Contaminated soils and sediments associated with Zn ore metallurgy near the São Francisco River, Minas Gerais (Brazil)

Abstract

Draining through industrial areas of the Minas Gerais mining state (Brazil), some tributaries of the São Francisco River constitute a potential environmental hazard for this great river and threaten the quality of the regional soils for agriculture and other activities. Extensive geochemistry and mineralogy of sediments, soils and alluvial plains from six selected areas within the Consciência drainage basin close to an important Zn-extraction plant, have been carried out. In this report, detailed mineralogy of those samples and supporting geochemical data are discussed, taking into account their specific climactic and environmental context. Petrographic and electron microprobe characterization of the sand-grained fraction of these materials was complemented by XRD on their finer fraction: the main contaminant minerals are willemite (one of the Zn ores used in the industrial plant) and jarosite, though their contents are quite variable in the studied areas and also with depth; minor amounts of Zn-, Pb-, Cd-, and Mn-bearing mineral phases are also frequent, usually as inclusions in willemite or in polycrystalline clasts, or adsorbed on the finer materials, such as clay minerals and associated Fe-hydroxides. Mineralogical contamination is responsible for high metal contents in the soils and sediments of the areas closer to the plant (e.g. Zn ≫ 2000 mg kg⁻¹ and Cd ≫ 20 mg kg⁻¹, which are the Intervention Values for Industrial Areas) and the greatest contamination risks are related to the more labile phases that circulate throughout the alluvial plains, the shallow sediments and the stream bed. Monitoring the mineral/chemical contamination and its extent also constitutes a useful basis for future proposals to remediate and recover this industrial area in order to decrease medium- and long-term negative impacts of metal contamination on the local and downstream environments.

Appendix: Petrographic characterization (coarse-grained fraction)

Area A1

Surface to 100-cm-deep sediment cores collected in area A1 (Table 1) mostly contain mineral phases resulting from mechanical and chemical weathering of local host rocks, namely abundant, relatively well-rounded medium- to fine-grained quartz, a few anhedral K-feldspar (more rarely albite) grains, and, in samples CA1-14 and CA1-51, fine-grained muscovite and partly chloritized biotite aggregates (Fig. 5a). These phyllosilicates sometimes include zircon, ilmenite (often with magnetite exsolution), rutile, or even Cd-rich sphalerite grains, as in sample CA1-51. Clay or sericitic material may also be present enveloping quartz grains, sometimes including barite, as in sample CA1-14 (Fig. 5b).

Besides the pervasive presence of fine-grained Fe oxides (~ 5 modal%, mostly magnetite, occasionally oxidized to haematite; Tables 4, 5), most of the samples in area A1 also contain irregular, rounded polymineralic clasts consisting of a microgranular mixture of quartz, Fe-hydroxides (Table 5), partly chloritized Fe-mica and feldspar. In some of these clasts, apatite, zircon and rutile grains were also identified. In sample CA1-51, an elongated polycrystalline clast of kaolinite was also identified (Table 4).

Microprobe-EDS was useful to identify apatite inclusions in some feldspars (sample CA1-26), chromite inclusions in a few quartz grains (sample CA1-29), and occasional inherited staurolite grains (sample CA1-29).

A few subhedral grains of dolomite and rounded polycrystalline carbonate clasts were found in samples CA1-26 and CA1-29.

Area A2

Two cores in the alluvia from area A2 provided several samples at different depths, some of which were selected for their significant Zn and Fe contents (Table 3).

Core CA2-10 mostly contains mineral phases inherited from the host rocks, after intensive weathering, and contains abundant, well-rounded quartz, accompanied by clasts of ferruginous material (usually fine-grained mixtures of Fe oxides or hydroxides, quartz and partly chloritized mica), with minor amounts of polycrystalline clasts of quartz + K-feldspar + mica ± ilmenite, apatite, chlorite, rutile, calcite, magnetite/haematite. Isolated grains of haematite and/or partially oxidized magnetite ± ilmenite also occur. Polycrystalline calcite clasts appear in the sample taken at depths of 20–40 cm, whereas in the surface sample of this core there are abundant crystals of Ca sulphate (gypsum and anhydrite; Table 4).

Most samples within core CA2-12 (covering depths from 20 to 100 cm) exhibit high Zn contents (Table 3) and were selected for this study. Quartz, very fine-grained, is abundant only in the most superficial levels (20–40 cm), often accompanied by some micaceous material. In the deeper levels of the sampled alluvia (40–100 cm), fine- to coarse-grained Fe-hydroxides predominate (50–90 modal%), usually goethite ± haematite ± relic magnetite (± very fine-grained quartz, micas, oxidized galena and pyrite, Cu sulphates, Mn–Pb–K–Ba-bearing oxides, monazite, willemite and sphalerite). The proportion of ferruginous materials seems to increase with depth.

Area A3

Sample CA3-18, collected at a depth of 20–40 cm, is an alluvial heterometric sediment (sand–silt–clay), mostly made of mineral phases resultant from weathering of local rocks: abundant fine- to medium-grained quartz, some K-feldspar (microcline) and albite, and clasts (~ 5 to 10 modal%) made of fine-grained mixture of muscovite, partly chloritized biotite and ferruginous material (Fe-hydroxides and residual Fe oxides, ± ilmenite ± zircon ± barite). Some willemite has also been identified in a few of these clasts.

Area A4

Sample CA4-41A (0–20 cm level) consists of minerals resulting from the weathering of local rocks, namely very abundant fine-grained quartz, muscovite and partly chloritized biotite, as well as of abundant heterometric clasts of K–Fe sulphate (jarosite, Table 4), sometimes containing quartz, magnetite ± zircon micro-inclusions. Rounded heterometric clasts of haematite and Fe-hydroxides are not very abundant, often showing micro-inclusions of quartz, muscovite, partly chloritized biotite, and some residual phases (Ti-magnetite, ilmenite, chromite, rutile and zircon).

Area A5

Two surface samples (0–10 cm deep) were collected in area A5.

Sample CA5-3 is mainly made up of fine- to medium-grained Fe oxides (70–80%), mostly magnetite, and willemite clasts (20–25%); a few quartz grains and a few small clasts of ferruginous material (Fe-hydroxides), often with quartz ± mica inclusions, and occasional monazite grains make up the rest of this sample.

Sample CA5-42 (0–10 cm deep) is almost exclusively made of willemite polycrystalline clasts, with only accessory amounts of Fe oxides, ferruginous clasts and fine-grained carbonate clasts, and rare quartz.