From the sixteenth century onward, mining towns in New Spain produced more than silver; they also led to the creation of new colonial communities and societies. The founding of mining towns outside of central Mexico served as catalysts for northern exploration, becoming and creating new borderlands in their wake. This chapter considers how mining towns constituted both geographical and social borderlands. It focuses on the roles and experience of indigenous, Spanish, African, and ethnically mixed descent individuals (castas) in Mexico’s northern silver mining district from 1540 to 1660. The colonization of the mining borderlands created new economic, social, and ethnic patterns shaped by population scarcity and instability, the labor needs of production, the incentives of the money economy, the lifeways and practices of indigenous populations, imbalanced sex ratios, and under-developed colonial institutions. Ultimately, the chapter argues that mining towns remained borderlands, sites of fluid cultural exchanges and social boundaries.