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Textures
Textures Textures offer valuable information about natural and industrial processes in which the material under investigation has formed. Although this book focuses on naturally occurring minerals, it also documents a small number of artificial mineral phases which may exist in nature, but are quite rare and were not accessible for investigation from natural sources. The word “texture” derives from Latin and relates to a weaved material. Thus, we shall investigate the composition and the fabric of ore materials at different scales (albeit here only under the microscope). For the purpose of this book, “texture” will only reflect the context of both the examined grain and its matrix. Grain shape and size, deformation, abundance, distribution, mineral association (not necessarily paragenesis), and replacements are the main descriptors for the characterisation of the intergranular relationships (Barton, 1991; Craig, 2001). Although there can be many ambiguities in the interpretation of the spatial relations between grains seen under the microscope (applies to macroscopic scales as well), textures disclose some genetic information and are crucial for mineral dressing, because the nature of the ore, grain properties, and intergrowths directly constrain processing techniques. However, is important to remember that the field of view under the microscope only constitutes a minute detail from a large ore body and should, therefore, very cautiously be taken into account for large-scale extrapolations. In ores and rocks, a succession of geological events, such as magmatic ore formation hydrothermal overprinting - metamorphism - exhumation leading to supergene alteration (weathering) may re-equilibrate the entire ore body and replace initial minerals, completely obliterating earlier textures, unless geochemically inert and physically stable minerals are preserved that reveal the true nature of the precursor (see photo below).
Right: Intercumulus chromite grain (medium grey) displaying embayments that are typical for the partial resorption of a chromite in disequilibrium with the surrounding ultramafic melt. Now, the grain is embedded in the sideritic matrix (brownish grey) of a listwaenite, which otherwise hardly bears any resemblance to the original dunitic rock (Fanja, SW of Muscat, Sultanate of Oman).
Even a slowly declining temperature during the cooling of a magma suffices to put an already established mineral into a state in which its lattice is no longer stable, leading to an exsolution process (e.g., hematite, rutile, and ulvöspinel separating from ilmenite). Fe-, Cu-, and Ag-containing sulfides are even more out of equilibrium than the oxides as soon as they come in contact with the atmosphere. These latter minerals will then start to oxidise and produce a large number of secondary phases. Such processes generate a vast array of minerals displaying various textures which, if understood, provide valuable clues on the origin of rocks, ores, and minerals and their respective formation conditions. As Barton (1991) states, “Ore deposits belong to the most complex inorganic features of our planet. If this complexity is reflected in multiple mineralisations overprinting each other but not necessarily producing a “valid” (meaningful) paragenesis, a determination of 9
the genesis of the ore is severely impeded and has to rely on individual mineralogical or geological markers (single grains may yield substantial information). In order to develop a “feeling” for the genetic hints “frozen” in a sample, it is necessary to delineate typical characteristics reflecting particular processes. Since a number of authors, such as Ramdohr (1969), Barton (1991), Craig (2001), and many others have compiled comprehensive accounts of textures, the descriptions below have been kept concise and are accompanied by typical images. The following section briefly outlines the most important textures from different geological environments: • Magmatic, • Post-magmatic (Hydrothermal), • Metamorphic, • Sedimentary & Soils, • Supergene, and • Biomineralisation. Footnote: All samples shown in the section below have been referenced in the description chapter with respect to their sample sources and show details of presented photos.
Magmatic Environments Cumulus During the cooling of a silicate magma, minerals with a high melting point start to crystallise and form euhedral to subhedral grains, because the surrounding melt poses few obstacles to the developing crystal. Thus, oxides such as chromite, ilmenite, and magnetite plus PGE alloys separate from the siliceous melt. Gravitation forces these dense minerals to sink and accumulate (cumulus phase) in deeper parts of the melt body (massive ores can form), a process that may also carry individual components into places in which the newly formed mineral is no longer in equilibrium with the melt. This, in turn, causes edge disintegration and results in rounded shapes where original crystal faces may or may no longer show, resorption embayments can also develop. Rapidly quenched extrusives (mainly basalts, but also industrial slags and alloys) often exhibit skeletal crystals, where the grains grow along preferred crystallographic orientations (axes) and do not find the time to fill interior gaps. In cases of a very fast cooling, not even crystal faces can develop and the grains have rounded outlines and dendritic arrangements. Poikilitic intergrowths between silicates, sulfides, and oxides are a common phenomenon.
