Summary:
The Mantoverde deposit has been defined as a Fe-Cu-Au type (IOCG), located in the Chilean Iron Belt, within the Atacama Fault System (AFS). Locally, it is located along the Mantoverde Fault (FMV) of general orientation N15°-20°W and dip varying from 40°E in the south to 65°E in the north.
Mineralization is hosted in Lower Cretaceous cataclastic andesitic and intrusive (dioritic porphyry) rocks, with three breccia units paralleling the NNW-striking, 40° to 50°E-dipping Mantoverde Fault, a 12 km long subsidiary structure between two N-S master faults of the Atacama Fault System. In the southern half of the Mantoverde Fault there is discontinuous copper mineralization in the form of tabular bodies dominated by specularite (Mantoverde and Laura), pipe breccias (Manto Monstruo and Manto Ruso) and irregular stockwork deposits (Montecristo).
Copper mineralization extends for 2.5 km in the EW, 600 m in NS and 800 m in depth. The deposit is oxidized down to 200 m depth. The breccias in the sloping fault block contain abundant haematite with brochantite, minor antlerite, chrysocolla, malachite and atacamite, occurring in veinlets, patches and disseminated in the specularite matrix.
Hypogene mineralization at depth occurs disseminated in the specularite matrix and consists of chalcopyrite and pyrite (chalcopyrite/pyrite = 5/1); same as Candelaria). Between the oxidized zone and the hypogene sulphide zone there is a thin sub-horizontal zone of weak supergene enrichment (3 m to 5 m thick) which contains native copper, cuprite, tenorite and chalcocite (±covellite) partially replacing the hypogene sulphides.
The volcanic and intrusive rocks at Mantoverde were affected by potassic alteration characterized by an intergrowth of K-feldspar and chlorite with minor quartz and hematite. Part of the breccias show K-feldspar and chlorite moderately altered to sericite and clays, accompanied by an increase in specularite, quartz and finely disseminated tourmaline. In addition, late calcite veining occurred. K-Ar ages of altered andesites and altered dikes have yielded 117±3 Ma and 121±3 Ma (Aptian), which have been assigned to the alteration-mineralization process.
The main oxide deposits at Mantoverde include:
• Franko: mineralization is controlled by the FMV, is associated with magnetite and occurs in the form of breccias, stockworks and disseminations;
• Laura and Kuroki: hosted in a tectonic breccia formed in a recumbent block of the FMV (i.e. under FMV). At depth the oxide gives way to chalcopyrite that is associated with hydrothermal breccias and specularite stockwork zones;
• Manto Ruso and Celso: oxide copper mineralized bodies are associated with hydrothermal specularite breccias that can grade into specularite stockwork zones;
• Rebosadero: located in a tectonic breccia generated by the intersection of the eastern branch of the ZFA 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.
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 Manto Ruso and Celso deposits.
Hypogene Deposit Description
Based on the different lithologies and location with respect to the FMV four 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 UG1 to UG4:
• UG1: magnetite zone that is found mainly in the south of the Mantoverde district, 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 district; forms small bodies in the southern portion of the area;
• UG4: pipe-like breccias that grade into areas of specularite stockwork; found in the Manto Ruso and Celso deposits.
Magnetite Zone (UG1)
The UG1 unit is a magnetite-rich zone hosting copper sulphides which can be located in both the hanging wall and foot wall of the FMV. It is characteristic of the Montecristo and Altavista areas of the Mantoverde Sur deposit. Two styles of mineralization are known:
• Magnetite stockworks and disseminations;
• Elongate magnetite-cemented breccia or massive magnetite bodies.
The rocks are cut by K-feldspar ±quartz, calcite, sericite and late specularite-calcite veins. The magnetite-rich rocks may be barren or may contain chalcopyrite (and/or pyrite), mainly in the form of disseminations, patches and discontinuous veining.
The magnetite zone is overlain or enveloped by pervasive argillic alteration in the Mantoverde Sur and Montecristo sectors. The argillic zone stretches from the present surface to the base of the oxidation zone. At Mantoverde Norte the magnetite zone occurs below 680 masl and is open at depth.
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. The matrix may also contain calcite, subordinate sericite and, locally, zeolite-group minerals (chabasite-phyllite) and small amounts of tourmaline. The rock is cut by low to moderately dense assemblages of K-feldspar quartz, calcite ±siderite, specularite ±quartz and sericite veins.
Rock fragments show pervasive K-feldspar alteration. Towards the surrounding country rocks the UG2 unit grades into a chlorite–quartz-rich stockwork. The UG2 is mostly barren, though locally it can host elevated copper–gold grades. In this case, mineralization is normally related to specularite veinlets.
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.
The rock is cut by veinlets of K-feldspar ±quartz, tourmaline or sericite. The last stage of hydrothermal activity consists of calcite and specularite veinlets.
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.