A rock magnetic fingerprint of hydrothermal alteration in volcanic rocks - An example from the Los Azufres Geothermal Field, Mexico

Abstract Drill rock cuttings from different depths of two geothermal wells (Az-26 and Az-49) from the Los Azufres Geothermal Field (LAGF), Mexico, were analyzed for rock magnetic properties (magnetic susceptibility, χ lf ; frequency-dependent susceptibility, χ fd; susceptibility of anhysteretic remanent magnetization, χ ARM ; isothermal remanent magnetization, IRM; saturation isothermal remanent magnetization, SIRM; “hard” isothermal remanent magnetization, HIRM) and their inter-parametric ratios (SIRM/ χ lf , χ ARM /SIRM and S-ratio) to explore the relationship, if any, between these properties and hydrothermal alteration. In both the geothermal wells, the intensity of hydrothermal alteration increases with depth and temperature of the well. The samples exhibit hydrothermal alteration-induced changes in several rock magnetic properties: (1) a decreasing trend in the values of concentration-dependent rock magnetic parameters ( χ lf ; χ fd, χ ARM , IRM 300mT , and SIRM); (2) a decrease in S-ratio (relative proportions of ferrimagnetic and anti-ferromagnetic minerals) values and an increase in HIRM (“hard” IRM; concentration of magnetically “hard” minerals, e.g., hematite) values; and (3) increasing SIRM/ χ lf  and χ ARM /SIRM ratio values. Most of the rock cuttings in both the wells are characterized by coarse SSD grains and they exhibit χ lf values between 1  ×  10 −6 m 3 kg −1 (acid igneous rocks) and 10 × 10 −6 m 3 kg −1 (SSD/MD grain size). Our study also indicates that the initial (primary) mineralogy of reservoir rocks may have an influence on the type of hydrothermal minerals that would form at low reservoir temperatures and/or with low intensity of hydrothermal alteration; however, this may not have any significant influence at high reservoir temperatures and/or high intensity of hydrothermal alteration. Our study suggests that in a homogenous litho-unit (rocks of the same type and age) of a large area, a zone characterized by anomalously low values of concentration-dependent rock magnetic properties (χ lf , χ fd, χ ARM , IRM 300mT and SIRM), high SIRM/ χ lf , χ ARM /SIRM and HIRM values and low S-ratio values may indicate intense hydrothermal alteration. Consequently, rock magnetic properties may be a simple, yet an important tool, to identify hydrothermally altered rocks and zones of hydrothermal alteration, particularly in the initial stages of geothermal exploration.