Chemical Characterization of Auriferous Ores from the Brazilian State of Paraiba ()
1. Introduction
In Cachoeira de Minas, gold occurs primarily in auriferous quartz veins, the distinguishing feature of which is the absence or scarcity of hydrothermal alterations in the host rock. In addition, minor occurrences of gold can be found in sulfide-rich shear zones created by hydrothermal alterations where no mining is performed. The dis- tribution of these different zones is determined by the tectonic and metamorphic architecture of the Formação Salinas (distal unit of the Macaúbas Group in the Araçuaí basin). Thus, the gold mineralization in Paraiba is represented mainly by quartz-gold-sulfide veins (lode gold), with primary gold located in rocks of Riacho Gravatá complex. The secondary source is the quartz veins and alluvial deposits, and the gold found in the micro- fractures of these veins is probably associated with hydrothermal processes that occurred in the area [3] .
2. Methodology
2.1. Sample Preparation
Samples of auriferous rock (100 kg) were collected in the region of Princesa Isabel, and subsequently crushed, ground in a disk mill and homogenized in stacks in order to obtain a representative sample of the study area. All transfer operations were carried out using an iron shovel. Screening assays were performed on 1.0 kg samples using standard Tyler sieves, with mesh openings of 0.600, 0.500, 0.300, 0.177, 0.104 and 0.074 mm, and a vi- bratory sieve shaker. A precision analytical scale with an accuracy of 1 mg was used to weigh the materials re- tained by the sieves.
2.2. Chemical and Mineralogical Analyses
Samples were subjected to elemental analysis by X-ray fluorescence (XRF) spectrometry employing a RIX- 3000 instrument (Rigaku, Tokyo, Japan) equipped with a Rh X-ray tube and six analyzing crystals. Crystalline structure was evaluated using a model D-5000 X-ray diffractometer (Siemens, Berlin, Germany) with Cu-Kα as the source of monochromatic radiation. For all samples, scattering intensities were recorded at room temperature over the angular range 2θ = 10˚ - 90˚ with a step size of 0.03˚ and a step time of 1.0 s. Infrared spectra of samples in the form of vacuum-pressed KBr pellets were measured in the range 4000 - 400 cm−1 using a Bomem (ABB-Bomem, Quebec, Canada) model MB-102 Fourier-transform infrared (FTIR) spectrometer. Rare earth elements were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using an ARL model 35,000 instrument (Applied Research Laboratories, Austin, TX, USA). Gold and other noble metals were characterized by the Fire Assay method and atomic absorption spectrometry (AAS) employing an Analyst model 100 spectrometer (Perkin Elmer, Waltham, MA, USA). The limits of detection of the ICP-AES and Fire Assay/AAS techniques were 0.008 - 0.02 ppm for light elements and 0.005 ppm for gold, respectively.
3. Results
Particle size classification of the bulk sample (Figure 1) revealed p80 and p50 values of 0.485 and 0.265 mm, indicating that 80% and 50% of the particles, respectively, passed through the 32- and 48-mesh sieves. In addition, 90% of particles passed through the 28-mesh sieve, whereas only 14.9% passed through the 200-mesh sieve. The mean values of the quantity of material retained in each of the sieves reflect the efficiency of the grinding method since the mechanical properties of the minerals present in the sample are distinct. For example, the host rock is more brittle and has high iron content, whereas the gold-bearing rock (quartz) is very hard.
XRF analysis verified the heterogeneous composition of the sample and confirmed that the levels of silicon dioxide (from the quartz), aluminum oxide and iron oxide were high, as expected (Table 1). The refractoriness of the ore was explained by the presence of arsenic trioxide and sulfur trioxide, which contribute to the difficulty in treating and processing the gold associated with pyrite, arsenopyrite, and chalcopyrite.
ICP-AES analysis (Table 1) revealed that the sample contained various economically important metals such as Cu, Zn and Ni that could be recovered by flotation and/or leaching techniques. The presence of rare earth elements in the ore suggests the possibility of greater exploitation in the longer term, since these materials have a number of applications in the computer industry, industrial engineering, renewable energy sciences and medi- cine.
According to the Fire Assay/AAS method, the concentration of gold in the ore sample was 4.422 ppm (assay detection limit 0.005 ppm), a value that is below the limit of detection of XRF (10 ppm or 0.01%). In this type of mineral ore, the gold is typically dissolved in the interstices, primarily of pyrite and secondarily of arsenopyrite [4] .
Since every crystalline material has a specific and characteristic X-ray diffraction pattern, it was possible toidentify the substances present in the sample by comparison of the diffractograms with those published in the literature. The diffraction profile presented in Figure 2 confirms the presence of gold in the study sample [5] - [7] , and reveals the face-centered cubic structure of the crystalline material. Additionally, the less intense peak observed at 64.1˚ is typical of Fe2O3 [8] .
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Figure 1. Particle size distribution of a representative sample of gold mining oreoriginating from Princesa Isabel, PB, Brazil.
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Table 1. Chemical profile of a representative sample of gold mining ore originating from Princesa Isabel, PB, Brazil.
aLoss on Ignition (LOI) = 2.52%.
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Figure 2. X-ray diffraction profile of a representative sample of gold mining ore originating from Princesa Isabel, PB, Brazil. Legend: Au (*); SiO2 (+); Fe2O3 (#).
Information regarding the SiO2 matrix could be obtained from the FTIR spectrum (Figure 3). Based on detailed interpretations of the IR spectra of various silica-containing structures [9] - [11] , a band attributable to the asymmetric stretching mode of Si-O bonds in an [SiO4]2− tetrahedron was observed around 1080 cm−1. Although this band could be overlapping with another band corresponding to g-Al2O3 [12] , the symmetric stretching modes of Si-O bonds could be readily observed at around 785 cm−1, and bands between 565 and 455 cm−1 could be assigned to the deformation vibrations of Si-O-Si bonds.
4. Discussion
In the recovery of gold from auriferous rock, the particle size at which the metal lies represents a key factor in determining the overall efficiency of the process. In this context, the crushing and grinding steps are extremely important since they determine the range of particle sizes generated from the ores. According to the literature [3] , 80% of gold can be collected on sieves ranging between 100 and 400 mesh. Hence, the aim of a particle size- study is to establish sieve sizes that allow the selection of particles with a high degree of release, thereby facilitating the separation of particles of interest from the gangue. Although ball mills are normally employed in grinding in gold mining operations [13] , a disk mill was used in the present study because the sample rocks ex-
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Figure 3. Fourier-Transform infrared spectrum of a representative sample of gold mining ore originating from Princesa Isabel, PB, Brazil.
hibited elevated hardness owing to the presence of quartz.
It may be inferred from the results presented herein that the gold in ores from the state of Paraiba is associated predominantly with pyrite, arsenopyrite and sulfide. Refractory ores of this type generally require treatment involving roasting, bacterial oxidation or fine grinding. The mechanism of incorporation of submicroscopic particles of gold into sulfides has not yet been elucidated, but it has been suggested that gold is assimilated as sulfide by chemisorption onto surfaces that are Fe-deficient and As-rich, where it exists as a metastable solid solution [14] . Simon et al. [15] demonstrated that gold was present in the structure of arsenopyrite as microinclusions of Au0 and Au1+, while Au0 concentrated in fine pyrite at lower temperatures. Cabri et al. [16] confirmed that gold is present in arsenopyrite either as microparticles of Au0 or in the form of covalently linked Au1+. These ores can become more reactive if the crystal lattice energy is reduced, and this can be achieved through mechanical grinding and/or vibration techniques. In this case, the reduction of the activation energy of the reaction is attributed to microcracks and structural defects.
In the last decade, numerous studies have been performed with the purpose of augmenting gold recovery and developing cleaner technologies, but few studies have focused on understanding the mineralogy of the deposits in which gold can be found. Knowledge of the composition of the rocks that encase the precious metal can contribute to the reduction in operational and production costs. In this context, our study provides essential data regarding a representative sample of auriferous ore from the state of Paraiba.
5. Conclusion
It is concluded that the submicroscopic gold particles incorporated into the crystal lattice of pyrite and arsenopyrite are progressive released as inclusions and fractures, and the presence of such particles is the main factor responsible for the refractoriness of the auriferous ore from Princesa Isabel, PB, Brazil. This information is of considerable importance since, in ores containing pyrite and arsenopyrite, the fine particles of gold are occluded and disseminated among the sulfide minerals and cannot be released solely by mechanical grinding. The flotation concentrates of this type of gold ore are usually pre-treated by pyrometallurgical or hydrometallurgical methods.
Acknowledgements
The authors wish to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; grant no. 550261/2010-9) for financial support and the miners from Princesa Isabel for assistance with sample collection.
NOTES
*Corresponding author.