Microstructural Control on Trace Element Distribution and Au Concentration in Pyrite Nodules
Riccardo Graziani et al.
Geochemistry
DOI: 10.1016/j.chemer.2024.126174
This recent publication in Geochemistry, led by researchers from Natural Resources Canada in collaboration with our icpTOF team, utilized the icpTOF 2R instrument to investigate gold-rich (Au) pyrite nodule from the Timmins-Porcupine Au camp in Ontario, Canada. Pyrite (FeSâ‚‚) nodules, commonly found in shales, siltstones, and ore deposits, are studied extensively to gain insights into ore system formation, as they often contain significant amounts of trace elements like Au, either bound within the crystal lattice or as microscopic inclusions.
The research employed advanced techniques such as Electron Backscatter Diffraction (EBSD) and Laser Ablation Inductively Coupled Plasma Time-of-Flight Mass Spectrometry (LA-ICP-TOFMS) using our icpTOF technology to analyze the nodule’s internal microstructure and trace element distribution, aiming to understand the factors influencing Au uptake. The study identified four distinct growth stages within the pyrite nodule, each characterized by variations in grain size, crystallographic orientation, crystal form, and trace element concentration. Specifically, Zones 1 and 2 exhibited non-oriented growth patterns, while Zones 3 and 4 displayed radial crystal growth with increasing grain size and a pronounced crystallographic preferred orientation (CPO).
LA-ICP-TOFMS results revealed that high concentrations of Au, arsenic (As), silver (Ag), and copper (Cu) were predominantly associated with areas containing dense subgrain boundaries, particularly within zones formed through octahedral facet growth. This suggests that trace element incorporation in pyrite is intrinsically linked to the crystal growth process and the development of specific microstructures, rather than resulting from secondary enrichment processes. The variation in crystal forms across different zones indicates fluctuating sulfur and iron concentrations in the source fluids during nodule formation.
These findings underscore the significant role of crystallographic controls in the distribution of trace elements within pyrite and have important implications for understanding the genesis of orogenic Au deposits. Enhanced knowledge of these processes is vital for assessing mineral potential and developing more effective exploration strategies for Au and other precious metals in metamorphic rock environments. Additionally, the study highlights the capability of the icpTOF 2R in providing detailed geochemical insights that contribute to the advancement of mineral exploration and economic geology.
