COPPER AND ZINC IN SILICATE AND OXIDE MINERALS IN IGNEOUS ROCKS FROM THE BINGHAM PARK CITY BELT, UTAH: SYNCHROTRON X-RAY-FLUORESCENCE DATA

Abstract

Synchrotron X-ray-fluorescence (SXRF) analyses for Cu, Zn and Fe were carried out on magmatic minerals from intrusive and volcanic rocks in the Bingham – Park City Belt, Utah. The main goals of the study were to determine average Cu and Zn contents and zoning patterns in magmatic minerals in order to shed light on the chemical behavior of Cu and Zn during crystallization of the magmas. The combination of low limits of detection and small analytical footprint makes SXRF the preferred method for tests of this type. Results of the study show that the Zn is homogeneously distributed through most ferromagnesian minerals, commonly following Fe, for which it probably substitutes. The distribution of Cu is more complex, and includes Cu-rich patches of as yet unknown mineralogy that are widespread in all minerals. Median Cu and Zn contents for magmatic minerals, estimated excluding the Cu-rich patches, are lower than and higher than, respectively, those of volcanic rocks in the area. Copper thus must have behaved incompatibly, whereas Zn behaved compatibly during magmatic crystallization. Decreasing Cu concentrations from core to rim were observed in biotite from the volcanic rocks, but Cu and Zn zoning is absent in ferromagnesian minerals in the intrusive rocks. Altered rims on biotite in the intrusive rocks are enriched in Cu and locally in Zn thought to reflect postcrystallization migration of a late magmatic vapor phase.