ГЕОЛОГИЯ РУДНЫХ МЕСТОРОЖДЕНИЙ, 2014, том 56, № 5, с. 440-452
УДК
THE FORMATION OF LOW-TEMPERATURE SEDIMENTARY PYRITE AND ITS RELATIONSHIP WITH BIOLOGICALLY-INDUCED PROCESSES © 2014 B. Cavalazzi*,**, A. Agangi*, R. Barbieri**, F. Franchi**, ***, G. Gasparotto**
*Department of Geology, University of Johannesburg 2006, Johannesburg, SOUTH AFRICA **Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Universita di Bologna
40126, Bologna, ITALY ***ISMAR-CNR, U.O.S. Bologna 40129, Bologna, ITALY Received February 10, 20141
This contribution is an updated review on sedimentary pyrite and on its role in well-consolidated research topics, such as the biogeochemical cycles and the studies on sediment-hosted ore deposit studies, as well as new frontiers of research, such as astrobiology. Textural and compositional information preserved in sedimentary pyrite from sediment-hosted ore deposits has contributed to elucidate their environment of formation. In particular, the content of redox-sensitive elements such as Ni, Co, Mo, and V has implications for defining the syn- and post-sedimentary conditions. In addition, the stable isotope compositions are useful indicators of the pathways of both biogenic and abiogenic pyrite formation. Despite the longstanding research on pyrite and the mechanism of its formation, there are still signicant gaps in our knowledge. In this non-exhaustive review, we briefly touch on different current aspects of research on sedimentary pyrite, exemplifying how sedimentary pyrite remains relevant to geoscientists, and becomes more and more relevant in understanding some basic aspects of knowledge, such as the origin of life and the search for extraterrestrial life, as well as aspect of classical applied science, such as the implications for ore deposition.
DOI: 10.7868/S0016777014050025
INTRODUCTION
Pyrite, cubic-FeS2, is the most abundant and widely occurring ferrous iron disulfide minerals at the Earth's crust near-surface conditions (Table 1), and an important phase in many economic ore bodies, the exploitation of which may represent an environmental issue. The natural ferrous metastable orthorhombic-2
FeS2, marcasite that inverts to pyrite under elevated temperatures and/or over time, is now estimated to be more abundant than previously thought (Schieber,
3
2007, 20111). Pyrrhotite group minerals, Fe1-xS, are more common in basic and mantle rocks, and represent, with pyrite, the most common sulfides in the bulk Earth (Rickard and Luther III, 2007).
1 Corresponding author: B. Cavalazzi. E-mail: barbara.cavalazzi@unibo.it
2 Marcasite, dimorph of pyrite, can be synthesized by reaction of Fe2+ and S species in acid aqueous solutions, and can form under low-temperature and low-pH (<6) conditions (e.g. Schoo-nen, 2004; Rickard and Luther III, 2007).
3 Stability of pyrrhotite at high temperature is well known (e.g. Scott, 1983). However, pyrrhotites have been experimentally observed to form in low temperature aqueous biologically-controlled and abiotic system, although its process of formation is not yet understood (Werner and Dove, 2003). As a consequence, pyrrhotite minerals might potentially form in low temperature sedimentary systems.
Pyrite forms under a wide variety of conditions, it occurs in many different geological contexts and different depositional environments, both marine and continental. It is a common iron sulfide in sulfide ore deposits and hydrothermal (quartz) veins, in many metamorphic and igneous rocks, in tills, coal beds, soils, mine tailings and hydrocarbon-seep deposits; it also occurs as diagenetic replacement in sedimentary rocks from a wide range of depositional environments, ranging from deep sea to non-marine, and has a role in fossilization processes (e.g. Berner, 2001; Wilkin and Barnes, 1996, 1997; Popa et al., 2004; Schoonen, 2004; Rickard and Luther III, 2007; El Albani et al., 2010; Schieber, 2011b; Agangi et al., 2013). Pyritized (mineralized or replaced by pyrite) is the ~2.1 Ga preserved fossil fauna from the Francevillian Basin in Gabon that recorded the remains of the alleged oldest multicellular life (El Albani et al., 2010). Pyrite has also been found in the spores of a lichen-like inclusion in Eocene amber (Garty et al., 1982), in ancient books as a product of sulfate reduction of the tannin-iron rich ink in the anoxic cellulose environment, and within Paleozoic fossil wood which is clearly hosted in the wood cells (Fig. 1) (Garcia-Guinea et al., 1997, 1998).
