La práctica social en el Modelo de Prácticas Anidadas

Authigenic pyrite forms following the reaction between Fe and H2S, which are produced by the reduction of dissolved sulphate (SO42-) by bacteria that use organic matter as a reducing

7 agent and an energy supply (Berner, 1970, 1984). The amount of pyrite formed initially relies on the availability of these parameters in the sediment or sea water controlled by the seafloor redox conditions. Authigenic pyrites are found in marine sediments and they often have a framboidal textures (Fig. 1.1). The term framboid is derived from “framboise” the French for raspberry. The framboidal pyrites commonly settle from the water column and grow within the sediments after deposition, reflecting pore water composition, which in turn depends primarily on the sea water composition and contained organic component. Diagenetic pyrites extract heavy metals from the pore waters during growth (Huerta-Diaz and Morse, 1990). However, syngenetic pyrites are smaller (i.e. a few microns) generally enriched in As, Mo, and Sb and under euxinic conditions precipitates above the sediment seawater boundary directly from the seawater, thus reflecting a direct seawater composition (Large et al., 2014).

The geochemical variation in these sedimentary pyrites thus reflects changes in sea water composition. Previous research has also documented that sedimentary pyrite can form syngenetically if the chemocline lies above the sediment-water interface, or diagenetically if the chemocline lies at or below the sediment–water interface (Tribovillar, 2006). A chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. Syngenetic pyrite consists of fine-grained, euhedral crystals (<5 µm in diameter), whereas diagenetic pyrite is made up of larger (>10 µm in diameter), spherical framboids (Wilkin et al., 1996; Tribovillar, 2006). Raiswell (1982); and Coleman and Raiswell (1995) have argued that framboidal pyrite forms during the early stage of diagenesis, with euhedral pyrite forming later in burial as dissolved sulphide concentrations decreased. Iron-sulphide commonly precipitates from solutions oversaturated in its components (Morse et al., 1987; Schoonen and Barnes, 1991), but may also form by in-situ replacement of solid Fe-bearing mineral phases in contact with aqueous sulphur species (Canfield and Raiswell, 1991; Raiswell and Canfield, 2012). Laboratory observations indicate that framboidal pyrite does not form directly, but via a metastable

8 precursor Fe-sulphide phase (‘amorphous’ FeS, mackinawite and greigite) from iron-rich waters where, locally, sulphide production rates were high enough to reach supersaturation as for FeS (Morse et al., 1987; Schoonen and Barnes, 1991; Wilkin et al., 1996; Benning et al., 2000; Rickard and Luther, 2007; Taylor and Macquaker, 2011).

Fig. 1.1. Pyrite microtextures. A) Diagenetic framboidal pyrite with microcrystals <1µm surrounded by shells within sapropel (Passier et al., 1997). B) Cluster of framboidal pyrite in clay-rich mudstone in the sediments of Cleverland Ironstone Formation (Taylor and Macquaker, 2000). C) Syngenetic euhedral pyrite cluster in clay within mudstone in the sediments of Cleverland Ironstone Formation (Taylor and Macquaker, 2000). D) Pyrite framboids with overgrowths from the euxinic sediments of Green Lake (Suits and Wilkin, 1998). E) Cluster of diagenetic framboidal pyrite discovered from the Derwent Estuary sediments, Tasmania (Gregory et al., 2014). F) Aggregate of pyrite framboid from the Huon Estuary, Tasmania (Gregory et al., 2014).

A B 10µm C D 10µm E F 3 µm

9 The transformation of mackinawite to greigite was proposed experimentally as follows by Lennie et al., 1995: 12FeS + 2O2 3Fe3S4 (greigite) + Fe3O4 and 12FeS + 4H2O 3Fe3S4+ Fe3O4 + 4H2. The transformation of FeS to pyrite is controlled by the availability of an oxidant to produce sulphur species with intermediate oxidation states (Schoonen and Barnes, 1991; Neumann et al., 2005). Lennie et al. (1997) argued that the transition from greigite to pyrite (FeS2) requires addition of S to greigite and reduction of Fe3+ to Fe2+ to form pyrite or marcasite. Raiswell and Canfield (2012) argued that available reactive iron minerals and reactive organic matter are essential during pyrite formation within the sediments. These two factors control the rate at which sulphide is produced by sulphate-reducing bacteria (Berner, 1970). Other experiments (Huerta-Diaz and Morse, 1992; Morse and Arakaki, 1993) have shown that a wide range of trace elements are incorporated into the precursor iron monosulphide (mackinawite or greigite) at an early stage, including As, Hg, Mo, Co, Cu, Mn, Ni, Cr, Pb, Zn and Cd, which are absorbed from seawater and local pore waters in seafloor muds (Large et al., 2007, 2009; Gregory et al., 2014). Work by Taylor and Macquaker (2000) shows that euhedral pyrite precipitates directly from pore waters oversaturated with respect to pyrite, but not Fe- monosulphides.

