Research ArticleGEOCHEMISTRY

Coupled partitioning of Au and As into pyrite controls formation of giant Au deposits

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Science Advances  01 May 2019:
Vol. 5, no. 5, eaav5891
DOI: 10.1126/sciadv.aav5891
  • Fig. 1 Pyrite formed by experimental replacement of siderite (FeCO3).

    In experiments (A) Sd2Py26 and (B) Sd2Py52. Euhedral to subhedral clusters of As- and Au-bearing pyrite (FeS2) are formed when fluid (rich in H2S)–mediated dissolution-reprecipitation of siderite occurs. Minor amounts of pyrrhotite (Po) needles formed interstitially between pyrite (Py) clusters, either in equilibrium with pyrite or as a late phase during cooling of the experiments.

  • Fig. 2 Compositions of experimental fluid and coexisting hydrothermal pyrite.

    Experimental products are coded by the same color. The black line is the empirically defined upper limit of Au+1 in pyrite as a function of As concentration of the pyrite (10, 11), pyrite falling above the limit contains Au as (nano)nuggets, and samples that plot below characteristically have Au dissolved as Au+1 in the pyrite structure (12). Natural pyrite compositions for CTGD from Nevada (•) (11) and Shuyindong (+) (22) are plotted together with limited published data for fluid inclusions (*) (9). Experimental fluids: Without As (gray), low As (bluish colors), and high As (reddish colors) produce pyrite that has As and Au concentrations that are orders of magnitude higher than the fluids and agrees well with natural pyrite.

  • Fig. 3 Gold partition coefficients (D values) between pyrite and fluid as a function of the As concentration in pyrite.

    Inset: Concentration of Au in the newly formed pyrite compared to the initial Au composition of the experimental fluid; color coding identical to Fig. 2. Circles represent Dopt values, error bars for D values represent Dmin and Dmax (see Materials and Methods), and error bars for As is the SD of LA-ICPMS measurements. Filled symbols represent pyrite composition falling below the Au+1 solubility limit for arsenian pyrite (11). Half-filled symbols represent pyrite above the limit, but LA-ICPMS analysis does not show substantial formation of Au0 nuggets. Empty symbols represent pyrite falls above limit, and LA-ICPMS analysis indicates Au0 nuggets (see fig. S1)

  • Fig. 4 Modeled evolution of H2S and Au in hydrothermal fluids and pyrite for CTGD systems.

    Reaction of hydrothermal fluids with reactive Fe at concentrations common in CTGD leading to Au-rich pyrite. Numerical model assumes complete consumption of H2S from the fluid to form pyrite. Au concentration depends either on partitioning (at low degrees of pyritization) or supersaturation due to sulfidation (at high degrees of pyritization). The gold solubility limit will decrease during pyritization as Au is consumed because of partitioning. The onset of sulfidation causes a marked increase in modeled Au concentration in pyrite. Under these conditions, typical compositions of CTGD pyrite are only formed at the very end of pyritization (gray field). When partitioning is the major ore-forming process, typical CTGD compositions are produced during the course of pyritization. Ore grades are calculated from amounts of fluid and rock and initial Au concentration (see the Supplementary Materials).

  • Table 1 Variables and conditions used in thermodynamic and mass balance models.
    VariableValueLiterature source
    H2S0.01–0.1 m(5)
    Temperature200°C(5)
    NaCl5 wt %(5) and (14)
    CO22 mol %(5)
    Reactive Fe in wall rock0.5–2 wt %(13); given as siderite in thermodydnamic modeling
    pH~5.6(5) and (14); buffered by calcite and siderite dissolution
    LogfO2~−45(5); buffered by calcite and siderite dissolution
    Au in fluid0.5–2 μg/g(9) and this study
    DAu100–6000This study

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaav5891/DC1

    Supplementary Text

    Fig. S1. Au nuggets formation on the outside of pyrite.

    Fig. S2. Time resolved LA-ICPMS spectra.

    Fig. S3. Dependency of the modeled Au evolution on D values and initial Au concentration.

    Fig. S4. Dependency of the modeled Au evolution depending on different Au solubilities calculated for different fO2 and pH and constant boundary conditions.

    Table S1. Experimental conditions.

    Table S2. As and Au concentrations (in μg/g) of experimental pyrite measured by LA-ICPMS and calculated D values.

    Table S3. Sources of thermodynamic data for species and minerals used in this study.

    References (4446)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Au nuggets formation on the outside of pyrite.
    • Fig. S2. Time resolved LA-ICPMS spectra.
    • Fig. S3. Dependency of the modeled Au evolution on D values and initial Au concentration.
    • Fig. S4. Dependency of the modeled Au evolution depending on different Au solubilities calculated for different fO2 and pH and constant boundary conditions.
    • Table S1. Experimental conditions.
    • Table S2. As and Au concentrations (in μg/g) of experimental pyrite measured by LA-ICPMS and calculated D values.
    • Table S3. Sources of thermodynamic data for species and minerals used in this study.
    • References (4446)

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