Pueblo Viejo

Overview

Pueblo Viejo is located in the central part of the Dominican Republic on the Caribbean island of Hispaniola in the province of Sanchez Ramirez. It is 15 km west of the provincial capital of Cotui and approximately 100 km northwest of the national capital of Santo Domingo. 

The Pueblo Viejo epithermal gold and silver deposit had been mined intermittently since the Pre-Columbian era. Open pit mining was executed by Rosario Dominicana between 1975 and 1999. In 24 years of production, the mine produced a total of 5.5 million of gold and 25.2 million ounces of silver. 

As a result of the historic mining activities important environmental liabilities exist in form of ARD that impacts the surface water and groundwater within the project area. The Dominican Republicis responsible for the hazardous waste cleanup, disposal and remediation. 

Gold ore will be mined by conventional open pit methods from two pits. The mines will follow a conventional drill-blast-load-haul mining sequence. 

The ore is refractory and needs special treatment. Processing consists of crushing, grinding, pressure oxidation, and CIL cyanidation. 

The maximum production rate from mining operations will greatly exceed the capacity of the processing plant and that would lead to mining operations being completed in 16 years as compared to the 26 years required for processing. 

There are good prospects to adding more reserves to the deposit. 

The pre-production capital budget was estimated at $3.0 billion. The project is scheduled to enter production in the fourth quarter of 2011. 

Operations workforce requirements vary between 156 and 471 persons over the life of mine (LOM). 

President Leonel Fernandez considers Pueblo Viejo as representing the largest project in the history of the Dominican Republic..

Location

Pueblo Viejo is located in the central part of the Dominican Republicon the Caribbean island of Hispaniola in the province of Sanchez Ramirez.  It is located 15 km west from provincial capital of Cotui and approximately 95 km northwest of the national capital of Santo Domingo. 

Dominican Republic is the second largest Caribbean country and shares the island with Haiti. It boasts a population of approximately 10 million Spanish speaking people. 

In 1492, Christopher Columbus reached the island of Hispaniola where the Spanish would soon establish the first permanent European settlement in Americas: the Santo Domingo. 

On the island colonizers found a well organized Indian population, the Tainos, which were most likely also involved in mining of the outcropping Pueblo Viejo gold deposit. The Spanish rulers mined the deposit in the 1505 to 1525 period - the mine was abandoned in favor of the newly discovered deposits of Mexico.

 The main port facility in theDominican Republicis Haina inSanto Domingo. Other port facilities are located at Puerto Plata, Boca Chica, and San Pedro de Macoris. 

The central region of theDominican Republicis dominated by the Cordillera Central mountain range, which runs from the Haitian border to the Caribbean Sea. The highest point in the Cordillera Centralis Pico Duarte at 3,175 m. Pueblo Viejo is located in the eastern portion of the Cordillera Central where local topography ranges from 565 m at Loma Cuaba to approximately 65 m at the Hatillo Reservoir. 

Two rivers run through the concession, the Margajita and the Maguaca. The flows of both rivers vary substantially during rainstorms. 

The Dominican Republichas a tropical climate with little fluctuation in seasonal temperatures, although August is generally the hottest month, and January and February are the coolest. Temperatures at the project site range from daytime highs of 32°C to nighttime lows of 18°C. Annual rainfall is approximately 1.8m, with May through October typically being the wettest months. The Dominican Republic is in a hurricane channel; the hurricane season is typically August to November.

As a result of previous mining and agriculture, there is little primary vegetation on the Pueblo Viejo site and surrounding concessions. Secondary vegetation is abundant outside of the excavated areas and can be quite dense.

Geology and Mineralization

Pueblo Viejo is hosted by the Lower Cretaceous Los Ranchos Formation, a series of volcanic and volcaniclastic rocks that extend across the eastern half of the Dominican Republic, generally striking northwest and dipping southwest. 

The setting is described as a maar-diatreme complex with the various deposits around the margins of the diatreme. The coarse-grained fragmental rocks that occur at depth in the deposit are considered by these investigators to be the product of an explosive volcanic eruption that fragmented the rocks and partially filled the crater. The crater was then completely filled with shallow, marine sedimentary rocks with variable amounts of fragmental rocks from nearby volcanoes. This sequence was then crosscut by later dikes and small dacite and andesite lava domes. 

Pueblo Viejo is classed as a high sulphidation, epithermal gold and silver deposit of the quartz-alunite style. 

