Composition changes in an ash-flow cooling unit during K metasomatism, west-central Arizona

Abstract

A variably altered rhyolitic ash-flow cooling unit in west-central Arizona was sampled and analyzed to gain a better understanding of compositional changes that took place during Tertiary K metasomatism. Mn, Cu, Zn, and other metals of economic importance were removed from the K-metasomatized rocks during alteration, making them a likely source for these metals in vein and stratiform Mn deposits and base metal ores related to detachment faults in western Arizona. Using a single cooling unit to understand the compositional changes during K metasomatism reduces the primary chemical variability inherent in the sampling of multiple units, a problem with most previous studies. The least altered samples (group 1) experienced a moderate degree of K metasomatism, having K 2 O/Na 2 O ratios of 3.1 to 5.5, compared to likely igneous values of <2. The most altered samples (group 2) were severely K metasomatized with K 2 O/Na 2 O ratios of 17.9 to 29.5. K 2 O, P 2 O 5 , and Ba increased in concentration substantially during K metasomatism, and MnO, Na 2 O, Cu, Zn, Sr, Mo, and Cs decreased. All immobile elements decreased in concentration by approximately 9 percent, which is attributed to a dilution effect caused by net mass gain in the group 2 rocks, principally as K 2 O and SiO 2 . Mineralogical zoning in small late veins seen in thin section is quartz + or - K feldspar + or - calcite from vein wall to center. The strong covariance in CaO and loss on ignition (L.O.I.) with observed calcite in thin section and the lack of correlation of CaO and L.O.I. with other components indicates that their variability is unrelated to K metasomatism. Calculations indicate that the loss of Mn, Cu, Zn, and other metals from K-metasomatized volcanics is easily large enough to account for middle Tertiary Mn and detachment-related Cu and Zn ore deposits in west-central Arizona.