Contrasting W-Mo-Cu and W-Sn-F Skarn Types and Related Granitoids
Tungsten replacement deposits fall into two general types: W-Mo-Cu types occur in oxidized (Fe3+/Fe2+ generally>1.0) skarns whereas W-Sn-F types occur in reduced (Fe3+/Fe2+ generally<1.0) skarns. These are related to the oxidized "I-type" granitoids and the reduced "A-type" granitoids respectively. In W-Mo-Cu skarns, interaction of ore solutions with low fo2 wall rock and/or later hydrous alteration overprints may result in lower Fe3+/Fe2+ but the main metallogenic characteristics of the skarn persist. Most of the world's largest tungsten deposits are W-Mo-Cu type skarns whereas the W-Sn-F skarns are usually of limited extent and commonly related to other styles of W-Sn-F mineralization such as quartz veins and greisens.