Vancouver — Catherine McLeod-Seltzer and David Lowell, who, a year ago, re-united to search for gold-silver deposits under the banner of
The mine-finders are probably best-known for their contribution to the Pierina gold discovery in Peru while running Arequipa Resources, which was taken out by
In order to acquire rights to the Toromocho property, they joined forces with Luis Baertl, past vice-president of
Lying 100 km east of Lima, Toromocho was worked by Cerro de Pasco from 1966 until 1974, when the property was nationalized. The Peruvian company tested a 1-by-1.5 km area centred on a 200-metre-high hill. In total, 143 drill holes were put down and four widely spaced adit levels covering a 1-km vertical extent were completed. At the end of the day, the company blocked out some 364 million tonnes of proven and probable reserves grading 0.67% copper, 0.4% zinc and 0.03% moly, plus 12 grams silver per tonne.
State-controlled Empresa Minera del Centro del Peru (Centromin) took over the ground in 1974 with Kaiser Engineers, completing a feasibility study on the project back in 1981. The study envisioned a conventional open-pit mine utilizing a 30,000-tonne-per-day sulphide flotation mill and simultaneously leaching 30,000 tonnes per day of low-grade oxide ore. Sulphide copper production was expected to hit 132.6 million lbs. per year in concentrates, plus 16.1 million lbs. per year of copper cathode from leaching. Capital costs at the time of the study were US$548.1 million. The report pegged the reserves at 217.5 million tonnes grading 0.67% copper in the sulphide portion, along with 212.9 million tonnes of leachable material grading 0.32% copper.
The project subsequently sat idle until May of this year, when the Peru Copper Syndicate won control of it. Under an agreement, the company is required to spend US$12 million over five years. The government retains a sliding-scale net smelter return royalty ranging from 0.51% when the price of copper is less than US80 per lb. to 1.1% when the copper price is US$1.20 or higher.
“More than two years ago, I started to search for large leachable chalcocite deposits in South America, and Toromocho was the best one,” Lowell tells The Northern Miner. “My interest came from the fact that there was much more chalcocite in the core than had been logged, and the core recovery was poor, . . . so I boldly predicated a 10% increase in the copper grade.”
Using assay reject samples collected from about half of the 143 historic drill holes, the company was able to take 10-metre composite samples and test them for leachable copper.
“The upper two-hundred and fifty metres had an average of sixty per cent of the copper mineral as chalcocite,” says Lowell. “From two-hundred and fifty to four-hundred metres, the chalcocite dropped to forty per cent, but below four-hundred metres the content rose to between sixty and seventy per cent chalcocite.”
The next step was to drill four holes collared only a few metres from the historical holes to obtain the better core recovery needed to determine a copper grade calibration factor. The preliminary results have proved Lowell correct, with the copper values increasing 9.3-11.3% from the historical results.
As well as improving the copper grade, the latest drilling confirmed that the chalcocite column extends to at least 600 metres, with the deepest hole extending to 583 metres.
The company is running arsenic and copper assays on every 1.5-metre sample, whereas gold, silver, moly and zinc are being run on every 10-metre composite.
Lowell says there are many similarities between Toromocho and the highly successful Cerro Verde mine. “It’s always hard finding comparable deposits — kind of like comparing apples and oranges — but our model is Cerro Verde.”
The Cerro Verde mine is 34 km from Peru’s second-largest city, Arequipa. It operates as two open pits, known as Cerro Verde and Santa Rosa. The copper operation was previously owned by Minero Peru, which was privatized and sold by the federal government in 1993 to Cyprus Amax Minerals (now part of
At an average grade of 0.52% copper, Cerro Verde produced 84,600 tons of copper cathodes last year and during the second quarter of this year; Cerro Verde’s earnings amounted to US$8 million.
With mining expected to last another 14 years, Phelps Dodge recently approved a US$15-million feasibility study for a primary sulphide leach project. This project would allow Cerro Verde to extend the mine life by an additional 15 years. The projected construction cost of doing so is in the range of US$400-700 million.
