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Greenschist-facies, Lower Proterozoic metasedimentary rocks of the Johnny Lyon Rills and Little Dragoon Mountains of southeastern Arizona were deposited prior to the intrusion of an approximately 1690 Ma rhyodacite pluton. Well-preserved primary structures indicate deposition by turbidity currents in an intermediate to neardistal setting. Sandstone compositions suggest derivation from either a complex, heterogeneous source or multiple source terranes that provided mature, quartzose sediment as well as lesser quantities of volcaniclastic detritus. Earliest deformation, predating both intrusion of the rhyodacite and metamorphism, produced sections of melange composed primarily of dismembered turbidite beds, but also incorporating large (up to several km long) blocks of deformed basalt. Subsequent deformation, in part post-dating intrusion of the rhyodacite and in part coinciding with metamorphism, affected both melange and coherent strata, and involved isoclinal folding and layerparallel faulting and shearing. It is proposed that turbidite deposition occurred in a trench associated with a north-dipping subduction zone or on ocean floor outboard of such a trench. Melange formed primarily by ductile disruption of unlithified sediments within the subduction zone. Basalt blocks incorporated within the melange represent fragments of oceanic crust or seamounts detached from the lower plate during subduction. Later deformation and intrusion of the rhyodacite occurred within an accretionary prism above the subduction zone. Deformation within the prism ended prior to intrusion of the 1625 ± 10 Ma posttectonic Johnny Lyon Granodiorite.

The United Verde orebody, a Proterozoic volcanogenic massive sulfide deposit, is hosted by the Cleopatra Formation. The Cleopatra Formation is subdivided into two distinct members, the Upper and Lower, on the basis of alteration facies, whole rock geochemistry and the chemistry of alteration minerals. The Lower member was deposited prior to ore deposition and consists of five major alteration facies. Facies Bl, the most distant from the orebody represents the recharge area for the ore-forming fluid. Facies B2 surrounds the major discharge area or the chlorite pipe. These three facies contain chlorite and quartz as alteration minerals in variable amounts. Two facies, 81 and S2, contain quartz and sericite as alteration minerals. Mass balance calculations show progressive removal of Na and Ca, and addition of MgO and FeO* from the area of recharge (facies Bl) to facies B2 to the chlorite pipe. Whole rock δ¹⁸O values are high relative to least altered Cleopatra Formation in the recharge area and low in the discharge zone. Mineralogy and geochemistry of samples from the Upper member indicate that it was deposited following ore deposition and interacted with fluids rich in silica and iron. The hydrothermal fluid, which is interpreted to have been seawater, evolved to a high temperature slightly acidic, reduced fluid during water-rock interaction(log a₀₂ = -33 to -41; log a(H2S) = -2.6 to -5.0). The fluid δ¹⁸O and δ¹³C₀₂ values increased. Calculated δ¹³C₀₂ and δ¹⁸O fluid values, and sphalerite and chlorite chemistry imply that mixing of the hydrothermal fluid with seawater occurred in the orebody. the upper The levels of the chlorite pipe and in limited range in δ³⁴S values of sulfides is consistent with the derivation of the oreforming fluids from the reduced basal layer of a stratified basin. The study area represents only a small part of the United Verde circulation cell. Increased δ¹⁸O values of the fluid, and the need for a source of Mg, Fe and other metals suggest that the fluids may have circulated into the Shea Basalt.

Numerical values for parameters which characterize melting regimes and differentiation history have been determined for a suite of genetically-related calc-alkaline rocks. Isotopic ratios of Nd and Sr vary sympathetically, with the least differentiated and oldest rocks having ε(Nd) = -4.3 and ⁸⁷/Sr⁸⁶Srₒ = 0.7069 and the most differentiated and youngest characterized by ε(Nd) = -8.5 and ⁸⁷Sr/⁸⁷/Srₒ = 0.7092. These observations imply that a process which might relate the various units is that of invasion of the crust by mantle-derived magmas and progressive assimilation. The most negative Nd values may well represent the isotopic signature of the crustal rocks which melted. Melting of a 1.8 Ga source region (an age characteristic of the basement in southeastern Arizona) which had a ¹⁴⁷Sm/¹⁴⁴Nd ratio of .13 would yield Laramide melts with and ε(Nd) of -8.5. This Sm/Nd ratio is characteristic of a somewhat mafic (lower?) crust, a character consistent with petrological and chemical evidence which suggest that the source was intermediate to mafic in composition and of amphibolite grade. Solution of the isotopic data yields a value of r (that instantaneous ratio of assimilated material to crystallizing magma) equal to 0.6 to 0.9. These values are reasonable considering heat budgets of crystallization and fusion in the lower crust. Solution of the set of equations for changes in concentration of the trace elements yields numeric values for the f, fraction of remaining melt for each unit in the series. The values are: andesite, f-0.63; granodiorite, f-0.42 to 0.32; and the final granite stocks and dikes, f-0.34. The modeling provides insight into the way an igneous suite intimately associated with copper mineralization has evolved. The porphyry ore bodies are related to long-lived and large magma systems. At the level of mineralization and observation, we sample only a small portion of the system. The importance of subduction to metallogenesis may be that it provides a heat source, in the form of mantle-derived magma, which allows extensive melting of hydrous crust. Thus, as is becoming evident from other studies as well, assimilation and crustal anatexis are major processes in generating granitoid rocks at convergent plate boundaries.

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