The Ore-forming Processes And Mineralization Of Caosiyao Superlarge Porphyry Mo Deposit,Inner Mongolia

The Caosiyao Mo deposit,located in the northern margin of the North China Craton,is a newly prospected Mo deposit in the Yan-Liao molybdenum belt.Orebodies of the deposit are mainly hosted in metamorphic rocks of the Mesoarchean Jining Group with a small amount of orebodies in the Late Jurassic syenogranite porphyry.The ore-forming process at the deposit can be divided into three stages:an early quartz-molybdenite±magnetite±pyrite stage,a middle quartz-molybdenitepyrite±pyrrhotite±chalcopyrite stage,and a late quartz-carbonatepyrite±fluorite±pyrrhotite stage.Molybdenite Re-Os dating indicates that the deposit formed at ca.150 Ma.The Mesozoic intrusive rocks in the Caosiyao ore district are mainly composed of syenogranite porphyries and monzogranite porphyries.Zircon LA-ICP-MS U-Pb ages of the former are 148.5 ± 0.9 Ma and 140.9 ± 1.6 Ma-140.1 ±1.7 Ma,and that of the latter is 145.0 ± 1.0 Ma,indicating that the syenogranite porphyries emplaced during the late Jurassic-early Cretaceous and that the monzogranite porphyries emplaced during the early Cretaceous.These late Jurassicearly Cretaceous granitic porphyries belong to the high-K calc-alkaline series and are characterized by high SiO2,Al2O3,K2O,Rb,Th,U,K,and Zr contents and low MgO,CaO,T1O2,P2O5,Ba,Nb,La,Cr,and Ni contents,with high K2O/Na2O ratios and medium-strong negative Eu anomalies.They are generally considerably enriched in Rb,Th,U,K,La,and Ce,extremely dedepleted in Ba,Nb,Sr,P,and Ti,and slightly dedepleted in Zr and Hf.All of these geochemical characters are similar to those of the granites occurring in a post-orogenic extension setting.These late Jurassic-early Cretaceous granitic porphyries have very high whole-rock initial 87Sr/86Sr ratios(varying from 0.710417 to 0.718568),very low zircon ?Hf(t)and whole-rock ?Nd(t)values(varying from-18.9 to-12.5 and-17.8 to-13.7,respectively),very old zircon Hf and whole-rock Nd single-stage and two-stage model ages(varying from 1280 to 1552 Ma and 1989 to 2393 Ma for zircon and from 1766 to 2431 Ma and 2046 to 2375 Ma for whole-rock,respectively),and low K-feldspar 206Pb/204Pb,207Pb/204Pb,and 208Pb/204Pb ratios(varying from 16.603 to 17.177,15.319 to 15.374,and 36.841 to 37.225,respectively).Taking these geochemical characteristics and Sr-Nd-Hf-Pb isotope compositions together,we suggest that the late Jurassic syenogranite porphyry sourced from a relatively high mature lower crust,source area of the early Cretaceous monzogranite porphyry belongs to an immature lower crust,and that the early Cretaceous syenogranite porphyry originated from a moderate mature lower crust.The origin of the late Jurassic-early Cretaceous granitic porphyries in this district is related to both the post-orogenic extension after the closure of the Mongol-Okhotsk Ocean and/or the subduction of the Paleo-Pacific plate.Four types of fluid inclusions(FIs)have been distinguished in quartz phenocryst and various quartz veins including liquid-rich,gas-rich,H2O-CO2,and daughter mineral-bearing inclusions.The FIs in the quartz phenocrysts of the syenogranite porphyry contain liquid-rich,gas-rich,H2O-CO2,and daughter mineral-bearing types;their homogenization temperatures and salinities vary from 385? to>550? and 6.5 wt%to 65.0 wt%NaCl eqv.,respectively.The FIs in the quartz of the early ore-forming stage are also mainly liquid-rich,gas-rich,H2O-CO2,and daughter mineral-bearing types;their homogenization temperatures and salinities vary from 243? to 401? and 6.0 wt%to 39.8 wt%NaCl eqv.,respectively.The FIs in the quartz of the middle ore-forming stage are mainly liquid-rich,H2O-CO2,and daughter mineral-bearing types,with a small amount of gas-rich type;their homogenization temperatures and salinities vary from 208? to 336? and 1.2 wt%to 34.1 wt%NaCl eqv.,respectively.The FIs in the quartz of the late ore-forming stage are only liquid-rich type;their homogenization temperatures and salinities vary from 124? to 196? and 3.9 wt%to 13.2 wt%NaCl eqv.,respectively.The ore-forming fluids of the molybdenite precipitation stage are characterized by moderate-high temperature and great fluctuating salinity,belonging to an H2O-NaCl-KCl-MgCl2-CO2±CH4 system.The ?18OH2O and ?DH2O values of the early and middle stages fluids vary from 3.5‰ to 7.4‰ and-96‰ to-77‰,respectively;the ?18OH2O and ?DH2O values of the late stage fluid vary from-2.9‰ to-0.6‰ and-112‰ to-77‰,respectively;indicating that the ore-forming fluids of the early and middle stages mainly consist of magmatic water and that the fluid in late stage is a mixture of magmatic and meteoric water.The ?13C values vary from-16.5‰to-8.8‰ for the early and middle stages and-11.4‰ to-5.3‰ for the late stage,suggesting that the carbon of the early and middle stages consists of a mixture of organic and magmatic sources and that the carbon in the late stage was mainly from a magmatic source.The ?34S values range from 4.0‰ to 6.1‰ with an average of 4.8‰.The 206Pb/204Pb,207Pb/204Pb,and 208Pb/204Pb ratios of the metallic minerals are in the ranges of 15.962-17.832,15.200-15.511,and 35.918-37.927,respectively.Both S and Pb isotopic systems indicate that the ore-forming metals came primarily from Mesozoic intermediate-felsic magma.The multi-stage boiling or immiscibility of the ore-forming fluids was the dominant mechanism for the deposition of ore-forming materials.