Sample 54: Sample taken at the contact between vein and shale in a hydraulic breccia. Within the
5. MINERALOGY AND GEOCHEMISTRY
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5. MINERALOGY AND GEOCHEMISTRY
A total of 36 samples were selected for carbon and oxygen analyses. Those samples correspond to vein
calcite, breccia and host rocks from the productive and non- productive areas in seven different mines from the WEB (Table 2). The separation of the samples was done manually.
The criteria for the classification of the samples according to their crystal habits, emerald potential zones and specific sample locations are set out below:
Crystal habit
Three different calcite crystal habits were recognized:
1. Fibrous calcite (Fig. 32): According to Giullini 1994, this calcite is typical of the first stage of the emerald mineralization and is not associated directly with the emeralds since the beryl precipitated at a later stage. However, previous authors (Torres Giraldo, 2004) have reported emerald
Table 2 Number of samples per mine.
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mineralization in this stage, concluding that the fibrous calcite is indeed an early stage, but afterwards in a later recrystallization of the emeralds, the calcite is replaced. The emerald in this stage is not economically viable. Field observations for this research did not note emeralds in paragenesis with the fibrous calcite. The miners in the area do not recognize the fibrous calcite as a mineral indicator for the presence of emerald, therefore this crystal habit is interpreted as non-productive.
Figure 32 Fibrous calcite.
1. Calcite Geode (Fig. 33): This occurrence was only found in Españoles mine. The reasons for
geode hosted mineralization are: 1. The very complex structural geology in the mine, where more recent faults cut previous folds and faults plus steep dip angles for the stratigraphy of 75- 85° in multiple orientations. Tight overturned, recumbent and asymmetrical anticlines are evident along the tunnels and galleries in Españoles. 2. The mine is probably located in the hinge of a large anticline where the pressure and temperatures are high enough to hydraulically
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fracture the shale, and generate relatively large cavities. The mineralizing fluid is mobilized to these cavities, where pressure decreases lead to carbonate crystallization in a geode. 3. This calcite is assumed to be deposited in an intermediate stage, as the fibrous calcite is deposited in the absence of hydraulic breccia (Giulliani 1996). Emerald productivity in this setting is still debatable.
2.
Figure 33 Calcite geode (taken from Torres Giraldo, 2004).
2. Rombohedral calcite (Fig. 34): For emerald exploration, the rombohedral calcite is a positive
indicator, since this mineral is deposited in the latest stage for the emerald precipitation.
Hydraulic breccias occur in high fluid-pressure zones related to stage 2 vein development (Giulliani 1996). This carbonate is present throughout the visited emerald mines and is a good proxy for emerald exploration. However, this proxy is not enough as not all the rombohedral
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carbonates are accompanied by green beryl. It is necessary to consider additional criteria in order to define a zone as a productive area.
3.
Figure 34 Rombohedral calcite with emerald
Emerald potential zones
In the field, the emerald potential for each sample was determined on the basis of the parameters defined by Mantilla et al., 2007. These ‘‘indicators of reliability and risk’’, would be a direct reflection of the degree of geological knowledge of the area of interest. The knowledge and experience of the miners are considered but sometimes these are not totally reliable since there are often contradictions between themselves.
The following are the field geological criteria to be considered to delimit areas with emerald potential (Modified from Mantilla et al., 2007).
1. Depocenter area formed during the lower Cretaceous: During the lower Cretaceous, this sector of the Colombian territory underwent a marine invasion of the
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Caribbean Sea, forming a bay (in a ‘’back-arc’’ environment), whose southern limits extended approximately until the present Ecuadorian territory. In this paleogeographic context, there was an important accumulation of sediments, especially in the area of the aforementioned depocenter, where the WEB is now located.
All the sampled mines are located in this area
2. Age of the rocks which host the hydrothermal manifestations: Geological mapping with an important bio-stratigraphic component, made by INGEOMINAS (Reyes et al., 2006), has allowed the identification of two important belts, where the emeralds are hosted. In the WEB area, the emerald deposits are present in two formations: limestone of the Rosablanca Formation (Valanginian) and Shales of the Muzo Formation (Hauterivian-Barremian) with a thickness of approximately 160 to 300m.
