Trace elements in fluid inclusions of sediment-hosted gold deposits indicate a magmatic-hydrothermal origin of the Carlin ore trend

Trace elements in native gold provide a “fingerprint” that tends to be unique to individual gold deposits. Fingerprinting can distinguish gold sources and potentially yield insights into geochemical processes operating during gold deposit formation. Native gold grains come from three historical gold ore deposits; Hollinger, McIntyre (quartz-vein ore), and Aunor near Timmins, Ontario, at the western end of the Porcupine gold camp and the south-western part of the Abitibi greenstone belt. Laser-ablation, inductively-coupled plasma mass spectrometry (LA ICP MS) trace element concentrations were determined on 20 to 25 µm wide, 300 µm long rastor trails in ~ 60 native gold grains. Analyses used Ag as an internal standard with Ag and Au determined by a scanning electron microscope with an energy dispersive spectrometer. The London Bullion Market AuRM2 reference material served as the external standard for 21 trace element analytes (Al, As, Bi, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Pd, Pt, Rh, Sb, Se, Si, Sn, Te, Ti, Zn; Se generally below detection in samples). Trace elements in native gold associate according to Goldschmidt’s classification of elements strongly suggesting that element behavior in native Au is not random. Such element behavior suggests that samples from each Timmins deposit formed under similar but slightly variable geochemical conditions. Chalcophile and siderophile elements provide the most compelling fingerprints of the three ore deposits and appear to be mostly in solid solution in Au. Lithophile elements are not very useful for distinguishing these deposits and element ABSTRACT CUT OFF BY SOFTWARE

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Fluid inclusion hardening: Nanoscale evidence from naturally deformed pyrite

10.5194/egusphere-egu21-894 ◽

2021 ◽

Author(s):

Anna Rogowitz ◽

Renelle Dubosq ◽

David Schneider ◽

Kevin Schweinar ◽

Baptiste Gault

Keyword(s):

Trace Elements ◽

Fluid Inclusions ◽

Electron Backscatter Diffraction ◽

Analytical Techniques ◽

Atom Probe ◽

Crystal Defects ◽

Atomic Scale ◽

Ductile Deformation ◽

Temperature Deformation ◽

Contrast Imaging

<p>The interaction of trace elements, fluids and crystal defects plays a vital role in a crystalline material&#8217;s response to an applied stress. For example, dislocations can be arrested by the strain field of immobile defects (i.e., particles or precipitates) or by the accumulation of mobile solutes in their cores, which can lead to strain hardening. The rheology of minerals is also strongly influenced by interactions with fluids, which are typically known to facilitate ductile deformation in geomaterials (i.e., hydrolytic weakening, dissolution creep). Investigation of these nanometer scale processes however, requires a correlative approach combining high-spatial resolution analytical techniques. In recent years, increasing developments in microscopy and microanalysis have allowed for the compositional measurements and spatial imaging of materials at the near-atomic scale. Herein, we have combined electron backscatter diffraction (EBSD) mapping, electron channeling contrast imaging (ECCI), scanning transmission electron microscopy (STEM) and atom probe tomography (APT) on a naturally deformed polycrystalline pyrite aggregate from the Abitibi Subprovince in Canada to investigate the role of fluid inclusions on mineral rheology. The combined EBSD and ECCI data reveal minor crystal misorientation and low-angle grain boundary development in the vicinity and at the tip of microfractures indicating a dominantly brittle regime with minor strain accommodation via crystal-plasticity where dislocations are mostly emitted by the propagating fracture. These interpretations are consistent with the peak temperature conditions of the sample estimated at 302 &#177; 27&#176;C, which falls within the lower range of the brittle to crystal-plastic behaviour of pyrite (260&#8211;450&#176;C). Nanoscale structural and chemical data reveal nanoscale fluid inclusions enriched in As, O, Na and K that are linked by As-enriched dislocations. Based on these results, we propose a model of fluid hardening whereby dislocations get pinned at fluid inclusions during crystal-plastic deformation, initiating pipe diffusion of trace elements from the fluid inclusions into dislocations that leads to their stabilization and local hardening. Although additional experiments are required on other mineral phases, our initial efforts advance the understanding of the interplay between nanostructures and impurities and its impact on the rheology of geomaterials during relatively low temperature deformation.</p><p>&#160;</p>

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