Tracing the Fe-enriched quartz vein-type wolframite mineralization: Wolframite U-Pb dating and compositions of wolframite and scheelite from the Anglonggangri and Jiaoxi areas, Xizang, China

Abstract

Critical factors controlling Fe-enriched wolframite formation in the quartz vein-type wolframite deposit are in controversy. The Jiaoxi and Anglonggangri are typical examples of quartz vein-type wolframite mineralization found in western Lhasa terrane. The Jiaoxi wolframite comprise dominantly early hübnerite with late ferberite, whereas, the Anglonggangri wolframite are mainly ferberite. Wolframite U-Pb dating yields direct timing of 11.0 ± 0.2 Ma and 9.7 ± 0.2 Ma for Jiaoxi and Anglonggangri tungsten mineralization, respectively. Ages together with enrichment of Nb, Ta, Ti, and Sc and depletion of Sr and most Eu anomaly within Anglonggangri wolframite confirm that metals and fluids were mainly derived from the ore-forming garnet-muscovite granite. Iron and Mn mapping of garnet in the garnet-muscovite granite suggests this granite has changed from the early Mn-enriched to late Fe-rich affinity. Therefore, initial ore-forming fluids exsolved from garnet-muscovite granite have Fe-rich composition, that was significantly modified during tungsten mineralization. Positive trend between Mg and Fe/(Fe + Mn) of wolframite indicates biotite breakdown from country rock of biotite-muscovite granite to add Fe and Mg into fluid during greisenization. Meanwhile, decomposition of plagioclase and K-feldspar provided Ca, Sr, and Eu to form late scheelite. Enrichment of Sr with positive Eu anomaly and decline of REE, Nb, and Ta are typical features of the highly evolved and modified fluid. Positive and negative Eu anomalies within the same wolframite grains reveal the oxidized character and fluctuating composition of fluid during wolframite precipitation. By contrast, negative Eu anomaly and low Mo abundance of scheelite indicate the reducing fluid during scheelite crystallization. Identification of metal and fluid sources for Anglonggangri ferberite highlights the evolved granites play more crucial roles on ferberite formation than early hübnerite crystallization and Fe addition from country rocks during hydrothermal interaction.