All the sampled mines are in shales of the Muzo Formation.
3. Tectonic and hydrothermal structures: The emerald mineralization in the Muzo Formation located in the WEB is related with a very strong metasomatism of the black shales caused by the alkaline and saline fluid expelled by the compressive tectonics in the Paleogene. The fluid was driven by thrust faulting, and due to the pressure:
hydraulic breccia, stockwork veins, geodes and hydrothermal breccias were developed.
The shale shows albitization and carbonization which, in a later stage, is altered to kaolin. Tight recumbent, asymmetrical and overturned folds are common in the area.
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4. Mineralogical and geochemical aspects: The emeralds in the WEB are in paragenesis with dolomite, ankerite, calcite and authigenic albite which precipitated after the interaction between the brines and the shale. Pyrite and chalcopyrite are not present everywhere but are important indicators for emeralds as those minerals represent sulphidation of the excess Fe available during the mineralising process, which limits the contamination and discolouration of the emeralds. The sulphides are precipitated due to the reduction of the fluid when it is in contact with the shale (Cheilletz et al., 1994). Quartz is also sometimes present, even forming druses. The presence of this mineral is assumed to be a result of an increase in the temperature within the hydrothermal system. All the mentioned minerals are emplaced in veins or breccia within the black shale. Some supergene minerals have been precipitated due to oxidation of primary minerals. Geochemical analyses in the electronic microscopy and RAMAN laboratory at the National Taiwan University identified: allophane Al2O3·(SiO2)1.3-2·(2.5-3)H2O, epsomite MgSO4·7H2O and ettringite Ca6Al2(SO4)3(OH)12·26H2O plus gypsum.
All the aforegoing minerals show the results of hydrothermal interaction between the host rocks and the brines and are positive indicators for emerald occurrence. All the mines show at least three of these minerals.
5. Debris flow vs. Hydrothermal breccia: The concept of hydrothermal breccia in the
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shales can be confused with old debris flow zones. This characteristic is discussed for the first time in this research
In the study area, it is possible to recognize both hydrothermal breccia and secondary debris flow in the weathering zone using field criteria. The hydrothermal breccia generally manifests in very soft and carbonaceous altered shale, and it gives a non-compact appearance to the rock. Debris flows are most common in areas of high slope angles. Both deposits in the field can be confused by the miners. In the case of the Monteblanco mine, located at the base of a mountain, close to a river, it is evident that the miners are exploring debris flow deposits, since there is no continuity to the veins or faults. This contrasts with the situation at La Pita, developed in outcrop
For this reason, the Monteblanco, Puerto Siad and much of Masato mines are considered as relatively low probability productive areas as they are interpreted as being hosted in debris flow deposits. Any emeralds found around those mines is interpreted to be deposited from up slope via eluvial gravity slides.
The above primary characteristics for the exploration of emeralds. In some mines one feature may prevail over another and some characteristics may be absent. For example, in the La Pita mine the hydrothermal alteration is very evident and prevails, the shale is highly altered.
However, in Españoles mine, the hydraulic breccia is dominant where the rock is highly fractured creating geodes. Both mines are productive.
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For this research, the productivity factor is divided into three different categories: 1. Productive calcite (P), this includes all the samples that fulfil all or almost all the geological characteristics for a prolific area; some of the samples were picked in a proved emerald zone; 2. Non-productive area (Np): includes all of the samples that do not fulfil the minimum geological criteria for a prolific area. 3. Not defined area (Nd); includes all the samples that fulfil some of the geological criteria but not most of them.
The classification of the samples according to their crystal habit, emerald potential and where the samples were found are summarized in Table 3.
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Productivity (Productive= P.
Nonproductive= Np, Not defined=
Nd)
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Table 3 Classification of the samples selected for carbon and oxygen analysis.
