AMD-precipitates formed in an arsenic-rich environment – the case of Penedono
These neoformation products are referred to as acid drainage precipitates (AMD-precipitates), in view of the conditions that provide their genesis. Globally, they result from the weathering processes, oxidation, leach- ing, transport and precipitation and/or neutralization occurring in the waste dumps. Therefore, AMD- precipitates result from a set of evolutionary processes, such as salinization and crustification, which, simulta- neously, control the composition of the leaching acidic solutions.
The major goals of the present work are: (i) to iden- tify neoformation processes in a sulfide-rich waste dumps; (ii) describe the mineralogy of salt efflores- cences and of crusts assemblages; (iii) describe compo- sition and morphology of nanoprecipitates; (iv) eva- luate the role of the neoformation processes in control- ling the mobility of metals and arsenic.
SITE DESCRIPTION
The waste dumps resulted from the exploitation of the Mine “Santo António de Penedono” is located in the Penedono municipality, Viseu (Portugal). It was an important gold mine, with gold occurring in quartz veins with sulfides, mainly pyrite and arsenopyrite, emplaced in a granite massif (Figure 1). The mine was also exploited for other elements, such as arsenic (Ma- tias et al., 2003). The waste dumps were formed by over 50 years of accumulation of thin wastes, resulting from ore treatment, mainly by hydrogravitic separation and flotation.
Currently, the mining complex shows strong signs of degradation, regarding industrial structures and also in terms of environmental contamination (Gomes, 2015).
The waste dumps have two main levels of waste accu- mulation and presents strong evidences of erosion.
There is an important ravine that serves as a testimony of the physical instability of the structure (Figure 1).
The mineral-water interaction is dependent on seasonal climate cycles. So, in climatic terms, the region is cha- racterized by summer warm periods and cold and wet winters. The annual rainfall is 700 mm (Roque, 2009).
The average annual temperature is 13.6 ° C. January is the coldest month with 6.9°C, while July and August show higher temperatures, around 21 °C. The first autumn rains occur by end of September-begin of Oc- tober.
SAMPLING AND ANALYTICAL METHODS For soluble salts, sampling covered plain surfaces, the main gullies illustrated in Figure 1, and other tem- poral varying terrain irregularities, in such a way to represent different conditions of sun exposure, humidi- ty and topography. Samples were also collected along surface seepages, mainly at the base of the waste dumps, where acid mine drainage emerges. The crusts were collected mainly at the base of the waste dumps where they are more abundant. Sampling intended to
cover the textural, color and compositional variability observed in the field.
Samples were stored in closed plastic bags and transported to the laboratory soon after the collection, in order to prevent mineralogical changes.
Mineralogical composition of soluble salts was ana- lyzed by X-ray powder diffraction (XRD) with a Phi- lips X'pert Pro-MPD diffractometer, using Cu-Kα radi- ation. Sample preparation procedures and the appro- priated XRD conditions for these kinds of samples, in particular leading with fine grain size, impurity of the assemblages and high hydration states are described in Valente et al. (2009, 2013). Morphology and chemical composition were studied by scanning electron micro- scopy (SEM).
FIGURE 1. Site location with a simplified geological map and an aerial image of the waste dumps (GoogleEarth@).
Bulk samples of crusts were firstly analyzed by XRD. Polished section were also prepared and ob- served in reflected light and by SEM (ES and ER mode).
The mine wastes were also submitted to sieving and to separation based on theoretical Stockes’ Law in order to obtain the < 2 µm fraction. This fraction was studied by XRD to achieve mineral identifications and, then, by transmission electronic microscopy (TEM), to study the properties of the AMD precipitates occurring in the nanoscale.
RESULTS AND DISCUSSION
Occurrence modes of AMD-precipitates
The AMD-precipitates define the following main types of occurrence modes: salt efflorescences (Figure 2a), thin coatings on the surface drainages (Figure 2b) and hard crusts cemented by neoformed products (Fig- ure 2c-d).
Salt efflorescences result from evaporative processes. They are ubiquitous, but are more abundant in relatively humid environments, such as the ravine zone, at the base of the waste dump, and at the AMD main channel, where AMD solution may evaporate.
Nevertheless, in winter periods, or in the sequence of rain events salt efflorescences are rae or absent, since they dissolve. On the other hand, crusts are persistent neoformation structures. They consist of the layering of minerals deposited successively. These layers include secondary phases as well as inherited primary minerals that are enclosed inside the hard crust. The agglutina- tion of the whole structure is promoted by the cement- ing power of the secondary phases. Depending on the type of crust, the cement is mainly composed by scoro- dite, jarosite or goethite, often combined ith each other.
The hard crusts occur at the base of the waste dumps and as cemented layers observed in the vertical profiles exposed by erosion. The thin coatings are distributed by seepages and by the channel beds that drain AMD solutions.
Composition and morphology of mineral assem- blages
The XRD combined with MEV analyses indicate that efflorescences are composed by sulfates minerals, mainly iron, calcium, and magnesium sulfates, and by the iron arsenate scorodite.
Mineral Ideal Formula Occurrence
mode/color/habit
Gypsum CaSO4 2H2O Ef./botryoidal
Pickeringite MgAl2(SO4)4 22H2O Ef./white acicular Halotrichite FeAl2(SO4)4 22H2O Ef./withe acicular Rozenite FeSO4 4H2O Ef./white powder Rhomboclase HFe(SO4)2 4(H2O) Ef./grayish powder
Copiapite Fe2+Fe3+4(SO4)6(OH)220(H2O) Ef./yellow powder Jarosite KFe3+3(OH)6(SO4)2 Hard crusts
Goethite FeOOH Hard crusts
Scorodite FeAsO4 2H2O
Hard crusts Coatings (blue) Ef (/blue-greenish
powder) TABLE 1 – Identified AMD-precipitates and respective occurrence modes and main habits. Ef. – Eflorescences.
