit types are porphyry,skarn,hydrothermal,and superimposed deposits.In the last two decades,a number of Ag-Pb-Zn and Fe-Sn deposits have been discovered in this region,making this area to be an important Ag-Pb-Zn-Sn producer throughout the SGXR.In this paper,we take the typical deposits(Haobugao deposit and Shuangjianzishan deposit)as the research object,a systematic study is carried out on the geodynamic setting,mineralization characteristics,mineraliza tion age,silver occurrence,mineral composition,ore-forming fluid characteristic and material sources,to reveal the mechanism of mineralization and to understand the deposit genesis,in order to summarize the regional metallogenic regularities,and point out the orientation for further ore hunting.The Haobugao Fe-Zn polymetallic deposit is a typical skarn type deposit,the deposit is occurred in the outer contact zone of the Early Cretaceous Wulanba biotite granite and Dashizhai Formation,and skarnization is the most important alteration type.The ore-bodies are layered and stratoid,and obviously controlled by marble.Metallogenic elements show Pb-Zn-Cu-Fe-Sn zoning characteristics from shallow to deep.Based on the field geological characteristics,mineral metasomatic relations and mineral structure,the Haobugao mineralization can be divided into two metallogenic epochs and five metallogenic stages:(1)early skarn stage;(2)late skarn stages;(3)oxide stage;(4)early sulfide stage;(5)late sulfide stage.The La-Icp-Ms U-Pb dating of Wulanba granite and diorite porphyrite cutting the ore bodies indicate that the mineralization age range from 134 to 139 Ma,which is consistent with the skarn-type molybdenite Re-Os isochron age(137.5 Ma).Three different type garnets can be identified: Grt-?,Grt-?and Grt-Alt.The major elements show that Grt-?are main andradite,however,Grt-?and Grt-Alt obviously present an increasing trend of grossularite.The rare earth elements(REE)display a light REE(LREE)decreasing trend from core(Grt-?)to rim(Grt-Alt);in the process of metasomatism,the Fe and Mg content in garnet were decreased,but Al and Mn content were increased.The composition and variation of major and trace elements indicate that the garnet growth is mainly controlled by the fluid composition,oxygen fugacity and temperature.Two kinds of magnetite(Mag1 and Mag2)including six sub-types can be distinguished based on the SEM-BSE images.EPMA and La-Icp-Ms were used to determine major and trace elements of different type magnetite.Mag1 has higher Ti and V concentrations than that in Mag2 indicating a relatively hotter formation temperature of Mag1,and Mag1 also contains relatively higher Mg and Mn concentrations,coupled with much lower Si and Al concentrations,which reflects a low fluid/rock ratio at the site of Mag1 deposition.The composition and variation of major and trace elements indicate that the magnetite growth is mainly controlled by the fluid/rock ratio and temperature.And the elemental characteristics of magnetite show that they belong to skarn genesis.Comprehensive studies on fluid inclusions show that the ore-forming fluid in skarn epoch is of high temperature(445-550?)and high salinity(51.44-58.16 wt.%NaC leqv)hydrothermal solutions.The fluid boiling plays an important role in the precipitation of Fe,Sn.The temperature and salinity of the ore-forming fluid in polymetallic sulfide stage are gradually decreased,which indicate the mixing of meteoric water.The reduction of temperature and change of mineralization environment play important role in the precipitation of Pb,Zn,and Cu.H-O isotope of quartz and C-O isotope of calcite also displays the ore-forming fluids are dominant by magmatic fluids in the early stage,and there is an addition of a large amount of meteoric water in the late stage.Therefore,the addition of meteoric water is closely related to the precipitation of metal elements.The sulfur isotope of sulfide(-2.7 – +0.1‰)and in-situ sulfur isotope(-3.0 –-1.0‰)show a similar distribution range,which suggests that the sulfur is of magmatic sulfur.The sulfide Pb isotope imply that the ore-forming materials posed a deep source feature.In brief,ore-forming fluids and materials of the Haobugao deposit show the magma source.The Shuangjianzishan Ag-Pb-Zn polymetallic deposit,discovered in recent years,is a super-large silver polymetallic deposit,which is a typical medium temperature magmatic hydrothermal deposit.The ore-bodies,composed of vein group,are hosted in the Permian Dashizhai Formation slate and controlled by NW-trending ductile shear zone and volcanic edifice.The mineralization of the Shuangjianzishan Ag-Pb-Zn polymetallic deposit can be divided into five stages:(1)pyrite-arsenopyrite stage;(2)sphalerite-quartz stage;(3)galena-sphalerite-silver mineral stage;(4)quartz-calcite-silver mineral stage;(5)pyrite-carbonate stage.The wall-rock alteration includes silicification,chloritization,calcitization and pyritization.The occurrence stage of silver in Shuangjianzishan deposit is preliminarily identified:(1)acicular or emulsion droplet occurred in galena;(2)irregularly shaped silver mineral rep lacing the sphalerite;(3)silver mineral aggregate rep lacing the sphalerite;(4)independent silver mineral wrapped by quartz particle or occurred along the quartz micro-fractures;(5)Ag in its ion state incorporated into galena.The zircon U-Pb age of the Shuangjianzishan mineralization-related granite porphyry is 130.9±0.7 Ma,consistent with the sphalerite Rb-Sr isochron age,which indicate that the Shuangjianzishan mineralization occurred in the Early Cretaceous.The study on the Shuangjianzishan fluid inclusions show that the ore-forming fluids in pyrite-arsenopyrite stage are characterized by high temperature(300 – 320?),low salinity and density;the Ag-Pb-Zn mineralization temperature range from 260? to 170?,which belongs to the middle temperature mineralization;and the hydrothermal fluids show the characteristic of low salinity(0.35 – 7.73 wt.