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 others (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. Table 1 consists of summaries, based largely upon a review of the published literature, of the geology of mid-Tertiary mineral districts of Arizona. Only mineral districts with greater than 100 tons recorded production are included in the compilation. Following each mineral district name is a list of the metals produced, in order of decreasing value at the time of production. A brief description of the geology of each mineral district is provided and succeeded by a discussion of the data constraining the age of each district. Many of the districts designated as mid-Tertiary by Keith and others (1983a,b) are thought by us to be older, but are included for completeness. The essential references for each mineral district are also provided. The references are cited by number and listed alphabetically and numerically at the end of this report. Keith and others (1983b) provide a comprehensive list of references for all metallic mineral districts in the Basin and Range Province of Arizona. Many of these references are of limited value; therefore, we list only those most useful in compiling the basic geologic and geochronologic information. Figure 1 is a map showing all the mineral districts in Arizona of presumed middle Tertiary age. We have included districts of problematic age on the figure for completeness. (88 pages)