Geochemical Modeling at Raft River

Abstract

Chemical analysis of water from three depth regimes at the Raft River KGRA indicate the presence of at least two distinct hydrothermal fluids. One fluid predominates in the fracture system on the west side of the valley, known as the Bridge Fault. This fluid is characterized by low conductivity (2,000 to 3,000 us) and 6 to 9 ug/ml F. The second fluid, encountered in the center of the valley, appears to be associated with the Narrows Structure and is characterized by a conductivity of 6,000 to 11,000 us and F of 33 to 6 ug/ml. Contour mapping of conductivity and C1/F ratios indicates upwelling of both deep geothermal fluids into the shallow system. This recharge into the intermediate and shallow zone produces high conductivity water which is used for irrigation. Application of a simple mixing model shows that all the water sampled in the intermediate and deep zones can be described by mixtures of two nearly pure fluids. One mechanism, consistent with the known data, is deep upwelling of a highly mineralized fluid which is heated by the basement rock and then penetrates sediment layers through fractures. The second fluid is relatively recent meteoric water conductivity heated by the basement rock.