Terrestrial heat flow in the alkaline intrusive complex of Poços de Caldas, Brazil

Abstract

Terrestrial heat flow measurements have been made in five localities within the alkaline intrusive complex of Poços de Caldas (Brasil) making use of exploratory boreholes drilled for uranium. Heat flow is found to be variable, from as low as 53 mW/m 2 to as high as 99.4 mW/m 2. The non-mineralized zones are found to be characterized by high heat flow values (70–99.4 mW/m 2) relative to those in mineralized areas (53–62 mW/m 2). The lower values in mineralized zones arc believed to be due to heat removal associated with groundwater circulation. Hence the mean value of 85.6 mW/m 2 for the non-mineralized zone is considered as representative of the true heat flow within the intrusion. The overall mean for the five localities within the intrusion is 74.4 mW/m 2, significantly higher than the mean value for seven localities in the surrounding shield areas. Thus the intrusive complex is characterized by a positive thermal anomaly relative to the adjacent shield. Radiogenic heat production values were calculated based on radiometric measurements of U, Th and K on thirteen samples of drill cores. Mean heat production value (excluding those samples from mineralized zones) calculated is 7.0 μW/m 3. On a reexamination of earlier heat flow and heat generation measurements on three other sites situated within the same age province (the Brazilian fold belt), along with the present values for Poços de Caldas, a case can be made for a linear heat flow-heat generation relation. This new relation defines a reduced heat flow of 53 mW/m 2 and a depth parameter of 4.8 km. The adjacent but older Trans-Amazonian fold belt seem to be characterized by a lower reduced heat flow ( Q o = 28.3 mW/m 2 ) and a higher depth parameter ( D = 13.1 km ).