Comparison Of Crust Evolution And Gold Deposits Genesis In The Bayan Obo-Alxa League Region,inner Mongolia

This paper focuses on the Bayan Obo-Alxa League region in Western Inner Mongolia.On the basis of sorting out and summarizing the evolution process of tectonic environment in this area from Mesoproterozoic to present,the black rock series type gold deposits,such as Haoyaoerhudong gold deposit and Tebai gold deposit,are taken as research objects.The genesis of the two gold deposits is analyzed in detail by means of Zircon U-Pb dating,rock geochemistry,isotope geochemistry,element content analysis structural attitude statistics and fluid inclusions measurement.On the basis of comparing the similarities and differences between the two gold deposits,and comparisons to other typical gold deposits in Western Inner Mongolia,the prospecting criteria of the black rock series type gold deposits in Western Inner Mongolia are preliminarily summarized.The main understandings obtained in this paper are as follows.1.The formation time of Tebai quartz diorite was 281.7 1.1Ma,which was formed in the subduction environment of the Holsen-Chaganchulu back-arc basin to the north of the Alxa Platform Uplift.It can stand for the lower limit of the closure time of the back-arc basin.The formation time of tube Guancaitaolugai granite porphyry was 272.6 0.8Ma,which was formed in the limited extension environment after the closure of Holsen-Chaganchulu back-arc basin.It can stand for the upper limit of the closure time of this back-arc basin.The closure time of the western side of the Holsen-Chacanchulu back-arc basin?282-272Ma?is consistent with that of its eastern side,indicating that the main tectonic framework of the Alxa Platform Uplift in Paleozoic is EW trending.Futhermore,it can be compared with the NCC north margin to the east of the Helan Mountain.2.There are two main enrichment processes of gold elements in the Haoyaoerhudong gold deposit.The first enrichment occurred in the diagenetic stage of the Bayan Obo Group,and the second enrichment occurred after the Hercynian magmatic activity.As the wall rock of ore body,black rock series is not only the provider of gold element,but also the interface of hydro-rock metasomatism of ore-bearing hydrothermal fluid,which plays an important role in gold element enrichment.The sources of gold elements include the black series of the Bilut Formation in the Bayan Obo Group,the mafic dikes and the acidic dikes.3.In the Haoyaoerhudong gold deposit,the Haoyaoerhudong syncline formed in Caledonian period controlled the distribution of shallow metamorphic ore bodies and restricted the distribution scale of the later gold-bearing quartz veins.The Gaoletu arc fault belt of the Hercynian period provides a migration channel for the ore-forming fluid.The ductile shear zone on the east side of the Gaoletu arc fault belt are the ore-hosting structure of the gold-bearing quartz veins.The occurrence of the fracture zone take advantage of the Caledonian extensional joints in the Haoyaoerhudong syncline with only small Angle changes.4.In the Haoyaoerhudong gold deposit,the basic dikes are the product of the Caledonian magmatic activity,which was formed in the transition environment which the regional stress field changed from compression state to relative extension.Acid dykes are the product of magmatic activity in post-collision tectonic environment.The pegmatitic dikes are the product of magmatic activity in Yanshanian period.The pyroxene porphyrite and biotite lamprophyre may have provided gold for the shallow metamorphic rock type ores,the granite aplite and biotite granite porphyry may have provided gold for the quartz vein type ores,and the gold provided by the Caledonian dike may have been reactivated and redistributed in the Hercynian magmatic activity.5.After the magmatic stage,the hydrothermal fluid migrated from the deep part of the Haoyaoerhudong region,and it passed through the Haoyaoerhudong region to the east and west along the south wing of the Haoyaoerhudong syncline,but mainly to the western core of the syncline.The ore-forming fluids of Haoya gold deposit,Naritu gold deposit,Zhamuhuduge gold mineral occurrence and Wuhuachaolu gold mineral occurrence are mainly provided by strata,and only the hydrothermal fluid of haoyaolhudong gold deposit is closely related to magmatic activity.The most important ore-forming fluid in Haoyaoerhudong region is postmagmatic hydrothermal rather than metamorphic hydrothermal.6.The Tebai gold deposit ore-forming fluid has low temperature?150-350??,low salinity NaCleqv?1-6%?,low density?0.7-0.9g/cm³?and high Zr/Hf,low Rb/Sr,low Nb contant,low Ta contant geochemical characteristics.The properties of fluids are very similar to postmagmatic hydrothermal fluids.When the fluid is deep underground,the fluid is characterized by reduction and low temperature.At this time,gold mainly migrates in the form of Au?HS?^2-complex,and the CO₂ in the fluid ensures the stability of Au?HS?^2-complex.With gradual cooling and shallow migration,the fluid become immiscibility,leading to the escape of CO₂ from the fluid which then damaging the stability of Au?HS?^2-complex.With the mixing of meteoric water,the gold precipitation was further intensified,and finally resulted in the positioning of the ore body.7.The main metallogenic control factors of Haoyaoerhudong gold deposit and Tebai gold deposit include:strata and alteration characteristics of black rock series,superimposition control and transformation of folds and faults,post-Hercynian collision alkaline granites close to ores.The two gold deposits have obvious comparabilities in geotectonic location,ore-hosting characteristics and metallogenic mechanism,which suggesting that the multi-stage metallogenic models of the two gold deposits are similar,and supporting the comparability of the evolution processes of the two regions.8.The prospecting indicators of the black rock series type gold deposit in the Bayan Obo-Alxa League region of Inner Mongolia are as follows:the ore body is bedded in the black rock series,and the distribution is controlled by the black rock series.Blurred boundary between ore body and non-ore-bearing black rock series.The black rock series has high metallogenic abundance and strongly transformed by tight fold structure.The gold-bearing quartz veins are often superimposed on the gold-bearing black rock series.The ore body is located at the superposition of fold structure and fault structure.The alkaline granitoids?veins?of the hercynian period in the collision tectonic environment are closely related to mineralization.The wall rocks alteration mainly include silicification,pyritization,chlorinization and carbonation.The mineral assemblage of the ores mainly include gold,pyrite,pyrrhotite,quartz,sericite,chlorite,carbonate and so on.