Geothermal gradients, recent climatic changes, and rate of sulfide oxidation in the San Manuel District, Arizona

Abstract

Thermal gradients were measured in eleven churn drill holes in the San Manuel district, 40 miles north of Tucson, Arizona, to determine if possible whether oxidizing sulfides undisturbed by mining operations gave off measurable amounts of heat. Nearly all the geothermal gradients have two inflection points and many of them have three.Comparison of the gradient in different rocks in the same hole showed that the conductivity of the different wall rocks, both wet and dry, differs by such small amounts that the effect on the thermal gradient was inconspicuous with the instruments used. The thermal conductivities of three representative samples were determined by Francis Birch and range from .0053 to .0085 (cal.cm (super -1) sec (super -1) ).Historical records and tree rings both indicate change from moist to dry climate about 65 years ago, but the accompanying temperature changes are unknown. The inflections in the depth-temperature curves above 500-foot depths harmonize with the assumption that the average surface temperature increased when the arid cycle began. The preceding moist climate had apparently continued without interruption from 1600 to about 1880 and had succeeded a devastating drought that lasted from 1570 to 1600. If this early drought was accompanied by a warmer climate the inflection on the depth-temperature curves would now be found at a depth of about 800 feet, approximately where the deepest inflections are found. Aside from these inflections, believed assignable to climatic changes, the most marked inflection is in the gradients of the holes where sulfides are at or above water level, and the departure in each of these indicates a heat source in the sulfides. The difference in slope above and below the inflection related to the sulfide zone is used to calculate the rate of oxidation of pyrite. The gradient due to oxidation is from 0.4 degrees to 0.6 degrees F per 100 feet and the heat liberated under each square centimeter of surface by oxidizing pyrite is approximately 28 calories per year. At this rate about 40,000 years would be required for the complete oxidation of the pyrite. The rocks are strongly fractured but the fractures are tight and the rock mass appears to have a low permeability; if more open fractures were present the rate of oxidation would probably be many times as fast and the gradient due to oxidation would then be several times as steep. In other localities horizontal gradients of several degrees per hundred feet have been observed underground near vertical sulfide veins, which are commonly marked by thermal highs 15 degrees to 40 degrees F above the normal rock temperature. Calculations show that more than 95 percent of the oxygen necessary for converting sulfide to oxide must come from air entering and leaving the voids as a result of changing barometric pressure.Using the least distorted geothermal gradients and the conductivity of the altered rocks as determined by Francis Birch, the heat flow where no oxidation is taking place is apparently equivalent to not less than 30 nor more than 46 calories per square centimeter per year.