Emerald Deposit Type And Occurrences Biology Essay

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Emerald occurs in several localities. According to Giuliani deposits of economical importance occurs in Afghanistan, Brazil, Colombia, Madagascar, Pakistan, Russia Zambia, and Zimbabwe with other occurrences in Australia, Austria, Bulgaria, Canada, China, Egypt, India, Mozambique, Nigeria, Norway, South-Africa, Somalia, Spain, Tanzania and the USA. Emerald origin and occurrences has been described by many researchers including ( e.g. Barton and Young, 2002; Zwaan, 2006; Giuliani et al, 1998; Kazmi et al, 1989, Ottaway et al, 1994 and many more) which has been briefly summarised by Groat et al (2008). The following section on some occurrences is adapted from these authors.

Columbia:

For centuries Colombia has produced the largest and finest quality emerald crystals in the world. The various emerald deposits in Colombia are located around two belts, the eastern and western side of the Cordillera Oriental. This is the easternmost section of three mountain ranges extending from the northern end of the Andes.

According to Branquet et al (1999a cited in Kathryn, 2000; Groat et al, 2008) the two belts are geochemically similar but differ in age and tectonic settings, the eastern side is about 65 millions year old and the western side about 32-38 millions year old. The Chivor, Gachalá, and Macanal deposits in the Guavió-Guatéque mining district is on the eastern side of the Cordillera Oriental (Giuliani et al., 1993). The western side of the Cordillera Oriental consists of Coscuez, Maripi, Muzo, La Pava, Peñas Blancas, and Yacopí deposits in the Vasquez-Yacopí mining district. Despite the rumour that these deposits are depleting, the La Pita find in the Maripi area in 1998 shows to be a promising deposit and should ensure that Colombian deposits remain a prolific source of emeralds for years to come ( Groat et al, 2008).

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Emerald in the Colombian deposit occurs in carbonate-silicate-pyrite veins, pockets, and breccias in Early Cretaceous black shale-limestone. On the eastern side the emerald deposits formed around the Cretaceous-Tertiary boundary during a soft tectonic event which was controlled by evaporite dissolution which is stimulated by gravity (Branquet et al., 1999a cited in Groat et al, 2008; Barton and Young 2002). On the western side the deposits are linked by faults and thrusts, which developed during a compressive tectonic period that occurred at the time of the Eocene-Oligocene boundary, before the major uplift of the Cordillera during the middle Miocene (Branquet et al., 1999a,b cited in Groat et al, 2008; Barton and Young 2002).

The unusual Columbian deposit is structurally related to tectonic blocks, 200 to 300 m wide, with no evidence of magmatic or pegmatitic activity and emerald mineralization is contained in black shales altered by Na and Ca metasomatism. The deposits are believed to have formed as a result of hydrothermal growth associated with tectonic activity (Ottaway et al., 1994; Giuliani et al., 1995; Branquet et al., 1999a, cited in Groat et al, 2008; Barton and Young 2002). It is suggested that these deposit have an evaporitic hydrothermal source, the deriving fluids from depths of 7 km and temperatures of at least 250oC to 400oC interacted with salt beds becoming highly alkaline (up to 40 wt.% NaCl). These saline fluids than migrated through sedimentary sequence along thrust planes before reaching black shales in the back arc basin (Groat et al, 2008; Barton and young, 2002). Ottaway (1991) as quoted in Groat et al (2008) reported precipitation temperatures of emerald at the Muzo deposit at about 325 °C, while Cheilletz et al. (1994) reported temperatures of 290 -360oC for Coscuez mine (Groat et al, 2008).

Ottaway et al. (1994), suggested the fluids transported evaporitic sulphate to promissing sites, where it was thermochemically reduced by bitumen-derived H2S to produce native sulphur and pyrite. The sulphur produced by this method reacted with the organic matter in the shales to release trapped Cr, V, and Be, which in turn enabled emerald formation (Ottaway et al., 1994). This idea of sulphur production by thermochemical reduction was argued by Chielletz and Giuliani (1996) and Giuliani et al. (2000b cited in Groat et al, 2008) but emphasized on the role of organic matter in the formation of Colombian emeralds. They proposed a redox reaction involving large organic molecules, a carbonic hydrate, and SO42−, this would generate large quantities of HCO3 − and H2S, which then reacted with Ca2+ and Fe2+ (extracted from the black shale by the hydrothermal fluid) to produce calcite and pyrite. These minerals are closely associated with the emeralds in the veins and breccias.

