Abstract

delta 18 O and/or delta D values were measured for quartz (11 samples), plagioclase (3), K-feldspar (3), biotite (4), hornblende (1), sericite (3), chlorite (1), kaolinite (2), and pyrophyllite (1) from the porphyry copper deposit at El Salvador, Chile. The samples analyzed were chosen to represent the hypogene evolution from early K-silicate alteration and quartz veining to late pyritic veining and sericitic alteration to very late advanced argillic alteration, as well as supergene kaolinization.The isotopic compositions of the biotites are identical to those from most other porphyry copper deposits, and the sericite compositions are similar to those from Santa Rita. The requirements for equilibrium isotopic geothermometry are met only for unmineralized "L" Porphyry and K-silicate alteration assemblages, yielding minimum temperatures of about 650 degrees and 525 degrees C, respectively. Temperatures estimated from various other geologic evidence are used to calculate the equilibrium isotopic compositions of hydrothermal solutions. These solutions show a distinct trend of 18 O depletion and deuterium enrichment from early to late stages and from high to low temperatures. The delta 18 O and delta D values of fluids responsible for Early K-silicate alteration assemblages are similar in isotopic composition to magmatic waters but are enriched in deuterium relative to the later unmineralized "L" Porphyry even though it is petrologically similar to the earlier mineralized porphyries. Later fluids are dominantly meteoric waters enriched in 18 O by reaction with the host rock. Late and presumably shallow advanced argillic assemblages developed from acid geothermal waters of meteoric origin that were enriched in deuterium and 18 O by near-surface evaporation processes. There was possibly some mixing between these fluids and the deeper meteoric-hydrothermal fluids. The similar isotopic compositions of both meteoric and magmatic waters and the variety of possible processes modifying their isotopic compositions prevent a quantitative analysis of the relative proportions of each at the different stages in the evolution of the hydrothermal fluids.

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