HEAT FLOW IN A “BLIND” GEOTHERMAL AREA NEAR MARYSVILLE, MONTANA

Abstract

A unique geothermal area has been discovered during the course of a regional heat‐flow study using holes drilled for mineral exploration. There are no surface manifestations of abnormal subsurface temperature in spite of the fact that at one locality a temperature of 58°C was measured at a depth of only 220 m. The area of anomalous heat flow straddles the Continental Divide near the old gold mining camp of Marysville, Montana about 30 km northwest of Helena. Measured values of conducted heat flow range from 3.2 to [Formula: see text] sec. The immediate source of the high heat flow is either an unexposed reservoir of thermal fluids or a very shallow still‐cooling magma chamber. At the present time the magma chamber model is preferred. The presence of additional similar areas in the western U.S. is suggested by the data from regional heat‐flow studies. However, in most of the other areas only single anomalous heat‐flow value is available, whereas at Marysville a region of several tens of [Formula: see text] is known to have abnormal heat‐flow values. It is suggested that temperature measurements should be made in available drill holes deeper than 30 m in the high heat‐flow regions of the western U.S. as an inexpensive way to explore for other “blind” geothermal reservoirs.