Exsolution of primary fluids from magma in the superlarge Dahutang tungsten deposit of Jiangxi Province: Records from the pegmatoid shell.

Located in the Jiuling ore concentration area of the middle Jiangnan orogenic belt, the Dahutang tungsten deposit discovered in recent year in Jiangxi Province is a famous producing area of world-class superlarge tungsten ore. Scheelite orebodies occurring as disseminated veins are dominantly hosted gently in the medium-coarse grained biotite granodiorite. Pegmatoid shell is well developed on the top of the mother rock in the Dahutang No. 1 ore block. A vertical section from the mother rock to the wall rock shows the following sequence: porphyritic biotite granite, felsic aplite, quartz-feldspar pegmatoid, feldspar-quartz pegmatoid, and scheelite-bearing biotite grandodiorite. Cathodoluminescence petrographic studies of the quartz from the pegmatoid show that most quartz megacrysts, phenocrysts, matrices and quartz veins experienced three evolution stages, i.e., the early stage of main crystallization, the middle stage of diffused metasomatism and the late stage of filtrated metasomatism. Trace elemental studies show that the growth of quartz at the early stage was a process of accelerated precipitation of fluids. Fluids of the middle and late stages exerted superimposition and transformation of the host quartz, causing decreasing trend of trace elements. Quartzs of all types are characterized by enriched alkali metal elements, middle-high temperature (t≥483℃), and low Li/Al ratios (0.01~0.50, most less than 0.24), which suggests that the fluids might have originated from magmatic water. The growth of quartzs shows a transition from magma to hydrothermal system. Comprehensive studies indicate that the pegmatoid shell in Dahutang was formed by exsolution of primary magmatic fluids, and the dynamic physical-chemical environment during the crystallization was favorable for the transport of W toward the wall rocks and enrichment of this element in these rocks.