Right (top): Rounded euhedral chromite in a silicate matrix (top) and Right (bottom): Massive chromite (medium grey) containing only minor amounts of silicates in fractures (darker grey).
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Cumulus (continued)
Right: Skeletal magnetite in the siliceous matrix of a basalt (light grey crystals).
Right: Dendritic magnetite in a historic Etruscan slag (lighter grey), again in a silicate matrix.
Intercumulus (Intergranular) During fractionated crystallisation, intergranular or intercumulus textures develop wherever a mineral solidifies/crystallizes later than the surrounding matrix, thus filling the left-over interstitial space. Examples include various Cr, Fe, and Ti oxides as well as sulfides, the latter crystallizing from melts immiscible with the dominantly silicic magma.
Right (top): Intergranular chromite (lighter grey) in ultramafic silicate matrix (dark grey). Right (bottom): Interstitial pyrite (cream) and connected droplet in a magnetite matrix (grey).
Exsolution Generally, exsolution textures imply slow or intermediate cooling rates, during which the respective solid phase re-equilibrates mineralogically and texturally (Barton, 1970). Miscibility gaps in the phase stability of a number of minerals show up during cooling: A homogenous single-phase high-temperature component breaks down into two or more distinct minerals. The result is an oriented intergrowth of those components that no longer fit into the same crystal lattice at the lower temperature; examples are, for instance, surplus Fe or Ti in ilmenite or extra Fe in Cu-sulfides, such as chalcopyrite. 11
Exsolution (continued) This process presents a record of the thermal history of the ore (Ramdohr, 1969). These exsolutions form discrete mineral grains along crystallographically preferred directions of the host from which they differentiated. Thus, oriented exsolution lamellae of rutile, ulvöspinel or ilmenite may occur in titanomagnetite. In other cases, more complex shapes develop. Right: Finely lamellated and spindle-like exsolution of ilmenite (brownish grey) in hematite crystal (lighter grey).
Right: Cross-like sphalerite exsolution (grey) in chalcopyrite matrix (yellowish).
Right: Patchy, almost myrmekitic rutile exsolution (slightly lighter grey) in ilmenite matrix (even grey surrounding of rutile).
Post-magmatic Environments Hydrothermal Examining hydrothermal textures, it is necessary to distinguish between vastly different types, although all are directly deriving from hydrothermal precipitation processes involving open space deposition. Right: Large open pore space of a black smoker wall with sphalerite (medium grey) and fine specs of galena (lightest grey), open pores (darkest grey).
The main textural types are: • Submarine environments at or close to the seafloor where quenching of the hydrothermal fluids produces sponge-like textures, often with skeletal mineral grains, • Fault-/fracture- or dissolution-related precipitates; temperature fluctuations in the fluids are often less pronounced than in the previous case; mineral precipitates are either very well crystallised or appear as colloform layers sub-parallel to the wall-rock exhibiting radial textures, 12
Hydrothermal (continued) • Replacements of a) sediments, where the hydrothermal ore minerals mirror the sedimentary textures or b) pre-existing minerals within hydrothermal ore deposits where, for instance, a brecciation has created pathways for successive infiltrating fluids or where a strongly differing fluid composition (often temperature-related) places existing minerals in disequilibrium with the surrounding environment. Common to all the described types is the fact that temperature variations in the fluid result in mineralogical variations that show distinct compositions of the precipitates. This could result in simple fluctuations, such as the Fe-content in sphalerite, or the formation of quite complex mineral suites as seen, for instance, in hydrothermal black smokers from inside (hot) to outside (cold): • Sphalerite-galena-pyrite-isocubanite-fahlore-enargite, • Stibnite-realgar-orpiment-marcasite-cinnabar, • Amorphous silica-barite. In general, such a succession can develop over large distances (tens or even hundreds of meters) or occur on an extremely fine scale (2-3 cm, as observed in black smokers); telescoping is the key term for this behaviour. Nevertheless, these types also share similar characteristics that can grade into each other (e.g., well crystallised and skeletal forms), making interpretationmore difficult. Right (top): Massive layered sphalerite (Schalenblende, various grey and yellowish to brown internal reflections) overgrowing jordanite (slightly anisotropic); dike filling. Right (second from top): Late proustite (red internal reflections) filling quartz druse (milky); dike filling. Right (third from top): Digital growth of orpiment (pinkish medium grey); from black smoker. Right (second from bottom): Syngenetic skeletal intergrowth of sphalerite (medium grey) and chalcopyrite (yellow); black smoker. Right (bottom): Fibrous radially crystallised kermesite (pink internal reflections); black smoker.