%NaC leqv)and density(0.80 – 0.95g/cm3).The calcite C-O isotope display a magmatic origin(?18OV-SMOW‰=5.41 – 5.86‰),accompanied with the addition of meteoric water.The in-situ sulfur isotopes of the Shuangjianzishan sulfide show a wide range(?34S=-37.8‰ – 2.2‰);the ?34S values of colloidal pyrite interior range from-22‰ to-8‰,and the ?34S values of colloidal pyrite edge range from-1‰ to 0.6‰,the variation values of ?34S suggest that the colloidal pyrite interior and edge are of sedimentary origin and magmatic origin,respectively.The in-situ S isotope values of other sulfides range from-4‰ to 2‰,and show the characteristics of tower distribution suggesting a magmatic origin.The ?34S values of pyrite range from-1.8‰ to +0.2‰,and the minimum ?34S values of galena can reach to-4.4%,close to the ?34S values of the Dashizhaizu Formation slate,which indicate that Ag,Pb,Zn mineralization related to the addition of strata sulfur.The sulfide Pb isotope imply that the ore-forming materials posed a deep source feature.In brief,ore-forming fluids and materials of the Shuangjianzishan deposit show a magma source,and the addition of strata sulfur and meteoric water play important roles in the precipitation of Ag,Pb,and Zn mineralization.In order to understand the geodynamic setting of metallogenesis and diagenesis,a systematic petrographical,geochronological,and geochemical study are carried out on the magmatic rocks associated with mineralization,and get the following conclusions:(1)the Haobugao biotite granite and Shuangjianzishan granite porphyry zircon U-Pb ages were formed in 139.1±2.2 Ma and 130.9±0.68 Ma,respectively;(2)major elements show that Si,Al,Na,and K are quite enrichment,but Mg,Fe,and Ca obvious depletion;(3)trace elements display that Ba,Sr,P,and Ti are strongly depletion,but Th,U,Zr obvious enrichment;and are characterized by high Rb,Y and low Sr contents;(4)rare earth elements have obvious characteristics of LREE enrichment and a strong negative Eu anomaly;(5)the magmatic genetic type belongs to A2 type granite,which formed in an extensional tectonic setting;(6)the magma Sr-Nd-Hf isotopes have low(87Sr/86Sr)i values(0.705206 – 0.705582),high ?Nd(t)(0.60 – 1.58)and ?Hf(t)values(4.9 – 8.9),and younger two stage Nd model ages(800 – 880 Ma)and Hf model ages(563 – 778 Ma),which reflect that these A-type granites were most likely dominated by a mixture of mantle-derived juvenile component(~80%)and the lower crust component(~20%),supported by the Pb isotopic compositions.A comparative study of two typical deposits shows that two deposits belong to Fe-Sn-Ag-Pb-Zn mineralization series related with the Early Cretaceous A2 type granite in extensional tectonic setting.Based on comprehensive research of geological features,magmatism,mineralization and tectonic setting about two typical deposits,and compared with the regional metallogenic regularities,the ore controlling condition,spatial and temporal distribution,and ore-prospecting potential are analyzed as fo llows:(1)In terms of ore controlling geological conditions,it mainly includes three aspects: strata,structure,and magmas.The strata can not only provide the ore-forming elements,but also change the fluid properties which can lead to the enrichment a nd precipitation of ore-forming elements;the structure mainly provides the leading and hosting space;magmatic rocks can provide the necessary metal elements and heat source for mineralization,and their physical and chemical properties(e.g.high oxygen fugacity)also affect the migration of ore-forming elements.(2)In terms of mineralization time regularities,the previous studies pay much attention to the temporal distribution regularity of ore deposit: the metallogenic epoch show a younger trend from northeast to southwest.However,for the single ore deposit,many deposits are characterized by muti epoch superimposed mineralization,for example Baiyinnuoer and Shuangjianzishan deposits are related with the superimposed of two metallogenic events.(3)In terms of the spatial distribution,the distribution of ore deposits is strictly controlled by the lattice structure framework;of the metallogenic elements,the Haobugao deposit show the superimposed mineralization of Fe-Sn-Cu-Mo high temperature metallogenic elements and Ag-Pb-Zn middle-low temperature elements;in terms of mineralization,metallogenic elements show Pb-Zn-Cu-Fe-Sn zoning characteristics from shallow to deep.(4)In terms of prospecting potential,the study area is not only a favorable target region for Ag-Pb-Zn deposit,but also should pay much attention to looking for Cu-Mo-Sn-Fe deposits in the periphery and deep.