矿床地球化学国家重点实验室知识库

Veinlet-disseminated and vein-type tungsten deposits are important tungsten resources in South China and show remarkable diversity in dominant tungsten minerals. To better understand their genesis, scheelite from the Shimensi veinlet-disseminated deposit and wolframite from the Xihuashan and Piaotang vein-type deposits were selected to conduct in-situ laser ablation-inductively coupled plasma-mass spectrometry trace element analyses. 

The Shimensi tungsten mineralization occurs in intensively altered granitic rocks and is characterized by scheelite. Two generations of scheelite in a single grain can be identified by cathodoluminescence (CL) imaging. The early scheelite (dark domains in CL images) is characterized by nearly flat chondrite-normalized REE (BEEN) patterns with significantly negative Eu anomalies, whereas the late one (bright domains in CL images) shows light rare earth element (LREE)-enriched BEEN patterns and obviously positive Eu anomalies. The former has higher REE, Na, Nb and Ta and lower Sr contents than the latter. The Xihuashan and Piaotang wolframite-quartz veins are developed in greisenized granite (type I) and metasedimentary rocks (type II). Both types of wolframite show LREE-depleted patterns, but type I exhibits strongly negative Eu anomalies and type II positive Eu anomalies. Type I contains higher REE, Nb and Ta concentrations, and lower Fe0/Mn0 ratios than type II. 

Variations of Eu anomalies and trace element compositions in both scheelite and wolframite can be used to decipher the origin and processes of tungsten mineralization. Both the early scheelite and wolframite (type I) display significantly negative Eu anomalies and have high REE, Nb and Ta contents, suggesting that the initial ore-forming fluids were of magmatic origin. Precipitation of tungsten minerals and alteration would effectively modify the composition of ore-forming fluids. Deposition of the early tungsten minerals would lower REE, Nb and Ta in the mineralizing fluids, leading to depletion of these elements in the late ones. Although both the type I and II wolframite have different REE contents and Eu anomalies, they show similar left-dipped BEEN patterns, implying that compositional variation of fluids is likely driven by crystallization of wolframite during the processes of fluid evolution. In contrast, the elevated LREE/HREE (heavy rare earth element) ratios, SEu (EuN/EuN") values and Sr abundances in the late scheelite are possibly caused by the decomposition of plagioclase and K feldspar. Alteration plays an important role in the formation of veinlet-disseminated scheelite deposits. It can be concluded that vein-type wolframite mineralization is mainly formed by filling and that veinlet-disseminated scheelite mineralization is associated with metasomatism.