EL INDIO: PRODUCTIVITY IS PRIORITY ONE; An underground and open

The El Indio mine sprawls across several peaks of the Chilean Andes some 4,000 m above sea level. A collection of profitable underground and open-pit mines, the complex lies approximately 190 km east of the Pacific coastal town of La Serena, the staging area for supplies and services destined for El Indio. Argentina borders the east only a few kilometres away. The Tortolas summit (elev. 6,330 m) marks the boundary. LAC Minerals acquired El Indio when it bought controlling interest in Bond International Gold in 1989.

First to work the deposits were Pirqineros, the Chilean equivalent of the Brazilean garimpeiros. They were hand-selecting ore back in 1967. St. Joe Minerals developed it into a modern mine and plant in the early 1980s. Today, El Indio, the first Chilean copper mine to have gold as more than just a minor by-product, is a 3,100-tonne-per-day operation yielding 216,000 oz. of gold and 27,000 tonnes of copper (1990 figures). The principal mining areas are the vein systems of El Indio and, 5 km. to the south, Tambo.

GEOLOGY

Regionally, the area is underlain by granite and granodiorite batholiths of the Paleozoic era. The El Indio and its adjacent deposit, the Tambo, occur in a north-south trending Tertiary volcanic belt. El Indio mineralization is epithermal in origin and hosted in dacitic rocks. It is characterized by extensive hydrothermal alteration. The main ore-bearing structure strikes north/northeast.

Two principal stages of mineralization are apparent. First are massive sulphide veins with high concentrations of copper and silver consisting primarily of enargite (sulphides of copper and arsenic) and pyrite. The second stage comprises gold quartz veins. These generally carry high gold values and are relatively low in copper and silver. The veins consist mainly of native gold and gold tellurides, finely dispersed veinlets of quartz and a mixture of enargite, pyrite and other sulphides.

Besides the El Indio system, there are related ones: Viento, mined predominantly from underground now, and the Campana, a narrow vein that may be a tensional system splayed off from El Indio. “We don’t know the connection between Campana and El Indio,” chief geologist Patricio Valenzuela told The Northern Miner Magazine during a tour.

Historically, El Indio has been considered a multiple-vein deposit. Current geological knowledge indicates there aren’t as many veins as previously thought. Rather, it is a multi-faulted and repeated vein system, which was given different names in the past.

Five kilometres away at Tambo, the secondary production area of the El Indio complex, the deposit is different geologically, Tambo being strictly a gold deposit. Native gold and some tellurides of gold and silver are contained principally in breccia pipes the approximate shape of inverted cones. The gold is associated with barite and alunite. Argillic alteration is evident in the hangingwall and footwall.

MINING

El Indio (which includes Viento) provides 78% of the total annual tonnage of nearly 1 million tonnes of ore. Tambo accounts for the remaining 22%, nearly all of it from open pit. There is also a heap leach section at Tambo recovering gold from low-grade rock that would otherwise be relegated to waste dumps.

El Indio is now mined almost exclusively from underground. There are three key mining methods: Longhole (conventional panel type and Avoca), which accounts for 52.6% of El Indio/Viento production; mechanized cut-and-fill; and an innovative method borrowed from South Africa called cut-and-fill ramp-in-vein.

The latter is likely the least known in the industry. At El Indio it is reserved for narrow vein systems, such as Campana, that erratically pinch and swell. “To develop the whole infrastructure (of ore access) in waste for Campana would have been too costly,” said Daniel Vanin, LAC’s former mine manager at Quebec’s Doyon mine who has been installed as the El Indio mine manager. With ramp-in-vein, all development occurs within mineralization, obviating the need for ramp access outside the ore. In essence, the vein is developed and mined sub-level on a 15% diagonal advance along strike. Once that one triangular portion (viewed longitudinally) is mined and filled from sub-level to sub-level, the remainder is worked from the same incline (see accompanying diagram). The low waste factor is partially offset by the lack of total mechanization. However, Campana grades average 8.2 g per tonne gold and 3.88% copper.

The more traditional cut-and-fill stopes, taken in 2.8-m lifts with stopers and 3-m lifts with Tamrock jumbos, are used in veins wider than Campana’s, anywhere from 2.5-6 m, but where longhole would over-dilute mined grade. Even so, the last 14 m or so are mined by longhole when possible, Vanin points out. The backfill, either sand or development waste, is backhauled or supplied through raises. Sub-level to sub-level is 40 m.

Long-hole stopes account for an annual 53% of production. There are two types: conventional longhole with a sequenced series of drilled, mined and backfilled panels; and an Avoca type. The latter incorporates 30-m sub-levels (usually) and requires unconsolidated fill. A prerequisite, however, is competent ground in both hangingwall and footwall. “We leave up to 40 m exposed as mining advances (horizontally) ahead of backfill,” Vanin noted. Should either side of the void start to deteriorate as mining proceeds, cemented fill is dumped in, allowing for conversion to another mining method. The void is backfilled to stabilize the ground and then the mine department either re-initiates the Avoca method or turns to conventional longhole.

In longhole stopes, Cubex down-the-hole machines drill 4.5-inch-diameter holes up to 50 m in depth. In ground prone to post-blast sloughing, the vertical dimension will not exceed 25 m. “Viento is generally good ground,” Vanin said, “but at 50 m high, blocks come off in the hangingwall and footwall. As well, the drilling and opening of slots are difficult.” A Dresser 500 raiseborer excavates ventilation raises and orepasses. Slotraises are the domain of the Quebec-made Machine Roger. Longhole drillers reach a rate of 110 m per 10-hour shift, a rate “that would challenge a few Canadian operations,” Vanin said.

