Molybdenite Re–Os age, H–O–C–S–Pb isotopes, and fluid inclusion study of the Caosiyao porphyry Mo deposit in Inner Mongolia, China

Abstract The giant Caosiyao porphyry Mo deposit, located in the northern margin of the North China Craton, is a newly discovered 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 of the deposit can be divided into three stages: an early quartz–molybdenite ± magnetite ± pyrite stage, a middle quartz–molybdenite–pyrite ± pyrrhotite ± chalcopyrite stage, and a late quartz–carbonate–pyrite ± fluorite ± pyrrhotite stage. Molybdenite Re–Os dating indicates that the Caosiyao deposit formed at ca. 150 Ma. Four types of fluid inclusions (FIs) have been distinguished in quartz phenocryst and various quartz veins including liquid-rich, gas-rich, H 2 O–CO 2 , and daughter mineral-bearing inclusions. The FIs in the quartz phenocrysts of the syenogranite porphyry contain liquid-rich, gas-rich, H 2 O–CO 2 , and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 385 °C to > 550 °C and 6.5 wt% to 65.0 wt% NaCl eqv., respectively. The FIs in the quartz of the early stage are also mainly liquid-rich, gas-rich, H 2 O–CO 2 , and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 243 °C to 401 °C and 6.0 wt% to 39.8 wt% NaCl eqv., respectively. The FIs in the quartz of the middle stage are mainly liquid-rich, H 2 O–CO 2 , and daughter mineral-bearing types; their homogenization temperatures and salinities vary from 208 °C to 336 °C and 1.2 wt% to 34.1 wt% NaCl eqv., respectively. The FIs in the quartz of the late stage are only liquid-rich type; their homogenization temperatures and salinities vary from 124 °C to 196 °C and 3.9 wt% to 13.2 wt% NaCl eqv., respectively. The ore-forming fluids of the Caosiyao deposit are characterized by moderate–high temperature and great fluctuating salinity, belonging to an H 2 O–NaCl–KCl–MgCl 2 –CO 2  ± CH 4 system. The δ 18 O H2O and δD H2O values of the early and middle stages vary from 3.5‰ to 7.4‰ and − 96‰ to − 77‰, respectively; the δ 18 O H2O and δD H2O values of the late stage 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 δ 13 C 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 δ 34 S values range from 4.0‰ to 6.1‰ with an average of 4.8‰. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb 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.