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Nassau (1983) suggested that Colombian emeralds have a deeper colour than Fe-bearing emeralds from other deposit with pegmatitic ultramafic environments because Fe3+ quenches the red fluorescence. Ottaway (1991 cited in Groat et al, 2008 and Ottaway, 1994) suggested that the removal of Fe from the system as pyrite is an important factor in the development of the spectacular colour of Colombian emeralds.

Australia

There are a few districts where emerald deposit occurs in Australia. The two famous Australian deposits are in the Emmervale-Torrington districts (related to pegmatitic intrusion without schist at contact zone) and the Poona district (related to pegmatitic intrusion with schist at contact zone).

Emerald in Australia was first discovered in 1890 in an abandon tin mine near Emmaville in northern New South Wales. This discovery wasn`t particularly economical and the mine was opened and closed several times, out of the 25,000 ct originally mined, only 0.01% to 0.02% were of gem-quality (Groat et al, 2008).

Both the close by Emmaville and Torrington deposits are related to the intrusion of pegmatites and aplites into mudstones and siltstones (Kazmi and Snee, 1989). According to the study by Groat et al (2002) the isotopic investigation on the fluids trapped by the emeralds indicates a magmatic origin. Mineralization at the Emmaville mine are in the pegmatite dikes with fluorite, beryl, quartz, topaz, arsenopyrite, and cassiterite. The crystals are embedded in granitic cavities that have weathered to kaolinite. At the Torrington mineralization occurs in quartz and pegmatite veins which are associated with quartz, feldspar, biotite, and wolframite.

The other economical and productive emerald district is a 10 km2 area around the village of Poona in Western Australia (Kazmi and Snee, 1989). It was reported by(Grundmann and Morteani, 1995 cited in Groat et al, 2008) that mineralization of emeralds in this deposit was a result of metasomatic reaction caused by low-grade metamorphism, between pre-existing quartz-muscovite or quartz-topaz greisens and ultramafic bodies. The formation of pale green emerald occurs with chrysoberyl, ruby, sapphire, margarite, topaz, fluorite, and quartz in a phlogopite "blackwall" zone. The chrysoberyl crystals in this deposit display the alexandrite effect.

Sandwana, Zimbabwe Deposit

In 1956 Emeralds were discovered on the southern slope of the Mweza Range, 360 km S of Harare in Zimbabwe. Majority of mined emeralds comes from the Sandawana (formerly Zeus) mine which covers more than 40 km of workings and depths reaching up to 152 m. Zwaan et al (1997) quoted in Zwaan (2006) reported that this deposit is the oldest known source available for emeralds, forming around 2.6 Ga. These emeralds have a bright green hue, but faceted gem quality stones are rare over 1.5 cts.

The mineralization process in this deposit began when pegmatites intruded the Mweza Greenstone Belt prior to and/or during a major deformation event. Then, a late-stage Na-rich solution containing Li, Be, F, P, and Cr was injected along shear zones, causing albitization of the pegmatite and phlogopitization of the wallrock (Zwaan, 2006). The deformation of pegmatites, showing differing layers associated with amphibole-phlogopite schist, and (micro) shear zones indicate ductile deformation. According to Zwaan (2006) this deformation led to the synkinematic growth of emerald, chromian ilmenorutile, fluorapatite, and holmquistite which indicates enrichment of Li, Be, F, Na, P, K, Rb, Nb, Cs, and Ta in the shear zones, this suggests that emerald formation in this deposit is closely related to syntectonic K-Na metasomatism, in which actinolite, albite, cummingtonite, emerald, fluorapatite, holmquistite, and phlogopite crystallized at the expense of microcline, oligoclase, quartz, and chlorite.

The saline brine fluid inclusion in emerald also indicates that a Na- and F-rich hydrous fluid was involved in the process which produced emerald. Crystallisation temperature due to contact metamorphism for this deposit is believed to be around 560 to 650 °C which was obtained by apatite-phlogopite thermometrical analysis (Zwaan, 2006).

Geochemistry of Be, Cr and V.

