The Geochemical Characteristics Study Of Dasuji Porphyry Mo Deposit In Inner Mongolia

Dasuji porphyry Mo deposit is located in the western part of Inner Mongolia fault uplift in the north of North China Craton,and its temporal and spatial distribution is closely related to the magmatic activity of Mesozoic calc-alkaline granite.The purpose of this study is to explore the rock geochemical characteristics,S-Pb isotope composition,Sr-Nd isotope characteristics,ore-forming material sources and ore-forming background in the Dasuji Mo deposit.It has important reference significance for the source of diagenetic and ore-forming materials and tectonic setting in the study of porphyry molybdenum deposits.The research method adopted is the combination of field geological investigation and laboratory experimental study,and the characteristics of major elements and trace elements in rocks are discussed by means of rock geochemistry.S-Pb-Sr-Nd isotope characteristics of the deposit are studied by means of isotope geochemistry,and its diagenetic and ore-forming process and material source are inferred.The granitic complex in the Dasuji Mo deposit has a Si O₂content of 67.61-79.32%and a total alkali content of 4.57-8.67%,which belongs to the high potassium calc-alkaline-potassium basalt series.The K₂O/Na₂O ratio is 1.87-75.73.A/CNK ratio is1.13-3.41,which belongs to strong peraluminous rocks.Quartz porphyry and granite porphyry are basically in accordance with the characteristics of Zr/Hf<38 and Nb/Ta<17,and have features of 10000Ga/Al>2.6,K₂O>4%,Si O₂>70%,and have the average value of Al₂O₃is between 12-13%.And then content of Sr,Ba,Ti,P in Quartz porphyry and granite porphyry is low.?Eu values are slightly more than 0.3 in Quartz porphyry and granite porphyry.Therefore,Quartz porphyry and granite porphyry belong to highly differentiated A-type granite.The mean value of Zr/Hf of alkali feldspar granite porphyry is less than 38,the mean value of Nb/Ta is greater than 17,the high Sr(mean value385.5×10^-6),the negative europium anomaly is small and partly slightly positive europium anomaly,Rb is positively correlated with Th,so it belongs to the type I granite with high degree of variation.The characteristics of main elements and trace elements show that granitic complex has the granite characteristics that plate collision turns to the extended environment of post-collisional,which is formed in extensional environment that plate collision turns to post-collisional.The?³⁴S of the ore metal sulfide is 2.5-5.8‰,which is distributed in a tower pattern,so the sulfur source is deep magma sulfur.The ²⁰⁶Pb/²⁰⁴Pb of sulfide in ore body is 16.811-18.035,²⁰⁷Pb/²⁰⁴Pb is 15.340-15.474,and ²⁰⁸Pb/²⁰⁴Pb is 36.945-38.631.The lead isotopic tectonic pattern shows that the lead source of Dasuji Mo ore lies between the orogenic belt and the lower crust,so the diagenetic material mainly comes from the lower crust and has the influence of the mantle.(⁸⁷Sr/⁸⁶Sr)_iof granite porphyry is 0.70497-0.75311(mean value 0.72334),showing crust-mantle mixed type.The?_Nd(t)values of granitic porphyry is-16.97--14.83(mean value-15.85),indicating the addition of ancient crustal components.The T_DM1(Nd)of granite porphyry is 2.165 Ga-1.761 Ga,and the T_DM2(Nd)is 2.378 Ga-2.207 Ga,indicating that the diagenetic material mainly comes from the partial melting of the lower crust in the early Paleoproterozoic to Mesoproterozoic.Therefore,in the late Triassic,Dasuji Mo deposit was located in the extensional tectonic environment from collision to post-collision.The retreating plate caused the upwelling of asthenosphere,which caused the lower crust of the early Paleoproterozoic to mesoproterozoic to be partially melted into magma,which was mixed with crustal materials in the process of magma rising,and molybdenum precipitation and mineralization occurred under appropriate conditions when rising to the shallow-ultra-shallow-forming environment along the deep large fault.