The La Coipa mine consists of eight deposits (notable deposits being Phase 7, Puren, Coipa Norte, Ladera Farellon and Can Can), which are owned by Compania Minera Mantos de Oro (“MDO”), a Chilean subsidiary of Kinross, except for Puren, which is owned through a joint venture between MDO and Codelco-Chile, with participation interests of 65% and 35%, respectively.
Summary:
The La Coipa mine consists of eight deposits (notable deposits being Phase 7, Puren, Coipa Norte, Ladera Farellon and Can Can).
The deposits in the La Coipa district are typical of high-sulphidation epithermal systems produced by volcanism-related hydrothermal activity at shallow depths and low temperatures. Deposition normally takes place within 1 km of the surface in the temperature range of 50°C to 200°C, although temperatures up to 400°C are not uncommon. Most deposits occur as siliceous vein fillings, irregular branching fissures, stockworks, breccia pipes, vesicle fillings and disseminations. The fissures have a direct connection with the surface, which allowed the ore-bearing fluids to flow with comparative ease. In many cases the deposits are related directly to deep seated intrusive bodies; it is typical for most ores to be in or near areas of Tertiary volcanism. The country rocks located near epithermal veins are commonly extensively altered. Relatively high porosity and open-channel permeability allow fluids to circulate in the wall rocks for great distances. Favourable temperature gradients promote reactions between cool host rocks and warm to hot invading solutions. As a result, wall-rock alteration is both widespread and conspicuous. Among the principal alteration products are chlorite, sericite, alunite, zeolites, clays, adularia, silica and pyrite.
The mineralization at La Coipa displays intense zoning and is characterized by an upper zone of secondary Ag enrichment, intermediate levels of Au-Ag in the oxidized zone and primary Au-Cu mineralization in the lower reduced zone. In the Au-Ag zone, mineralization forms disseminated and structurally controlled deposits. Disseminated deposits occur within both the volcanic and the sedimentary basement rocks, closely associated with silicic and advanced argillic alteration. Structurally controlled deposits typically formed at lower elevations, but still occur within the precious metal zone. They are often hosted by basement metamorphic rocks (lutites and sandstones) and often display higher grades. Different controls on the mineralization are consistent with what has been observed at the deeper levels of Cu-Au mineralization at Ladera-Farellon and Can-Can. Grades of up to 4% Cu is common in veins and manto like bodies.
The mineralogy of the precious metals mineralization within the oxidized and enrichment zone consists mainly of cerargyrite, native Au, native Ag and electrum with minimal embolite, iodargyrite and argentojarosite. Within the sulphide zone, enargite and native Au are the dominant minerals, but variable amounts of chalcopyrite, bornite, covellite, mineralization tenantite-tetrahedrite, sphalerite and galena are also present. In general these sulphides appear as fine veinlets in weakly altered black shales.
Gold mineralization is found generally in association with powdery alunite, distributed along stockwork fractures varying from 50 µm to fractions of a micron. Jarosite and scorodite are common minerals in gold-bearing alunitic stockworks. Gold is normally in the native state, although electrum is detected locally.
High gold values in the eastern portion of Coipa Norte are in direct association with advanced argillic alteration. Higher silver grades accompanied by little or no gold, especially those in western Coipa Norte, are associated mainly with silicification and less commonly with advanced argillic assemblages.