ABSTRACT

We present new major and trace elements and Sr, Nd, Pb, and O isotopic data on basaltic andesite to rhyolitic volcanic rocks from surface outcrops as well as drill wells from the Los Azufres geothermal field (LAGF). With a total installed capacity for electricity production of about 100 MW, LAGF is the second most important geothermal field in Mexico. Hydrothermal alteration has affected most subsurface (drill well) rocks, ranging from partial to complete alteration. The alteration mineralogy with increasing depth (and temperature) is: argillitization/silicification, zeolite/calcite formation, sericitization/chloritization, chloritization/epidotization. The Sr, Nd and Pb isotopic ratios from the LAGF show the following ranges: for surface rocks (⁸⁷Sr/⁸⁶Sr)_i ≈ 0.70344-0.7048; (¹⁴³Nd/¹⁴⁴Nd)_i ≈ 0.512745-0.512908; ²⁰⁶Pb/²⁰⁴Pb ≈ 18.695-18.725; ²⁰⁷Pb/²⁰⁴Pb ≈ 15.585-15.612; ²⁰⁸Pb/²⁰⁴Pb ≈ 38.406-38.522; whereas for drill well rocks (⁸⁷Sr/⁸⁶Sr)_i ≈ 0.70374-0.70462; (¹⁴³Nd/¹⁴⁴Nd)_i ≈ 0.512675-0.512862; ²⁰⁶Pb/²⁰⁴Pb ≈ 18.643-18.770; ²⁰⁷Pb/²⁰⁴Pb ≈ 15.590-15.621; ²⁰⁸Pb/²⁰⁴Pb ≈ 38.390-38.595. An objective statistical methodology, based on F-ratio and Student-t tests, used for the comparison of chemistry of surface fresh rocks (from the LAGF and the surrounding area), to that of shallow (<1000 m depth) drill well altered rocks reveals that hydrothermal alteration causes a loss of total alkalis in most rock types. High-field strength elements such as Zr, TiO₂, and P₂O₅, generally considered as immobile elements, also show significant hydrothermal alteration-related changes. Finally, Pb isotopic ratios of drill well altered rocks significantly decrease and ¹⁴³Nd/¹⁴⁴Nd significantly increase with increasing SiO₂. Such a trend of decreasing Pb isotopic ratios and increasing ¹⁴³Nd/¹⁴⁴Nd is also observed for hydrothermally-altered rhyolites as compared to fresh rhyolites.