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Post-magmatic Environments Hydrothermal Replacements A relatively common hydrothermal replacement has been named “chalcopyrite disease” after the fine-grained impregnation of chalcopyrite in sphalerite (Barton and Bethke, 1987). This texture was originally thought to be the result of exsolution and/or co-precipitation. More recent research, however, revealed that it is related to solid-state diffusion reactions (DIS = diffusion induced segregation) by which sphalerite is replaced by various Cu-Fe-S-phases (e.g., chalcopyrite bornite, cubanite, and others) in a bi-directional redox reaction that first converts Fe2+ into Fe3+ (by increasing S-fugacity) and then flips it back to Fe2+ (Cu reacts with Fe3+), Zn2+ is liberated in the process (Blesgen et al., 2004). The replacement occurs at elevated temperatures (still within the hydrothermal frame) but without fluids being involved; rather, chalcopyrite or bornite grains adjacent to the sphalerite directly act as Cu source. Comparable with the reactions described above, there are cases of single or multiple reaction rims forming along a grain boundary. Again, we must consider geochemical gradients leading to such zonal replacements. At the moment, however, it is not clear whether during any of these re-equilibrations water is essential (i.e., if the mechanism is purely solid-state driven or not). Nevertheless, wherever the two proto-minerals are associated in the hydrothermal system, it can be assumed that re-equilibration starts soon after the establishment of these phases, thus leading to the described rims. Replacement reactions (not only in the hydrothermal environments), often focus on cracks and cleavages, along which the respective mineral is then replaced. Right (top): “Dusty” chalcopyrite (yellow) in sphalerite (grey); black smoker. Right (second from top): Larger patches of chalcopyrite DIS (yellow) in sphalerite (grey), pyrite matrix (light yellow).
Right (second from bottom): Bornite (violet grey) and chalcopyrite (yellow) act as Cu source for DIS in sphalerite (grey). Right (bottom): Native silver (white) reacts with galena (medium grey, large grain) to form an argentite/acanthite reaction rim (darker grey); the formation of such rims may extend into the supergene conditions.
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Hydrothermal Replacements (continued)
Right: Chalcocite (bluish grey) irregularly replaces chalcopyrite (yellow) along fractures.
Metamorphic Environments Recrystallisation Hydrothermal minerals, especially when they develop during fluid quenching on the oceanfloor, may precipitate as more or less amorphous colloform materials. Metamorphic/metasomatic overprinting then presents the kinetic energy for re-equilibration and re-crystallisation. Existing minerals re-adjust by increasing their grain-size and straight boundaries form that meet at specific angles (often 120º). Right (top): Recrystallised euhedral quartz (dark grey) in pyrite (creme) and chalcopyrite (yellow) from a fossil Kuroko-type deposit, as seen on the left; colloform amorphous silica (light internal reflections) next to sphalerite (dark, brown internal reflections) from a modern Kuroko-type marine hydrothermal deposit, as seen on the right. Right (bottom): Four vonsenite grains (dark pinkish to greenish greys) show the typical metamorphic angles and straight boundaries.