The Pueblo Viejo property comprises several high sulphidation (or acid-sulphate) epithermal deposits of which Moore and Monte Negro are the largest. The deposits form funnel shaped envelopes of advanced argillic alteration where hydrothermal fluids migrated upwards and laterally along permeable horizons. 

The oxide was formed where surface oxidation removed sulphide minerals and carbon from the host sediments, leaving silicified host rock and massive jasperoid with jarosite, goethite, and local hematite mineralization. 

Gold is intimately associated with pyrite veins, disseminations, replacements, and layers within the zones of advanced argillic alteration. Gold values are generally the highest in zones of silicification or strong quartz-pyrophyllite alteration. These gold-bearing alteration zones are widely distributed in the upper parts of the deposits and tend to funnel into narrow feeder zones. 

Pueblo Viejo’s 2010 reserves were estimated at 252.5 Mt grading 2.93 g/t gold, 17.5 g/t silver and 0.09% copper for 23.7 Moz gold, 141.8 Moz silver, 505 Mlbs copper. There are good prospects to adding more reserves to the deposit.

Mining & Operation 

Gold ore will be mined by conventional open pit methods from two pits: Moore and Monte Negro. 

Past mining operations have stripped the deposit areas of any surface oxidation and the oxide mineralization is now depleted. 

The mines will follow a conventional drill-blast-load-haul mining sequence. A 10 m bench height has been selected for the pits. 

The ore from the mines will be hauled to a crusher near the plant site or to the low-grade stockpile. Approximately 205,016 tonnes of ore would be mined over the life-of-mine (LOM). 

Orefrom several different areas of the mine will be mined concurrently and stockpiled according to both gold content and sulphur grade.Orewith a higher gold grade will be mined and processed in the earlier years to benefit project economics. Similarly, stockpiling ore according to sulphur grade is intended to allow blending for processing at around an average sulphur content of 6.75%, a necessary strategy to achieve the planned daily processing output. 

The waste dump is located within the tailings dam impoundment area. 

Mining equipment include trucks loaded by 15 m3 hydraulic shovels and 19 m3 front-end loaders. 

The maximum production rate from mining operations will greatly exceed the capacity of the processing plant, viz. approx.40,000 t/d mining vs. 24,000 t/d processing. This will influence the project in several ways mining operations will be completed in 16 years as compared to the 26 years required for processing. 

Significant amounts of limestone (121,361 t) will be required for processing, and construction of dams and roads. The necessary limestone will be taken from a series of quarries adjacent to the ore mining operations. 

Pit dewatering will be required to manage the large amounts of surface water that will affect the area in the rainy season, with diverting ditches needed to minimize surface water entering the stockpile and pit areas. 

The Hatillo and Hondo Reservoirs will supply fresh water to the site. Reclaimed water from El Llagal tailings containment pond will only be used as a supplementary water supply under drought and flood situations. 

The power supply for the mine and process plant would be provided by a third party. The plant will receive power from the generating plant via a dual incoming 230 kV power line.

Processing

The ore from both pits is refractory, mainly due to gold encapsulation in the pyrite minerals, but also due to preg-robbing carbonaceous materials and to cyanide-consuming copper and zinc minerals. Pyrite and sphalerite are the two main sulphide minerals, both occurring in veins and disseminated within the host rock. 

Five metallurgical ore types have been defined based on lithological and mineralization criteria. 

The crushing and grinding circuit uses high-pressure grinding rolls (HPGR) and ball mills. The ore is delivered by 135 t haul trucks from the open pit. 

Run-of-mine (ROM) crushing will be done at the primary crushing station which will consist of a primary 24,000 t/d gyratory crusher equipped with a hydraulic rock breaker and vibrating screens. As the ore is weakly acidic, about 5 kg/t limestone will be added to the crusher feed to minimize equipment corrosion. Limestone will be mined in three separate quarries. A secondary MP800 cone crusher will be installed to crush the plus 50 mm material to 100% passing 50 mm. 

Run-of-mine (ROM) limestone is crushed to minus 85 mm (P80) in a gyratory crusher equipped with a rock breaker. The limestone grinding circuit will consist of a SAG mill (6.70 m dia x 3.65 m EGL) driven by a 2,610 kW synchronous motor with VFD and a ball mill (4.88 m dia x 9.80 m EGL) driven by a 3,540 kW synchronous motor. 

