Fluid inclusion studies on the porphyry-type ore deposits at Bingham, Utah, Butte, Montana, and Climax, Colorado

Abstract

Data are given on the composition, temperature, pressure, and density of the hydrothermal fluids present in the central Cu-Mo core of the deposit at Bingham, Utah, and in its related but not necessarily coeval peripheral Pb-Zn deposits. These data are based on a study of primary and secondary fluid inclusions in transparent ore and gangue minerals that included the use of freezing, heating, and crushing microscope stages. The composition of the hydrothermal fluids at various stages in the mineralization and repeated later fracturing and rehealing ranged from nearly fresh water to water containing more than 60 weight percent salts in solution--virtually a hydrous saline melt--in some quartz-molybdenite-chalcopyrite veins from the core. Most of these salts have crystallized out as daughter minerals on cooling, forming major amounts of halite and sylvite and minor amounts of anhydrite (?), hematite (?) and several unidentified phases. These highly saline fluids occur only in the core. Some of them have apparently boiled, forming bubbles of a relatively low density CO 2 -rich "steam" containing only a few percent NaCl. These low-density fluids have also been trapped as inclusions. The fluids that formed the peripheral deposits had low salinities, and some of these also have apparently boiled. A few were very high in hydrogen sulfide. Inclusions from a quartz pod and in the quartz-molybdenite-chalcopyrite veins from the core yield the highest temperatures, 640 ° -725 ° C; most inclusions from the core homogenize at temperatures above 400 ° C. Samples from the peripheral deposits were uniformly lower, in the range 294 ° -330 ° C. The abundant evidence of intermittent boiling of these solutions is important because it places limits on the pressure at the time of trapping, it results in there being little or no need for a pressure correction to the homogenization temperatures, and it indicates that the pressure has varied with time. Although some of the homogenization temperatures are very high, the high salinity causes the vapor pressures at homogenization to be relatively low, from about 80 to a maximum of about 1,100 atmospheres. The density of the hydrothermal fluids is of great concern in any consideration of flow patterns, and particularly in the inevitable mixing with possibly heated ground waters. Steam inclusions from the core had gross densities of 0.3 to 0.1 g · cm -3 , but many of the highly saline inclusions in the core contain fluids whose density at trapping was as high as 1.3 g · cm -3 . The fluids trapped in inclusions in the peripheral deposits had densities of 0.75-0.95 g · cm -3 , well below that of the surrounding cold ground water. Hydraulic pressure gradients from these density differences, and the vapor pressures involved, must also have varied with time in any given location, particularly when boiling occurred, and thus the circulation patterns could have been very complex. The more highly saline fluids are believed to be of truly magmatic origin, and not merely heated ground water from the area at the time of the intrusion. The great abundance throughout the core of planes of secondary inclusions in which individual planes are uniform but adjacent planes have widely varying composition, density, and homogenization temperature, is evidence of thorough and repeated fracturing of these rocks under hydrothermal conditions. Inclusions were also examined in some samples from Butte, Montana, Climax, Colorado, and several Arizona porphyry copper deposits. The ranges of temperature, composition, and density found were similar but smaller than at Bingham. This might be simply a result of insufficient sampling.