Given its abundance and distribution on the Earth's crust, the issue of the pyrite origin has been widely studied for more than two centuries since Hatchett (1804) determined its stoichiometric composition, and
Non-exhaustive list of the most common iron-sulfide solid phases (with a wide range of stoichiometric formulae and different crystalline structures) at, or near the Earth's surface. List compiled from many sources
Sulfides Crystallography Formula Common impurities
Amorphous Amorphous FeS
FeS less stable Fe(II)S amorphous form
Greigite Cubic; isometric Fe3S4 Cu, Ni, Zn, Mn,
metastable FeII/FeIIIsulfide hexoctahedral Sb, As
Mackinawite Tetragonal; FeSi-x Co, Cu, Ni
metal rich, sulfur deficient metastable iron sulfide; it is the major con- ditetragonal;
stituent of the FeS precipitated from aqueous solutions dipyramidal
Marcasite* Orthorhombic; FeS2 Co, Ni, As, Cu
(metastable) iron(II) disulfide dipyramidal
Pyrite Cubic; isometric; FeS2 As, Co, Cu, Ni,
stable iron(II) disulfide diploidal Mn, Sb, Zn, Au, Tl,
Se, Te
Pyrrhotite Monoclinic; Fei- XS Ni, Co, Cu
iron-deficient hexagonal
Smythite Hexagonal Fe9S11 Ni
metastable phase related to the Fe1 __ xSgroup
Troilite Hexagonal FeS
not stable in terrestrial environment; present in meteorites and common
in Early terrestrial crust
* Dimorph of pyrite.
Bakewell (1815) and Allen et al. (1912) first experimentally synthesized the pyrite. One century ago X-ray diffraction was applied for the first time on the pyrite mineral structure (Bragg, 1914). Early studies on sedimentary pyrite were centered on the understanding of a possible relationship between mineral texture and biological processes. To date, the possibility that micrometer-scale aggregates of pyrite crystallites such as framboids (microscopic spheroidal aggregates of microcrystals) represent fossilized microorganisms or cell colonies (e.g. Schneiderhohn, 1923; Love, 1957; Massaad, 1974; Chen et al., 2007; Gong et al.,
2008) is still debated. The importance of biological processes in driving pyrite formation and the role of pyrite in global Earth's surface processes and in Fe—S geochemical systems are, however, commonly accepted (e.g. Canfield and Raiswell, 1999; Shen et al., 2001; Goldhaber, 2003; Ohfuji and Rickard, 2005; Rickard and Luther III, 2007). Due to its surface reactivity, pyrite oxidation can seriously affect the surrounding environment (Murphy and Strongin, 2009). For instance, pyrite oxidation has important consequences for the environment due to acid mine drainage and consequent metallic pollution (e.g. Amils et al., 2008;
Fig. 1. Transmitted (a) and reflected (b) light photomicrographs of a cross section of fossilized wood fragment embedded in phosphate-rich black shale nodule from Late Carboniferous deposits, Main Karoo Basin, South Africa. The images show individual wood cells and circular resin canals filled with polyframboid aggregates of pyrite (arrow). Scale bar: (a) 500 ^m; (b) 200 ^m.
Fig. 2. SEM photomicrograph of cubic pyrite (bright minerals) in black siltstone-sandstone (SOCOBA quarry, Pale-oproterozoic, Francevillian Basin, Gabon). The host-matrix is mostly made of illite and chlorite clay minerals. Scale bar: 100 ^m (Image courtesy of F.-G. Ossa Ossa, Department of Geology, University of Johannesburg, South Africa).
Bower et al., 2008). Pyrite reactivity is also considered in precious metals recovery (Suzuki, 2001), and in beneficial commercial processes such as mineral benefaction, which can range from the desulfurization of coal to the recovery of copper or gold from low concentration ores. Iron sulfide species such as pyrite have been known to cause problems in the oil industry. Deposits of iron sulfide can form in wellbores, water injectors and gas wells as a result of the reaction between iron (leached from minerals present in the formation) and hydrogen sulfide that can derive from thermal decomposition of drilling mud additives, sulfate reducing bacteria, and thermo-catalytic reduction of the sulfate ion (e.g. Nasr-El-Din et al., 2001).
At present pyrite is not of high economic importance with the exception of the production of sulfur dioxide (e.g. for paper industry) or sulfuric acid (e.g. for modern industries of chemicals, fertilizers, paints and textiles) (Habashi, 2012). Although such applications are declining in importance, its association with metal deposits (copper, lead-zinc, gold and other metallic ore deposits, such as Ag and Mo) has made pyrite a prime target of ore studies (Habashi, 2012). In fact, its refractory nature allows pyrite to record signicant parts of the deformation and metamorphic history not preserved in the softer economic sulfides such as chal-copyrite and galena (e.g. Craig and Vokes, 1993). Many sulfide ore deposits form at plate margins, and are often involved in later orogenic events resulting in deformation and/or metamorphism of the ores (Allen et al., 2002; Scott et al., 2009). Thus, micro- and nano-scale study of deformation mechanisms that operate in pyrite (e.g. Barrie et al., 2010) is considered
the key to understand the genesis of deformed ore bodies and their evolution through time, which is important for their successful exploitation (Freitag et al.,
2004).
Pyrite formation and burial in sedimentary environments are important processes in several global geochemical and biogeochemical cycles and processes throughout most of geological time
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