Metasomatized Tertiary lavas with anomalously high K2O and low Na2O content are distributed within the northwest-trending Miocene extensional terrane of southwestern Arizona. These rocks are common near core-complex–related detachment faults at Picacho Peak and the Harcuvar Mountains and in listric-faulted terrane at the Vulture Mountains. In addition to systematic changes in K2O and Na2O, the rocks have been enriched in Zr and depleted in MgO.

Secondary, introduced minerals include orthoclase, quartz, and calcite. Fine-grained, euhedral orthoclase (var. adularia), from 2 to 10 µm, is the dominant potassium mineral.

Metasomatic changes at Picacho Peak are spatially associated with a major detachment fault. Thus, it is interpreted that detachment provided a conduit for hydrothermal fluids that altered the initial chemical composition of the Tertiary volcanics by potassium metasomatism and charged the upper-plate rocks with mineralizing fluids that carried Zr and Ba, along with Au, Ag, and Cu, during detachment 17–18 Ma.

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