Stable isotopes and amphibole chemistry on hydrothermally altered granitoids in the North Chilean Precordillera: a limited role for meteoric water?

Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tert iary plutonic rocks contain important ore deposits and frequently display c ompositional and textural evidence of hydrothermal alteration even in barre n rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compo sitions are not significantly affected, compositional variations in amphibo les suggest two types of hydrothermal alteration. Hornblende with actinolit ic patches and rims and tight compositional trends from hornblende to Mg-ri ch actinolite indicate increasing oxygen fugacity from magmatic to hydrothe rmal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distributio n and variable Al content are interpreted as reflecting subsolidus hydratio n reactions at low temperatures. The FD values of hydrous silicates vary fr om -63 to -105 parts per thousand. Most delta(18)O values of whole rocks ar e in the range of 5.7 to 7.7%, and are considered normal for igneous rocks in the Andes. These delta(18)O values also coincide well with the oxygen is otope composition of geochemically similar recent volcanics from the Centra l Andean Volcanic Zone (delta(18)O=7.0-7.4 parts per thousand,). Only one s ample in this study (delta(18)O=3.0 parts per thousand,) appears to be depl eted by isotope exchange with light meteoric water at high temperatures. Th e formation of secondary minerals in all other intrusions is mainly the pro duct of deuteric alteration. This also holds true for the sample from El Ab ra, the only pluton associated with mineralization. This indicates the domi nant role of a magmatic rather than a meteoric fluid in the alteration of t he Cretaceous and Tertiary granitoids in northern Chile.