“Gold and Emerald Deposits from Middle Cauca Belt and Muzo Region, Northern Andes, Colombia”
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“Gold and emerald deposits from Middle Cauca Belt and Muzo region, Northern Andes, Colombia” April-May 2019 Objective: The aim of this field trip is to provide an overview of the general geological characteristics, as mineralization, alteration and structural settings of intermediate-to-low sulfidation epithermal and porphyry systems from Middle Cauca Belt (MCB) and hydrothermal emerald deposits from Muzo, in the Northern Andes, Colombia. Gold and emeralds are some of the most important Colombian commodities. The Antioquia region, where the MCB is located, represents half of Colombia’s total gold production and Colombian emeralds are known as the best emeralds in the world. Given their importance and the absence of these mineralization styles in Brazil, the UFRJ SEG Student Chapter committee chose the the main operations and mines to organize a 7-day field-based activity, from April 28th to May 4th. The UFRJ SEG Student Chapter field trip ‘18 program brought together technical mine visits to some of the most important gold and emerald operations in Colombia comprising Buriticá and Marmato Au-Ag epithermal deposits and emerald mines Muzo and Coscuez. We invited industry professionals from Continental Gold Inc., Gran Colombia Gold Inc., Minería Texas Colombia S.A., Fura Gems Inc. and one professor as mentors in order to provide to all participants a great geological understanding of tectonic context and genetic models of deposits controlled by the Andean orogeny (intrusion-related and hydrothermal deposits). The UFRJ SEG Student Chapter proved that this field trip was a great opportunity to sign future partnerships with international companies and to provide networking opportunities and professional development to graduate and undergraduate students. Field Trip Participants: DSc. José Carlos Seoane Academic Advisor SEG Fellow Lethicia Carlos Ferreira Undergraduate Student SEG Member Taís Ferreira Martins Postgraduate student SEG Member Eric Bernard Gonçalves Postgraduate student SEG Member Alvaro Balmant Pessamilio Postgraduate student SEG Member Vivian Fernandes Moutinho Undergraduate Student Not associated Gabriel Machado da Silva Undergraduate Student SEG Member Beatriz Caetano Undergraduate Student SEG Member Charlys Vaz Undergraduate Student SEG Member Luiza Tinoco Undergraduate Student SEG Member Mayara Marinho Undergraduate Student Not associated Locality: The deposits in the Middle Cauca Belt and Muzo mines are located in West central and Northwest of Colombia (Fig. 1) in Antioquia and Boyacá departments, respectively. The 7-day field based activity roadmap is divided in two parts, the first part is located in San Pablo de Borbur, Boyacá situated 200 km far from Bogotá and near the cities Muzo and Coscuez. The second part is located in Caramanta and Santa Fé de Antioquia within a radius of 100 km from Medellín and near the cities Marmato and Buriticá (Fig. 2). Figure 1 – Northwest region of Colombia visited during the field trip highlighted by the black polygon (Image obtained from Google Earth Satellite Image). Figure 2 – Locations visited during the 7-day field trip in Colombia (Image obtained from Google Earth Satellite Image). Day by Day Trip Itinerary: DATE Activities April, 28th Arrival in Bogotá and departure to Boyacá April, 29th Visit to Coscuez Mine - Ore: Emerald April, 30th Visit to Muzo Mine - Ore: Emerald May, 1st Road trip to Antioquia May, 2nd Visit to Marmato Mine - Ore: Gold May, 3rd Visit to Buriticá Project - Ore: Gold May, 4th Departure to Bogotá and return to Rio de Janeiro Geological Overview: 1 – The Middle Cauca Belt, Marmato and Buriticá gold deposits: The Middle Cauca Belt (MCB) is defined as a Late-Miocene belt of gold-rich porphyry and low-sulfidation epithermal systems in Northern Cordillera (Fig. 3). It is hosted in the Romeral melange, West Central Colombia, and comprises low-grade metamorphosed ocean floor rocks of basaltic-gabbroic origin from Quebradona Complex and local fragments of Paleozoic and Permo-Triassic schists, gneisses and amphibolites from Arquía Complex. The basement of MCB is overlain by the Amagá Formation, Upper Oligocene to Lower Miocene siliciclastic rocks, and by the Combia Formation, Middle to Upper Miocene mafic to felsic volcanic rocks. Figure 3 – Middle Cauca belt, Colombia, showing known porphyry and epithermal systems, modified from Sillitoe (2008). The red line shows the limit of underlying oceanic crust and continental crust and tends to follow the trace of the Romeral fault system. The Marmato deposit (Fig. 4) is the largest gold producer in the MCB with ca. 2.3 Moz produced and published resources of 14.4 Moz of Au and 90 Moz of Ag. Diorite-granodiorite porphyries of 6.9 to 6.0 Ma intrude the Arquía Complex and host low-sulfidation epithermal mineralization of 5.6 Ma. The Buriticá deposit is located outside the geological limits of Romeral melange (Fig. 4) but of similar age of MCB porphyries and epithermal systems. It is a high grade low-to- intermediate