Stratigraphy and alteration of the host rocks, United Verde massive sulfide deposit, Jerome, Arizona

Abstract

The early Proterozoic volcanogenic United Verde massive sulfide orebody is situated within the Cleopatra Member of the Deception Rhyolite. The Cleopatra Member, a porphyritic rhyodacite, has been subdivided into an upper unit and a lower unit. The United Verde orebody, previously interpreted to lie at the top of the Cleopatra Member, is now recognized as lying within the basal section of the upper unit of the Cleopatra Member.The upper unit and lower unit are differentiated on the basis of alteration mineralogy, mineral and rock geochemistry, and lithologic peculiarities. The contact between the two units is marked by sediments at the base of the upper unit, including a distinct green calcitebearing feldspar tuff, chert and hematitic chert layers, and a quartz porphyry conglomerate.The Cleopatra Member contains six distinct alteration types. Five alteration types characterize the lower unit and include two types of moderate chloritic and sericitic alteration (designated B1 and B2), two spatially distinct types of sericitic and silicic alteration (designated S1 and S2), and an area of intense chloritic alteration (designated C). These five alteration types record the effects of hydrothermal alteration by the massive sulfide ore-forming system and of a regional greenschist facies metamorphic event. The upper unit contains the least altered rocks of the Cleopatra Member and the bulk of the sixth alteration type, designated H. The rocks included within H-type alteration have disseminated hematite and/or hematitic chert veining.Whole-rock and mineral geochemistry of the lower unit may be used to establish position within the palcohydrothermal system. The distal area of moderate alteration (B1) contains samples enriched in MgO, slightly depleted in alkalies, and having ratios of Fe 2 O (sub 3 (sub (total)) ) /Fe 2 O (sub 3 (sub (total)) ) + MgO for whole rocks and Fe (sub (total)) /Fe (sub (total)) + Mg for chlorites that are low relative to other Cleopatra Member samples. These attributes are suggestive of the low-temperature seawater-rock interaction expected in the domain of fluid recharge. Rocks of alteration type C experienced significant additions of iron and magnesium, and losses of CaO, Na 2 O, and K 2 O, presumed to reflect alteration in the main discharge conduit for the hydrothermal fluid. Chlorite and whole-rock iron to iron-plus-magnesium ratios decrease within the area of alteration type C toward the sediment-water interface. This is consistent with mixing of an evolved ore-forming fluid with seawater in the upper levels of the area of alteration type C. Samples from the area of alteration type B2, which is intermediate between the recharge (B1) and discharge areas (C), are greatly depleted in CaO and Na 2 O, and are enriched in iron. Alteration types S1 and S2 are interpreted to reflect localized sites of fluid discharge where SiO 2 and K 2 O have been added and other major elements, except for Al 2 O 3 , have been removed.Whole-rock and mineral chemistry of the alteration types record progressive changes in the hydrothermal fluid from seawater to an ore-forming fluid. The fluid that was discharging and depositing ore apparently had a log a (sub O 2 ) between -41.7 and -35.5, a log a (sub H 2 S) between -5 and -2.6, and was saturated with respect to iron sulfides, magnesium silicates, and ore-forming metals.