Analyses of stable isotopes of C and O
The study of the fractionation of stable isotopes was done mainly to understand the paleo-hydrogeological history of the study area, especially to main source of the fluid(s) which caused the emerald and associated mineralization (pyrite, calcite, chalcopyrite, quartz, albite, etc.). This is a fundamental tool to understand the physical and chemical changes of rocks and fluids, and in general to characterize these processes of fluid-rock interaction (Rollinson 1993).
The isotopic compositions were normalized to the Vienna Pee Dee Belemnite (V-PDB) for δ13C and
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the Vienna Standard Mean Ocean Water (V-SMOW) for δ18O and δ13C. The δ notation is defined as:
δ(‰) = [(Rsample ⁄Rstandard) -1] ´ 1000, where R is the ratio of either 13C⁄ 12C or 18O⁄16O. The analytical precision (1s), based on replicate analyses of the carbonate standards, was 0.03 ‰ and 0.06
‰ for carbon and oxygen isotopes, respectively (Lu et al., 2017).
The δ18O and δ13C composition of the analyzed samples (in ‰), is presented in the Table 4. Using V-PDB and V-SMOW standards.
The following formula was used to convert the values from V-PDB to V-SMOW scale:
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Table 4 δ18O and δ13C composition of the analyzed samples for different mines (‰).
For the δ18O composition of the measured samples the values ranged between 17.54 to 25.05‰ in V-SMOW standards. The data can be plotted in a diagram for natural oxygen reservoirs created by Rollinson in 1993. The values are very positive and suggest that magmatic reservoirs were not involved, instead suggesting sedimentary and metamorphic origins (Fig. 35).
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Figure 35 Natural Oxygen reservoir. Modified from Rollinson 1993.
The composition of δ13C for the same samples vary in a range between -7.49 to 1.91‰ in V-PDB standards. Rollinson 1993 also presented information from different authors to design a graph for natural oxygen reservoirs. The range for the samples from the WEB is plotted in Figure 36.
The fractionation of carbon isotopes is controlled by both equilibrium and kinetic processes. In many cases the fractionation of δ13C is strongly temperature dependant. But, dissolution and reprecipitation processes do not fractionate carbonates (Rollinson 1993).
Thesis samples range
range
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Isotopic composition of H2O (δ18OH2O) and CO2 (δ13CCO2) in equilibrium with the calcites.
One of objectives of this study is to determine the original isotopic compositions of the fluids which mineralized the calcites. The resultant signature will help define the fluid character in a δ13C (PDB) vs δ18O (PDB – SMOW) diagram (Rollinson 1993). It is possible to calculate these parameters assuming different temperatures for the mineral formation. According to Cheilletz et al., 1994 and Mantilla, 2007 who studied primary fluid inclusions in calcites and emeralds, the temperature for emeralds formation is between 290- 360oC. Therefore, for the present research, in order to calculate the original isotopic composition of H2O and CO2 in contact with calcites for the original fluid, the assumed temperatures
Thesis samples range
range
Figure 36 Natural Carbon reservoir. Modified from Rollinson 1993.
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will be 290 oC, 315 oC, and 350oC and the different isotopic fractionation changes for the given temperatures are estimated.
The stable isotope fractionation calculation for H2O and CO2 in contact with calcite was made using different formulas given by different authors. Below, detailed information explaining the procedure is presented. Simple programs to perform these calculations can be done directly on the following website: http://www.ggl.ulaval.ca/cgibin/Isotope / generisotope_4alpha.cgi.
- Stable Isotope Fractionation Calculation
The isotopic composition of water (δ18OH2O) in equilibrium with calcites was calculated using Kim, S.-T. & O'Neil, J.R. (1997) formula with temperatures between 25-350 oC:
Equation: calcite<=>H2O; T = 25-350 oC
E: 18.030; F: -32.420
T = 290 oC
1000 lnα = -0.4
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Figure 37 1000 lnα = -0.4 for T= 290°C.
T = 320 oC
1000 ln α = -2.0
Figure 38 1000 ln α = -2.0 for T= 320 °C.