Recent research work has shown a wide range of pyrite textures, which have been discussed in terms of their origins and chemical characteristics (Guy et al., 2010; Thomas et al., 2011, Large et al., 2011; Pisarzowska et al., 2014). Guy et al. (2010) have documented three main types of pyrites including detrital, diagenetic, and epigenetic pyrites (Fig. 1.2). All captions in this Figure come from the work of Guy et al. (2010). Detrital pyrites (Fig. 1.2A) were found in conglomerate, sandstone, siltstone, wackestone and diamictite. They are inclusion free, compact non-porous and often show rounded cubes to well-rounded grains. The irregular microcrystal pyrite aggregates (Fig. 1.2B) are framboid-like and irregular. The framboid-like patches are 15 µm in diameter and contain cemented 0.5µm sized microcrystals.

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Fig. 1.2. Pyrite textures from the MESOARCHEAN Witwatersrand Supergroup, South Africa (Data and descriptions from Guy et al., 2010). A) Detrital pyrite. B) Diagenetic, microcrystal aggregates of pyrite. C) Diagenetic, small euhedral crystals in mudstone. D) Diagenetic, inclusion-rich euhedral pyrite. E) Inclusion-rich overgrowth in carbonate-oxide BIF. F) Inclusion-free overgrowth around a tiny pyrite nodule. G) Epigenetic, pyrite formed by biogenic replacement of quartz. H) Epigenetic pyrite consists of crystallographically aligned porous overgrowths. A B C D E F G H 50µm 20µm 20µm 50µm 50µm 20µm 500µm 100µm

11 These pyrite grains were found in carbonaceous shales. They are interpreted to be of diagenetic origin. The small euhedral crystals are of syngenetic origin (Fig. 1.2C). The euhedral to subhedral pyrite crystals (Fig. 1.2D) which are inclusion-rich were found in sandstone, siltstone,

diamictite, and carbonaceous mudstone.They are interpreted to have formed at a later stage of

diagenesis marked by migrating pore waters within permeable siltstone beds or laminae (Alonso- Azcarate et al., 1999). Pyrite overgrowths, inclusion-rich and inclusion-free (Figs. 1.2E-F) often form at a later stage of metamorphism and/or hydrothermal activity (Wagner and Boyce, 2006); however, these pyrite overgrowths were interpreted to have formed during diagenesis as they have been found in impermeable beds (e.g., mudstones). Epigenetic pyrite types which are documented herein are non-porous (or inclusion-free) and porous (Figs. 1.2G-H). These pyrite types are commonly found in sandstone and siltstone and they are larger than diagenetic pyrite with size ranging up to 500 µm or more in diameter. They contain crystallographically aligned pores indicative of fast growth and metamorphism (Ramdohr, 1958; Craig et al., 1998). Additionally, non-porous pyrite may reach similar sizes especially when they are in the form of veinlets, individual crystals or aggregates.

Some of the epigenetic pyrite crystals may occur near early diagenetic pyrite. These crystals were interpreted to represent remobilized/recrystallized early diagenetic pyrite (Wagner and Boyce, 2006). Pyrite overgrowths (Fig. 1.2H) were observed close to diagenetic pyrite. They were interpreted to be of epigenetic origin. Other pyrite types such as pyrite veinlets or replacive pyrite were all interpreted to be of epigenetic origin (Guy et al., 2010). Work by Thomas et al. (2011) adds to that of Guy et al. (2010) and presents textural characteristics of pyrite at Bendigo Gold Mine, Australia. Some key textures of diagenetic pyrites are fine acicular diagenetic pyrite hosted in black shales (Fig. 1.3A), and fine-grained pyrite nodules (Fig. 1.3B). Diagenetic pyrite cores (py1) followed by later diagenetic overgrowths (py2) which in turn is overgrown by euhedral metamorphic-hydrothermal pyrite (py3) (Fig. 1.3C). Metamorphic-hydrothermal

12 pyrites are often coarse grained euhedral with an outermost rim (Figs. 1.3D-E) and euhedral zoned in the overgrowth pyrite (Fig. 1.3F).

Fig. 1.3. Pyrite textures in sediments from the Bendigo Gold Mine, Australia (Data from Thomas et al., 2011). A) Fine acicular grained pyrite, diagenetic py1 after marcasite. B) Fine-grained diagenetic pyrite in shale. C) Early diagenetic core of py1, followed by a later diagenetic overgrowth nodule (py2), overgrown by euhedral metamorphic-hydrothermal py3. D) Coarse-grained euhedral metamorphic-hydrothermal py3, with an outermost rim. E) Coarse-grained euhedral metamorphic-hydrothermal pyrite aggregate overgrowing a silt-shale boundary. F) Zoned euhedral metamorphic-hydrothermal pyrite showing the metamorphic fabric of the shale preserved in the overgrowth pyrite; revealed by acid etch (Data and descriptions from Thomas et al., 2011).

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