Impacts of Hydrothermal Alteration on Magnetic Susceptibility and Some Geochemical Properties of Volcanic Rocks from Geothermal Areas

Abstract

In geothermal areas, identification and distribution of hydrothermally altered rocks provide the information about the size of a geothermal system and the thermal conditions prevailing at the region. Though there are several geochemical, petrographic, and mineralogical methods successfully applied in identification of hydrothermally altered rocks, new reliable methods are simple to measure and fast are presently needed for the geothermal industry. Chemical weathering indices (CWIs), loss-on-ignition (LOI) and sulfur (S) contents, and magnetic susceptibility (χ) are some of the methods extensively and successfully applied in the studies of weathering processes and soil formation. Considering the several similarities between the weathering and hydrothermal alteration processes, the present work is carried out to evaluate whether these methods may also be successfully applied in identification of hydrothermally altered rocks in geothermal areas. The surface and subsurface rocks of Los Azufres Geothermal Field (LAGF) are considered for this purpose. The present study has indicated that the surface rocks from the LAGF are fresh/least altered as these rocks are characterized by comparatively low values (<60) of Chemical Index of Alteration (CIA), loss-on-ignition (LOI; < 2 wt%), low content of sulfur (S), and presence of several primary minerals. In the case of subsurface (well) rocks, the high values of CIA (>60) and LOI (>2 wt%) and almost absence of primary minerals throughout the depths in all the three studied wells (Az-3, Az-26 and Az-52), except in the top 500 m depth of the well Az-26, suggest that all the rocks of the three wells are altered, whereas the majority rocks in the < 500 m depth of the well Az-26 are fresh/least altered. This may be because all the three wells are characterized by same lithology (intermediate rocks), whereas acid rocks consist in the < 500 m depth in the well Az-26. This suggest that there exists a lithological influence in their values, and hence, comparison of these parameters of heterogeneous geological formations may result in erroneous conclusions. The rock samples throughout the depths in the three wells are intermediate rocks, except the rocks from < 500 m depth in the well Az-26 are acid rocks. The LOI and CIA values increase steeply in all the three wells, except in the top 500 m depth of the well Az-26, resulting into comparably higher intensity of alteration (intermediate magma). The changes in the CIA values in the top 500 m depth in the well Az-26 may be assigned to the rocks of different type (acidic rocks). Below 1000 m depths in the wells Az-3 and Az-52, the rocks are plotted very near to the boundary line differentiating fresh and altered rocks. Even in the well Az-26, the LOI values are not very high. Similarly, even though the CIA values are > 60 in the three wells (except top 500 m of the well Az-26) but not very high (mostly < 75) which indicate that the intensity of alteration has affected the alteration of the rocks in all the three wells (except top 500 m in the well Az-26) but the intensity of alteration is not very intensive. These results are in consistent with those reported in the literature for the studied samples, which says that there is a relatively low element mobility during alteration, a weak silicification, and low mobility of alkalis.