Making a mine is difficult enough without additional distractions. Having endured protests from illegal gold miners and the nuisance of a protracted challenge to its land title,
The Las Cristinas concession is about 8 km northwest of Kilometre 88, the town named after a milepost on Venezuela’s Highway 10. Organized mining began in the 1930s, when placer operators from British Guiana moved up the Cuyuni River and established two small gravity mills. Those mines shut down a short time later, but in the late 1970s the area was active once again, this time as the centre of a garimpeiro gold rush.
Craft miners from Venezuela, Brazil and Guyana swarmed into the rain forests south of the Cuyuni and Yuruan Rivers; excavating the weathered tropical soils with hand tools and gasoline-powered pumps, and recovering the free gold with batea pans and mercury, the garimpeiros left behind a badly deforested and eroded landscape, and ultimately were cleared off by the Venezuelan government, which backed up its environmental and mining regulations with the army.
In the early 1990s came the next gold rush, an influx of foreign companies, some in league with the Venezuelan development company Corporacion Venezolana de Guyana. Among the most favorable ground was the Las Cristinas area, where gold occurs in northeasterly striking shears and on foliation planes in greenstone-facies volcanic rocks.
CVG dealt the property to Placer Dome in 1991; to operate the four Las Cristinas concessions, the partners formed Minera Las Cristinas (Minca), owned 70% by Placer Dome and 30% by CVG. Placer Dome funds construction of the mine, and CVG can then maintain its interest in Minca by paying its share of those costs.
At the same time as CVG and Placer Dome were inking their deal, another Canadian-based company,
Thus began the Las Cristinas soap opera: Crystallex had bought a Venezuelan company, Inversora Mael, which claimed to hold rights to mine on the Las Cristinas concessions. A series of legal challenges to CVG’s title to the ground ended in June 1998, when the Venezuelan Supreme Court refused to hear claims brought by Mael and Crystallex.
The direct effect on the project was that Placer Dome pulled the hand brake in January 1998, suspending construction activity until the property’s legal hassles were out of the way. During the hiatus, the project’s feasibility study has been revised, with a different mining schedule and a larger mill capacity.
The new feasibility study, made public in October, recommends production of 530,000 oz. gold annually, averaged over the first 10 years of operation.
Over the 20-year mine life, Las Cristinas should produce an average of 470,000 oz. a year. Annual copper production should average about 16,000 tonnes over the life of the mine.
The estimated capital cost is US$575 million, which includes running a transmission line from a power sub-station now under construction just north of Kilometre 88. Bolivar state, which has numerous dams on tributaries of the Orinoco River, is a major exporter of hydroelectric power, so connecting to the national power grid was no problem.
Another US$300 million in capital investment and deferred stripping costs is expected to show up down the line, and cash production costs are expected to run around US$155 per oz. after credits for copper.
Once CVG and Placer Dome make the decision to start work, building the mine and mill is expected to take about two years. A camp for construction crews is ready, and most permits are either in place or awaiting approval.
Las Cristinas is in the large Imataca forest reserve, Venezuela’s principal protected rain-forest area. Concerns that mining might destroy sensitive areas in the Imataca have been taken up by groups in other countries, and recently have surfaced in Venezuela as well.
Members of the Patriotic Pole party, whose presidential candidate, Hugo Chavez Frias, came to power in early December, had previously suggested scrapping the previous government’s plans to allow mining in the Imataca.
But faced with severe government and trade deficits, Chavez Frias may have little choice but to adopt the policy.
The mine planned for Las Cristinas will ultimately consist of two open pits: one on the Mesones and Sofia zones to the north, and the other, which is much larger, on the Conductora and Cuatro Muertos zones to the south. The two pits will feed the mill 44,000 tonnes of ore daily.
Placer Dome’s latest feasibility study recommended that mining proceed in stages, starting with the most profitable phases. In the first stage of production, Minca will mine in the smaller northern pit, Mesones-Sofia.
The Mesones reserve contains 37 million tonnes grading 1.1 grams gold per tonne and 0.35% copper, and this higher copper grade is expected to push down initial production costs. As well, the Mesones pit is drier, which highlights the principal challenge at Las Cristinas — water.
