The link between fluid evolution and vertical zonation at the Maoping tungsten deposit, Southern Jiangxi, China: Fluid inclusion and stable isotope evidence

The Maoping tungsten deposit is a wolframite-quartz vein type deposit located in southern Jiangxi province, China. Both steeply and gently dipping veins are developed and are characteristics of vertical zonation that can be divided into five zones depending on the thickness of the veins from the top to the bottom: (I) thread zone, (II) veinlet zone, (III) moderate vein zone, (IV) thick vein zone, and (V) thin out zone. Quartz from zones III-V contains two-phase aqueous inclusions (type Iq) and two- or three-phase C-O2-bearing inclusions (type IIq). However, quartz from zone I contains only type Iq inclusions. Wolframite from zones III-V contains ubiquitous two-phase aqueous inclusions (type Iw), with rare CO2-bearing inclusions (type IIw) occurring in wolframite from zone IV. In quartz from zones III-V, coexisting type Iq and type IIq inclusions as well as their microthermometric behaviors suggest that fluid immiscibility had occurred during quartz deposition. Fluid inclusion data obtained from zone I show a trend of temperature decrease along with decreasing salinity, indicating that fluid mixing took place. In contrast, type Iw inclusions in wolframite from zones III-V record a distribution pattern of large homogenization temperature range but relatively constant salinity. We therefore propose that wolframite precipitation was predominantly triggered by simple cooling rather than immiscibility or fluid mixing which has been recorded by fluid inclusions in coexisting quartz. Spatially, the widespread type Iq inclusions in quartz from zone V up to zone I exhibit an overall slight and continuous decrease of homogenization temperature and salinity, whereas type Iw inclusions in wolframite from these zones are identical regardless of the depth. This may imply that wolframite was precipitated from an early, hot and short-lived fluid pulse that flushed out of the magma into hydrothermal system via fractures and nucleated wolframite crystals along the vein edges by simple cooling. The vein quartz however, was formed by subsequent fluid pulses which may reside for a longer time until the depletion of the heat source. Hydrogen and oxygen isotope data of quartz (delta O-18(fluid) = 2.4 parts per thousand to 7.1 parts per thousand V-SMOW and delta D = -43 parts per thousand to - 83 parts per thousand V-SMOW) and wolframite (delta O-18(fluid) = 6.1 parts per thousand to 8 parts per thousand V-SMOW and delta D = - 45 parts per thousand to - 64 parts per thousand V-SMOW) indicate ore-forming fluids had a magmatic source, with limited input of meteoric water occurring only in zone I. By integrating the deposit geology, mineral assemblages, fluid inclusion and H-O isotopes, we propose that the gently dipping veins in the Maoping tungsten deposit are consistent with the "five floor" model that may have broader applications to exploration of wolframite-quartz vein type deposits.