Abstract

Three zones of progressively increasing alteration occur in the Deception Rhyolite associated with the United Verde massive sulfide deposit of early Proterozoic age near Jerome, Arizona. Zone 1 (most intense alteration) is comprised chiefly of chlorite and exhibits apparent losses of SiO 2 , Na 2 O, CaO, and K 2 O; enrichments in MgO, Fe 2 O (sub 3 total ) , 1 Al 2 O 3 , volatile constituents (chiefly H 2 O), and relatively unchanged TiO 2 , which is used to monitor changes in other major elements. SiO 2 and K 2 O losses are most prominent in zone 1. Quartz, chlorite, and sericite are the dominant secondary minerals in zones 2 and 3. These zones also show major losses of Na 2 O and CaO and variable but small enrichments in MgO, Fe 2 O (sub 3 total ) , 1 SiO 2 , and locally in K 2 O. Zone 1 is characterized by major decreases in K, Rb, Sr, and Ba and increases in Cs, U, Th, Mn, Sc, Co, rare earth elements, and high field strength elements (Zr, Nb, Y, Ta, Hf). Increases in Cu, Zn, and Pb occur in all zones but are greatest in zone 1. With the exception of light rare earth elements, Nb, and Y in zone 1, ratios of high field strength elements (viz. Ti/Zr, Ta/Hf, Hf/Th) and rare earth element distribution patterns are relatively unchanged during alteration and provide a means of identifying protoliths.Alteration at Jerome is complex, involving both submarine diagenetic, hydrothermal, and possibly metamorphic processes. Zone 1 formed at the highest temperatures and at high water/rock ratios. The geochemical character and lithologic association of the volcanic host rocks are consistent with a back-are basin tectonic setting for the United Verde deposit. 1 Fe 2 O (sub 3 total ) = total Fe expressed as Fe 2 O 3 .

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