A visit to a small Argentine mine — New zone developed at

A new zone is being developed at the Farallon Negro epithermal gold-silver-manganese deposit, deep within the foothills of the Argentine Andes.

About 300 tons per day are mined from the No. 1 vein system, which has been developed via two shafts to a depth of 220 metres. A limited amount is also mined from a small surface pit.

The new zone, called the No. 2 vein system, is 50 metres north of the current workings and is being developed via an inclined ramp.

Annual production totals about 490,000 grams gold (15,750 oz.) and 4.2 million grams silver (135,000 oz.). With development of the No. 2 system, gold production is estimated to increase by more than 71%.

Although operating costs were not available, power consumption and water demand render them relatively high, The Northern Miner learned on a site visit.

The veins are hosted in the Farallon Negro volcanic complex in association with the Alto de la Blenda monzonitic stock. Both systems consist of a 3-to-8-metre-wide quartz-carbonate vein that contains native gold, silver and 13-16% manganese. The veins trend east-west and have a strike length of about 200 metres.

Proven reserves in the No. 1 zone are 16,711 tons grading 5.77 grams gold and 104.7 grams silver per ton with an additional 180,000 tons of inferred reserves of similar grade. Preliminary reserves in the No. 2 system are estimated at two million tons grading seven grams gold and 200 grams silver. The veins are selectively mined. Ore from the lower-grade hangingwall and footwall is heap-leached while the richer, central portions of the veins are milled using flotation and carbon-in-pulp technology. About 100 tons per day are sent to the heap leach pads which have a capacity of 11,000 tons. Roughly 35 days are required to process 1,000 tons on the pads, with recoveries in the order of 70% for gold and 30% for silver.

Recoveries in the mill average 92% for gold and 45% for silver. Unfortunately, the manganese cannot be recovered.

Farallon Negro, which began producing in 1980, is about 1,100 km northwest of Buenos Aires. It is owned by Yacimientos Mineros de Agua de Dionisio, a quasi-government mining company 60%-owned by the province of Catamarca and 40% by the University of Tucuman (in Tucuman province). Efforts are under way to privatize the deposit.