Geology, geochronology, fluid inclusion and h-o isotope geochemistry of the luoboling porphyry cu-mo deposit, zijinshan orefield, fujian province, china

The Luoboling Cu-Mo deposit in the Zijinshan Orefield, Fujian province, southeastern China, is a large porphyry deposit hosted by the Sifang granodiorite and the Luoboling granodiorite porphyry. The largest Cu-Mo orebody is saddle-shaped with various types of hydrothermal veinlets. Intensive hydrothermal alteration in the deposit is characterized by outward zoning from potassic, overprinted by phyllic alteration, to phyllic and alunite-dickite alteration. Based on the mineral assemblages and crosscutting relationships of veins, the ore-forming process can be divided into three stages, namely: an early-stage K-feldspar+quartz±magnetite±molybdenite veins associated with potassic alteration; a middle-stage quartz+molybdenite+chalcopyrite+pyrite veins in phyllic zone; and a late-stage quartz±gypsum veins in the phyllic and alunite-dickite alteration zones. Six molybdenite separates yield a Re-Os isochron age 104.6±1.0Ma, which is identical to the age of emplacement of the Sifang and Luoboling granodiorite porphyries. Three types of fluid inclusions (FIs) were observed at the Luoboling deposit: 1) NaCl-H2O (aqueous), 2) daughter mineral-bearing and 3) CO2-H2O fluid inclusions. FIs of the early and middle stages are predominantly vapor-rich aqueous and daughter mineral-bearing inclusions, together with minor CO2-rich and liquid-rich aqueous inclusions; whereas the late-stage minerals only contain liquid-rich aqueous inclusions. Homogenization temperatures and salinities of FIs trapped in the early-stage minerals range from 420 to 540°C and 0.4 to 62.9wt.% NaCl equiv., respectively. FIs of the middle-stage yield homogenization temperatures of 340 to 480°C and salinities of 0.5 to 56.0wt.% NaCl equiv. CO2 content and the oxygen fugacity (indicated by daughter minerals) of FIs trapped in middle-stage minerals are lower than those in the early stage. The liquid-rich aqueous inclusions of the late-stage homogenize at temperatures of 140 to 280°C, yielding salinities of 0.4 to 8.4wt.% NaCl equiv. The minimum estimated pressures of the three stages are 30-70MPa, 10-40MPa and 1-10MPa, respectively, corresponding to minimum ore-forming depths of 1-2.8km. Fluids trapped in early, middle and late stages yield δD values of -67‰ to -54‰, -54‰ to -70‰, and -62‰, and δ18O values of 5.4‰ to 6.7‰, 2.8‰ to 4.2‰, and -2.1‰, respectively. Fluid boiling, which resulted in the formation of stockworks and the precipitation of sulfides, occurred in the early and middle stages. The fluids subsequently evolved into a low temperature, low salinity system in the late stage, along with an input of meteoric water. The Luoboling porphyry Cu-Mo system was developed in a transition from continental arc to back-arc extension region, which was related to the westward subduction of the paleo-Pacific plate beneath the Huanan Orogen. •The Luoboling porphyry Cu-Mo deposit, Zijinshan Orefield, occurs in Huanan Orogen.•The deposit developed in a transition from continental arc to back-arc extension at ~105Ma.•It is shallow-seated and contains CO2-rich, daughter mineral-bearing and aqueous inclusions.•Ore fluids evolved from hypothermal, saline and CO2-rich to epithermal, dilute, CO2-poor.•H-O isotope signatures show the fluids evolved from magmatic to meteoric in origin. © 2013 Elsevier B.V.