Description

The Northern Plomosa district in west-central Arizona is a mid-Miocene Au-Cu district hosted by Miocene lacustrine sedimentary and volcanic rocks and their Proterozoic crystalline depositional basement. The host rocks are in the upper plate of the Plomosa detachment fault and are broken and tilted to the southwest by numerous northwest-striking listric normal faults which are the main controls on mineralization. The primary ore minerals are native gold, chrysocolla, and malachite. Gangue minerals include specular and earthy hematite, quartz, barite, fluorite, calcite and manganese oxides. Although open-space filling textures are locally abundant, most of the economic mineralization occurs as fault-controlled replacements in calcareous sediments. Fluid inclusions indicate that the mineralizing system involved low temperature (150 to 250°C), high salinity (17 to 26 eq. wt. % NaCI) fluids. Oxygen, sulfur, and carbon isotopes from the hydrothermal minerals are consistently heavy and, combined with fluid salinities, suggest that the mineralizing fluids were basin brines. (55 pages)