Candelaria copper-gold mine is located in Atacama province in northern Chile, in the Cordillera Mountains on the coast, 20km south of Copiapo. The deposit is the western extension of Punta del Cobre mining area and located in the east of Copiapo River (Figure 1 1-3 1).
Fig. 1 1-3 1 induced polarization response of concealed copper-gold deposits in candelaria, Chile and its surrounding areas.
(quoted from P.J.Ryan et al., 1995)
Copper-gold mineralization occurs in andesite dominated by lava flows and pyroclastic rocks in the early Cretaceous, concealed under a set of thick limestone, and its stratigraphic position is similar to that of copper-gold deposits outcropped in the eastern part of the mining area. Because the bedrock less than 1km exposed to the west is in place, these rocks are subjected to contact metamorphism and metasomatism. However, the exact intrusive body causing mineralization is still unknown, because no other evidence has been found in or near the ore body except the rock wall.
The copper-gold deposit is basically stratabound in morphology, and the thickness of its middle part is over 350m m. Compared with other copper-gold deposits in Punta del Covre mining area, structural control may be important. The ore is in the form of brass veins, veinlets, breccia fillings and coarse-grained to fine-grained disseminated, accompanied by magnetite, accounting for 10% ~ 15% by volume, followed by pyrite and a small amount of pyrrhotite. The associated alteration has no obvious zoning phenomenon, and the alteration is mainly manifested by the development of biotite and calcium silicate, which are common in the lower 70% ore bodies, and the latter is mainly actinolite and andalusite. The overlying limestone has undergone calcium silicate alteration, but the associated copper-gold mineralization is relatively limited. However, there are small gold-rich copper-bearing skarns exposed in Bronce and Lal (Figure 1 1-3 1), which are located above the candelaria deposit.
The recoverable reserves of candelaria deposit are 366 million t, the copper grade is 1.08%, and the gold grade is 0.26× 10-6, which is defined by 0.4% copper cut-off grade. 366 million tons of ore contains 3,952,800 tons of copper and 95. 16 tons of gold. The geological reserve of the same grade ore is 600 million t.
2. Exploration and discovery
Copper production in Punta del Covre mining area began in17th century. Since19th century, several Chilean companies have operated several small underground mines in this mining area. 198 1 year, Phelps Dodge Mining Company purchased 66% of the shares of one of the companies (Ojos del salado Mining Company), and purchased the remaining shares of the company four years later. Phelps Dodge Company immediately decided to use part of the profits of Ojos del salado Company for the exploration of Punta del Covre mining area after owning 0/00% equity of/kloc. The exploration target is copper sulfide ore with high gold content to help make up for the downturn of copper price 1985. Several exploration areas investigated include Lal mine, which is a small copper oxide-Jinxika deposit (about 200,000 t), facing several production mines across the river (Figure 1 1-3 1). Ojos-Del salado Company once carried out small-scale mining of oxidized ore (about 198 1 ~ 1984), but later abandoned the mine site because of unsatisfactory gold recovery. At the end of 1985, Lal mine was selected again, impact drilling was carried out, and sulfide zone was detected and quantitatively identified along the inclined downward direction. In the middle section of 1986, after reaching the predetermined depth, the No.33 borehole continued drilling, passing through 70m thick andesite, and finally reaching 20m thick sulfide and magnetite mineralization. The average grade of copper is 1%, and the average grade of gold is 0.3× 10-6.
At present, three induced polarization profiles have been tested at the line spacing of Lal Mine 100m, and a moderate polarization anomaly has been delineated (Figure 1 1-3 1). Therefore, Ojos del salado bought a second-hand drilling rig to verify the abnormal situation. At the beginning of 1987, drill the first hole at the same position as hole 33 (Figure 1 1-3 1). It is found that the thickness of ore is more than 50m, the average grade of copper is nearly 2%, and there is a considerable amount of gold. Copper mineralization was also found in the boreholes in the north and south of Borehole 33. So far, the candelaria deposit in the north has been discovered.
During the period of 1987, 25 boreholes were drilled, and the hole net was 50m×50m. Core drilling and impact drilling were carried out in the ore belt. At the same time, dipole-dipole induced polarization measurement and ground magnetic measurement were carried out in the whole region, and both of them obtained great anomalies. * * * Complete induced polarization and ground magnetic survey of about 2 15km, with the distance of 100m ... The dipole spacing is100m at first, then 50m, and the magnetic reading spacing is 25m. The polarization response (Figure 1 1-3 1) does not help to delineate the northern part of the deposit (this part of the deposit is hidden under the thick limestone overburden), but it leads to the discovery of the southern part of the deposit, which is some hidden outcrops under the alluvium with a thickness of only about 40 meters. Further west, some large-scale polarizability anomalies are similar to those above the Leah deposit in Candeira (figure 1 1-3 1), but they are all caused by pyrite in the contact halo of bedrock. A wide range of high ground magnetic force is not helpful for delineating ore or determining the location of drilling holes.
After the deposit is discovered, the geological mapping of the mining area 1: 5000 and the mapping of the deposit itself 1: 2000 will be completed. During the period of 1988, aeromagnetic survey was carried out in the whole mining area, and it was found that there was a strong gradient anomaly similar to the ground survey results in candelaria area. Later, one of the magnetic highlands was drilled and the Alcaparrosa satellite deposit in the northeast was discovered.
During the exploration and delineation of candelaria deposit in 1987 ~ 1990, 325 boreholes were drilled, with a total footage of 98,600 meters for core drilling and 36,600 meters for impact drilling. The borehole grid in the ore body is 50m, and the borehole spacing in the surrounding area is100 ~ 400m. In order to prove the continuity of ore body and collect metallurgical test samples, a 2090-meter inclined shaft was drilled. At 1989, representative samples of different ore types (total 72455t) were sent to Ojos Del salado flotation plant for treatment. The results show that the recovery rate of copper can reach over 90%.
199 1 Mid-term, sammito Mo Metal Mining Company purchased 20% shares in candelaria deposit, and the open-pit/flotation plant started production at the end of 1994.
3. Summary
The copper and gold deposits in candelaria were discovered after the first year of the exploration plan for gold-rich copper deposits in the whole mining area. However, in this discovery, the luck of accidentally discovering the treasure is an important factor, because no reason or concept has been put forward in advance to drill the ore-free rocks under the evaluated copper-gold deposit. In fact, no one expected that there might be Punta del Covre-type ore bodies under the ore-free limestone west of Copiapo River.
Drilling, geological simulation and induced polarization geophysical methods have promoted the exploration of candelaria deposit. Geochemistry didn't play any role because there were no outcrops in the deposit. Although mineral exploration was postponed in the early 1990s due to mineral rights disputes, it took only eight years from discovery to production.