The use of microtextural and arsenic markers in the study of pyrite framboids and microcrystals

Abstract

We characterize the morphology and arsenic contents of pyrite framboids versus pyrite microcrystals in the Upper Cambrian pyrite stratiform deposit of the Wiśniówka area (south-central Poland). The framboids form isolated spherical or ellipsoidal aggregates varying from 4 to 70 µm across or irregular agglomerations (polyframboidal aggregations) attaining 150 µm in size. Each of them is made up of loosely- or closely-packed, equidimensional, equimorphic, locally multifaceted nano- and microcrystallites (nano- and microglobules) ranging mostly from tens of nanometres to 1.5 µm in diameter. Some framboids show a well-preserved pyritized spider-web-like matrix (organic biofilm) with embedded pyrite crystallites. Unlike pyrite microcrystals, framboids lack mineral inclusions and do not form paragenetic associations with hydrothermal index minerals. Some corroded framboids are completely overgrown by pyrite microcrystals averaging 10–15 µm in diameter. In contrast to pyrite microcrystals enriched in As (0.06–6.33 wt.%), framboid occurrences are distinctly depleted in this metalloid which varies from <0.015 to 0.43 wt.% in recrystallized forms of primary framboids. In pyrite framboids cobalt usually predominates over nickel (with a Co/Ni median ratio of ~3) as opposed to pyrite microcrystals in which cobalt is barely traceable. This study indirectly shows that framboids represent products of microbial activity presumably as low-temperature bacterial sulphate reduction. Their formation may have preceded high-temperature multi-stage influxes of metal(loid)- and H2S-rich hydrothermal fluids into the depositional basin giving rise to crystallization of pyrite microcrystals and their aggregations that vary from tens to hundreds of micrometres in diameter. This pyrite type prevails over framboids in the upper parts of the Podwiśniówka-Wiśniówka Duża profile. The morphological, microtextural and geochemical relationships between pyrite framboids and pyrite microcrystals suggest two different pathways of pyrite formation.