Fracturing & Deformation Various kinds of metamorphic processes, such as orogenic regional metamorphism or auto-metasomatic serpentinisation, involve small- to large-scale displacements and the related stress. In the most simple case, this leads to a brecciation of rigid ore minerals; mineralising fluids can subsequently pass through the cracks and deposit later mineral phases or lead to an alteration of the primary mineral. Where malleable or flexible minerals are involved, foliation will develop. Right: Brecciated pyrite (creme) with vein-filling of chalcopyrite (yellow).
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Fracturing & Deformation (continued) Further stress, even on pliant minerals, ultimately results in crenulations and kink-banding (see also pressure twins). Right: Galena (light grey) fills gaps produced by cleavage of foliated muscovite (dark).
Right: Kink-banding of layered molybdite (olive to brown pleochroism) and molybdenite (grey bireflectance).
Sedimentary Environments & Soils Transport In sedimentary environments, ore textures principally reflect the following: • Transport that shaped rock/mineral fragments (sharp-edged brecciated or rounded conglomeratic), • Physical movement during the chemical precipitation which formed oolitic/pisolitic grains (shallow marine) as well as nodular spherulitic particles during “soil creep”, • Stagnant but more or less unrestricted growth space, where chemical precipitates display continuous banding, botryoidal or vermiform aggregates (e.g., sinters), and • Cementation/Alteration binding and replacing sedimentary components, concretionary accumulations are encompassed here as well. Right: Conglomerate of uraninite (grey rounded grains) in a massive matrix of pyrobitumen (almost the same colour as uraninite), rounded pyrite (creme) with a thin overgrowth of secondary pyrite, and irregular late stage pore-fillings of gold (yellow).
Right: Finely laminated iron oolith with a relatively coarse core of oolite fragment. Such a core, which can be composed of any sedimentary material, such as sand grains or fossil relics, is characteristic for oolitic/pisolitic materials. Layering is always very fine, more than 100 layers have been counted in a manganiferous pisolith of ~2 cm diameter.
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Transport (continued)
Right: Soil spherulites are vastly different in their microscopic appearance when compared with the previous texture. They generally bind rounded to sharp-edged particles in a globular shape (often not entirely regular). A fine lamination is missing, although occasionally a crude irregular cm-thick layering can be observed (see dashed yellow line).
Sedimentary Replacements Original sedimentary textures, such as layering, may be preserved during replacement processes, even if a mild metamorphic/metasomatic overprinting occurs. A special case of replacement in sediments is the fossilisation of organic/organogenic remains by ore minerals. Organic carbon compounds (acting like activated charcoal) attract and concentrate many base metals in a reducing environment. An elevated sulfur fugacity then converts these elements to sulfide minerals, replacing internal and external shapes of the organism. Right (top): Relict textures of clay-rich sediments infiltrated by sulfide carrying solutions show sheet silicates (dark grey), sphalerite (medium grey), chalcopyrite (yellow) and pyrite (creme). Right (center): Plant cells are replaced by pyrite (creme). Right (bottom): Polychaeta relict (diagonal cut through tube worm wall) from the former surface of a black smoker is replaced by pyrite (creme) and overgrown by chalcopyrite during the growth of the smoker.
Supergene Replacement Supergene weathering (oxidation, hydration) products and textures can be as versatile as there are ore and secondary minerals. Thus, only a few are presented here. In sulfidic systems, the replacements strongly depend on the prevailing redox conditions that will either support or hinder the formation of particular minerals. The associated textures vary from patchy, irregular crack fillings, regular linear features relating to internal structures or cleavages to full replacements (mostly as pseudomorphs, less common as paramorphs).
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Supergene Replacement (continued)
Right: A characteristic replacement of chalcopyrite often encompasses multiple replacement products coinciding with a loss of Fe and an increasing oxidation potential: bornitechalcocite/covellite-tenorite and/or malachite plus azurite; native copper and cuprite are less common in this suite.
Right (center): A fine irregular network of cryptomelane replaces a siliceous rock; even quartz grains of a sandstone have been seen to be corroded in this way.
Right (bottom): Hematite replaces siderite along its cleavage planes in a regular network, leaving some unaltered carbonate in interstitial spaces.
