Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

Abstract

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

Trace-element remobilisation from W–Sn–U–Pb zoned hematite: Nanoscale insights into a mineral geochronometer behaviour during interaction with fluids

“…As shown by Verdugo-Ihl et al . (2019), the reprecipitated hematite records two-fold superstructuring close to the inclusion trails.…”

“…Each of these displays complex composition and includes non-formula elements introduced by the fluid. The inclusions along the trails are comparable to those interpreted as fluid inclusions in Cu–As-zoned hematite (Verdugo-Ihl et al ., 2019), and represent a further example of how transient porosity, which is intrinsic to CDRR mechanisms, can trap metals and fluid produced during the reaction (Putnis et al ., 2005). As shown by Verdugo-Ihl et al .…”

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

“…Changes in mineral signatures, phase associations, and zoning patterns are interpreted in terms of partitioning and cooling paths in magmatic systems [24,28]. The presence of critical metals, invisible gold in sulphides, and fluid-rock interactions in magmatic-hydrothermal systems are addressed for single deposits or ore provinces [23,25,27,[29][30][31][32]. The mapping of trace elements in ore minerals using nanoSIMS is revolutionary in terms of its detection limits and spatial resolution, but also permits the visualization of transient radionuclides at ultra-trace concentrations [33].…”

“…Verdugo-Ihl et al [31] use HAADF STEM imaging and STEM EDS mapping and spot analysis on foils prepared in-situ by focused ion beam methods to characterize atypical Cu-As-zoned and weave-twinned hematite from Olympic Dam. The study reveals Cu-and Cu-As-nanoparticles, also containing mappable K, Cl, and C. The presence of internal voids with rounded morphologies, clearly revealed by high-resolution imaging, allows the nanoparticles to be interpreted as attachments to the walls of fluid inclusions, the products of replacement processes driven by coupled dissolution and reprecipitation reactions.…”

Nanoscale Study of Titanomagnetite from the Panzhihua Layered Intrusion, Southwest China: Multistage Exsolutions Record Ore Formation

“…Considering the fine and ultrafine nature of such exsolutions, as well as the various crystal structural modifications invoked in the debate about their genesis, nanoscale studies are most suitable for solving the nature of the species present in such assemblages. Advanced microscopy, and particularly Z-contrast imaging by scanning transmission electron microscopy (STEM) on samples obtained by focused ion beam (FIB) from a site of petrogenetic interest previously assessed by complementary techniques [19,20], provides unparalleled insights into understanding mineralization processes down to the nanoscale [18,[21][22][23][24][25]. Important for the research reported here are the discovery of Si-Fe-nanoprecipitates or nanoparticles in "silician magnetite" from different deposit types [26,27], imaging of nanoscale-intergrowths between different olivine derivative minerals (laihunite series [28] that could be present in magmatic iron ores), and the definition of nanominerals, such as luogufengite, a new Al-bearing Fe 2 O 3 polymorph, ε-Fe 2 O 3 [29].…”