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The goal of this study was to produce an internally-consistent set of mineralogical data on near-surface samples collected from the Normandie, King Beaver and BC I tailings piles in the Thetford Mines area in Québec. The hope is that it might provide pertinent information that can help in the identification of realistic optimal strategies for any economically-viable, environmentally-responsible, and safe chrysotile tailings management solutions. The sampling strategy consisted in collecting near-surface residue material from the flat summit where the waste is considered to have had significant residence time and also at the base of the steep flank that consists of material remobilized mainly through water erosion.
The three tailings piles show chemical characteristics suggesting that the serpentinized ore from which chrysotile was mined remain compositionally very close to the harzburgite precursor, although evidences of contribution from a feldspar-rich contaminant, most likely fragments of silica-rich intrusive material crosscutting the peridotite, are found in the samples from Normandie. These intrusive fragments are also the source of actinolite observed in very small amounts and displaying prismatic to acicular habit.
Lizardite is clearly the major serpentine phase in all samples, and its proportion relative to chrysotile seems fairly constant on the summit as well as on the flank. The Cr content of serpentine is heterogeneous and can account for about 25% of the total Cr. In the case of Ni, a fairly gradual distribution with averages of 0.1 wt% for Normandie and King Beaver and 0.06 wt% for BC I are recorded. The significantly lower Ni average in serpentine from BC I can be explained by higher abundance of preserved Ni-bearing forsterite. The observed proportion of Ni carried by serpentine represents, at best, slightly less than half the total amount of Ni in the tailings.
Coarse Cr-rich spinel grains surrounded by magnetite rims are present in all samples. The cores show compositions comparable to those of spinel from the harzburgite host. These coarse spinel grains represents the only Cr-carrier phase of interest, carrying about 75% of the total Cr, and they are amenable to concentration through grinding and physical separation.
Awaruite occurs in two habits, either as coarse grains (>5 µm) or as a multitude of nanometric particles distributed throughout large serpentine grains. Reduction of NiO in solid solution in forsterite during the oceanic serpentinization event is likely the source of nanometric awaruite, whereas coarse awaruite possibly formed through desulfurization of primary sulfides. The contribution of coarse awaruite to the total Ni budget is estimated at about 10%, whereas nanometric awaruite could account for up to 50% or even more. Considering that serpentine and nanometric awaruite both host about half of the total Ni, a global leaching approach would seem to be the most efficient in the context of a potential interest in extracting Ni from the tailings piles. Acid leaching with HCl will effectively recovered most of the Ni from serpentine and awaruite, but involves the formation of vast amount of MgCl and significant acid consumption. Selectively leaching Ni from awaruite represents another alternative, but the optimal total recovery is evidently much less (≈ 50%) and, in some cases, potential Ni retention by serpentine in the residue represents a potential problem.
Textural evidence of very efficient leaching of brucite at the near surface of all tailings piles and the ability to reproduce these textures through leaching experiments using carbonated distilled water strongly suggest that brucite represents the main source of Mg to produce most of the hydromagnesite cementation observed. A control on carbonation by brucite has significant implication for predicting long-term cementation rates, as well as on the time needed for reworked material, in which brucite has been consumed, to produce a new crust.
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