Phelps Dodge has an 82.5% stake in the mine, with 9.2% held by the Buenaventura family group and 8.3%, by minor shareholders.
Copper leaching is nothing new, but the economics of the process require favourable mineralogy. In the early days, leachable ore was confined to high-grade concentrates of oxidized copper — namely, malachite, azurite and tenorite. Today’s mine-for-leach includes both oxide and sulphide ore minerals.
In the leaching process, an acidic solution percolates through large piles of crushed ore, dissolving the copper from the rock. Fines from the crushing operation, which are leached along with the ore, are agglomerated using an acidic solution. The copper-laden solution is recovered from the bottom of the piles and stripped of its copper in a solvent-extraction process. Copper is exchanged for hydrogen in the solvent using an organic extractant, and the resulting acid solution, now stripped of copper, is returned to the leach pile for further leaching. Copper is stripped from the organic extractant using depleted electrolyte. This copper-bearing aqueous stream is then pumped to the tank house, where high-purity copper is recovered by electrowinning. Since copper is exchanged for hydrogen at the cathode, the electrolyte becomes acidified as it is depleted of copper. The depleted electrolyte is then pumped to the solvent-extraction plant, where it represents the primary source of acid used in the process.
The leaching of sulphide ores is difficult, and often needs the addition of a bacterial process. The bacteria are used to oxidize the sulphide minerals, including primary chalcocite and pyrite, forming ferric sulphate and sulphuric acid. The ferric sulphate, in turn, oxidizes the copper minerals and enables them to be leached by the acid. Air is blown into the piles to speed up the oxidation process.
No two deposits have the exact same mineralogy, making metallurgical tests the key to determining the economics of leaching the copper. Says Lowell: “The Toromocho project is based as much on metallurgy as it is on geology.”
The Peru Copper Syndicate is taking a 3-pronged approach to evaluating the metallurgy: pure floatation, a hybrid of floatation-leaching, and finally leaching.
Arizona-based Metcon Research is running column leach testing on selected reject samples. Bottle-roll tests on material from each of the new drill holes are also planned, as is a complete prefeasibility study.
Mineralization at Toromocho is hosted in Jurassic limestone of the Pucara formation and Tertiary intrusives. The higher-grade zones occur as skarns. The alteration halo is typical for a copper porphyry deposit with a central potassic zone comprising secondary biotite, quartz and pyrite surrounded by a quartz-sericite phyllic zone. The outer zone consists of epidote, chlorite propylitic alteration.
Metal zonation shows a central core of copper-molybdenum mineralization surrounded by a ring of lead-zinc, mostly as vein occurrences.
The deposit forms a roughly cylindrical body hosting a series of higher-grade skarn bodies where intrusions cut dipping limestone beds. Some enargite is found in the upper part of the deposit, suggesting a telescoping of mineralizing events. A leach capping has been eroded from 66% of the deposit, and the distribution of chalcocite exceeds 600 metres in depth, with chalcopyrite present through much of this interval.
“Toromocho looks like a large breccia pipe,” says Lowell. “On one side, the veins dip west toward the orebody, and on the other side, they dip east. You expect the veins to intersect somewhere below in a high-grade breccia.”
Based on the mineralization, the company believes that only the top part of the system has been tested. The deposit could extend for hundreds of metres below the historical exploration.
Exploration potential is believed to be high in the northeastern part of the property, where skarn-hosted sulphides have been identified over a 600-by-800-metre area down to depths of 300 metres. Cerro de Pasco completed a series of tunnels and adits to exploit higher-grade veins in the area but did not evaluate the disseminated, polymetallic mineralization.
In order to move the project forward, the private group will need additional capital. “We have financing to carry the project for a year, and then we can either seek [the participation of] a major company or form a public company,” says Lowell. “Going public is an exciting option, but it’s not the only one.”
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