Areas of known metallic mineralization in Arizona, with recorded production, have been divided into 453 mineral districts, nearly 180 of which are known or suspected to be mid-Tertiary in age (Keith and others, 1983a, b). This report briefly describes the geology of mid-Tertiary mineral districts in Arizona that were considered by Keith and other (1983a, b) to be mid-Tertiary, and examines the basis of age determinations for each of them. This report is preliminary in nature, and Bureau-directed mapping and research continues in order to clarify the nature of mid-Tertiary metallogenesis.