Fluid inclusions in amethyst quartz of different geological environments from Brazil

Abstract

Fluid-inclusion studies were conducted on amethyst quartz from three different geological environments: basalt cavities; hydrothermal veins; and granitic pegmatites of Eastern Brazil, to understand the conditions of amethyst crystallisation in each of these environments. In samples from basalt cavities, fluid inclusions are exclusively one-phase aqueous types suggesting a low-temperature formation environment. Crystals from the two other environments show that fluid inclusions can be either one-phase aqueous, two-phase aqueous, three-phase aqueous carbonic, or three-phase aqueous with the presence of precipitated solid halite. The carbonic composition of the system H2O-CO2-NaCl was confirmed by Raman spectroscopic analysis and suggests a metamorphic or magmatic fluid source. Fluid inclusions trapped in samples from hydrothermal veins reveal at least two different fluid generations based on homogenisation temperatures. The first generation has minimum trapping temperatures between 249 degrees C and 391 degrees C. The second generation of lower temperature fluids has minimum trapping temperatures varying from 82 degrees C to 203 degrees C. Fluid inclusions of this group record eutectic temperatures that indicate the presence of Ca and Fe cations in addition to Na. Fluid inclusions trapped in amethyst from a pegmatite body have moderate salinity values between 15 and 25 eq. wt.% NaCl, thus reflecting the elevated salt content in pegmatite-forming fluids. In this sample, the first fluid generation is represented by aqueous fluid inclusions with minimum trapping temperatures ranging from 268 degrees C to 375 degrees C. The estimated eutectic temperatures, generally below -50 degrees C, indicate the presence of Ca cations in addition to Na. Minimum trapping temperatures correspond to temperatures of late-to-post-pegmatitic hydrothermal stages. The second generation records minimum trapping temperatures between 125 degrees C and 247 degrees C. Amethyst from both hydrothermal veins and pegmatite environments contain solid inclusions of hematite, an indication that the mineralising fluid was Fe rich and thus, possibly magmatic in origin.