Hunt for additional reserves continues at La Coipa mine

Partners Placer Dome (TSE) and TVX Gold (TSE) are planning to spend $1 million on exploration this year to delineate additional reserves at La Coipa mine in northern Chile.

Exploration will focus on targets adjacent to the Ladera/Farellon and Coipa Norte orebodies, as well as new target areas on La Coipa property. The project is owned and operated by Compania Minera Mantos de Oro, a Chilean company in which Placer has a 50% interest through a wholly owned subsidiary. TVX holds the remaining interest.

The mine, 500 miles north of Santiago on the southern edge of the Atacama Desert, began producing in July, 1989, at 1,100 tons per day. In October, 1991, a US$218-million facility was commissioned and production was expanded to 16,500 tons per day. The new plant, designed as a zero-discharge facility, uses 12 horizontal belt filters to recover 85% of the moisture in the mill tailings. The recovered water is then recycled and the resultant “dry” tailings delivered to a disposal area.

Last year, the mine produced 196,182 oz. gold and more than 16 million oz. silver at a cash cost of US$137 per oz. gold equivalent, making it Chile’s largest precious metal mine. Average recoveries were 83.9% for gold and 78.1% for silver. Owing to the refractory nature of the ore, recoveries typically range from 76% to 84% for gold and 68% to 80% for silver.

Production comes from the Ladera/Farellon deposit where ore is mined via open pit. Mining has yet to begin at the Coipa Norte orebody which lies two miles north of the Ladera/Farellon pit.

The orebodies at La Coipa are hosted in Triassic sediments and overlying Tertiary volcanics. In the Ladera and Coipa Norte areas, mineralization occurs primarily in altered volcanics. The principal alteration minerals are quartz, sericite and alunite.

The Farellon orebody and a portion of the mineralization at Coipa Norte occur within the sedimen-tary sequence. Mineralization is associated with breccia zones and accompanied by strong silicification and alunization. Gold occurs in quartz with secondary oxides and in jarosite (a potassium-iron sulphate). Silver occurs in native form and as halides. Other accessory elements include copper, lead, zinc, arsenic and mercury.

In 1992, exploration drilling resumed at La Coipa for the first time since 1984. The program tested targets near the Ladera/Farellon pit and identified an ore zone known as the Farellon Bajo which contains 4.2 million tons with a gold equivalent of 0.07 oz. per ton.

This year, a 30,000-ft. program will test several additional targets on the property. Drilling will be carried out to evaluate the continuity of mineralization in the Connecting zone, situated between La Coipa mineralized area and the Brecha Norte outcrops.

At the Cerros Bravos prospect, three miles north of Ladera/Farellon, a 1.25×1-mile hydrothermal alteration zone with anomalous gold, silver and molybdenum values will be tested. A second alteration zone with anomalous values will be examined at the Indagua-Los Burros prospect in the extreme northeast portion of the property. Additional drilling will be done at the Margarita area, southwest of the Coipa Norte, to evaluate anomalous gold values in structures and associated volcanic rocks.

At year-end, proven and probable reserves were 69,476,000 tons of oxide material grading 0.037 oz. gold per ton and 2.187 oz. silver, adequate for 11.5 years of production. An additional mineral resource of 17.9 million tons of oxide material, grading 0.034 oz. gold and 2.32 oz. silver, has also been identified.