Episodic fluid evolution in the formation of the large scale Luoyang Fe deposit, Fujian, eastern China

Abstract

The Luoyang Fe skarn deposit is located in the Yong’an–Meizhou depression belt in Fujian Province, eastern China. Skarn alteration and mineralization are concentrated in a zone adjacent to an early Yanshanian granite porphyry (146 Ma), within limestone of the upper Carboniferous Chuanshan and lower Permian Qixia formations. Petrographic observations reveal five paragenetic stages of skarn formation and ore deposition: Stage 1 (early skarn; andradite); Stage 2 (diopside + magnetite); Stage 3 (late skarn; actinolite + chlorite + tremolite + magnetite + pyrite); Stage 4 (quartz + calcite + fluorite + pyrite + hematite + sphalerite + galena + chalcopyrite); and Stage 5 (calcite + quartz). We analyzed fluid inclusions in garnet, diopside, calcite, and quartz from stages 1, 2, 4, and 5, and measured H–O isotopic compositions of fluid inclusions in garnet, magnetite, and quartz from stages 1, 2, and 4.Observations of fluid compositions and phase assemblages allow three types (and two subtypes) of fluid inclusions to be distinguished: halite-bearing (type 1a) and hematite-bearing (type 1b), vapor-rich (type 2), and liquid-rich aqueous inclusions (type 3). In stage 1 garnet contains type-1a, -1b, and -2 primary fluid inclusions, which comprise two immiscible phases that formed through boiling. In stage 2 diopside contains type-1a and -2 fluid inclusions that also record boiling, which resulted in the precipitation of magnetite. In stage 4 quartz contains type-3 primary inclusions that are related to sulfide mineralization. In stage 5 calcite and quartz contain type-3 fluid inclusions, which represent the post-sulfide mineralization fluids.Microthermometric and H–O isotopic data indicate that multiple episodes of fluid flow occurred in the Luoyang deposit. During the early skarn stage, an initial supercritical fluid that exsolved during crystallization of the parent granitic magma ascended and underwent a first boiling event at 488 °C–536 °C and 380–438 bar (lithostatic pressure), which produced two fluids with differing salinities, represented by type-1a and -2 inclusions. This stage of boiling resulted in minor magnetite precipitation and type 1b fluid inclusions associated with this stage contain hematite. The magnetite stage was associated with a second episode of fluid ascent as the parent magma cooled to 454 °C–498 °C, which triggered a second boiling event and abundant magnetite precipitation at 336–449 bar (lithostatic pressure). The sulfide stage is associated to a third period of fluid ascent and occurred at 220 °C–261 °C at the same pressure conditions as the second boiling event (~380 bar). This stage included the input of meteoric water, which resulted in sulfide precipitation. Late-stage carbonate crystallization involved the influx of further meteoric water at lower temperatures (144 °C–193 °C) and the same pressure (~380 bar), representing the collapse of the skarn system. These observations suggest that the Luoyang deposit formed at a shallower depth than the Makeng deposit, which is located within the same belt. Variations in skarn alteration and magnetite mineralization are ascribed to the difference in depth between the two skarn systems.