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T = 350 oC
1000 ln α = -3.5
Figure 39 1000 ln α = -3.5 for T= 350 oC.
The isotopic composition of Carbon dioxide (δ13CCO2) in equilibrium with calcites was calculated using Ohmoto y Rye (1976) formula with temperatures up to 600°C:
Equation: calcite<=>CO2; T < 600 °C
C: -0.8910; D: 8.557; E: -18.110; F: 8.270
1000 ln α = -1.9
T = 290 oC
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Figure 40 1000 ln α = -1.9 for T= 290oC.
T = 320 oC
1000 ln α = -2.2
Figure 41 1000 ln α = -2.2 for T= 320oC.
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T = 350 oC
1000 ln α = -2.4
Figure 42 1000 ln α = -2.4 for T= 360 oC.
After the stable isotope fractionation calculation for H2O and CO2 in contact with calcite, it is possible to estimate the original isotopic composition for each sample at different temperatures (290 oC, 320 oC, and 350oC). The isotopic composition values of H2O and CO2 in equilibrium with the calcites for each mine are summarized and represented in different tables and figures; providing an understanding of the changes in isotopic composition with temperature.
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1. La Pita:
Sample Name d18O_SMOW (ametur°C
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Figure 43 Isotopic composition of H2O in equilibrium with calcites for La Pita at different T.
Sample
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Table 6 Isotopic composition values of CO2 in equilibrium with calcites for La Pita at different T.
Figure 44 isotopic composition of CO2 in equilibrium with calcites for La Pita at different T.
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Table 7 isotopic composition values of H2O in equilibrium with calcites for Coscuez at different T.
Figure 45 Isotopic composition of H2O in equilibrium with calcites for Coscuez at different T.
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Table 8 Isotopic composition values of CO2 in equilibrium with calcites for Coscuez at different T.
Figure 46 Isotopic composition of CO2 in equilibrium with calcites for Coscuez at different T.
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Table 9 Isotopic composition values of H2O in equilibrium with calcites for Monteblanco at different T.
Figure 47 Isotopic composition of H2O in equilibrium with calcites for Monteblanco at different T.
Sample Name d13C_PDB
Table 10 Isotopic composition values of CO2 in equilibrium with calcites for Monteblanco at different T.
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Figure 48 Isotopic composition of CO2 in equilibrium with calcites for Monteblanco at different T.
2. Cunas
Table 11 Isotopic composition values of H2O in equilibrium with calcites for Cunas at different T.
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Figure 49 Isotopic composition of H2O in equilibrium with calcites for Cunas at different T.
Sample
Table 12 Isotopic composition values of CO2 in equilibrium with calcites for Cunas at different T.
Figure 50 Isotopic composition of CO2 in equilibrium with calcites for Cunas at different T.
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Table 13 Isotopic composition values of CO2 in equilibrium with calcites for Españoles at different T.
Figure 51 Isotopic composition of H2O in equilibrium with calcites for Españoles at different T.
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Figure 52 Isotopic composition of CO2 in equilibrium with calcites for Españoles at different T.
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Table 14 Isotopic composition values of CO2 in equilibrium with calcites for Españoles at different T.
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Table 15 Isotopic composition values of H20 in equilibrium with calcites for Puerto Siad at different T
Figure 53 Isotopic composition of H2O in equilibrium with calcites for Puerto Siad at different T.
Sample
Table 16 Isotopic composition values of CO2 in equilibrium with calcites for Puerto Siad at different T.
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Figure 54 Isotopic composition of CO2 in equilibrium with calcites for Puerto Siad at different T.
7. Masato
Table 17 Isotopic composition values of H20 in equilibrium with calcites for Masato at different T.
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Figure 55 Isotopic composition values of CO2 in equilibrium with calcites for Puerto Said at different T.\
Table 18 Isotopic composition of CO2 in equilibrium with calcites for Masato at different T.
0
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Figure 56 Isotopic composition of CO2 in equilibrium with calcites for Masato at different T.