Geochemical and Mineralogical Investigation of Mine Tailings and Mine-Impacted Sediments at Long Lake near Sudbury, Ontario

Abstract

The former Long Lake Gold Mine (1908-1939) near Sudbury, Ontario, produced sulfide-bearing tailings that were discharged directly to shallow wetland areas without containment, and oxidation of sulfide minerals (mainly pyrite and arsenopyrite) has resulted in elevated aqueous arsenic (As) concentrations downstream in Luke Creek and Long Lake. Over time, erosion and transport of tailings-derived solids, and their deposition in the lake formed a sediment delta at the mouth of the creek. In the early 1970s, the Ontario government constructed a sand cover (~ 0.3 m thick) to stabilize the tailings surface and the sand cover now contains cemented crusts formed by precipitation of secondary minerals. The cemented sand layer has eroded at several locations and the transport of the eroded material by surface-water runoff has become a secondary source of pollution to the surrounding environment. The main objectives of this research were to characterize the nature and distribution of cemented crusts in the tailings and the sand cover, and to understand diagenetic transformations that affect tailings-derived oxidation products following burial in the sediment delta. Vertical cores were collected from the tailings and the sediment delta. They were dried anaerobically, and polished thin sections were prepared for examination by SEM-EDS, EPMA, and (S)TEM-EDS/EELS. Elevated concentrations of solid-phase As (1.41 wt.%), Fe (7.62 wt.%), and S (2.88 wt.%) in the sand layer indicate upward transport of sulfide-oxidation products from the underlying tailings. The mineralogical observations indicate moderate to high degrees of sulfide-mineral oxidation in the shallow (< 0.7 m depth) tailings, and secondary minerals in the tailings and the sand layer mainly consist of ferric-arsenates (principally scorodite) and K/Na-jarosite. The shallow porewater is acidic (pH 4 - 6) and contains elevated concentrations of As (up to 346 mg/L), Fe (up to 1844 mg/L), and SO₄ (up to 12,000 mg/L). The capillary rise of tailings porewater led to the precipitation of ferric-arsenates and K/Na-jarosite that cemented the sand particles and formed cemented crusts. The distribution of secondary-mineral precipitates is variable in the sand cover, leading to a distribution of cemented crusts that is discontinuous both vertically and horizontally. The lake sediment contains numerous fragments of cemented crusts in the mine-impacted section (maximum depth 1.9 m). Compared to the cemented crusts that occur in the tailings areas, these fragments display partial-dissolution features. The partial-dissolution of scorodite and K/Na-jarosite contribute to elevated aqueous concentrations of As, Fe, and SO₄ near the sediment-water interface, creating concentration gradients that can drive diffusive transport upward into the lake water and downward into the underlying natural sediment. In comparison to the crusts in the tailings, the sediment-hosted crusts have relatively low and variable As/Fe ratios, implying selective As-depletion. The depletion and mobilization of As leaves residual ferric-arsenates progressively enriched with Fe, and as a result, Fe-oxyhydroxides (nm to um size) form as the main diagenetic product in the sediment. They occur throughout the mine-impacted sediment, from the surface to depths where Fe- and As-sulfides form in response to sulfate reduction in suboxic-anoxic conditions. Fe-oxyhydroxides exhibit diverse morphologies, with some consisting of aggregates of lepidocrocite microcrystals arranged in spherulitic form with silica in the interstitial spaces. The lepidocrocite microcrystals contain a mixture of Fe(III) and Fe(II), with Fe(III)/ΣFe = approximately 0.8. The mineralogical forms of As-bearing phases that control As concentrations in the tailings and sediment porewater can have implications for design of remediation plans. Future remediation efforts may benefit from knowledge of the mineralogical forms of As, and understanding of the erosion and transport of tailings-derived solids and their diagenesis upon burial in the lake sediment.