Biomineralisation Biomineralisation has been identified in various environments, but in the context of ore minerals they are especially intriguing in relation to heavy metal mineralisation. They are often associated with filamentous bacteria, cyanobacteria or fungi (coatings, pseudomorphs). Nano-minerals, such as magnetite from magnetotactic bacteria, are not discussed here because the crystallites are too fine-grained to be directly observed. A case in which biomineralisation has been rejected is framboidal pyrite. It is now thought to form from magnetic precursors, such as greigite (iron-thiospinel; Fe3S4) which, during mild pore water movement, accumulates into globular structures of various sizes. Right (top): Cinnabar (red internal reflections) replaces fungal filaments; from a black smoker rim. Right (bottom): Framboidal pyrite (creme). 18
Right: Intercumulus chromite grain (medium grey) displaying embayments that are typical for the partial resorption of a chromite in disequilibrium with the surrounding ultramafic melt. Now, the grain is embedded in the sideritic matrix (brownish grey) of a listwaenite, which otherwise hardly bears any resemblance to the original dunitic rock (Fanja, SW of Muscat, Sultanate of Oman).
Even a slowly declining temperature during the cooling of a magma suffices to put an already established mineral into a state in which its lattice is no longer stable, leading to an exsolution process (e.g., hematite, rutile, and ulvöspinel separating from ilmenite). Fe-, Cu-, and Ag-containing sulfides are even more out of equilibrium than the oxides as soon as they come in contact with the atmosphere. These latter minerals will then start to oxidise and produce a large number of secondary phases. Such processes generate a vast array of minerals displaying various textures which, if understood, provide valuable clues on the origin of rocks, ores, and minerals and their respective formation conditions. As Barton (1991) states, “Ore deposits belong to the most complex inorganic features of our planet. If this complexity is reflected in multiple mineralisations overprinting each other but not necessarily producing a “valid” (meaningful) paragenesis, a determination of 9
the genesis of the ore is severely impeded and has to rely on individual mineralogical or geological markers (single grains may yield substantial information). In order to develop a “feeling” for the genetic hints “frozen” in a sample, it is necessary to delineate typical characteristics reflecting particular processes. Since a number of authors, such as Ramdohr (1969), Barton (1991), Craig (2001), and many others have compiled comprehensive accounts of textures, the descriptions below have been kept concise and are accompanied by typical images. The following section briefly outlines the most important textures from different geological environments: • Magmatic, • Post-magmatic (Hydrothermal), • Metamorphic, • Sedimentary & Soils, • Supergene, and • Biomineralisation. Footnote: All samples shown in the section below have been referenced in the description chapter with respect to their sample sources and show details of presented photos.
Magmatic Environments Cumulus During the cooling of a silicate magma, minerals with a high melting point start to crystallise and form euhedral to subhedral grains, because the surrounding melt poses few obstacles to the developing crystal. Thus, oxides such as chromite, ilmenite, and magnetite plus PGE alloys separate from the siliceous melt. Gravitation forces these dense minerals to sink and accumulate (cumulus phase) in deeper parts of the melt body (massive ores can form), a process that may also carry individual components into places in which the newly formed mineral is no longer in equilibrium with the melt. This, in turn, causes edge disintegration and results in rounded shapes where original crystal faces may or may no longer show, resorption embayments can also develop. Rapidly quenched extrusives (mainly basalts, but also industrial slags and alloys) often exhibit skeletal crystals, where the grains grow along preferred crystallographic orientations (axes) and do not find the time to fill interior gaps. In cases of a very fast cooling, not even crystal faces can develop and the grains have rounded outlines and dendritic arrangements. Poikilitic intergrowths between silicates, sulfides, and oxides are a common phenomenon.
Right (top): Rounded euhedral chromite in a silicate matrix (top) and Right (bottom): Massive chromite (medium grey) containing only minor amounts of silicates in fractures (darker grey).
10
Cumulus (continued)
Right: Skeletal magnetite in the siliceous matrix of a basalt (light grey crystals).
Right: Dendritic magnetite in a historic Etruscan slag (lighter grey), again in a silicate matrix.
Intercumulus (Intergranular) During fractionated crystallisation, intergranular or intercumulus textures develop wherever a mineral solidifies/crystallizes later than the surrounding matrix, thus filling the left-over interstitial space. Examples include various Cr, Fe, and Ti oxides as well as sulfides, the latter crystallizing from melts immiscible with the dominantly silicic magma.
Right (top): Intergranular chromite (lighter grey) in ultramafic silicate matrix (dark grey). Right (bottom): Interstitial pyrite (cream) and connected droplet in a magnetite matrix (grey).
Exsolution Generally, exsolution textures imply slow or intermediate cooling rates, during which the respective solid phase re-equilibrates mineralogically and texturally (Barton, 1970). Miscibility gaps in the phase stability of a number of minerals show up during cooling: A homogenous single-phase high-temperature component breaks down into two or more distinct minerals. The result is an oriented intergrowth of those components that no longer fit into the same crystal lattice at the lower temperature; examples are, for instance, surplus Fe or Ti in ilmenite or extra Fe in Cu-sulfides, such as chalcopyrite. 11
Exsolution (continued) This process presents a record of the thermal history of the ore (Ramdohr, 1969). These exsolutions form discrete mineral grains along crystallographically preferred directions of the host from which they differentiated. Thus, oriented exsolution lamellae of rutile, ulvöspinel or ilmenite may occur in titanomagnetite. In other cases, more complex shapes develop. Right: Finely lamellated and spindle-like exsolution of ilmenite (brownish grey) in hematite crystal (lighter grey).
Right: Cross-like sphalerite exsolution (grey) in chalcopyrite matrix (yellowish).
Right: Patchy, almost myrmekitic rutile exsolution (slightly lighter grey) in ilmenite matrix (even grey surrounding of rutile).
Post-magmatic Environments Hydrothermal Examining hydrothermal textures, it is necessary to distinguish between vastly different types, although all are directly deriving from hydrothermal precipitation processes involving open space deposition. Right: Large open pore space of a black smoker wall with sphalerite (medium grey) and fine specs of galena (lightest grey), open pores (darkest grey).
The main textural types are: • Submarine environments at or close to the seafloor where quenching of the hydrothermal fluids produces sponge-like textures, often with skeletal mineral grains, • Fault-/fracture- or dissolution-related precipitates; temperature fluctuations in the fluids are often less pronounced than in the previous case; mineral precipitates are either very well crystallised or appear as colloform layers sub-parallel to the wall-rock exhibiting radial textures, 12
Hydrothermal (continued) • Replacements of a) sediments, where the hydrothermal ore minerals mirror the sedimentary textures or b) pre-existing minerals within hydrothermal ore deposits where, for instance, a brecciation has created pathways for successive infiltrating fluids or where a strongly differing fluid composition (often temperature-related) places existing minerals in disequilibrium with the surrounding environment. Common to all the described types is the fact that temperature variations in the fluid result in mineralogical variations that show distinct compositions of the precipitates. This could result in simple fluctuations, such as the Fe-content in sphalerite, or the formation of quite complex mineral suites as seen, for instance, in hydrothermal black smokers from inside (hot) to outside (cold): • Sphalerite-galena-pyrite-isocubanite-fahlore-enargite, • Stibnite-realgar-orpiment-marcasite-cinnabar, • Amorphous silica-barite. In general, such a succession can develop over large distances (tens or even hundreds of meters) or occur on an extremely fine scale (2-3 cm, as observed in black smokers); telescoping is the key term for this behaviour. Nevertheless, these types also share similar characteristics that can grade into each other (e.g., well crystallised and skeletal forms), making interpretationmore difficult. Right (top): Massive layered sphalerite (Schalenblende, various grey and yellowish to brown internal reflections) overgrowing jordanite (slightly anisotropic); dike filling. Right (second from top): Late proustite (red internal reflections) filling quartz druse (milky); dike filling. Right (third from top): Digital growth of orpiment (pinkish medium grey); from black smoker. Right (second from bottom): Syngenetic skeletal intergrowth of sphalerite (medium grey) and chalcopyrite (yellow); black smoker. Right (bottom): Fibrous radially crystallised kermesite (pink internal reflections); black smoker.
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Post-magmatic Environments Hydrothermal Replacements A relatively common hydrothermal replacement has been named “chalcopyrite disease” after the fine-grained impregnation of chalcopyrite in sphalerite (Barton and Bethke, 1987). This texture was originally thought to be the result of exsolution and/or co-precipitation. More recent research, however, revealed that it is related to solid-state diffusion reactions (DIS = diffusion induced segregation) by which sphalerite is replaced by various Cu-Fe-S-phases (e.g., chalcopyrite bornite, cubanite, and others) in a bi-directional redox reaction that first converts Fe2+ into Fe3+ (by increasing S-fugacity) and then flips it back to Fe2+ (Cu reacts with Fe3+), Zn2+ is liberated in the process (Blesgen et al., 2004). The replacement occurs at elevated temperatures (still within the hydrothermal frame) but without fluids being involved; rather, chalcopyrite or bornite grains adjacent to the sphalerite directly act as Cu source. Comparable with the reactions described above, there are cases of single or multiple reaction rims forming along a grain boundary. Again, we must consider geochemical gradients leading to such zonal replacements. At the moment, however, it is not clear whether during any of these re-equilibrations water is essential (i.e., if the mechanism is purely solid-state driven or not). Nevertheless, wherever the two proto-minerals are associated in the hydrothermal system, it can be assumed that re-equilibration starts soon after the establishment of these phases, thus leading to the described rims. Replacement reactions (not only in the hydrothermal environments), often focus on cracks and cleavages, along which the respective mineral is then replaced. Right (top): “Dusty” chalcopyrite (yellow) in sphalerite (grey); black smoker. Right (second from top): Larger patches of chalcopyrite DIS (yellow) in sphalerite (grey), pyrite matrix (light yellow).
Right (second from bottom): Bornite (violet grey) and chalcopyrite (yellow) act as Cu source for DIS in sphalerite (grey). Right (bottom): Native silver (white) reacts with galena (medium grey, large grain) to form an argentite/acanthite reaction rim (darker grey); the formation of such rims may extend into the supergene conditions.
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Hydrothermal Replacements (continued)
Right: Chalcocite (bluish grey) irregularly replaces chalcopyrite (yellow) along fractures.
Metamorphic Environments Recrystallisation Hydrothermal minerals, especially when they develop during fluid quenching on the oceanfloor, may precipitate as more or less amorphous colloform materials. Metamorphic/metasomatic overprinting then presents the kinetic energy for re-equilibration and re-crystallisation. Existing minerals re-adjust by increasing their grain-size and straight boundaries form that meet at specific angles (often 120º). Right (top): Recrystallised euhedral quartz (dark grey) in pyrite (creme) and chalcopyrite (yellow) from a fossil Kuroko-type deposit, as seen on the left; colloform amorphous silica (light internal reflections) next to sphalerite (dark, brown internal reflections) from a modern Kuroko-type marine hydrothermal deposit, as seen on the right. Right (bottom): Four vonsenite grains (dark pinkish to greenish greys) show the typical metamorphic angles and straight boundaries.
Fracturing & Deformation Various kinds of metamorphic processes, such as orogenic regional metamorphism or auto-metasomatic serpentinisation, involve small- to large-scale displacements and the related stress. In the most simple case, this leads to a brecciation of rigid ore minerals; mineralising fluids can subsequently pass through the cracks and deposit later mineral phases or lead to an alteration of the primary mineral. Where malleable or flexible minerals are involved, foliation will develop. Right: Brecciated pyrite (creme) with vein-filling of chalcopyrite (yellow).
15
Fracturing & Deformation (continued) Further stress, even on pliant minerals, ultimately results in crenulations and kink-banding (see also pressure twins). Right: Galena (light grey) fills gaps produced by cleavage of foliated muscovite (dark).
Right: Kink-banding of layered molybdite (olive to brown pleochroism) and molybdenite (grey bireflectance).
