The Anderson Mine (Arizona)--An early diagenetic uranium deposit in Miocene Lake Sediments

The Anderson mine is located in Yavapai County, West-Central Arizona, an area forming part of the Basin and Range province. The uranium deposit occurs in the Miocene Chapin Wash Formation, a sequence of interbedded lacustrine and alluvial sediments. Uranium mineralization is confined to the lacustrine facies of Chapin Wash sediments, composed of tuffaceous mudstones, fine-grained sandstones, and silicified marlstones. South of the Anderson mine outcrop zone, the lacustrine sequence reaches an average thickness of 150 m, including two basal carbonaceous horizons of 37 and 35 m in thickness. Uranium is locally associated with high values of Mo, As, and V. In drill core samples, highgrade mineralization (up to 2.2% U) is confined to individual seams of lignitic coal, forms halos around rootlet remains, and fills burrows or irregular fractures. Spotlike concentrations are associated with framboidal pyrite spheres. Uranyl-bearing opaline silica and colloform coffinite have been identified as main ore minerals. Uranium was preconcentrated during early diagenesis by sorption onto colloidal humic substance, silica gel, and zeolites. Subsequent precipitation as coffinite was mainly controlled by bacteriogenic H2S gas. At present, both mineralized carbonaceous horizons are in a state of radiogenic equilibrium. Uranium mineralization was displaced by late Miocene to early Pliocene tectonic movements along Basin and Range normal faults. The Anderson mine may be classified as a strata-bound, low-temperature deposit in lacustrine sediments formed by complex sorption and precipitation processes in an early diagenetic environment