Barite is distributed over much of the western United States. Its distribution is related to large-scale uplifted structural features; either domal or linear. However, the features are only mineralized where they occur within non-basaltic or non-alkaline igneous provinces. There is a strong correlation between the occurrence of barite and regions characterized by quartz-diorite, granodiorite, and adamellite intrusives. These rocks have intermediate Niggli k-values (between 0.2 and 0.4 on Moore's regional map normalized to 60% SiO 2 ). Following the conclusions of Moore, it is suggested that both the igneous rocks and the barite deposits are controlled by magmatic activity at depth. Barium is enriched in melts that crystallize plagioclase first, thus making it available for incorporation in the vapor phase of magmatic activity. Hence, the bulk composition of the magma and the time during crystallization when a vapor phase is formed control whether or not a barium-rich fluid is evolved. The large-scale uplifts may control the plumbing system.