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Sediment-Derived Euclase Mineral Characterization and Its EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 1 SEDIMENT-DERIVED EUCLASE MINERAL CHARACTERIZATION AND ITS IMPLICATIONS FOR THE EVOLUTION OF THE COLOMBIAN EMERALD DEPOSITS Author: Julián Mauricio Reyes Álvarez Director: Idael Francisco Blanco Quintero Co-Director: Edwin Yovanny Alba Vasquez Universidad de Los Andes EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 2 EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 3 SEDIMENT-DERIVED EUCLASE MINERAL CHARACTERIZATION AND ITS IMPLICATIONS FOR THE EVOLUTION OF THE COLOMBIAN EMERALD DEPOSITS _____________________________ Julián Mauricio Reyes Álvarez Estudiante _____________________________ _____________________________ Idael Francisco Blanco Quintero Edwin Yovanny Alba Vasquez Director Co-Director Universidad de Los Andes Facultad de Ciencias Departamento de Geociencias Septiembre 2016 EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 4 Author Note Julián M. Reyes and Idael Blanco, Department of Geosciences, School of Sciences, Universidad de Los Andes. This research was presented as an undergraduated thesis project for Julián M. Reyes at Universidad de Los Andes, Bogotá, Colombia. This research was supported by the Fondo Educativo Howard T. Corrigan grant from the Asociación Colombiana de Geólogos y Geofísicos del Petróleo (ACGGP) and Corporación Geológica Ares. Questions concerning this article should be written to Julián M. Reyes. E-mail: [email protected] EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 5 Contents Abstract ............................................................................................................................... 7 Introduction ......................................................................................................................... 8 Conceptual Framework ....................................................................................................... 9 Fluid inclusions (FI) ........................................................................................................ 9 Microthermometry ........................................................................................................ 12 Hydrothermal ore deposit ............................................................................................. 13 Albitization ................................................................................................................... 14 Geological Framework...................................................................................................... 15 Methodology and Analytical techniques ........................................................................... 20 Field work ..................................................................................................................... 20 Laboratory work............................................................................................................ 22 Petrography. .............................................................................................................. 22 Whole Rock Chemistry. ............................................................................................ 22 SEM & X-Ray spectroscopy. .................................................................................... 22 Microthermometry. ................................................................................................... 23 Results ............................................................................................................................... 23 Petrography ................................................................................................................... 23 Whole rock chemistry ................................................................................................... 24 SEM & X-Ray spectroscopy ......................................................................................... 28 EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 6 Microthermometry ........................................................................................................ 32 Discussion and Conclusion ............................................................................................... 33 Acknowledgements ........................................................................................................... 42 References ......................................................................................................................... 44 Tables ................................................................................................................................ 51 EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 7 Abstract Euclase is a precious stone commonly found in igneous or metamorphic rocks formed after dissolution/reprecipitation of beryl. However, it is found in cretaceous black shales, of the Paja formation, in the Colombian Emerald Belt. The field occurrence of euclase is in albitite veins, perpendicular to large fault systems (dip 144°/43°). The mineralization of these veins is composed of euclase + apatite + albite, but emerald (beryl) coexisting with euclase is scarce and generally has not been identified in most deposits. To determine the conditions of formation in a sedimentary environment, fluid and solid inclusions have been investigated by microthermometric studies, micro-Raman spectroscopy, X-ray spectroscopy and electron microscopy. Further, the host rock is analyzed to investigate the interaction between the hydrothermal fluids with the black shales and the effect in the mineralization. Petrography studied in thin sections show that the main vein deforms the surrounding crystals. The information gathered is compared to the reported for emerald deposits. The fluid on euclase has a mean eutectic temperature of -96.5 ºC and a minimum temperature of formation of 338 ºC, with similar temperatures for apatite and albite, moreover all inclusions have high salinity ~40 % in weight of NaCl eq. Mineral inclusions inside the main phases (euclase, apatite and albite) are quartz, plagioclase, phengite, pyrite, phenakite, rutile and diaspore. Whole rock chemistry shows that the host rock, close to the mineralization, is depleted in most of the elements. The data here presented shown that the fluids that produce the euclase in the sedimentary deposits of Colombia has similar temperature and salinity of the emerald; precluding the formation by dissolution/reprecipitation of the former by the second. Alternatively, the data support the crystallization of euclase instead emerald by change in the activity of some major components like Al and/or Si. Keywords: Euclase, Colombian Emerald Belt, Albitization, Mineralogy. EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 8 Introduction The Colombian Emerald Belt (CEB) is an area with hydrothermal veins of low-T which crystalizes a variety of gemstones of great quality, such as emerald, parisite, codazzite and euclase (Kozlowski et al., 1988; Cheilletz et al., 1994; Giuliani et al., 1995, 1999; Banks et al., 2000). The CEB is located in the center part of the Eastern Cordillera, from the south of Santander to the north of Cundinamarca departments, mainly in the department of Boyacá. This zone is divided into two regions, the Western and Eastern Emerald Belts. Each one is subdivided into mining districts where the most important are Muzo, Coscuez and Peñas Blancas from the western and Chivor, Gachala to the eastern districts (Maya et al., 2004). Probably the most important feature of the CEB is the sedimentary origin for these gemstone crystals. The CEB has been known for the great quality of gems found through all the area. One of this gemstones is the euclase (AlBeSiO4(OH)) which is a greenish blue, pale blue mineral related to emerald (Al2Be3Si6O18(Cr,V)) (Rubiano, 1990; Chavez-Gil et al., 1997). Formally euclase is defined as an orthosilicate of beryllium and aluminum with a radical (OH), crystallizes in the monoclinic system (Gossner & Mussgnug, 1929; Biscoe & Warren, 1933 in: Vlasov, 1966) Generally, euclase is found in pegmatites, a high-T environment associated with an intrusion of igneous body (mostly of acid composition), characterized by the large size of the crystals (bigger than 2 cm; Gallagher & Hawkes, 1966; Strand, 1953). Euclase normally is considered as formed in pegmatites after dissolution of beryl and then crystalized in a different position. (Strand, 1953). However, the genesis of euclase in low-T (sedimentary) systems is not well reported. Hence, the main objective of this thesis is to determine how the euclase crystals form in a low-T environment, the conditions (temperature and composition) of the fluid that crystallizes it, and how does his formation affects the evolution of the emerald deposits. To accomplish the EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 9 research, the fluid and solid inclusions of the euclase are going to be studied, such as those found in related minerals of apatite and calcite. Also geochemistry and petrography of the vein and host rock is going to be done, combined with Scanning Electron Microscope and Raman Spectroscopic techniques. The starting materials for the studies are samples collected in the La Marina mine located in the Western Emerald Belt. Conceptual Framework Fluid inclusions (FI) A fluid inclusion is a small quantity of fluid that got trapped in imperfections of a crystal while it was crystalizing or as product of the healing of fractures (Bodnar, 2003). Commonly the size of an inclusion is very low, less than 1mm, and generally between 1 – 10 µm. There is relation between the size of the inclusions and the quantity
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