Role of impurities in the semiconducting properties of natural pyrite: Implications for the electrochemical accumulation of visible gold and formation of hydrothermal gold deposits
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Abstract Pyrite (FeS2), the most abundant sulfide mineral on Earth, typically contains a host of minor and trace elements, including As, Co, Ni, and Au. It is an important semiconductor with unique structural properties markedly influenced by elemental impurities. However, whether the change in semiconducting properties of natural pyrite is caused by the type and concentration of trace elements or by a non-stoichiometry-related doping mechanism remains uncertain. Moreover, the effect of semiconducting properties on the enrichment mechanism of Au has not been well addressed. Here, we investigate microscopic pyrite crystals from the Heilangou gold field (HGF) in the eastern Jiaodong Peninsula using field emission scanning electron microscopy (SEM), electron probe microanalysis (EPMA), in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), potential-Seebeck microprobe (PSM), and thermoelectric measurements. The results demonstrate that pyrite grains show either p- or n-type conductivity depending on chemical compositions. Pyrite enriched in As, which typically substitutes for S in the crystal structure, tends to be p-type with a positive Seebeck coefficient, whereas pyrite crystals enriched in Co, Ni, Cu, and Zn, as well as those depleted in As, are typically n-type. Moreover, As shows the strongest influence on the semiconducting properties of natural pyrite crystals and a strong positive correlation with Au. We observed that visible Au grains are preferentially accumulated on individual domains of sulfides (e.g., As-rich pyrite) that act as cathodes, suggesting that electrical p-n junctions in sulfides drive electrochemical reactions with ore-forming fluids, resulting in the deposition of visible Au. The electrochemical precipitation mechanism of Au may account for the formation of other types of hydrothermal Au deposits.The major objectives of this work are (1) to determine the Eh-pH conditions under which pyrite is stable, (2) to determine the mechanism of the initial stages of pyrite oxidation and (3) to determine if the semi-conducting properties of pyrite effects its oxidation behavior. It is known that moderate oxidation of pyrite produces a hydrophobic surface product. This hydrophobic product makes it extremely difficult to depress pyrite in coal flotation circuits. The eventual objective of this work is to prevent pyrite oxidation in order to better depress pyrite in coal flotation circuits. In this work clean, unoxidized pyrite surfaces are being produced by fracturing pyrite electrodes in an electrochemical cell. It has been shown that pyrite assumes a unique potential referred to as the ``stable potential`` at the instance it is fractured and that this potential is several hundred millivolts more negative than the steady state mixed potential of pyrite. It has also been shown that by holding the potential of pyrite at its stable potential during fracture, pyrite undergoes neither oxidation nor reduction. It has also been found that fresh pyrite surfaces created by fracture in an electrochemical begin to oxidize at potentials that are about 200 mV more negative than the potentials reported in the literature for pyrite oxidation. This is attributed to the fact that most work on pyrite has employed polished electrodes that have pre-existing oxidation products on the surface. The existence of a pH dependent stable potential for freshly fractured pyrite electrodes was based on studies conducted mainly on pyrite from Peru.
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Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.
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Many different types of pyrite have been observed in the mineral processing industry, varying in texture, chemical composition, electrochemical properties, and flotation response. One particular type of pyrite has been noted for its unusual behaviour is framboidal or "carbonaceous" pyrite. Specifically, a long history of carbonaceous pyrite presence has been recorded and studied at Glencore's Mount Isa Mines. This work is a detailed examination of the mineralogical characteristics of ore samples containing different pyrite types, to determine the underlying drivers of natural pyrite flotation. The work also revisits the original hypothesis that pyrite floatability is facilitated by the presence of carbonaceous material within the mineral matrix. Overall, this work closely mirrored that of previous studies, with the results closely aligned. The main difference is that when multiple ore domains are subjected to the same analysis, the flotation behaviour of carbonaceous content and pyrite appear to be independent. Instead, excessive natural flotation recoveries of pyrite are likely to be driven by factors that include the effect of galvanic interactions between chalcopyrite and pyrite.
