Eden Roc exploration expands reserves

Long before West Africa caught the attention of mineral explorers and investors, Eden Roc Minerals (TSE) began searching for gold in the cocoa plantations and tropical jungles of Ivory Coast.

Their search, while logistically difficult, was ultimately successful. Eden Roc, through its 68%-owned subsidiary Societe des Mines d’Afeme (SOMIAF), operates one of only two producing gold mines in the country. The Aniuri open pit is projected to produce 30,000 oz. this year and the company hopes that, by 1997, production from this and other deposits will have increased to 100,000 oz.

A highlight of The Northern Miner’s recent visit to the Aniuri mine site and the surrounding property was the pouring of a 31.3-kg dore bar estimated to contain 98% gold and 1% silver.

Situated 40 km east of the regional centre of Aboisso and 150 km east of the capital of Abidjan, the mine is accessible by car along a paved highway, 1.5 hours from the capital.

Eden Roc became involved in the West African property in 1981, following an airborne geophysical survey funded by the Canadian International Development Agency.

It was then that the company acquired a large, northeast-southwest-trending concession straddling a 35-km strike length of the Afema shear zone. This shear zone hosts several past-producing gold mines in neighboring Ghana and is strikingly similar to another, parallel structure — the Ashanti shear zone, which hosts the Ashanti gold mine, a 770,000-oz.-per-year producer owned by Lonrho, a British company, and the Ashanti government. Ashanti has been in continuous production since 1897 and is currently estimated to contain 18.1 million oz. gold.

Three types of mineralization exist on Eden Roc’s permit area: oxide, transitional and sulphide.

Currently, most production is from the highly oxidized, soft, lateritic ore near the surface. Minable grades range between 2 and 9 grams per tonne and persist to a depth of 25 metres below surface. Transitional ore, making up a small part of the production, is found 25-35 metres below surface. Generally, transitional ore is saprolitic and has a lower recovery rate as a result of its higher sulphide content. Sulphide ore associated with quartz veins is thought to persist at depth and it is this that represents potential growth for the operation.

Recently, two permits were approved, extending to 2,180 sq. km the total land package held by Eden Roc and its 58% owner, Marshall Minerals (ASE). The property consists of two exploration permits (PRA-33 and PRA-51), as well as a 124-sq.-km exploitation permit (CM-29). The exploitation permit, operated by SOMIAF, contains the Aniuri mine as well as the other deposits scheduled for production, including Asupiri, Brahima, Adiopan and several anomalies in the area known as Grid-X.

Old workings indicate mining and exploration occurred near the turn of the century, yet no modern mining or processing of oxide ores has taken place. The most recent documented work on the property was conducted between 1938 and 1958 by the Bureau de Recherches Geologiques et Minieres (BRGM), the French government-owned research organization. Its work consisted of regional geochemical and geophysical surveys followed by trenching, diamond drilling and, in some cases, underground development of high-grade gold deposits. Along with the permits, Eden Roc acquired all historical data compiled by BRGM — a considerable advantage, as they enabled the company to define and select several preliminary targets.

Said Eden Roc President Hermann Derbuch: “We became a junior mining company with a senior mining database.”

For the next seven years, the company focused on developing reserves at the Asupiri deposit. Previously, it had completed more than 2 km of underground development, yet no mining had taken place.

Eden Roc drilled the deposit and determined there was a probable reserve of 223,607 tonnes averaging 2.9 grams. But by that time, seven years of exploration had diminished the junior’s cash position and the company opted to enter into a partnership with Marshall Minerals.

In 1989, a large area of oxide mineralization, at surface and amenable to heap leaching, was identified by a soil geochemical survey. This discovery ultimately became the Aniuri gold mine, with gold reserves in excess of 2.5 million tonnes averaging 4.25 grams.

Mining operations started in 1992 and the first gold was poured in February, 1992. Since then, SOMIAF has produced more than a tonne (32,150 oz.) of the yellow metal.

The Aniuri pit measures about 700 by 25 metres and is about 30 metres deep. A D8 dozer was used to strip off vegetation and mine the lateritic and saprolitic material. Trucks are loaded with either waste or ore by means of wheel loaders or backhoes. The latter are used to mine the narrow, high-grade, saprolitic quartz veins while elevating scrappers are used for bulk removal of saprolitic and lateritic waste. Blasting is not often performed, owing to the soft nature of the oxide ore.

Following mining, the saprolitic quartz ore and soft oxide ore are crushed to minus 3/4 inches in a jaw crusher and hammer-type mill, respectively. These ores are then combined, mixed with 10 kg cement per tonne of ore, and wetted with a cyanide solution. The material is then agglomerated and fed by a series of conveyors to the permanent leach pad, where it is stacked eight metres high and sprayed with cyanide for 120 days. Results from column leaching tests show that more than 80% of the gold can be recovered within 35-40 days.

The pad currently in use is almost filled up, and another is being constructed, which will handle as much as 600,000 tonnes. This pad will be used to process the remaining ore from the Aniuri, as well as ore from the Brahima and Adiopan deposits.

Once the material is leached, the pregnant solution collects in ponds. It is then processed by a carbon-in-pulp method and the gold is precipitated onto steel wool and smelted.

The average operating cost is in the range of US$180-200 per oz., but the company hopes this will drop to US$120 per oz. with the planned increase in production.

Eden Roc thinks there is potential to find other deposits, perhaps larger than the Aniuri, and has developed a systematic approach to exploring for oxide orebodies.

Initially, a regional geochemical survey is completed over airborne or previously indicated anomalies. This is followed by a detailed soil geochemical survey and, if warranted, by trenching at 40-metre spacings. Two sets of horizontal samples are taken in the 2-metre-deep trenches and once a zone has been defined, shallow drilling on 20-metre sections is completed to upgrade reserves to the probable from the possible category. The company operates one assay lab, which processes 300 samples a day. It also sends samples out to six labs in Ghana for confirmation of results. This approach has led to the discovery of other deposits scheduled to go into production, as well as less-advanced exploration targets such as the Hermann and Study Area 9.

Currently, the company is using two drills on the CM-29 permit area. One is being used to drill exploration targets while the other is in use at the mine site. Eden Roc has arranged to have four more drills moved onto the property; they will be shipped from Canada to Abidjan and trucked to the site. Three will be used for exploration while the other will be used for parts. Joseph Hinzer, senior vice-president of exploration for Eden Roc, thinks the exploration potential for the three permit areas is high. “Not only do we have the makings of a good mine,” he said, “but we feel we have the potential for a very substantial mining camp, similar to the Malartic camp or the Ashanti mining area.”

So far, efforts have been focused on finding near-surface, heap-leachable gold deposits. According to Hinzer: “We have only just scratched the surface — our deepest hole is only about 100 metres down.” Eden Roc, however, is confident that below the oxide orebodies lie rich, gold-bearing, transitional orebodies and sulphide orebodies associated with quartz veins.
However, these types of ore are not amenable to heap leaching, so more conventional techniques will be needed to process the ore.

Once the new drills arrive, exploration will have been completed for the purpose of increasing oxide reserves and delineating deeper, transitional and sulphide reserves. Reserves now stand at 5.5 million tonnes averaging 4.5 grams gold per tonne, enough for 2-3 years of production. It is hoped that, by 1996, as new oxide reserves are discovered, daily production will rise to 1,500 from 500 tonnes per day.

The company has budgeted almost $3.1 million for exploration in 1994, more than half of which will finance definition drilling at the Brahima and Grid-X zones.