Copper-arsenic Nanoparticles in hematite: fingerprinting fluid-mineral interaction

摘要: Metal nanoparticles (NP) in minerals are an emerging field of research. Development advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, technique provides a link between micron-scale textures chemical patterns if sample is extracted situ from location petrogenetic interest. Here we use HAADF STEM energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned weave-twinned hematite Olympic Dam deposit, South Australia. We aim determine role solid-solution versus presence discrete included NPs observed zoning understand Cu-As-enrichment processes. Relative grain surface, Cu-As bands extend depth (sub)vertical trails opposite orientation, with Si-bearing NP inclusions one side coarser cavities (up hundreds nm) other. The latter host Cu NPs, contain mappable K, Cl, C, display internal voids rounded morphologies. Aside STEM-EDX mapping, agglomeration native copper was also assessed by high-resolution imaging. Collectively, characteristics, corroborated geometrical outlines negative crystal shapes cavities, infer that these opened fluid attached inclusion walls. Hematite along features distinct nanoscale domains lattice defects (twins, 2-fold superstructuring) relative outside trails, indicating this nanoprecipitate formed during replacement processes, i.e., coupled dissolution reprecipitation reactions (CDRR). Transient porosity intrinsically developed CDRR can trap fluids metals. Needle-shaped platelet (sub)horizontal which Si, Al K traceable margins. same signature depicted nm-wide planes crosscutting 60° offsetting (012)-twins hematite. High-resolution shows linear planar defects, kink deformation twin planes, misorientation dilation around duplexes Si-Al-K-planes. Such evidence strain, induced percolation channels become wider sericite platelets, well Cl-K-bearing inclusions, comparable those hematite, although without metal NPs. Cu-As-bands mapped correspond interaction changed composition alkali-silicic Cl- metal-bearing brines, rates evolved slow infiltration erratic inflow controlled fault-valve mechanism pumping. This explains hosted either Si-Al-K-planes (fluid supersaturation), or (phase separation depressurisation) common signatures variable degrees fluid-mineral interaction. invoked typical hydrolytic alteration pumping feasible via fault (re)activation. Using approach, show be fingerprinted (atomic) scale element exchange occurs.