sulfidation Au epithermal deposit with 3.7 Moz Au and 10.7 Moz Ag, hosted in granodiorite porphyry of 7.41 Ma in age intruded in the Early-mid Cretaceous Cañas Gordas terrane, which comprises oceanic volcanic and sedimentary rocks. The intermediate sulfidation epithermal mineralization is coeval with porphyry intrusion. Based on whole-rock geochemical and mineralogical characteristics, these mineralizing porphyries are water-rich and oxidized, and thus similar to other porphyry provinces around the world. What makes this an unique deposit is the Au-rich nature of mineralization. This high Au/Cu ratios are largely attributed to shallow emplacement of porphyry systems. Figure 4 – Lithotectonic map of the Northern Andes showing the location of the Late-Miocene Buriticá, Quinchía, Marmato, and La Colosa gold deposits that have strong structural controls (modified from Cediel et al., 2003). 2 - Muzo and Coscuez Colombian emerald: The Colombian emerald deposits are hosted in the central part of the Triassic- Paleogene Eastern Cordillera back arc basin. The sediments were deposited in two different basins in Early Cretaceous, the Tablazo-Magdalena basin to the West and the Cocuy basin to the East. Both of them underwent extension accompanied by mafic magmatism in Cretaceous and Tertiary times. During the Upper Miocene, the Triassic-Paleogene Eastern Cordillera back arc basin was inverted and now corresponds to a fold belt that thrusts over the Magdalena basin to the West and the Llanos basin to the East (Fig. 5). The Colombian emerald deposits were emplaced along two major polyphase thrust limits of the Eastern Cordillera defining two narrow emerald zones that correspond to the eastern and western limits of the Cretaceous basins. Muzo and Coscuez districts are located in the western zone and the deposits are hosted in carbonate-bearing and siliceous black shales and intercalated dolomitic limestones. The emerald deposits of western zone formed from 38-32 Ma during a compressive tectonic phase. The Colombian emerald deposits are very important, due to their quality, abundance and also non-conventional origin. Most of the deposits in the world have an igneous genesis, but the colombian ones are associated with the interaction of basinal fluids with organic-rich black shales and evaporites at ~300°C. Figure 5 – Generalized geological map of the Eastern Cordillera of Colombia with locations of the main emerald mines (modified from the Geological Map of Colombia, 1988, scale 1:500,000, Ingeominas, Bogotá). Muzo is one of the most famous with its emeralds related to carbonate-bearing black shales. Trip summary: April, 28th On the first day, we visited Coscuez emerald mine, owned and operated by Fura Gems Inc. Coscuez is the largest emerald underground mine in the world, with over 46 acres and 40 km of horizontal underground tunnels. In terms of production, this is the third largest in Colombia. The emerald mineralization is related to calcite and pyrite veins that are controlled by two large-scale vertical faults. The veins cross-cut the black shales from Muzo Formation and the potential for emeralds is directly related to the carbon rate in the shales. During the visit, we were introduced to the history of local emerald mining, to the geological context, the company mining operations and the drilling plan. We had the opportunity to discuss with Fura Gems Inc. geologists the characteristics of the deposit, such as: mineralization, structural controls, hydrothermal alteration, origin of mineralizing fluids and prospective methods. We visited the old open pit, which has a well-preserved exposure of the carbonaceous and siliceous shales, breccias, folds and faults cut by calcitic veins, and also the drill core shack. During the visit to the underground mine, the varieties of calcitic veins and their relationship with mineralization were highlighted. In addition, the geotechnical aspects of the old tunnels were discussed such as the mine tunnels reinforcement with wooden supports. We were also introduced to the process of emerald separation, which is handcrafted. Figure 6 – Chapter’s members, academic sponsor José Carlos Seoane and Fura Gem’s geologist Vasco Azinhaga at Coscuez underground mine entrance. Figure 7 – Chapter’s members, academic sponsor José Carlos Seoane and geologist Vasco Azinhaga on Fura Gems’ premises. Figure 8 – Outcrops of black shales from Muzo Formation cut by calcite and pyrite veins at Coscuez Mine. April, 29th On the second day we visited Minería Texas Colombia S.A., which is the first place where Colombian emerald were ever extracted. The Colombian emeralds are considered to be resulted from the interaction between basinal fluids with black shales. The fluid remobilized major and trace elements like Be, Cr and V and precipitated emeralds in areas with properly physical chemical conditions generally in the intersection of faults. During our visit, we had the opportunity to get in the underground mine, where we could recognize the host rocks, which consisted of black shales breccias cut by calcite veins. We also discussed with Minería Texas Colombia S.A.