“Getting through water-related issues is going to be the biggest problem,” says Murray Canfield, Minca’s chief engineer. Starting work in the drier Mesones pit will permit drivers and equipment operators to learn what can and cannot be done in the tropical soil that makes up much of the mineralized material at Las Cristinas.
Deep tropical weathering has left behind a residual soil profile 30-100 metres thick. Most of that profile is clayey saprolite, which is easy to dig but difficult to work with when wet.
Moreover, there is little or no dry land at Las Cristinas. “The permanent water table is basically at ground elevation,” says Canfield. This has forced Minca to plan most of the mine around drainage, rather than planning drainage around the mine.
A case in point: the mill will be built on high ground immediately west of the main Conductora pit, a mineralized area that could have been brought into the property’s gold resource. “The value of the high ground above the flood plain is greater than the value of gold in the ground in this area,” states Canfield.
Surface water also has to be kept out: four small streams are being diverted with a 9-km ditch that will carry their flows to a tributary of the Cuyuni, an exercise that consists largely of connecting up existing ponds and swamps.
The average 3.5-metre annual rainfall means plenty of pumping in the wet season — sumps in the pits are designed for a 60-mm rain event, yet the operation is still budgeting for 55 rain days per year, including those when the pit is flooded and other days when rain will make it dangerous to drive trucks on the saprolite pit floors.
A network of perimeter wells, pumping to the diversion channel at rates of up to 1 cubic metre per second, will be built to handle groundwater. A part of the groundwater may also be routed to the mill as process water. Some 70-80 wells may be needed at any one time to keep the pit floors dry. “In the rock it’s important,” says Canfield, “but in the saprolite it’s critical.”
Another concern in working in saprolite is slope stability. In rock, the pits will be sloped at the conventional 45, but gentler slopes — 25 to 35 — are going to be needed in the weathered material.
Mining at the larger southern pit, Conductora-Cuatros Muertos, starts in the operation’s second phase. The pit, which will be about 3 km from its southern to its northern end, contains a massive but low-grade resource of 286 million tonnes grading 1.1 gram gold per tonne and 0.11% copper. The pit will have a stripping ratio of around 1.1, compared to Mesones’ ratio of just under 2.
Minca will use diesel-powered hydraulic excavators in the initial stages of the pits, where saprolite can be ripped easily
. Trucks will have rubber “trampoline” bed liners to make it easier to dump sticky saprolite. As the pit progresses, larger rope-shovel units will be phased in to replace the diesel excavators.
There are three types of ore: saprolite-oxide, saprolite-sulphide and hard rock. The saprolite oxide material, which makes up about 10% of the reserve, and the fresh rock, which makes up about 80%, are both neutral or acid-consuming; only the saprolite-sulphide material, where carbonate minerals have been leached out but the sulphides remain, is acid-generating.
Acid-generating mine wastes will be encapsulated in saprolite-oxide material, then covered with rock fill to ensure a reducing environment.
Saprolite will go to units that feed a semi-autogenous grinder, with rock going through a crusher and ball mills. The first recovery of gold will be in a gravity circuit, but the three ore types will each require separate processes. Gold can be recovered by direct cyanidation of the oxide ore in a carbon-in-pulp circuit, but the copper mineralization in the sulphide types will require Minca to recover a flotation concentrate, then send the cleaner flotation tailings to a carbon-in-leach circuit.
The different ore types are also expected to give substantially different gold recoveries. The sulphide ores are expected to yield about 80% of their gold, but the oxide should show recoveries in the low 90s. Slightly more than half the gold will be shipped as a copper-gold concentrate.
Much of the cyanide is to be recovered and returned to the process, but waste process water will be routed to a cyanide-destruction plant that is expected to bring the total cyanide concentration down to 1 microgram per litre, the concentration at which water can be discharged to watercourses.
But the water will first go to a tailings pond about 7 sq. km in area, behind an 11-km-long perimeter dam made of saprolite.
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