Emerald formation necessitates Be which is rare and is usually concentrated in granites, pegmatites, black shales or their metamorphic equivalent. The Total abundance of Be in continental crust is about 1.9 ppm (Rudnick et al, 2003). The more common Cr and V (92 and 97 ppm) are usually concentrated in dunites -peridoties -basalts of the oceanic crust -upper mantle or their metamorphic equivalents in the upper continental crust (Rudnick et al, 2003). Studies by Schwarz et al (2002) has shown high concentrations of Cr and / V can also occur in sedimentary rocks, particularly black shales. Be apart from being rare is also an incompatible element due to its small ionic radius (0.3 Angstrom as BeO) therefore it is excluded by many minerals during crystallization. Be will stay in the melt as long as possible, -Fluorine (F) -Lithium (Li) -Boron (B) and -Phosphorous (P) also encourages the retention of Be in the melt during crystallisation. Though Be solubility can be increased by high levels of aSiO2 and aAl2O3. In hydrothermal fluids the complexing agents that control Be solubility are F, CO3, OH, F-CO3 and F-OH. Most Be deposits are associated with F phases, suggesting that F plays an important role Be solubility. Be being incompatible will concentrate in the late stage water rich melt from which pegmatites crystallise. Be may also fractionate into the late stage hydrothermal fluids - in quartz-rich veins. This makes emerald quite rare as unusual geological environment is required for Be, Cr and / V to meet.

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Classification

The diverse emerald mineralization is somewhat unique to its locality or particular geological environment. Therefore a number of genetic classifications have been put together over the years by different researchers to simplify and better understand the diverse petrogenesis of emerald minerlization. A universal classification is usually based on similarities or trends of one type or another over the amount of analysis in total or based on different petrogenetic groupings, some of the many modals has been mentioned in a number of studies (e.g. Zwaan 2006, Groat et al, 2008) for emerald deposits around the world. Dreppe et al (2002) based his classification on artificial neural network, Barton and Young (2002) associated the deposits with direct or indirect igneous connection, the indirect connection is further subdivided by host rock and / association of magma, Schwarz and Giuliani (2001) and Schwarz et al. (2001) based their classification on granitic intrusions or tectonic systems. Sabot (2002) based his classification using geochemical data to differentiate deposits. Although this modal can in future lead to potential universal classification but much work needs to be done.

Dreppe et al. (2002) classified the deposits using artificial neural network (ANN) from 450 electron microprobe analysis from different localities. The artificial neural network (ANN) is basically a simple mathematical algorithm defining distribution from the inputted data. Dreppe et al (2002) managed to classify them in five categories starting with 1. -granitic pegmatite intrusions and hydrothermal veins in mafic-ultramafic rocks 2.-tectonism( thrusts, shear, zones, faults) in mafic-ultramafic rocks 3.-Oceanic suture zones 4.- thrusts and faults in sedimentary "black shale" rocks and 5.- " Granite-cupola-type". This classification also has a "bad score" system for misclassification, an example of this is category 3 the oceanic suture zone where it has 12.5% in the bad score coloumn this refers to the fact that Afghan emeralds originate from quartz-ankerite-pyrite veins, whereas emeralds from the Swat Valley are found in an (Fe+Mg)-rich environment (talc-carbonate-mica schists)

The classification by Barton and Young (2002) on emerald deposits is association of direct or indirect igneous intrusion. Further subdivision of indirect igneous intrusion (i.e. metasomatic) was associated with magma and / or host rock. Certain deposits are also grouped by origin. The difficulty for this system was that certain deposit cannot be classified because there is interrelation between the direct and indirect igneous intrusion e.g. The swat valley deposit are classed as carbonate hosted but the source of the Cr is the associated with ophiolitic melange. For this reason the Swat Valley deposit could also be classfied as mafic / ultramafic (Barton and Young, 2002).

The classification by Schwarz and Giulani (2001) and Schwarz et al (2001) recognized two main types of emerald deposit which this study is based on are -Type 1 those related to granitic intrusions and -type 2 related to tectonic structures, such as thrust faults and shear zones. The majority of emerald deposits fall within the first category and are subdivided on the basis of pegmatites with or without schist at contact. Type 2 deposits are subdivided into schists without pegmatites and black shales with veins and breccias.

According to Zwaans (2006) argument where he pointed out that certain deposits cannot be classified clearly using these classification modals, as the mineralization does not rule out the connection of igneous intrusion in suture or shear zones. For instance the Egyptian deposit though it is classified as schist without pegmatites, but the mineralization is closely related to that of pegmatite veins (Abdulla and Mohamed, 1999) and the Swat Valley deposit where there is a latent link between fluids deriving from pegmatitic origin and mineralization in suture zones. In addition to the latter model (by Schwarz and Giuliani 2001 and Schwarz et al 2001) Zwaan (2006) mentioned that Carnaiba, Brazil and Poona, India deposits maybe have been influenced by tectonism. Zwaan (2006) also argued that the existing classification modals are ambiguous therefore it is not particularly useful when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit.