The two major MTV deposits, Don Gabriel (open pit) and Papomono (underground) (Figure 1), are located in a regional horst–graben system, formed by a 10 km wide corridor of middle to upper Cretaceous volcanic rocks, and bounded by kilometre-scale north–south-trending faults. They are examples of stratabound, mantos-type copper deposits. To the west and east of the horst–graben complex, are intrusive granodioritic to dioritic rocks of the Early Cretaceous Illapel Supergroup.
Figure 1: Deposit Plan
The volcanic units consist of a thick package of flat-lying or gently-dipping beds of lava, pyroclastic and epiclastic rocks. Valley slopes host pediments of the Barremian–Albian Quebrada Marquesa Formation. In its type location, the Quebrada Marquesa Formation consists of a 1,900 m thick succession of continental coarse and fine sedimentary deposits, volcaniclastic deposits and lavas, with a marine calcareous and fossiliferous intercalation near the base (Charrier et al., 2007). In the Salamanca–Illapel region, the major lithologies belong to the Quellen Member of the Quebrada Marquesa Formation. These consist of intercalations of andesitic lava, volcanic breccias and agglomerates, with subordinate sandstones. The Quebrada Marquesa Formation is the main mineralization host in the area of MTV’s two deposits.
Overlying the Quebrada Marquesa Formation is the Upper Cretaceous Viñita Formation; the contact may be conformable or unconformable (Charrier et al., 2007). The Viñita Formation consists of andesites, basaltic andesites and abundant coarse and fine pyroclastic intercalations. It is divided into two members, the Santa Virginia Member, consisting primarily of oxidized sandstones and conglomerates, and the Rio Manque Member, consisting of lava and volcanoclastic units.
In the MTV project area, the grey-coloured Quebrada Marquesa Formation forms pediments on valley slopes. The reddish-hued Viñita Formation forms plateaus, such as the Llanos de Talhuen located north of the Chalinga Valley, and can also form cliffs and escarpments.
Overlying these units are remnant Tertiary tuffs, which form isolated outcrops in the upper parts of the plateaus. Quaternary fluvial sediments infill river valleys.
Two major faults have been delineated, the Manquehua and Llimpo Faults. The faults result in a sharp contact between intrusive and volcanic rocks, and are also visible in aeromagnetic survey data (Figure 2).
Figure 2: Geology Map, Illapel Region
Note: Left figure: horst–graben structure, producing a long regional belt of volcanic rocks bounded by batholitic intrusives. Mapping by Sernageomin, 1991. Right figure: reduced to pole airborne geophysical data showing the regional north–northwest to south–south-east trending faults cut by a trans-Andean east-northeast to west–southwest-trending regional structure
The geophysical data also indicate a number of east–northeast to west–southwest trans-Andean structures that are coincident with some valleys (e.g. Chalinga Valley). These structures are often associated with Chilean porphyry copper deposits.
The known mantos-style mineralization within the MTV project area is close to the western Manquehua Fault (Figure 3). The Llimpo Fault, to the east, is less evident and to date, only skarn or vein copper deposits have been found in the fault vicinity.
Figure 3: Regional Geology Plan
In the Manquehua Valley, developed laterally to the Manquehua Fault trace, a sigmoidal, 6 km long, northwest–southeast-oriented structural corridor has been identified. The Papomono deposit is at the north of the sigmoid, and the Don Gabriel deposit to the south. Within the valley, several smaller deposits have been identified, and may be related to north–south, west–northwest to east–southeast, or east–west structures.
The stratigraphy observed in the Papomono deposit consists of a sequence of andesites interbedded with pyroclastic rocks of the Quebrada Marquesa Formation.
The dominant hypogene sulphides in the deposit are in decreasing order: chalcocite–covellite (~85%), bornite (~6%), enargite (~5%) and chalcopyrite (~4%). Supergene chalcocite is not common. In places where intense fracturing and faulting occurs, the action of oxidizing agents generates oxide copper minerals including chrysocolla, malachite, brochantite and atacamite. For this reason, the traditional vertical zonation of oxides on the top of a deposit and sulphides at depth does not occur at Papomono.
Don Gabriel Manto occur in a 110 m thick package of andesites. The mantos are continuous along strike and dip but the thickness can vary from 20–60 m. The mantos can merge and split: typically, there are two main mantos that may join up to form a single manto, and there may be minor mantos adjacent to the main mantos. The mineralized zone is outlined by a 0.2% TCu isograd.
The 100 m thick upper manto zone consists of stratabound, finely disseminated chalcocite mineralization dipping to the south–southwest at 30°. The mineralized zone is developed in amygdaloidal andesites.
In Don Gabriel Vetas, the mineralization is not vein hosted, but has a vein-type shape, with high-grade copper mineralization following contacts of sub-vertical microdioritic dikes. These dikes are 1–8 m-thick and, in general have N45°W strike, dipping 50° to 85° to the northeast or southwest. A minor low-grade manto has also been identified.
The lower, vein-shaped zone consists of high-grade stockwork or sheeted veinlets, and sulphide-bearing knots and disseminations. The primary copper minerals include chalcocite, digenite and some bornite. The host rock to mineralization is a medium- to coarse-textured porphyritic andesite and/or dioritic to microdioritic dikes.
Five lithological units are found in the Don Gabriel deposit area, all of which are upper members of the Quebrada Marquesa Formation. The units strike N60°W, and dip to the southwest at 25°.
In addition to the major mantos-style mineralization, the MTV Project hosts cupriferous epithermal veins. These are examples of high sulphidation epithermal deposits.
See the National Instrument 43-101 technical report entitled Minera Tres Valles Copper Project Salamanca, Coquimbo Region, Chile” with an effective date of October 4, 2018 and dated December 14, 2018 for complete details, available on the Company’s website at srhi.ca or Sedar profile at sedar.com.