Mineralogical, chemical, and Sr-Nd isotopic effects of hydrothermal alteration of near-surface rhyolite in the Los Azufres geothermal field, Mexico

Abstract

We present a combined mineralogical, chemical, and Sr-Nd isotopic study of hydrothermal alteration effects in near-surface Pleistocene rhyolite sampled at two distinct localities from the Los Azufres geothermal field (LAGF), Mexico. The alteration mineralogy of the near-surface rocks is dominated by silica polymorph minerals (cristobalite, tridymite, opal, and quartz), and kaolinite, showing an intense silicification in most altered samples. In some samples, alteration minerals berlinite, alunite, opal, and wollastonite, are also present. Sulfur (80–28, 300 μg/g) and LOI (1.36–12.18%) contents were used to indicate the intensity of alteration for the LAGF rock samples. The changes were considered as significant when they exceeded the analytical errors. Two main types of chemical and isotopic effects were documented: (1) small loss of both SiO2 and alkalis represented by one pair of samples, which did not show significant changes in the most major elements, REEs, Nb and Ta negative anomalies, nor in the normalized multiple-element LILE/REE, LILE/HFSE, and REE/HFSE ratio parameters, but indicated significant increase in Al, Fe, Ba, Ga, and Pb; and (2) significant gain of SiO2 accompanied by loss of alkalis for the rest of the rock samples collected from a different site showed significant decrease of most major elements, all REE concentrations and normalized multiple-element ratio parameters as well as changes in numerous trace element concentrations. During this alteration, the size of the Nb and Ta negative anomalies became smaller as a result of the alteration of the second type. Both types of alteration showed significant changes in 87Sr/86Sr, but generally not in 143Nd/144Nd, probably related to the involvement of highly heterogeneous crust in terms of 87Sr/86Sr.