Experimental evidence for a protracted enrichment of tungsten in evolving granitic melts: implications for scheelite mineralization

Abstract

The solubility of scheelite in evolved granitic magmas (Qz-Ab-Or-An system with minor FeOtotal, TiO2, and CaO added) was studied experimentally at 200 MPa, 750–850 °C and relatively oxidizing condition (logfO2 = NNO + 2.3, where NNO is Ni-NiO oxygen buffer). Water-saturated granitic melts have been equilibrated with seeds of scheelite crystals. The resulted WO3 contents in the melts vary only slightly from 0.21 to 0.32 wt.% WO3 over the investigated temperature and compositional range (0.7 to 1.4 wt.% CaO) but tends to increase with increasing temperature and decreasing CaO concentration. One important message conveyed from the study is that WO3 concentrations at scheelite saturation are more dependent on temperature in evolved Ca-poor melts than in Ca-rich melts. Natural granitic rocks associated with scheelite mineralization and associated melt inclusions hosted in quartz have much lower W contents than the experimental melts equilibrated with scheelite. This implies that enrichment of tungsten (W) at magmatic stages is not sufficient to produce significant scheelite mineralization and confirms the important role of W mobilization by magmatic-hydrothermal fluids in the formation of scheelite deposits.