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Metavolcanic and metasedimentary rocks of the Yavapai Series (Anderson and Creasey, 1958) crop out within the area described in this study. These rocks were originally subdivided into the Ash Creek Group and Alder Group (Anderson and Blacet, 1972). Isotopic data led Anderson and others (1971) to replace the name Alder Group with the Big Bug Group, which includes all the rocks of the Yavapai Series exposed in the Prescott-Jerome area.

The Big Bug Group is divided into three formations. From oldest to youngest they are: 1) Green Gulch Volcanics; 2) Spud Mountain Volcanics; and 3) Iron King Volcanics (Anderson and Silver, 1976). The Green Gulch Volcanics are not present in the area of this study and will not be discussed. The thickness of the Big Bug Group cannot be measured with confidence because of the probability that unrecognized small folds on the flanks of major folds duplicate the section, and because of probable thinning and thickening of units during deformation (Anderson and Blacet, 1972c).

The Iron King Volcanics are composed of a thick sequence of metamorphosed, pillowed and amygdaloidal andesitic and basaltic flows. Locally the Iron King contains thin interbeds of metamorphosed rhyolitic tuff and flows units. The youngest part of the Iron King is a mixed metatuff of andesitic to rhyolitic composition that crops out in the trough of an overturned syncline.

The Iron King Volcanics have been informally subdivided into members (O'Hara, 1981; unpublished map) in order to facilitate mapping for industry. The area encompassing the Montezuma, Swindler

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