Chemical and isotopic evolution of hydrothermal solutions at Bingham, Utah

Univ. Utah, Dep. Geol. and Geophys., Salt Lake City, UT, United States

Systematic changes in mineral assemblages, mineral compositions, fluid inclusion characteristics, and hydrogen and oxygen isotope compositions in altered igneous rocks in the Bingham, Utah, porphyry copper deposit record the chemical and isotopic evolution of the hydrothermal solutions. Hydrothermal alteration in and surrounding the Bingham porphyry copper deposit consists of an outer propylitic zone of chlorite, epidote, and actinolite; a transition zone of hydrothermal biotite, actinolite, epidote, and chlorite; and an inner potassic zone of hydrothermal biotite and K-feldspar. Later sericitic and argillic alteration are superimposed on early potassic and propylitic alteration.Compositions of alteration minerals determined by electron microprobe methods are: biotite, mole fraction phlogopite (Mg/Mg + Fe) 0.58 to 0.83; actinolite, mole fraction tremolite (Mg/Mg + Fe) 0.58 to 0.79; epidote, mole fraction pistacite (Fe/Fe + Al) 0.27 to 0.30; and chlorite, mole fraction clinochlore (Mg/Mg + Fe) 0.66 to 0.76. Mineral compositions, especially MnO and MgO in chlorite, MnO in epidote, and TiO ₂ in biotite, vary systematically with distance from disseminated ore. MgO in chlorite increases from 20.6 to 24.6 wt percent. MnO in chlorite and vein epidote increases from less than 0.1 to 0.59 wt percent and from 0.08 to 0.73 wt percent, respectively. The average mole proportion Ti in biotite increases from 0.1 to 0.24.Temperatures and salinities from fluid inclusions range from greater than 600 degrees C and 50 equiv wt percent NaCl for the potassic zone to approximately 370 degrees C and 33 wt percent for the propylitic zone. Coexisting liquid-rich and vapor-rich inclusions which homogenize at temperatures similar to the liquid and vapor phase, respectively, suggest boiling from the potassic core out to the potassic-propylitic transition zone. Compositions and homogenization temperatures for fluid inclusions suggest average P (sub (f)) -T conditions which vary smoothly from the potassic core to the propylitic fringe: 600 degrees C and 800 bars for the potassic zone, 450 degrees C and 500 bars for the transition zone, and 350 degrees C and 200 bars for the outer propylitic zone.Thermochemical calculations show that log f (sub H ₂ O) decreases from 2.74 in the potassic core to 2.17 in the propylitic zone fringe, log f (sub S ₂ ) decreases from -1 to -3.3 in the core to -8.7 in the propylitic zone, and log f (sub O ₂ ) decreases from -16 to -17.5 in the core to -29 in the same interval. The log activity ratios of Ca, Mg, and K to hydrogen ion increase from the potassic to the propylitic zone. K (super +) /H (super +) increases from 3.1 to 3.4, Mg (super +2) /(H (super +) ) ² ) increases from -2.25 to 4.25, and Ca (super +2) /(H (super +) ) ² increases from less than 1 to 7.75.The extent of oxygen isotope exchange between the hydrothermal vein fluids and the igneous rock matrix in the Bingham stock increases with increasing temperature, abundance of alteration and quartz veins, thickness of veins, and fracture intensity. All these parameters generally increase inward from the propylitic zone into the core of the potassic zone. Within the potassic zone, there is pervasive exchange equilibrium between vein fluids and the selvage and rock matrix domains of the highly altered, veined, and fractured igneous rock. In contrast, vein fluids closely approach exchange equilibrium only with vein selvage in the propylitic zone; the igneous rock matrix has not equilibrated extensively, as might be expected in this lower temperature, less altered, and veined portion of the hydrothermal system.Calculated delta ¹⁸ O and delta D values of early fluids range from 6.8, -67 per mil in the potassic core to 5.1, -41 per mil in the outer propylitic zone, showing a systematic trend of ¹⁸ O depletion and deuterium enrichment with decreasing temperature outward from the innermost potassic zone. This trend is inconsistent with the progressive influx of local meteoric waters (delta D < -80ppm). Either of two alternative hypotheses may account for the observed trend. First, the isotopic compositions of the early potassic and propylitic fluids are consistent with derivation from magmatic fluid (7.0, -70ppm), mixing with progressively larger fractions of formation water (4.2, -40ppm) with increasing distance from the potassic core. Second, the trend in calculated delta ¹⁸ O and delta D values for these fluids could have resulted from isotopic exchange between meteoric water and igneous rock at very low water (W)/rock (R) ratios over a range of temperature (> or =600 degrees -350 degrees C). Such water/rock exchange likely would be confined to deeper parts of the system with the exchanged fluids subsequently focused up into the present exposure of the deposit.Late-stage fluids responsible for sericitic and argillic alteration are progressively depleted in ¹⁸ O and deuterium, consistent with an increasing, and finally dominant, meteoric water component. If fluid-rock isotopic exchange has been the dominant mechanism in defining the isotopic compositions of the hydrothermal fluids at Bingham, then this later hydrothermal system was characterized by shallower circulation and higher water/rock ratios than was the system responsible for the development of the early hydrothermal fluids.

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