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This work examines the hydrogeochemical evolution of Tucson basin groundwater, including isotope hydrology, geochemistry and age determinations. Results of mineralogic investigation on basin fill were used to constrain water-rock geochemical reactions. Examination of 45 years of water quality data shows that groundwater mining has affected water quality. Stable isotopes of carbon, oxygen, hydrogen, sulfur, and chlorine and radiocarbon, tritium and radon determinations refine the interpretation of hydrogeochemical evolution of Tucson basin groundwater as modelled with NETPATH. Two distinct sampling periods, the first in 1965 and the second between 1984 and 1989, resulted in the determination of groundwater ages for water mined two decades apart. Isotope hydrology and geochemical modelling suggest that much of the water presently mined from the Tucson basin has a component recharged during the last 50 years. Increased sulfate concentrations suggest that heavy pumping in the northeastern basin may have induced increased leakage from lower units. Results of geochemical modelling indicate an average of 5 percent mountain-front recharge to the Ft. Lowell Formation along the northern aquifer margin. An increase in dissolved solids along the basin margin implies that this component to recharge has increased in the past decade. The radiocarbon age of the basin groundwater was compared with the temporal movement of water as modelled with MODFLOW and PATH3D. In general, the hydrologic simulation agrees with both the distribution of tritium and the exponentially modelled water age, as determined with bomb-derived radiocarbon, for areas of the Tucson basin that contain water less than 50 years in age. Hydrologic modelling failed to predict the antiquity of recently sampled water in the central basin but is similar to age determinations on waters collected in 1965.

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.

The concept of hydrochemical facies is used to study the distribution and, indirectly, to identify the origin of the chemical character of ground water in the basin-fill aquifer of the Tucson Basin in relationship to the hydrogeologic framework. Hydrochemical fades of ground water is defined operationally in terms of the lateral (horizontal) variation of chemical quality. The following chemical constituents are included in this study: total dissolved solids, calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, pH, nitrate, fluoride, strontium and zinc. A conceptual process-response model relates the regional distribution of dissolved constituents to the following hydrogeologic controls: (a) the chemical composition of the rock and soil in the drainage area before recharge to the ground-water basin and conditions at the recharge sites, (b) the lithology of the basin-fill aquifer, and (c) the direction of groundwater flow within the aquifer itself. Trend surface analysis suggests that the regional distribution patterns of total dissolved solids, calcium, sodium, sulfate and strontium show a tendency to parallel the direction of ground-water flow. The distribution pattern of chloride ions based on old analyses shows a trend opposite to the distribution pattern of chloride ions based on new analyses from the same area. Nitrate content of ground water and specific capacity of wells seem to be related. Q-factor analysis of data from the basin-fill aquifer demonstrates that the overall chemical character of the ground water does not change substantially as it moves through the basin. It is concluded that the chemical character of ground water in the basin-fill aquifer of the Tucson Basin was acquired mainly during contact with various rock types in the drainage basin before recharge. The lithology of the aquifer, presumably, only plays a secondary role in determining the overall chemical composition of the ground water. After recharge to the basin-fill aquifer the distribution of dissolved constituents is controlled primarily by the flow pattern. A chemical equilibrium model of calcite and water is used to approach the problem of determining whether precipitation or dissolution of calcite takes place in the aquifer. Measured calcium ion concentrations and pH values are compared to calcium ion concentrations and pH values computed for the equilibrium model. Assuming that the equilibrium model represents actual conditions in the aquifer, departures from the equilibrium model may be used to predict the chemical behaviour of calcite In the basin-fill aquifer.

A geochemical approach to the evaluation of the chemistry of natural recharge processes in the Tucson basin was used to identify the major minerals controlling the evolution of ground water chemistry and to assess the viability of recharging imported Central Arizona Project water supplies. Well cuttings analyses and water quality samples from over 65 wells in the basin were used as input to the geochemical computer model PATH4 (Helgeson, 1970) and the sequence of aqueous species and mineral production in a recharge reference volume examined. The study reveals that natural processes in the basin lead to the increase in dissolved solids content in ground water over time and the production of secondary minerals such as calcite, calcium montmorillonite, kaolinite and poorly crystallized alumino-silicate phases. Secondary minerals grow into aquifer pore spaces and may, over time, be responsible for the reduction in aquifer porosity and the specific capacity of wells. The recharge of imported Central Arizona Project water will lead to an increase in the dissolved solids content of ground water and may, in certain areas of the basin, lead to the enhanced production of secondary minerals. The use of CAP water as a recharge source must be guided by the geochemical factors which influence the nature and scope of reactions between CAP water and the Tucson aquifer matrix. The study demonstrates the need for and identifies water quality and aquifer matrix criteria for the assessment of sources of recharge water and recharge facility sites. The use of geochemistry as a tool for quantitatively assessing ground water quality is demonstrated.

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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.

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