Trace metal nanoparticles in pyrite

Hydrothermal pyrite contains significant amounts of minor and trace elements including As, Pb, Sb, Bi, Cu, Co,Ni, Zn, Au, Ag, Se and Te, which can be incorporated into nanoparticles (NPs). NP-bearing pyrite is mostcommon in hydrothermal ore deposits that contain a wide range of trace elements, especially deposits thatformed at low temperatures. In this study, we have characterized the chemical composition and structure ofthese NPs and their host pyrite with high-resolution transmission electron microscopy (HRTEM), selectedarea electron diffraction (SAED), high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM), analytical electron microscopy (AEM), and electron microprobe analysis (EMPA). Pyritecontaining the NPs comes from two types of common low-temperature deposits, Carlin-type (Lone Tree,Screamer, Deep Star (Nevada, USA)), and epithermal (Pueblo Viejo (Dominican Republic) and Porgera (PapuaNew-Guinea)).EMPA analysesofthepyrite show maximum concentrations of As(11.2),Ni(3.04), Cu (2.99), Sb(2.24),Pb(0.99),Co (0.58), Se (0.2), Au (0.19), Hg (0.19), Ag (0.16), Zn (0.04), and Te (0.04) (in wt.%). Three types of pyrite havebeen investigated:“pure”or“barren”pyrite, Cu-rich pyrite and As-rich pyrite. Arsenic in pyrite from Carlin-typedeposits and the Porgera epithermal deposit is negatively correlated with S, whereas some (colloform) pyritefrom Pueblo Viejo shows a negative correlation between As+Cu and Fe. HRTEM observations and SAED patternsconfirm that almost all NPs are crystalline and that their size varies from 5 to 100 nm (except for NPs of galena,which have diameters of up to 500 nm). NPs can be divided into three groups on the basis of their chemicalcomposition: (i) native metals: Au, Ag, Ag–Au (electrum); (ii) sulfides and sulfosalts: PbS (galena), HgS(cinnabar), Pb–Sb–S, Ag–Pb–S, Pb–Ag–Sb–S, Pb–Sb–Bi–Ag–Te–S, Pb–Te–Sb–Au–Ag–Bi–S, Cu–Fe–SNPs,andAu–Ag–As–Ni–S; and (iii) Fe-bearing NPs: Fe–As–Ag–Ni–S, Fe–As–Sb–Pb–Ni–Au–S, all of which are in a matrix ofdistorted and polycrystalline pyrite. TEM-EDX spectra collected from the NPs and pyrite matrix documentpreferentialpartitioningoftracemetalsincludingPb,Bi,Sb,Au,Ag,Ni,Te,andAsintotheNPs.TheNPsformedduetoexsolutionfromthepyritematrix,mostcommonlyforNPslessthan10 nminsize,anddirectprecipitationfromthe hydrothermalfluid and deposition into the growing pyrite, most commonly for thoseN20 nm in size. NPscontaining numerous heavy metals are likely to be found in pyrite and/or other sulfides in various hydrothermal,diagenetic and groundwater systems dominated by reducing conditions