The grinding circuit consists of two HPGRs in parallel which feed two ball mills (7,000 kW each, c/w VFD) in series, two cyclopaks (6 of 838 mm diameter for the first ball mill, and 8 of 660 mm diameter for the second ball mill), two vibrating screens for the first ball discharge, and two vibrating trash screens. 

The ball mill will be in closed circuit with a cluster of fifteen cyclones, expandable to eighteen. 

To treat 1,200 tonnes of sulphur per calendar day three autoclave trains will be installed. The autoclaves operate at 230°C and a gauge pressure of 500 psi (3,450 kPa).

The oxidized slurry is maintained at an elevated temperature and ambient pressure for several hours tin four stainless steel tanks to dissolve the basic ferric sulphate that has formed in the autoclave. Hot cured slurry is pumped into a cooling tower. The CCD washing circuit consists of three high capacity stainless steel thickeners 65 m in diameter. The CIL circuit consists of six carbon steel tanks with a total of 18 h to 24 h retention time. Each tank is equipped with an agitator and two KEMIX interstage screens sizing at 28 mesh (600 ìm).

The loaded carbon from No.1 CIL tank is separated and washed on two vibrating screens, and after washing, is transferred to the acid wash vessels in the refinery circuit. The loaded carbon is acid washed in four vessels. Each coned vessel is capable of treating 12 tonnes of carbon in a batch. The pregnant electrolyte is pumped from the pregnant holding tank to four parallel electrowinning (EW) trains, each having three EW cells connected in series. The filtered gold sludge is loaded into boats, and then dried and heated to 1,000°C in an electrically dried mercury retort furnace. The dried cathode sludge is then mixed with fluxes and smelted in three 500 kg capacity electric furnaces rated at 200 kW. The furnace melt is poured to make gold doré. 

The detoxified leach residue is combined with the sludge from the neutralization circuit for disposal to the El Llagal tailings storage facility. Containment earth berms will be installed alongside the tailings pipeline. Any spillage will be directed toward and stored in the collection ponds. 

In addition to the tailings, the El Llagal pond will also be used to store potentially acid generating mine waste rock. The material will be trucked to the storage site by way of a haul road. To prevent ARD formation, the waste rock will be kept submerged. 

The proposed tailings and waste rock storage facility will be located in the El Llagal valley 3.5 km south of the plant site and will be constructed to store tailings from the CIL circuit blended with sludge from the neutralization circuit and also waste rock from the open pits. 

The impoundment is designed as two cells contained by cross valley dams. Storage of tailings and waste rock under a permanent water cover will prevent the onset of ARD.

Conceptual designs for the 138 m high Lower LLagal (LL) dam and the 196 m highUpper LLagal(UL-2) dam have been developed.

Environment & Community

The ARD impacts on surface water and groundwater within the project area have developed from historical mining operations. The Mejita and Las Lagunas tailings storage facilities were constructed during the previous mine operation. Large amounts of hazardous waste materials have been identified on the mine site. The waste products include rusting machinery, hydrocarbon contaminated soils, mercury contaminated materials, asbestos, and tailings that have escaped into neighbouring watersheds. 

Weathering and bacteria have created a large and continuing amount of acid rock drainage that contaminates area streams and rivers. The karstic nature of the surrounding Hatillo limestone formation presents a potential problem in the way of seepage losses from waste/tailings impoundments or for tailings dam construction. 

Wastes from previous mining operation are located throughout the mine site. In accordance with the SLA theDominican Republicis responsible for the hazardous waste cleanup, disposal and remediation. 

No fish were found in the Margajita area and fish habitat is highly degraded. The absence of small fish in theMaguacaRiveris indicative of historical mining impacts. Little vegetation cover was found in the pit areas and most of the surrounding area is forested. 

Groundwater samples indicate that groundwater contamination is limited to the area of the Cumba Pit draining towards the Arroyo El Rey and Maguaca River, and to the area of Monte Negro. 

The results of a socio-economic baseline study show poverty and low levels of literacy in the towns and local communities around the mine site, together with significant unemployment. Potable water, energy and sewage systems are non-existent. Elementary and high school education is available in local towns as well as basic medical facilities. 

A Resettlement Action Plan (RAP), prepared for the government with the support of PVDC and with assistance from expert technical personnel, local consultants and local personnel, was developed in accordance with World Bank Standards. The RAP was approved and signed on September 25, 2007 by representatives of the three local communities affected by the plan, the Dominican State, PVDC and the Catholic Church. 

Operations workforce requirements vary between 156 and 471 persons over the LOM.