From BIF to Iron Ore - A journey from nanocrystals to huge iron ore deposits of the Hamersley Province, Australia
Creators: name: Egglseder, Mathias
Publisher: name: Monash University
Year: 2017
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences

description: Banded iron formations (BIFs) are extraordinary rocks because they provide important information about the Precambrian atmosphere and hydrosphere, the global rise of oxygen, and they host the world’s major iron ore deposits. The unique texture and petrology of BIF, which mainly comprise alternating layers of iron oxides and chert remain poorly understood. Although the mineral assemblage is of diagenetic or metamorphic origin, BIF textures are interpreted as sedimentary structures because no comprehensive study has addressed the processes that control their compaction, diagenesis and later orogenic overprints. Iron ore genesis models face similar challenges to explain the replacement and removal of chert bands by poorly understood fluids, which dissolve around 50% of the rock volume to form giant residual, structurally-controlled iron ore deposits. <br> The aim of this thesis is to determine the coupled nm- to km-scale processes that controlled the deformation of BIF of the Hamersley Province (Western Australia), and to evaluate the significance of these processes for the evolution of BIF textures and iron ore genesis. A multidisciplinary approach was adopted to characterise BIF and iron ore mineralisation that combines classical geological disciplines (e.g., field geology, petrography, structural geology, economic geology) with state-of-the-art microscopy (SEM: scanning electron microscopy; 3D XRT: 3D X-ray tomography) and 3D geological modelling. <br> Deformation and its effects played a fundamental role in the evolution of BIF and in the formation of iron ore deposits. Chert, composed of microcrystalline quartz, contains abundant iron oxide nanocrystals, which explains the red, brown or yellow colours of chert bands. Deformation-induced dissolution–precipitation creep led to the dissolution of quartz during burial and subsequent reprecipitation as secondary quartz layers with strong crystallographic preferred orientations. During this process the iron oxide nanoparticles encapsulated within chert accumulate residually and finally transform to iron oxide bands. Instead of recrystallising via a dissolution/precipitation stage, the iron nanoparticles mainly build individual iron oxide crystals through the oriented attachment of nanoparticles. This solid-state mechanism is known as non-classical crystallisation and is well-known in material science, environmental science, crystal chemistry and nanotechnology but it has not yet been applied to BIF. Later large-scale orogenic events led to a reactivation of dissolution–precipitation creep, leading to the dissolution of quartz, liberation of nanoparticles and proximal deposition and transformation to secondary iron oxides. Instead of replacing chert bands, the chemistry of the associated fluids control the amount of quartz dissolution, nanoparticle transport and their transformation to form either hematite or goethite ore deposits. <br> This multiscale and multidisciplinary study provides new insights into all aspects of BIF and iron ore research. It shows that the iron oxide layers in BIF formed from nano-sized iron oxide particles, liberated from dissolved chert layers during deformation. The same physico-chemical processes were later reactivated and led to the formation of giant iron ore deposits.<br><br><b>Additional material(s) submitted with thesis</b><br>