Characterization of micropyrite populations in low-grade metamorphic slate: A study using high-resolution X-ray tomography

Abstract

Measuring the abundance and size distribution of framboidal and euhedral microscopic crystals of pyrite has proven to be an effective method to determine the paleoenvironmental conditions in sedimentary rocks bearing these minerals. However, micropyrites can be found in other geological environments as well, such as low-grade metamorphic and hydrothermal. In these cases, their sizes and distributions are expected to be different from those deposited in a sedimentary environment, due to the different genetic processes involved. This paper analyzes the size and distribution of the micropyrite population in a low-degree metamorphic slate, originally formed under anoxic to low-oxygen conditions. The geological record of this rock suggests that micropyrite formation took place in different stages, resulting in different population distributions.Traditionally, SEM has been used to characterize micropyrite populations. However, for this study, a new technique in this field was applied, namely, high-resolution X-ray tomography (micro-CT), which allows for 3D visualization and analysis of the stone's structure, based on the differences in the X-ray attenuation coefficient of the component minerals. As complementary technique, transmitted and reflected optical microscopy was used. Size and population distributions of the micropyrites were measured using SEM and micro-CT and then statistically analyzed. SEM was able to determine the morphology and size of micropyrites in 2D, while micro-CT provided statistically relevant information about the size histogram of the micropyrites. The data obtained indicate the existence of three overlapping populations of micropyrites. Two of them have small sizes and a narrow range of distribution, which can be attributed to sedimentary processes, and the other have larger sizes and a broader range of distribution, attributed to the effect of metamorphic processes. In summary, micro-CT coupled with SEM and accurate statistical analysis proved capable of differentiating between overlapping micropyrite populations in a sample of low-grade metamorphic slate.