Textural and Chemical Constraints on the Formation of Disseminated Granite-hosted W-Ta-Nb Mineralization at the Dajishan Deposit, Nanling Range, Southeastern China

The Dajishan deposit, located in the Nanling Range of southeastern China, is the largest wolframite deposit in the world. Approximately two-thirds of the tungsten resource is contained in large quartz-wolframite-K-feldspar-muscovite-scheelite-sulfide-(beryl) veins, but the remainder occurs as wolframite and lesser scheelite, disseminated through the Nb-Ta mineralized No. 69 granite, which is characterized by abundant snowball quartz crystals. Disseminated granite-hosted wolframite mostly occurs as a replacement after lath-shaped muscovite, although an interstitial variety is present in some samples. The major element chemistry of the disseminated and vein-hosted wolframite is similar, however, the former has one to two orders of magnitude higher concentrations of high field strength elements (HFSE), including Nb and Ta. Disseminated muscovite in the No. 69 granite is Fe and Mg rich. Replacement rims on these muscovite crystals are enriched in Ta, Sn, and Cs, but more depleted in Nb, related to the original phase. Vein-hosted muscovite is characterized by lower Nb, Ta, Rb, and Cs contents than the granite-hosted muscovite, although Sn and W concentrations are comparable. Columbite-(Mn) is the principal Nb-Ta ore mineral and occurs disseminated through the No. 69 granite. Where it is hosted by snowball quartz crystals, some of the columbite has been altered and overgrown by wodginite. The restriction of these altered columbite crystals to within snowball quartz, along with the Ta and Sn enrichment of the alteration rims on muscovite, tie the formation of snowball quartz to a Ta-(Sn) metasomatic event. High Ge/Ti and Al/Ti ratios in the snowball quartz relative to interstitial quartz are also consistent with a late stage of formation for the former variety. The disseminated, granite-hosted W mineralization formed through fluid-rock interaction. The fluids that precipitated the disseminated wolframite were most likely those that formed the veins, but were modified through reaction with the HFSE-enriched No. 69 granite. Nb-Ta mineralization, however, is represented by magmatic columbite and by a later metasomatic stage of wodginite. The low solubility of Ta in aqueous fluids, combined with the evidence for Ta-(Sn) metasomatism, suggests that the Ta-(Sn) metasomatic event was caused by hydrosilicate liquids, a transitional fluid that lies compositionally between silicate magmas and hydrothermal fluids. Whole-rock and mineral chemistry both suggest that deep-level fractional crystallization of the No. 69 granite was required to elevate Ta and Nb concentrations to allow saturation of columbite.