Unfortunately, productivity at the complex as a whole is not where he and Nibaldo Cordova, vice-president of operations, would like it to be. At El Indio it is 2.6 tonnes per manshift; the North American average might be closer to four or six tonnes. Said Cordova: “Our most important project is to improve productivity. That is our preoccupation.” In its defence, El Indio has a few strikes against it: the infrastructure is complex; there are a multitude of mine workings; the roadworks are many and hazardous at times; the location is remote; altitude, where ambient oxygen content is 60%-70% that of sea level, is a factor (ask any sea-level dweller after about an hour into a mine tour); the metallurgy and, therefore, the mill are complicated. Few mines must maintain, in the event of emergencies such as a severe winter storm, a fully-equipped subsurface refuge station, complete with a dormitory, pantry and dining quarters to shelter and feed 240 workers. And elevation alone means that diesel-powered trucks and scoops, altitude-tuned and turbo-charged as they are, run at about 70%-80% efficiency.

But the drive to improve productivity is already underway, and part of it is a function of reorganization. For example, the Santiago offices were closed and the 80 people there were either laid off or relocated to La Serena. The accountants and human resource people at the downtown La Serena office were given desk space at the administrative/maintenance offices on the outskirts of the city. The result, two fewer offices (the Santiago one, no doubt, very expensive).

At the mine itself, changes are occurring. Maintenance is being centralized in a single, surface office. Separate quality control labs for the mine and mill are now under one roof. A less expensive private electrical supplier has been retained in favor of t
he government source. All the ore is now washed in the mill (not just a third as before), which has decreased lime consumption.

More significant, perhaps, is the modification of the mine workday to 10-hour shifts from the former eight hours. This has resulted in less absenteeism (formerly running at 19% and now 12%), reduced overtime, and a decrease in the mine labor force — only two shifts are required as opposed to the three, 8-hour shifts in the past.

“The key words,” said Cordova, “are consolidate, improve, optimize.” Cordova is aiming for a net cash cost of (US)$140 per oz.

MILLING

Coming as it does from several different types of ores, millfeed at El Indio requires blending. The main underground haulageway is the San Jose drift at the 3,900-m level, which exits adjacent to the mill. Almost all the El Indio ore reaches 3900 through a series of orepasses, while Tambo ore is trucked under contract to the mill. Blending is rather straightforward, accomplished by a Caterpillar front-end loader according to grade requirements. The ore is then reduced to 3/8 inch by standard 3-stage crushing followed by washing in a rotating drum to remove soluble salts. The slimes fraction from the washing drum goes to acidic flotation, and the coarse fraction, stored in a fine ore bin, feeds a pair of ball mills. It is then classified in closed-circuit cyclones. The combined float products from both the alkaline and acid flotation is a copper concentrate, filtered, dried, and sent to roasting. The roasting stage is specifically designed to remove arsenic and about half the antimony. Final concentrate product is a calcine which typically assays 45 g per tonne gold, 400 g per tonne silver and 28% copper. Copper flotation tails are scavenged by sodium cyanide leaching in a carbon-in-pulp circuit to recover residual gold.

During our tour it was apparent certain circuits in the concentrator were poorly laid out. For example, after crushing and washing, the ore is conveyed to the opposite end of the building for storage in a bin and then conveyed back toward the grinding circuit, which is close to the crusher. But because of space limitations — the mill fits snugly between two mountains — it is difficult to see how any other arrangement might have been made. As well, since its construction in the early 1980s, the concentrator has gone through several limited expansions, which have led to operational difficulties. “We are making a major commitment to debottlenecking the plant,” Vanin said. During the tour, refurbishing was well underway. Dust collectors along conveyors were being installed. The pinions on the washing circuit were being replaced. And workers were even applying a new coat of paint to railings.

Formerly, capacity restrictions in the roaster circuit required that some concentrates be toll-roasted. Because of high freight and toll-roasting costs this was an expensive proposition. Now, however, El Indio has a third roaster operating. It was built this year at a cost of US$19 million, making self-sufficiency posssible.

The calcined product is trucked under contract in daily convoys to the port of Coquimbo, 5 km south of La Serena or to national smelters. In Coquimbo, it is stored in two warehouses dockside. The drive to greater efficiency and productivity is also evident portside. For example, a mechanical Tennant sweeper carefully vacuums the paved apron outside the warehouse doors, retrieving any fine concentrate spillage. The $25,000 sweeper has paid for itself in a single year, said Jorge Balada, who supervises the port facilities and trucking.

As well, arsenic trioxide, a revenue-generating byproduct of the El Indio ores is currently shipped to chemical companies in the U.S. in cumbersome 45-gallon drums. Balada is overseeing a conversion to supersacks that can carry a ton of the trioxide at a time. “We’ll make more money from the trioxide once the conversion is done,” Balada said.

From the Coquimbo warehouses, the calcine is shipped by boat principally to Japan for smelting. About half the annual 108,000 tonnes of calcined product is shipped overseas. The remainder is smelted and refined in Chile. However, El Indio is currently studying the possibility of building a smelter in Chile in a joint-venture arrangement with several other companies. The product not exported would be smelted and refined in this new smelter. A feasibility study on a new smelter has been commissioned by the partners. Cordova feels that an El Indio-owned smelter would not only be welcome in the sense that transport and smelting charges would be lower, but also that it might become essential to assure product marketing. Smelters in Chile and worldwide are becoming less inclined to accept such concentrates as El Indios because of its impurities — arsenic and antimony being the culprits for El Indio.