Tiny particles building huge ore deposits - Particle-based crystallisation in banded iron formation-hosted iron ore deposits (Hamersley Province, Australia)

The world's major source of iron ore is hosted in Precambrian banded iron formations. These chemical (meta-) sedimentary rocks are composed of alternating laminae of iron oxide minerals and chert. Despite the economic significance of high-grade iron ore deposits, controversy persists after decades of research on how banded iron formations became upgraded to form iron ore. The fundamental requirement for iron ore formation is the removal of vast amounts of chert coupled with an increased concentration of iron oxide minerals. Here, we assess the fate of colloidal hematite inclusions encapsulated in chert after quartz dissolution and examine their role in the formation of hematite ore. We have analysed hematite ores from the Hamersley Province (Australia) using a combination of petrography, high-resolution electron microscopy and X-ray diffraction. These techniques reveal the presence of abundant nano and microscale hematite particles that we suggest form the building blocks of larger hematite crystals within the iron ore. Our textural observations indicate that hematite colloids that were previously encapsulated inside the microcrystalline quartz grains in chert layers are released during quartz dissolution, and subsequently reassemble in a self-similar fashion to from new microplaty hematite crystals via non-classical crystallisation pathways. Progressive growth and fusion leads to the transformation of hematite microplates into hematite bands, which resemble pre-existing iron oxide laminae of banded iron formations. In contrast to previous models, we observe the direct transformation of banded iron formations to microplaty hematite and have not found evidence that significant amounts of hematite formed through metamorphism of goethite or that intermediate carbonate minerals were involved during the upgrading of banded iron formations to pure hematite ore. Given the strong evidence for hypogene fluids found in many deposits, we have also assessed the role that such warm, highly saline fluids may have played during the evolution of iron ore. Using insights from crystal chemistry we conclude that fluid infiltration impacts many aspects of iron ore formation by controlling hematite colloid liberation and aggregation, and finally controlling the transformation of the colloids into macroscopic crystals of hematite via non-classical crystallisation mechanisms. Our study underlines the significance of hypogene fluids in the upgrading of banded iron formation to iron ore, however, we suggest that their influence was mainly passive as hematite was not precipitated directly from these fluids.