Magmatism and mineralization of the Ash Peak area, Arizona: Petrochemical interpretations

Andesitic and rhyolitic magmatism was active during the mid-Tertiary (Early Miocene) of the Ash Peak area, southeastern Arizona. Andesitic magmas of similar composition both preceded and followed the low- and high-silica rhyolitic magmas. The changes from andesitic to rhyolitic and back to andesitic volcanism is postulated to be the result of local variations in the tectonic regime.;Parental basalts formed by either processes of subduction or extension may have ascended through the crust at differing rates in response to the tectonic regime. Under pre-extensional and post-extensional conditions, parental basaltic magmas may have ascended slowly relative to extensional conditions. Loss of heat would induce crystallization of the basalt and drive the composition toward intermediate compositions. Elevated abundances of trace elements in the andesitic rocks suggest that the fractionating magma underwent cycles of fractionation punctuated by replenishment of parental basaltic magma.;Extensional tectonic conditions may have allowed the parental basaltic magmas to ascend rapidly to the crustal level at which they became bouyantly compensated. They may then have acted as sources of heat and volatiles for the partial melting of the crust to produce parental rhyolitic magmas similar in composition to biotite rhyolite. Crystal fractionation models provide a reasonable representation of the observed petrochemical abundances of the later rhyolites. Petrochemical abundances of the rhyolites suggest that the suites of biotite rhyolite to biotite tuff/crystal-rich rhyolite to Ash Peak Glass follows a crystal fractionation trend. The crystal-poor rhyolites are modelled by mixing parental magma into the Ash Peak Glass magma chamber followed by crystal fractionation. Porphyritic rhyolites were modelled as hybrid magmas formed by the mixing of either biotite rhyolite or upper andesite magmas with crystal-poor rhyolites and compensating for the observed phenocryst assemblage.;Abundances and patterns of trace elements in ore and gangue minerals of gold-silver-carbonate-silica and carbonate-manganese oxide epithermal vein deposits of Ash Peak suggest that they crystallized from a common hydrothermal fluid. The fluid is proposed to be an aqueous phase that separated from the biotite rhyolite magma. Oxygenated meteoric waters increasingly depleted the hydrothermal fluid of Ce as it ascended to higher levels.