Formation and evolution of multistage magmatic-hydrothermal fluids at the Yulong porphyry Cu-Mo deposit, eastern Tibet: Insights from LA-ICP-MS analysis of fluid inclusions. (1st July 2018)

• Record Type: Journal Article
• Authors: Chang, Jia; Li, Jian-Wei; Audétat, Andreas
• Is Part Of: Geochimica et cosmochimica acta. Volume 232(2018)
• Page Start: 181
• Page End: 205
• Publication Date: 2018-07-01
• Subjects: Porphyry Cu -- Fluid inclusions -- LA-ICP-MS microanalysis -- Cu-Mo decoupling -- Sulfur-rich apatite; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2018.04.009
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The giant Eocene Yulong porphyry Cu-Mo deposit in eastern Tibet, China was produced by multistage veining, Cu-Mo sulfide deposition, and hydrothermal alteration associated with pulsed magmatic intrusions (i.e., early, transitional, and late stages) lasting for a prolonged period of time. In this study, fluid inclusion microthermometry and laser ablation-ICP-MS microanalysis are combined with textural observations on magmatic apatite and various hydrothermal quartz vein assemblages to constrain the formation and evolution of the multistage magmatic-hydrothermal fluids producing the Yulong porphyry Cu-Mo deposit. Fluid inclusions hosted in quartz veins of the early and transitional stages have very similar compositions, indicating that the initial single-phase intermediate-density (ID) ore-forming fluids of these two mineralizing stages were exsolved from similarly evolved magma reservoirs in the underlying magma chamber. During their ascent, decompression and cooling, the single-phase ID input fluids (∼9 wt% NaClequiv. with ∼1000 ppm Cu and ∼20 ppm Mo) entered the two-phase field and condensed into a small amount of metal-rich brines (∼42 wt% NaClequiv. with ∼9300 ppm Cu and ∼330 ppm Mo) coexisting with a large amount of vapors (vapor/brine mass ratio of ∼4). The condensation of brines and their accumulation at shallow level could be an efficient mechanism to concentrate and then precipitate Cu ± Mo in a small rock volume as represented by the mineralized porphyries. The sequential deposition of Mo and Cu from the condensed brine phase of the early stage caused the local Cu-Mo decoupling in shallow parts of the deposit, whereas the deeply located Mo-rich mineralization of the transitional stage was probably caused by the early precipitation of molybdenite from the single-phase ID fluids before phase separation. It is concluded that the formation of a late Mo-rich mineralization (similar to the transitional-stage mineralization at Yulong) in many other porphyry Cu deposits is due to the subsolidus hydrothermal processes rather than the progressive increase of the Mo/Cu ratio in residual parental melts as magma crystallization proceeds. The high Cs + Rb ± B concentrations of melt and fluid inclusions in sulfur-rich magmatic apatite (0.3–1.4 wt% SO3 ) suggest a pegmatitic environment for the exsolution of the fluids. The similar element/K ratios of liquid-rich inclusions in the late-stage pyrite-quartz veins to the fluid inclusions in the sulfur-rich apatite suggest that the late-stage fluids were also likely derived from highly evolved melt fractions in felsic magmas during the solidification of the magma chamber.