Geochemical Journal, Vol. 39, pp. 141 to 163, 2005
Hydrothermal alteration effects in geochemistry and Sr, Nd, Pb, and O isotopes of magmas from the Los Azufres geothermal field (Mexico): A statistical approach SURENDRA P. VERMA,1* IGNACIO S. TORRES-A LVARADO,1 MUHARREM SATIR2 and P ATRICK F. DOBSON3 1
Centro de Investigación en Energía, UNAM, Priv. Xochicalco s/no., Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, Mexico 2 Institut für Mineralogie, Petrologie und Geochemie, Universität Tübingen, Wilhelm Str. 56, 72074 Tübingen, Germany 3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. (Received November 17, 2003; Accepted August 19, 2004) 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 ª 0.70344–0.7048; (143Nd/144Nd)i ª 0.512745–0.512908; 206Pb/204Pb ª 18.695– 18.725; 207Pb/204Pb ª 15.585–15.612; 208Pb/204Pb ª 38.406–38.522; whereas for drill well rocks (87Sr/ 86Sr)i ª 0.70374– 0.70462; (143Nd/144Nd)i ª 0.512675–0.512862; 206Pb/204Pb ª 18.643-18.770; 207Pb/ 204Pb ª 15.590–15.621; 208Pb/204Pb ª 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 (