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Liptinite
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One of the most difficult separations in minerals processing involves the differential flotation of pyrite and coal. Under practical flotation conditions, they are both hydrophobic and no cost-effective method has been developed to efficiently reject the pyrite. The problem arises from inherent floatability of coal and pyrite. Coal is naturally hydrophobic and remains so under practical flotation. Although pyrite is believed to be naturally hydrophilic under practical flotation conditions it undergoes a relatively rapid incipient oxidation reaction that causes self-induced'' flotation. The oxidation product responsible for self-induced'' flotation is believed to be a metal polysulfide, excess sulfur in the lattice, or in some cases elemental sulfur. It is believed that if incipient oxidation of pyrite could be prevented, good pyrite rejection could be obtained. In order to gain a better understanding of how pyrite oxidizes, a new method of preparing fresh, unoxidized pyrite surfaces and a new method of studying pyrite oxidation have been developed this reporting period.
Polysulfide
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One of the most difficult separations in minerals processing involves the differential flotation of pyrite and coal. Under practical flotation conditions, they are both hydrophobic and no cost-effective method has been developed to efficiently reject the pyrite. The problem arises from inherent floatability of coal and pyrite. Coal is naturally hydrophobic and remains so under practical flotation. Although pyrite is believed to be naturally hydrophilic under practical flotation conditions it undergoes a relatively rapid incipient oxidation reaction that causes ``self-induced`` flotation. The oxidation product responsible for ``self-induced`` flotation is believed to be a metal polysulfide, excess sulfur in the lattice, or in some cases elemental sulfur. It is believed that if incipient oxidation of pyrite could be prevented, good pyrite rejection could be obtained. In order to gain a better understanding of how pyrite oxidizes, a new method of preparing fresh, unoxidized pyrite surfaces and a new method of studying pyrite oxidation have been developed this reporting period.
Polysulfide
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Pyrite often forms in organic-rich marine sediments through bacterial action. Bacteria reduce organics to form bisulphide, which reacts in turn with dissolved iron in seawater to form pyrite. Consequently, it is frequently found in stratigraphic formations that consist of marine clays. Pyrite can be involved in fossilisation through a number of ways, including complete replacement, as infill in permineralised bone, or simply as microcrystals finely disseminated through the fossil and matrix. Replacement by pyrite can often be easily recognised: the fossil may have a gold or brassy metallic lustre. Infill is harder to recognise, and disseminated pyrite even more so. Pyrite in fossils may be stable or unstable- in its stable form, pyritised fossils will generally retain their shiny, metallic appearance. Pyrite preservation and stability can vary even within the same specimen. Fossils preserved in pyrite can be prone to oxidation, particularly at high relative humidities. There are a number of signs that indicate oxidation is occurring, depending on the severity of the condition. One or more of the following may be present: a sulphurous smell, white or yellow powdery crystals on the surface of the specimen, expansion cracks, as well as acid burns on associated labels, boxes and drawers. Such burns often have a characteristic ovoid appearance. If left unchecked, pyrite oxidation can completely destroy a specimen and its labels.
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To better understand the flotation behavior of coal pyrite, studies have been initiated to characterize the floatability of coal pyrite and mineral pyrite. The hydrophobicity of coal material pyrite was examined over a range of pH and oxidation times. The results indicate that surface oxidation plays an important role in coal and mineral pyrite hydrophobicity. The hydrophobicity of mineral pyrite decreases with increasing oxidation time (20 min. to 5 hr.) and increasing pH (pH 4.6 to 9.2), with maximum depression occurring at pH 9.2. However, coal pyrite exhibited low floatability, even at the lowest oxidation time, over the entire pH range. X-ray photoelectron spectroscopy (XPS) results suggest the growth of an oxidized iron layer as being responsible for the deterioration in floatability, while a sulfur-containing species present on the sample surfaces may promote floatability. Preliminary studies of the effect of frother indicate an enhancement in the floatability of both coal and mineral pyrite over the entire pH range.
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To better understand the flotation behavior of coal pyrite, studies have been initiated to characterize the floatability of coal pyrite and mineral pyrite. The hydrophobicity of coal material pyrite was examined over a range of pH and oxidation times. The results indicate that surface oxidation plays an important role in coal and mineral pyrite hydrophobicity. The hydrophobicity of mineral pyrite decreases with increasing oxidation time (20 min. to 5 hr.) and increasing pH (pH 4.6 to 9.2), with maximum depression occurring at pH 9.2. However, coal pyrite exhibited low floatability, even at the lowest oxidation time, over the entire pH range. X-ray photoelectron spectroscopy (XPS) results suggest the growth of an oxidized iron layer as being responsible for the deterioration in floatability, while a sulfur-containing species present on the sample surfaces may promote floatability. Preliminary studies of the effect of frother indicate an enhancement in the floatability of both coal and mineral pyrite over the entire pH range.
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