Four zones are located withing three feet of the fumarolic vent, and are delineated and characterized in the field by color differences. These zones average 700°C, 540°C, 370°C, and 320°C. Each is characterized by specific mineral occurences.
High lead and arsenic contents in the sublimates (as high as 4% Pb and 2% As) at the Caliente Crater on the crest is the most unusual chemical feature compared to the other fumaroles at Santiaguito. Since April 1967, the pyroclastic activity in the crater has prevented further study of fumarolic activity. Large volumes of volcanic gas and vapor still come from Caliente Crater, but the pattern of fumarolic sublimates has been obscured by great thicknesses of newly erupted ash.
100 meters south of the Caliente Crater on the crest of a ridge trending east-west is a low temperature fumarole which produces large quantities of vapor. The chemistry of sublimates and condensates from this fumarole is relatively simple: only sulfur and gypsum are depositing and the alkali and alkaline earth content of the condensate is not greatly different from local rainwater. The absence of chlorides at this fumarole suggests that ground water may be important in acting as a solvent for some of the constituents of the volcanic gas.
East of the Caliente Crater on the slope of Santiaguito facing Santa María is another active fumarolic area. Temperatures ranged 90-400°C and fumarolic incrustations which contained alunite and cristobalite were found (Zies, 1951). Other important minerals are anatase, ahydrite, rutile, and pyrite.
There are numerous steamy weak fumaroles on the central part of Santiaguito which have low temperatures (60-80°C) and are depositing no sublimates. Condensates from a fumarole of this type is chemically very similar to local rainwater.
On the summit ridge of Santiaguito between the El Brujo and Caliente vents are three important fumaroles. These fumaroles are all at the foot of steep linear fractures which have extruded spines or slabs associated with them.
One of the summit fumaroles is perhaps the largest single fumarole at Santiaguito, located at the foot of a high cliff along a major fracture near th ehighest point of the mountain. In spite of a large colume of vapor, this fumarole produces a small volume of sublimates. Temperatures are low and cristobalite, ralstonite, and amorphous iron chloride are the major sublimate minerals. SOme sulfur and gypsum are found at one of the smaller fumaroles nearby.
One hundred meters to the west at the base of a scarp forming the wall of a circular depression on the crest of the dome is another important fumarole. A zonal sequence of minerals is found at this fumarole. The hottest zone (350°C) contains anhydrite and sodium aluminum sulfate. Thin green and blue crusts on the surfaces of these sulfates are concentrations of minor elements. At 150°C soda-alum is the important sublimate. The lowest temperature zone (60-80°C) is characterized by yellow amorphous iron chloride, cristobalite, and minor amounts of ralstonite. This fumarole was under observation for more than three years, and during much of the time a continuous temperate recorder was operating. The temperature of the fumarole was shown to be sensitive to seasonal and even daily rainfall, but there is not evidence for any changes independent of this influence.
Four hundred meters further west, toward the El Brujo vent, is the third fumarolic area, which is quite similar to the other two. Anhydrite and the usual halide assemblage are the sublimate minerals.
(Stoiber and Rose, 1969)