Minerallogy,Geochemistry,and Metallogeny Of The Yichun And The Dajishan Deposits

The No. 414 deposit is the largest granite-hosted Ta-Nb-Li deposit in China, and develops a marginal pegmatite that has received little attention. In an examination of ore minerals in the marginal pegmatite, we found abundant Ta-Nb minerals that could represent magmatic and post-magmatic rare-metal mineralization. Our study shows that the marginal pegmatite can be divided into 4 zones, among which only the intermediate zone and the aplite zone host Ta-Nb oxides. Specifically, most fibrous columbite-(Mn) aggregates in the intermediate zone are enclosed by quartz that are interstitial to pegmatitic triplite. Fibrous columbite-(Mn), hosted by fluorapatite veins that crosscut the triplite is also present. These fibrous crystals are associated with molybdenite, lepidolite, zircon, uraninite, and native bismuth crystals, and are hydrothermal in origin. The medium that precipitated this mineral assemblage is most likely the fluid phase during the magmatic-hydrothermal transition. External fluids that contain Mo and S are envisaged to explain the occurrence of molybdenite. In the aplite, magmatic columbite-group minerals are altered and overgrown by wodginite-group minerals that were most likely produced by a metasomatic melt.Theses two types of Ta-Nb oxides are of approximately equal abundance, which means that the intensity of post-magmatic Ta mineralization is stronger than the magmatic one. The aplite-hosted columbite-tantalite is Fe- and Mo-rich, which indicates the injection of external fluids during its formation.At Dajishan, 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, including Nb and Ta. 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 disseminated, granite-hosted W mineralization formed through fluid-rock interaction. The fluids that precipitated the 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 metasomatic event was caused by a transitional fluid that lies compositionally between silicate magmas and hydrothermal fluids. The snowball quartz at the No. 414 deposit encloses only magmatic columbite-(Mn) but no magmatic tantalite-(Mn) crystals, hence an early magmatic mineral. At Dajishan,the restriction of altered columbite crystals to within snowball quartz 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.