Recently, the demand for lithium (Li) as an energy storage element has increased, owing to the rapid increase in the number of electric vehicles. To meet this demand, Li exploration has become increasingly important. The Boam deposit is located in the Uljin area of eastern South Korea, where several rare-element pegmatites (0.24% Li) intrude the Precambrian Janggun Limestone Formation. In this study, we performed petrographical and geometrical analyses of the rare-element pegmatites recognized in the vicinity of the Boam deposit, through which the Li-mineralization process was identified and the factors controlling intrusion studied. Our results are summarized as follows: (1) the pegmatites exhibit regional and internal zoning based on their mineral assemblages; (2) Li mineralization is restricted to pegmatites; (3) the geometry and distribution of the pegmatites are strongly controlled by fracture, bed contact, and post-intrusive deformation; and (4) exploration should be concentrated in the ENE–WSW-trending zone of the upper part of the Janggun Limestone Formation. These results provide valuable understanding to guide the development of strategies for early-stage mineral exploration in the Uljin area.