Summary:
The Mantoverde deposit has been defined as an iron-oxide-copper-gold type (IOCG), located in the Chilean Iron Belt associated with the Atacama Fault System (AFS). Locally, it is located along the Mantoverde Fault (FMV), which is a 12 km long subsidiary structure between two north-south main branches of the Atacama Fault System. The FMV has a general orientation N15°-20°W and dip varying from 40°E in the south to 65°E in the north of the deposit.
Mineralization in Mantoverde is mostly hosted by cataclastic andesitic and dioritic porphyry rocks, with three breccia units paralleling the FMV. The copper mineralization is hosted in at least three geological environments: i) tabular bodies along the FMV with discontinuous copper mineralization and dominated by magnetite and specularite (Mantoverde Sur and Mantoverde Norte respectively); ii) Pipe breccias (Celso and Mantoruso) in the north west of the area; and iii) irregular west dipping tectonic breccias (Montecristo).
Copper mineralization extends for 7.5 km in the NS, 250 m in EW and 800 m in depth. The deposit is oxidized to 200 m depth. Breccias in the sloping fault block contain abundant specular hematite with brochantite, minor antlerite, chrysocolla, malachite and atacamite, occurring in veinlets, patches and disseminated in the specularite-rich matrix.
Hypogene mineralization at depth is disseminated within the specularite-rich matrix and consists of chalcopyrite and pyrite (chalcopyrite/pyrite = 5/1).
These two main sulfides are almost exclusively associated with gold and cobalt, respectively.
Cobalt concentrations in Mantoverde are primarily associated with the southern sector of the deposit, with a clear boundary at the local coordinate 101,000N.
Gold mineralization, appears closely associated with Cu (chalcopyrite), as indicated by the positive correlation between Au and Cu concentrations in the ore and the limited presence of native gold or gold alloys.
Between the oxidized zone and the hypogene sulphide zone there is a thin sub-horizontal mixed zone. The mixed zone has a thickness ranging from 15 m to 45 m depending on its proximity to the Mantoverde fault.
The main oxide deposits at Mantoverde include:
• Mantoverde Norte: The first exploited pit hosts copper mineralization controlled by the FMV, associated with specularite. It occurs in the form of breccias, stockworks and disseminations.
• Mantoverde Sur: The mineralization is controlled by the FMV and unnamed NE structures. The copper mineralization is associated with magnetite breccias, stockworks and disseminations.
• Franko: Mineralization is emplaced along the Franko fault, sub parallel to the FMV (hanging wall of the FMV) withing a tectonic breccia, with disseminated specularite and/or magnetite.
• Laura and Kuroki: hosted in a tectonic breccia formed in a recumbent block of the FMV (i.e. below the FMV). At depth, the oxide gives way to chalcopyrite associated with hydrothermal breccias and specularite stockwork zones.
• Mantoruso and Celso: oxide copper mineralized bodies are associated with hydrothermal specularite breccias that may grade into specularite stockwork zones.
• Rebosadero: located in a tectonic breccia generated by the intersection of the eastern branch of the AFS with a set of northwest–southeast and northeast–southwest-trending structures. It consists of a tabular body with oxide copper mineralization in the upper levels, and pyrite and chalcopyrite at depth.
The first four previously exposed copper oxide deposits transition to hypogene sulphide mineralization at depth.
Hypogene Deposit Description
Hypogene sulphide mineralization consists mainly of chalcopyrite and pyrite with lesser amounts of chalcocite, covellite and traces of bornite within specularite and magnetite-cemented breccias and associated stockworks. Magnetite is predominant in the Mantoverde deposits and hematite dominates in the Mantoruso and Celso deposits, exhibiting a relatively well-defined iron oxide zoning.
Based on the different lithologies and location with respect to the FMV seven main geological units (UG) have been defined that host mineralization in the Mantoverde deposit (Vila et al., 1996; Rieger et al., 2010). These units are known as geological units UG1 to UG7:
- UG1: Magnetite zone that is found mainly in the south of the Mantoverde deposit, also forms small subordinate bodies. In the central and northern areas, the UG1 unit occurs below 650 masl
- UG2: Green breccia, found in all deposit areas.
- UG3: Hydrothermal breccia, found mainly in the centre of the deposit; forms small bodies in the southern portion of the area.
- UG4: The specularite stockwork is associated with the distal zones of the hydrothermal breccia found in the center of the deposit. This unit is also called the transition zone because of its position between the mineralized UG3 and the waste wall rock in the hanging wall of the Mantoverde fault.
- UG5: Tectonic breccia, found along the footwall of the Mantoverde fault.
- UG6 and 7: pipe-like breccias that grade into areas of specularite stockwork; found in the Mantoruso and Celso deposits.
Magnetite Zone (UG1)
The UG1, which combines the “Magnetite Unit” and “Magnetite Green Breccia”, is a magnetite-rich unit hosting copper sulphides which can occur in both the hanging wall and footwall of the FMV. It is characteristic of the Montecristo and Altavista areas of the Mantoverde Sur deposit. Two styles of mineralization are identified:
• Magnetite stockworks and disseminations.
• Elongate magnetite-cemented breccia or massive magnetite bodies.
Green Breccia (UG2)
The UG2 unit consists of silicified clasts of volcanic and intrusive rocks of andesitic composition in an altered chlorite-silica-feldspar matrix.
Hydrothermal Breccia (UG3)
The UG3 unit is a roughly tabular hydrothermal breccia, with an average thickness of about 80 m, but can be up to 200 m thick. Rock fragments range from a few millimeters to about 35 cm in diameter.
Below the oxidation zone (below mining level 780 masl), the specularite-rich matrix contains pyrite and chalcopyrite. Digenite and bornite can locally replace the chalcopyrite.
Manto Ruso (UG4)
The UG4 unit is found in the Manto Ruso deposit that has been subject to small-scale mining activity. Prior to mining, the deposit was a 400 m long and 100 m wide body, oriented N70° to 80°E, it was covered by unconsolidated material to the south. The thickness of the oxidation zone is variable and lies between 40 m and 150 m below surface level (800 masl to 850 masl). Hypogene copper mineralization is present at depth.
The Manto Ruso deposit is hosted in andesitic volcanic–volcaniclastic rocks and intrusive diorite of the Sierra Dieciocho complex; the latter outcrops immediately to the east of the deposit. The deposit consists of a sub-horizontal specularite-cemented hydrothermal breccia.
Celso (UG4)
The UG4 unit is also found in the Celso area, east of Manto Ruso, this area was previously mined for iron (Ferrífera Celso). The martitized magnetite bodies appear to be controlled by north and northwest-trending structural corridors and are located mainly at the contact between volcanic rocks and diorites. The copper mineralization is related to a 20 m to 60 m thick, N 40° to 55° W trending and 55° to 70° W dipping, specularite-cemented hydrothermal breccia body (the Celso breccia). The breccia is surrounded by a specularite stockwork hosted predominantly in diorites. The oxidation level extends to 100 m to 150 m depth (820 masl to 920 masl). Below these levels hypogene sulphides occur over a vertical distance of at least 300 m.