Chile Quebrada Blanca copper-molybdenum deposit

1. Geological background

The Quebrada Blanca deposit is a porphyry copper-molybdenum deposit located about 250km southeast of the port of Iquique in northern Chile. high altitude area. The deposit is part of a striking late Eocene-early Oligocene porphyry copper belt. Potassium silicate alteration and late sericite alteration, with accompanying sulfide mineralization, occur in equigranular diorite, which is intruded by quartz monzonite porphyry, which in turn forms The mine is cut by quartz and feldspathic porphyry dykes and hydrothermal breccias. Prior to the mining of the deposit, these rocks were partially buried beneath a thin layer of Miocene foothill gravels.

The ore body consists of a chalcocite-enriched layer with an average thickness of 80m, a total reserve of 89 million tons, a copper grade of 1.3%, and a copper content of about 1.157 million tons; above the ore body is a 100m thick The copper-free hematite iron cap, underneath is the endogenous epigenetic zone ore body, with at least 500 million tons of ore, with an average copper content of 0.5%, about 2.5 million tons of copper, and the total of the two amounts to 365 Ten thousand tons.

2. Exploration and discovery

About the first decades of the 20th century, a small amount of oxidized rock was mined from some shallow pits in Quebrada Blanca. Copper mine. In 1957, geologists from the Chilean Exploration Company (a subsidiary of Anaconda in Chile) conducted ground inspections of regional aerial color photogrammetry and identified the mineral property as a porphyry copper system. The company staked out the property and assessed the potential of supergene enrichment zones, but no drilling has taken place. In 1971, Anaconda's assets in Chile were confiscated and the Quebrada Blanca mine was transferred to the state.

During 1973-1974, the Geological Survey of Chile carried out geological, geochemical and geophysical (induced polarization) research in Quebrada Blanca. Based on the findings, state-owned mining agency CODELCO of Chile drilled a core borehole in 1975. The hole drilled into a chalcocite-enriched layer about 12m thick, with an average copper content of 1.39%; however, it was later confirmed that it had only cut through the edge of the enriched layer.

In 1975, the Superior oil-Falconbridge group obtained earlier information about Quebrada Blanca and conducted an investigation into the mineral property. interest. In 1977, it signed a foreign investment agreement with the Chilean government and won a 51% stake in the project. The group established a new Chilean subsidiary, the Dona Ines Mineral Exploration Company, and began geological work in Quebrada Blanca at the end of 1976. Within a year, it drilled its first borehole, The 32m thick high-grade chalcocite enrichment layer contains an average of 2.17% copper and 0.01% molybdenum.

After completing the 1:10,000 scale mining area mapping, more detailed geological and alteration mapping and iron cap interpretation were carried out in the exposed parts of the Quebrada Blanca alteration zone. Work. Ironcap studies include: limonite mapping, geochemical analysis of Cu, Mo, Au, Ag and K2O, determination of the overall alteration mineral assemblage based on thin section and X-ray diffraction testing, and studies of residual sulfides. Residual sulfides in more than 500 surface samples were studied. Residual sulfides in the iron cap are those that have generally escaped epigenetic oxidation (and generally not participated in chalcocite enrichment) because they were encased in quartz. Residual sulfides were identified and counted in oil-immersion light films using high-power microscopy. Zones with significantly higher (chalcopyrite ± bornite)/pyrite ratios determined using residual sulfides may have had sufficient primary copper ore to undergo supergene enrichment.

A set of 1:2000 overlay maps were compiled for all geological parameters. The map parameters were obtained using outcrop mapping and sample measurements in Quebrada Blanca. They are: rock type, structure, alteration mineral assemblage observed with the naked eye and microscopically, limonite type, type (A, B, D) and abundance of quartz veinlets, proportion and abundance of residual sulfides, Geochemical analysis results. This basic data is then used to predict hidden chalcocite-enriched zones and thereby determine initial drilling locations. Hunter et al. (1983, 1985) pointed out that the overlap zone of quartz veinlets and potassium silicate with strong development of sericite alteration and copper-bearing residual sulfides (especially bornite) is a high-grade (>1% ) is the most important sign of copper enrichment. These parameters are important criteria for selecting drilling hole locations.

At the end of 1982, the enrichment layer was delineated based on 180 core boreholes on the surface and 54 core boreholes in underground tunnels, with a total drilling footage of nearly 44,000m. Construction of the underground tunnel began in mid-1979, including a 2.6km long adit and 12 patios near the bottom of the enrichment layer. The purpose was to identify the continuity of geology and grade and provide metallurgical samples.

The significant drop in world copper prices in 1983 made the Quebrada Blanca deposit, which used flotation to recover sulfide, uneconomical, so the resource was mothballed. Superior Oil Company was absorbed by Mobil Oil Company, which decided to abandon the project in 1984. Quebrada Blanca has since been returned to a Chilean government agency, the National Mining Corporation (ENAMI), which launched an international tender for the deposit in 1988.

Cominco won the bid. The Quebrada Blanca deposit is now controlled by Cominco-Teck Group, SMP and ENAMI. The new exploration work of the Cominco-Tektronix Group is not large, but stimulated polarization measurements have been carried out. Polarizability anomalies were observed above the pyrite zone at the edge of the ore body. From 1990 to 1991, metallurgical test work was carried out using SMP's thin-layer bacterial heap leaching technology, and new feasibility studies were carried out. The deposit was open-pit mined in 1994 and copper was recovered using solution extraction electrowinning (SX-EW).

3. Summary

It took 20 years from the first identification of the Quebrada Blanca color anomaly on aerial photos to the final discovery of copper-rich porphyry deposits. It took more than 17 years before the deposit produced its first batch of copper. Detailed geological mapping and interpretation of iron caps are helpful in accurately delineating rich mineral deposits.