Sulfidation and Oxidation States of Calc-Alkaline Granitoids

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Essene EAR- 0230098 This work will evaluate magmatic sulfidation and oxidation states in calc-alkaline granitic rocks. These measurements will be carried out on granitoid suites from the Bingham-Park City belt of Utah, the central Chile porphyry copper belt, and the Providencia-Concepcion del Oro area in the central Sierra Madre Occidental of Mexico. Calculation of oxygen fugacity ranges over five orders of magnitude and sulfur fugacity over three orders of magnitude in different calc-alkaline granitoids. Mossbauer determination of Fe3+/Fe2+ and measurement of H2O in biotites will constrain redox reactions for biotite and provide another way to evaluate the redox state of these granitoids. In addition, systematic determinations of Ce4+/Ce3+ will be undertaken on zircons from the same suites of calc-alkaline granitoids. It is anticipated that these results will show that the granitoids possessed their unique redox signature throughout their crystallization history. The oxidation state of the magmas is most likely inherited from source regions rather than being imposed by late stage crystallization, wall-rock contamination, hydrothermal alteration or degassing history. Results of this study are expected to shed light on the source regions and evolution of calc-alkaline granitoids. Broader implications of this study relate to (1) impacts on climate by sulfur-bearing volcanic eruptions that are related to these granitoids, and (2) formation of porphyry copper ore bodies during their crystallization. The highly oxidized granitoids have sulfur mainly as sulfate, which is released in major volcanic eruptions to form sulfuric acid. These releases had a measurable impact on earth's climate during major eruptions of Mt. Pinutubo in the Philippines and El Chichon in Mexico. Porphyry copper ore deposits in the rock record are strongly correlated with emplacement of oxidized granitoids. Understanding this relationship is important for location of copper ores, most of which occur in these ore deposits.