Geochemistry and a metallogenic model for Nb-Ta-bearing granitic pegmatites from the northern Qaidam Basin

The Palaeoproterozoic Dakendaba Group and the Cambrian-Ordovician Tanjianshan Group occur on the northern margin of the Qaidam Basin. Their fracture structure is well developed, and intermediate-acid rocks are widely distributed, providing an ideal setting for pegmatite mineralization. Field investigation has identified 200 pegmatite veins, of which more than 40 have various degrees of Nb and Ta mineralization. These pegmatites can be classified into four types, based on distribution, mineral assemblages, and a genesis sequence of ? microcline, ? microcline-albite, ? albite, and ? muscovite-albite. Pegmatite types ? and ? are typically mineralized, with Nb and Ta occurring as niobite and columbite-tantalite. These pegmatites have high silica and alkali content and are Al-rich. The aluminium saturation index (A/CNK) is 1.36-1.60, and (A/NK) is 1.45-1.93. Their Sigma REE values are 2.61-23.95x10(-6). Other REE ratios have values of (La/Yb)(N)=2.02-8.04, (La/Sm)(N)=0.47-3.20, and (Gd/Yb)(N)=1.34-4.93. Their REE distribution pattern is right-inclined, with slight LREE enrichment. Negative Eu anomalies are apparent in types ? and ? but not in types ? and ?. High field strength elements, such as Nb, Ta, Zr, and Hf, have low contents, with Y depletion. Concentrations of REE are generally low, although Sr and Ba are slightly enriched. Thus, pegmatites of this area have low Ca, K, and Al but high Na and Si and are rich in rare metal elements and poor in REEs. This suggests liquid immiscibility was involved in their metallogenic evolution process.