Surficial weathering of iron sulfide mine tailings under semi-arid climate

Abstract

Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering under semi-arid climate at an EPA Superfund Site in semi-arid central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2 m of tailings. Despite nearly invariant Fe and S concentration with depth (130-140 and 100-120 g kg^-1, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5 m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in the lowest pH samples, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.

Keywords: QXRD; XAS; iron XANES; semi-arid mine tailings; sulfur XANES.

Figure 1

Map and cross-section of Iron King Superfund site (Dewy-Humbolt, AZ). A) Aerial view of tailings pile (from Google Earth^™), B) pit to 55 cm, C) cores to 185 cm, D) site map and E) cross section of waste pile containing ca. 3.9 Mm³ of tailings.

Figure 2

(a) Chemical depletion/enrichment plot showing Ti normalized S (open) and Fe (closed) mass concentrations in the weathering profile relative to parent material taken as 180 cm sample (see Eq. 1), the dashed line and shaded regions represent the redox boundary. (b) Sequential extraction results for iron as a function of depth with error bars representing standard deviation of triplicate measurements (see online version for color coding).

Figure 3

X-ray diffraction data and fits. Synchrotron transmission XRD patterns (black lines) converted to Cu Kα scale of IKMHSS pit samples at depths A–G and SSE samples, with corresponding Rietveld models (gray lines). Lower diffractograms represent the simulated patterns of selected phases used in the Rietveld models. Data are normalized to quartz. All jarosite group minerals (e.g., plumbojarosite, hydronium jarosite, jarosite) are modeled here using jarosite. Quantitative results are displayed in Table 4.

Figure 4

Sulfur XANES. The S oxidation state was directly probed with S K-edge XANES (fits shown as dashed lines). Fits (range 2465–2515 eV) are tabulated in Table 5. Arrows indicate subtle sulfate features that differentiate gypsum and (Pb)jarosite.

Figure 5

Iron XANES. A) IKMHSS normalized XANES from the shallow pit (0–55 cm) and deeper core (to 180cm) from mine tailings (fits calculated from first-derivative fits shown in dashed lines), B) IKMHSS first derivative sample spectra (fits shown in dashed lines), and C) Fe first-derivative XANES of reference minerals used in XANES linear combination fits. The Fe XANES show a gradual transition from ferric oxide to ferrous sulfide phases from the surface to deep tailings.

Figure 6

Comparison of apparent pyrite concentration from linear combination fits to S XANES and Fe XANES. Moles of pyrite per kg of tailings were calculated from the pyrite fractional fit from Fe and S XANES spectra and the moles of Fe and S in the tailings respectively. The solid line is the linear correlation (r²= 0.944), the dashed line is the Fe:2S line, accounting for stoichiometry of FeS₂. Error bars are from the calculated error estimates from the S and Fe fits and not from replicate spectra. The pH is shown in circles for each associated sample.

Figure 7

Activity-activity diagrams, shown with the energetically favorable hematite and goethite suppressed to illustrate the metastable Fe and S phases. The Eh-pH diagram shows Pb-jarosite, schwertmannite and ferrihydrite as the meta-stable iron phases in oxic-environments. The model was constrained by XANES and XRD, and elemental activities are given in inset.