Study On Geological Characteristics And Genesis Of Maozangsi Mo(Cu) Polymetallic Deposit In The Eastern Part Of North Qilian

The Maozangsi Mo(Cu)polymetallic deposit in the eastern part of North Qilian was found during the work of “ Maozangsi fluorite mine detailed investigation,Tianzhu County,Gansu Province,” by Geological exploration in the Chinese building materials center team in Gansu province.Up to now,the research degree of this deposit is low.The mine is located in the axis of the Maozangsi anticlinal fold in Lenglongling area,the eastern part of North Qilian Caledonian orogenic belt.The ore bodies mainly occur in the contact part of the Middle Ordovician Zhongbu group and the monzonitic porphyritic granite.The ore bodies are layered,lenticular,vein parallel arrangement.The ore is vein,fine vein disseminated,spotted,net vein shaped.The wall rock alteration is roughly zonal:it can be roughly divided into the potassic zone,the pyrite-sericite zone and the meta-andesitization zone in the granite;the hornstone zone in the sandstone.The ?34S result of S isotope of the sample in the mining area is more concentrated,and it varies from-3‰ to +3‰,which shows that the sulfur comes from deep magma.The average values of ?34S in different minerals show a decreasing sequence of molybdenite,pyrite and chalcopyrite,which is consistent with the decreasing sequence of the ?34S value in the sulfur isotope equilibrium.It is generally believed that the molybdenite,pyrite and chalcopyrite are in the state of S isotope equilibrium.The result of Pb isotope of the sample:206Pb/204 Pb ratio:18.274~19.472;207Pb/204 Pb ratio:15.594~15.747;208Pb/204 Pb ratio:38.397~ 40.096.The correlation diagram shows that the sources lead are more complex which might come from magma and the upper crust composition was joined,and may be related to orogeny.The petrographic characteristics and the results of microscopic temperature measurement of the fluid inclusions in the quartz formed during the main mineralization stage show that the primary fluid inclusions can be divided into three types: liquid-rich two-phase inclusions,gas-rich two-phase inclusions,and CO2-contained three-phase inclusions.The ore-forming fluid is H2O-NaCl-CO2(CH4)fluid system with medium-low temperature(175?~315?),low salinity(3~5wt%NaCleqv)and medium density(0.7~0.9g/cm3).The ore-forming pressure is concentrated in 15MPa~28MPa,and the metallogenic depth is0.39km~1.06 km,which shows the deposit is a hypabyssal deposit.The results of H-O isotopeshows that the ore-forming fluid mainly comes from magmatic water,and mixed with meteoric water later.The monzonitic porphyritic granite which has close relationship with mineralization is calc alkalic series and has the enrichment in large ion lithophile elements(LILE)such Rb?Th?U?K,has the obvious depletion in Nb?Ta?P?Ti elements.The granite is rich in LREE and depleted of HREE.The monzonitic porphyritic granite has the characteristics of high Si adakitic rock,which probably originated from the partial melting of the subducted oceanic crust and a certain degree of interaction with the mantle peridotite.The main residues in the source region are garnet(±hornblende).Zircon U-Pb dating shows that the diagenetic age of ore bearing rocks(monzonitic porphyritic granite)is(440.9 + 1.8)Ma,so the granite intruded in the Late Ordovician.The Re-Os isotopic model age of molybdenite is(423.3±6.7)Ma,which is in the Middle-Late Silurian,later form monzonitic porphyritic granite era(Late Ordovician).The Maozangsi Mo(Cu)polymetallic deposit is a typical middle-low temperature hydrothermal deposit which is related to the hypabyssal porphyry.The deposit was formed in the subduction of the North Qilian oceanic crust in Late-Ordovician to Early-Silurian when the North Qilian Ocean was at the conversion stage from oceanic crust subduction to continental crust collision.