“Gold and deposits from Middle Cauca Belt and region, Northern , ”

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 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 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 , Boyacá situated 200 km far from Bogotá and near the cities Muzo and Coscuez. The second part is located in 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. geologists the geotechnical structures used to ensure the security and the stability of the tunnels. After visiting the underground mine, we were guided to the office of the company, where we attended to a brief presentation about the deposit of Muzo and the origin of the emerald deposits in Colombia. There we could discuss the strong structural control of the deposit, the use of minerals as exploration guide like parisite (Ca-REE fluorcarbonate), pyrite and calcite and we could see some of the emerald specimens.

May, 2nd

On our fifth day we visit the Marmato gold deposit hosted by Gran Colombian Gold Inc. This deposit is the most important gold producer in the Middle Cauca Belt, with resources of 13.14 Moz of Au and 90 Moz of Ag. It consists of a structurally controlled low-to-intermediate sulfidation epithermal system, which is hosted by five porphyry bodies of dacite to andesite composition and ages between 6.58 to 5.74 Ma. The gold mineralization occurs less than 1 Ma after the porphyry intrusions in veins and veinlets with NW trends. This orientation is related to the NW-SE sinistral shear zone developed by the regional tectonic forces which also caused the rotation of the fracture system. The mainly mineral assemblage of the veins are composed of quartz, chalcopyrite, pyrite, marmatite (Fe-rich sphalerite), calcite, pyrrhotite and gold.The deposit shows a intense hydrothermal alteration marked by a pervasive and regional propylitic alteration and phyllic and intermediate argillic alteration more locally and close to the gold mineralized veins. During the visit we had the opportunity to visit the underground mine and the drill core shacks, where we observed the complex structural control of the deposit, the mineralogy of the ore breccias, veins, veinlets and different types of hydrothermal alteration that occurs close and distant to the gold mineralization.

Figure 9 – (a) Core with mineralized vein. (b) Mineralized vein at the mine front. (c) Chapter’s members at the drill core shacks.

Figure 10 – Chapter’s members at Marmato underground mine.

Figure 11 – Core samples showing textures and alteration in epithermal deposit Marmato. (a) crustiform- banding quartz (b) potassic alteration halo around mineralized quartz veinlet in porphyritic dacite showing pervasive albitization.

May, 3th

On the last day we visited the Continental Gold Inc. Buriticá Project, which is a high- grade gold project, with proven and probable reserves of 3.71 Moz @ 8.4 g/t (gold) and of 10.7 Moz @ 24.3 g/t (silver). We were hosted at one of the project’s offices, where geologists working on exploration team presented us the general aspects of the Buriticá Project, the main geological features of the deposit and showed the Western Cordillera view (Fig. 6). The Buriticá Project is 57% complete and on schedule for starting production, of up to 300,000 oz/year, at 06/2020, with predicted mine life of 14 years. Until the date of the visit, almost 11 km of underground development had been completed. One of the Buriticá Project concerns is to accomplish modern mining and obtaining the social license to operate, by adopting innovative plans and practices, such as a Local dialogue platform (Plan Buriticá) and an agro- development fund (“Siembra Futuro”), and adopting a dry mining tailing system, which drastically reduces environmental and local population risks.

Figure 12 – Western Cordillera of Colombia, sight from the Buriticá Project’s office. The deposit belongs to a porphyry system with associated veins, located in the Western Cordillera in Colombia, where porphyry deposits are much richer in gold than other porphyry systems due to the existence of host carbonaceous rocks that act as traps, favouring Au concentration. The basement is composed of Jurassic to Lower Cretaceous rocks from the Barroso Formation, which comprises basalts intercalated to siliceous and carbonaceous mudstones. It also includes diorites, tonalites and gabbro from the Upper Cretaceous. The porphyry stocks were formed by several pulses and are mostly diorites and monzodiorites from the Miocene. The main mineralization was originated around 7.6 M.a. in the form of mineralized veins within the stock, and the deposit consists of low-to-intermediate sulfidation. The most Au-rich zone in the system is the intersection zone of structures from two different mineralizing stages, stages 1 and 2, and the highest grade ore bodies, occurring in cigar shapes, are situated in the intersection of main fault planes. We also had the opportunity to visit the drill core shacks, where we could learn about the company’s quality control procedures concerning core sampling and assay. The project has already performed more than 350 km of exploration boreholes, and for each 20 samples sent to be analysed at the ASL Colombia laboratory, nearly 6 of them are rock quality control standards, blanks and duplicates ensuring better control of the assay quality. Furthermore we could observe, in core samples, the gold mineralization and several lithologies from the stocks and the basement. After the visit, we travelled back to Bogotá to return to Rio de Janeiro on the morning of May 4th.

Figure 13 – Field trip participants and Continental Gold Inc. professionals during our visit to the Buriticá Project.

Financial Report:

Student Chapter Activities US$1780

Stuart Wallace Funding US$1400

Members Monthly Payment US$285

Codelco do Brasil sponsorship US$470

Nexa Resources sponsorship US$945

Total Collected: US$4880

Airline tickets US$3910

Car Rent US$820

Accommodation US$380

Fuel, toll and emergency costs US$370

Total Spent: US$5480

*An additional US$60 and food costs were covered by the students.

Acknowledgments:

We are grateful for the experience of visiting unique geological features, networking with industry geologists and for the excellent cultural exchanges. All of these were definitely possible due the financial support received from SEG via Stewart R. Wallace funding, from NEXA Resources S.A, and from Codelco do Brasil Mineração LTDA. Besides that, our student chapter is putting a lot of effort into the realization of low-cost events related to economic geology, which are always supported by our Industrial sponsor, Marcos André Gonçalves and our Academic sponsor, José Carlos Sícoli Seoane. We also thank Fura Gems Inc., Minería Texas Colombia S.A., Continental Gold Inc. and Gran Colombian Gold Inc. for receiving us during the technical visits and providing us a unique geological knowledge.