In situ oxygen isotope, trace element, and fluid inclusion evidence for a primary magmatic fluid origin for the shell-shaped pegmatoid zone within the giant Dahutang tungsten deposit, Jiangxi Province, South China

Abstract

The world-class Dahutang tungsten deposit is located within the Jiuling mining district of the central Jiangnan orogenic belt, Jiangxi Province, China. The deposit consists of a massive and gently dipping scheelite orebody that contains disseminated and veinlet-hosted mineralization located within a medium- to coarse-grained biotite granodiorite. The No. 1 ore block of the deposit also contains a 1.50–1.75 m thick shell-shaped pegmatoid zone that defines an inner contact zone between the mineralization-related porphyritic-like biotite granite and strongly mineralized biotite granodiorite that hosts the tungsten deposit. This pegmatoid zone contains very low concentrations of tungsten, but records the processes involved in the migration and enrichment of this element within the deposit. The pegmatoid contrasts with typical hydrothermal pegmatite-type ore in that it often contains megacrystic to giant alkali feldspar, quartz, and muscovite, and is spatially zoned in terms of unique texture and composition. The pegmatoid is zoned from a felsic aplite zone with a layered texture proximal to the intrusion through quartz–feldspar pegmatoid and feldspar–quartz pegmatoid zones to a final distal quartz-dominated zone. The quartz–feldspar pegmatoid is striped and contains megacrystic feldspars that have grown nearly perpendicular to the edge of the pegmatoid, indicating the direction of movement of early exsolved magmatic fluids. Cathodoluminescence imaging indicates that there are five types of quartz within the feldspar–quartz pegmatoid: megacrystic, large phenocrystic, small phenocrystic, matrix, and vein-hosted quartz. Each of these generations of quartz crystallized early, with the disseminated metasomatic quartz that crystallized during the middle stages of pegmatoid generation being followed by the uniform crystallization of late-stage, space-filling quartz. In situ oxygen isotopic and trace element analysis indicates that all of the quartz within the pegmatoid records accelerated growth, with the middle-stage disseminated metasomatic quartz associated with an increase in δ18Oquartz values, and the late-stage space-filling quartz associated with a decrease in δ18Oquartz values. All of this quartz contains elevated concentrations of alkali metals and has low Li/Al ratios (generally <0.35), indicating the majority of this quartz has an igneous origin, although some quartz formed from magmatic–hydrothermal or hydrothermal fluids. All the quartz formed under medium–high temperatures (≥483 °C) and elevated δ18O (11.05‰ ≤ δ18Oquartz ≤ 15.07‰) conditions. The majority of δ18Oquartz–water values are >10‰, reflecting the fact that the fluids that formed these minerals were of the peraluminous granite water (PGW). The trace element compositions of the samples analyzed during this study also indicate a drop in the pH of the pegmatoid-forming fluids over time. The quartz within the feldspar–quartz pegmatoid also contains melt and gas–liquid fluid inclusions, with the latter containing significant amounts of gas-phase CH4, indicating that the pegmatoid formed from low oxygen fugacity fluids. Our data indicate that the shell-shaped pegmatoid zone within the Dahutang No. 1 ore block formed from primary PGW exsolved from the hosting intrusion, with the pegmatoid recording the transition from magmatic to hydrothermal processes during the continuous but multi-stage evolution of the Dahutang deposit. The late-stage, high-temperature, water-rich, high δ18O, alkali-metal-rich, low oxygen fugacity, and acidic nature of the hydrothermal fluids that formed the deposit promoted the transportation and further deposition of tungsten.