Arc-related porphyry molybdenum deposits form in the same tectonic environment as many porphyry copper deposits (Seedorff and others, 2005; John and others, 2010), but are mined principally for molybdenum (Mo) and not for copper (Cu). The principal ore mineral containing molybdenum is molybdenite (MoS2). Deposits are typically large, commonly greater than 50 million tonnes (Mt) and low-grade (0.03 to 0.22 percent Mo). They are hosted by hydrothermally altered porphyritic intermediate to felsic granitoid plutons and the flanking country rocks. These calc-alkaline granitoid intrusions are genetically, temporally, and spatially related to the molybdenum mineralization.