The Del Bac Hills are a northeast-trending chain composed of mid-Tertiary Turkey Track porphyry dikes and flows, overlain by potassic basaltic andesites intercalated with sediments. These hills are considered the southernmost extension of the Tucson Mountains and lie along a northeast-trending basement scarp which extends across the Tucson Basin. Turkey Track porphyry intrudes the San Xavier conglomerate as two parallel dikes, closely associated in time but with different magnetic orientations. Potassium-argon dating of a flow associated with the older dike yielded a mean of 26.8 m.y. for plagioclase phenocrysts and groundmass. Plagioclase of the younger dike sampled 50 feet from the contact with the San Xavier conglomerate and at a depth of approximately 200 feet yielded an anomalous age of 46.4 m.y., representing 0.29 x 10^-10 moles per gram excess Ar4° in comparison with the extrusive. The 21 basaltic andesite flows are essentially contemporaneous at 24-25 m.y. Tight grouping of K/Rb and Rb/Sr for the basaltic andesites confirms the field inference of two separate sources. Specific gravity and initial Sr^87/Sr^86 determinations for the porphyry and basaltic andesites do not preclude a comagmatic origin for the two rock types. Post-extrusive faulting along three principal directions has established the present configuration of the hills.

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Hydrogeochemical studies are generally qualitative in nature. The goal of this study is to investigate the possibility of quantitative interpretation of hydrogeochemistry by considering the chemical characteristics and the isotopic compositions of oxygen, hydrogen, and carbon of the water. This study examines ephemeral stream and well waters from Canada del Oro valley, southern Arizona. By chemical and isotopic considerations, this study finds that the change of chemical composition of the wash water was mainly due to water-rock interaction. The concentrations of dissolved constituents increase between 10 to 50% from upstream to downstream samples, while the evaporation loss of water is less than 3%. By chemical and isotopic considerations of the well waters, this study identifies three recharge waters in the CDO ground-water system. The chemical and water isotopic compositions of the well waters are results of mixing between these three recharge waters and subsequent dissolution of the aquifer. By thermodynamic consideration, albite, kaolinite, montmorillonite, and calcite are the main phases that influence the chemical characteristics of this ground-water system. Simulations with the computer program PHREEQE verifies the above conclusions. The mechanisms that influence the chemical and carbon isotopic compositions of the water are quite different in a system open to a CO2 gas reservoir than in a closed system. Deines, Langmuir, and Harmon (1974) derived a set of chemical-isotopic equations to calculate the carbon isotopic composition of water under open system condition. Wigley, Plummer, and Pearson (1978) formulated a mass transfer equation to calculate the change of carbon isotopic composition of natural water in closed system environment. This study implements these two type of equations as a subroutine--CSOTOP to the computer program PHREEQE. With this PHREEQE-CSOTOP package, the evolution of carbon chemical and isotopic composition of natural water can be conveniently modeled from open to closed system conditions. This study also uses this package to date water samples from the Tucson basin, and finds that choice of reaction path may cause a difference in carbon-14 age of up to a few thousand years. This study concludes that it is possible to rigorously interpret hydrogeochemistry in a quantitative way. With sufficient measurements to define the reaction path, followed by thermodynamic consideration, chemical-isotopic evaluation, and computer modeling, one should be able to achieve this goal.

Potassium metasomatism is a widespread alteration type in porphyry copper deposits and is often spatially associated with hypogene sulfide ore formation. At Ajo, potassic alteration composes the dominant alteration type and is spatially, and to some extent, temporally, associated with chalcopyrite and bornite mineralization. Physiochemical conditions prevailing during potassic alteration thus describe a significant portion of the hydrothermal ore-forming process. Studies of fractures, fluid inclusions in quartz, and structural and compositional characteristics of K-feldspar, all in the potassic alteration zone at Ajo, indicate that: fracturing was strongest during potassic alteration and sulfide deposition, pressure were at approximately 650 bars, temperatures started a a minimum of 580°C, continued through 470°C, log aK+/aH+ changed from 2.6 at 580°C to 3.2 at 470°C, log aNa/aH+ changed from 2.7 at 580°C to 3.8 at 470°C, and the ratio aK+/aNa+ radically decreased from 0.8 to 0.3 in this temperature range.