Table 1 resumes the identified phases, indicating the main occurrence modes. The minerals from the group halotrichite-pickeringite show a distal distribu- tion (downstream from the waste dumps), covering the exposed river beds. The remaining phases occur in the waste dumps. A typical assemblage is composed by rhomboclase+copiapite+gypsum, typically found in the ravine zone. Jarosite, goethite, and scorodite may occur as monomineralic aggregates.
FIGURE 2. Atlas of main occurrence modes of AMD-precipitates in Penedono waste-dumps. a) salt efflorescences, mainly composed by sulfate phases. b) thin coatings, manly of scorodite. c) Hard crusts, mainly of jarositic (left) and scoroditic (right) nature. d) Mixed composed hard crusts: jarosite+goethite(left) and scorodite+jarosite (right).
However, Jarosite+scorodite form one of the most typical assemblages in the waste dumps. In addition to these mineral phases there are also amorphous iron arsenates, and very poorly crystalline iron oxyhydrox-
d) c) b) a)
ides, enriched in arsenic, as presented by Valente et al.
(2015).
Figure 3 shows morphological and compositional aspects of jarosite and scorodite. Jarosite presents nor- mally perfect hexagonal or pseudohexagonal crystals (Figure 3a). Often, it incorporates Al and Na in its composition. In addition, TEM study revealed that often, jarosite is enriched in arsenic, indicating its role in retaining this toxic element. Scorodite appears often as platy mass of pyramidal crystals, showing their typical composition in iron and arsenic (Figure 3b).
FIGURE 3 – Morphology and composition of jarosite, showing hexagonal crystals (above,) and of scorodite occurring as platy mass (below). SEM images (mode ES) with respective EDS spectra for chemical composition. The analyses were performed on gold coated samples.
CONCLUSION
In the Penedono mine waste dumps distinctive types of supergenic neoformation processes lead to AMD-precipitates. As a result, there are typical mineral assemblages in the waste dumps and downstream, in the fluvial system. At the waste dumps, iron sulfates and scorodite are the main components of the efflores- cences. For distal positions, efflorescences are com- posed by Al-sulfates from the halotrichite-pickeringite group. Crustification process corresponds to a more developed neoformation stage. It promotes the forma-
tion of hard structures mainly composed by jarosite and scorodite. These two mineral phases assure the aggluti- nation of the crusts, enclosing the reactive minerals.
Therefore, hard crusts are the most effective AMD- precipitates in retaining acidity and controlling the mobility of toxic elements.
REFERENCES
Bowell RJ, Rees SB, Parshley JV (2000) Geochemical predictions of metal leaching and acid generation:
geologic controls and baseline assessment. In:
Symposium Proceedings of Geology and Ore De- posits: The Great Basin and Beyond (Eds. J.K.
Cluer; J.G. Price; E.M. Struhsacker; R.F. Hardyman
& C.L. Morris), Geological Society of Nevada, pp.
799-823.
Gomes P, Valente T, Sequeira Braga MA, Grande JA, de la Torre ML (2015) Enrichment of trace ele- ments in the clay size fraction of mining soils Envi- ronmental Science and Pollution Research. DOI:
10.1007/s11356-015-4236-x.
Keith CN, Vaughan DJ (2000) Mechanisms and rates of sulphide oxidation in relation to the problems of acid rock (mine) drainage. In: Campbell LS, Valsa- mi-Jones E, Batchelder M, editors. Environmental Mineralogy: microbial interactions, anthropogenic influences, contaminated land and waste, vol. 9.
The Mineralogical Society Series; 2000. p. 117–39.
Matias MJ, Abreu M, Santos Oliveira J, Magalhães M, Basto MJ, Ávila P, Joaquim C (2003) Avaliação preliminar dos impactos ambientais resultantes da exploração e abandono da mina de ouro de Santo António - Penedono. Memórias e Notícias, 2, 301- 314.
Nordstrom DK, Alpers CN (1999) Negative pH, efflo- rescent mineralogy, and consequences for environ- mental restoration at the Iron Mountain Superfund site, California. Proc Natl Acad Sci US ;96:3455–
Roque M 62. (2009) Estudos de caracterização de áreas mineiras degradadas. Proposta de metodologia com aplicação à área mineira de Santo António, Penedono. Tese de doutoramento, Universidade de Lisboa (não publicada), 524 p.
Valente T, Leal Gomes C (2009) Occurrence, properties and pollution potential of environmental minerals in acid mine drainage. Science of the Total Environment 407, 1135–1152.
Valente T, Grande JA, de la Torre ML, Santisteban M, Cerón JC (2013).Mineralogy and environmental re- levance of AMD-precipitates from the Tharsis mines, Iberian Pyrite Belt (SW, Spain). Appl. Geo- chem. 39, 11–25.
Valente T, Gomes P, Sequeira Braga MA, Dionísio A, Pamplona J, Grande J A (2015) Iron and arsenic- rich nanoprecipitates associated to clay minerals in sulfide-rich waste dumps. Catena, DOI:10.1016/
j.catena.2015.03.009.