Sedimentary Environments & Soils Transport In sedimentary environments, ore textures principally reflect the following: • Transport that shaped rock/mineral fragments (sharp-edged brecciated or rounded conglomeratic), • Physical movement during the chemical precipitation which formed oolitic/pisolitic grains (shallow marine) as well as nodular spherulitic particles during “soil creep”, • Stagnant but more or less unrestricted growth space, where chemical precipitates display continuous banding, botryoidal or vermiform aggregates (e.g., sinters), and • Cementation/Alteration binding and replacing sedimentary components, concretionary accumulations are encompassed here as well. Right: Conglomerate of uraninite (grey rounded grains) in a massive matrix of pyrobitumen (almost the same colour as uraninite), rounded pyrite (creme) with a thin overgrowth of secondary pyrite, and irregular late stage pore-fillings of gold (yellow).
Right: Finely laminated iron oolith with a relatively coarse core of oolite fragment. Such a core, which can be composed of any sedimentary material, such as sand grains or fossil relics, is characteristic for oolitic/pisolitic materials. Layering is always very fine, more than 100 layers have been counted in a manganiferous pisolith of ~2 cm diameter.
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Transport (continued)
Right: Soil spherulites are vastly different in their microscopic appearance when compared with the previous texture. They generally bind rounded to sharp-edged particles in a globular shape (often not entirely regular). A fine lamination is missing, although occasionally a crude irregular cm-thick layering can be observed (see dashed yellow line).
Sedimentary Replacements Original sedimentary textures, such as layering, may be preserved during replacement processes, even if a mild metamorphic/metasomatic overprinting occurs. A special case of replacement in sediments is the fossilisation of organic/organogenic remains by ore minerals. Organic carbon compounds (acting like activated charcoal) attract and concentrate many base metals in a reducing environment. An elevated sulfur fugacity then converts these elements to sulfide minerals, replacing internal and external shapes of the organism. Right (top): Relict textures of clay-rich sediments infiltrated by sulfide carrying solutions show sheet silicates (dark grey), sphalerite (medium grey), chalcopyrite (yellow) and pyrite (creme). Right (center): Plant cells are replaced by pyrite (creme). Right (bottom): Polychaeta relict (diagonal cut through tube worm wall) from the former surface of a black smoker is replaced by pyrite (creme) and overgrown by chalcopyrite during the growth of the smoker.
Supergene Replacement Supergene weathering (oxidation, hydration) products and textures can be as versatile as there are ore and secondary minerals. Thus, only a few are presented here. In sulfidic systems, the replacements strongly depend on the prevailing redox conditions that will either support or hinder the formation of particular minerals. The associated textures vary from patchy, irregular crack fillings, regular linear features relating to internal structures or cleavages to full replacements (mostly as pseudomorphs, less common as paramorphs).
17
Supergene Replacement (continued)
Right: A characteristic replacement of chalcopyrite often encompasses multiple replacement products coinciding with a loss of Fe and an increasing oxidation potential: bornitechalcocite/covellite-tenorite and/or malachite plus azurite; native copper and cuprite are less common in this suite.
Right (center): A fine irregular network of cryptomelane replaces a siliceous rock; even quartz grains of a sandstone have been seen to be corroded in this way.
Right (bottom): Hematite replaces siderite along its cleavage planes in a regular network, leaving some unaltered carbonate in interstitial spaces.
Biomineralisation Biomineralisation has been identified in various environments, but in the context of ore minerals they are especially intriguing in relation to heavy metal mineralisation. They are often associated with filamentous bacteria, cyanobacteria or fungi (coatings, pseudomorphs). Nano-minerals, such as magnetite from magnetotactic bacteria, are not discussed here because the crystallites are too fine-grained to be directly observed. A case in which biomineralisation has been rejected is framboidal pyrite. It is now thought to form from magnetic precursors, such as greigite (iron-thiospinel; Fe3S4) which, during mild pore water movement, accumulates into globular structures of various sizes. Right (top): Cinnabar (red internal reflections) replaces fungal filaments; from a black smoker rim. Right (bottom): Framboidal pyrite (creme). 18