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 +/- 1.0 Ma, 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 62.9 wt% NaCl equiv., respectively. FIs of the middle-stage yield homogenization temperatures of 340 to 480 degrees C and salinities of 0.5 to 56.0 wt.% 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 degrees C, yielding salinities of 0.4 to 8.4 wt.% NaCl equiv. The minimum estimated pressures of the three stages are 30-70 MPa, 10-40 MPa and 1-10 MPa, respectively, corresponding to minimum ore-forming depths of 1-2.8 km. Fluids trapped in early, middle and late stages yield delta D values of -67 parts per thousand to -54 parts per thousand, -54 parts per thousand to -70 parts per thousand, and -62 parts per thousand, and delta O-18 values of 5.4 parts per thousand. to 6.7 parts per thousand, 2. to 4.2 parts per thousand, and -2.1 parts per thousand 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. (C) 2013 Elsevier B.V. All rights reserved.