Summary:
The Filo del Sol deposit includes both porphyry Cu-Au and high-sulphidation epithermal Cu-Au-Ag mineralization. The mineralized system in its entirety represents a telescoped porphyry–epithermal system with multiple intrusive and breccia centres, and so combines aspects of both deposit types. The deeper porphyry mineralization contains both disseminated sulphides and various veinlet and stockwork systems that also host sulphides. The upper-level epithermal style mineralization includes siliceous vein fillings, irregular branching fissures, stockworks, breccia pipes, vesicle fillings and disseminations. The defined mineral resource is best classified as the upper oxidized part of the high-sulphidation epithermal Cu-Au-Ag part of the deposit.
The Filo del Sol deposit comprises a large porphyry Cu-Au and high-sulphidation epithermal Cu-Au-Ag system that formed during rapid uplift and erosion in the Middle Miocene. A north-northeast trending alignment of porphyry intrusions at least 3 km long in the deposit area includes an older, more deeply eroded porphyry Cu-Au mineralized domain in the Tamberías area, with slightly younger, partly blind to the surface porphyry Cu-Au mineralized intrusions in the Filo del Sol-Aurora zone to the north. Extreme telescoping in the Filo del Sol-Aurora zone has led to the overprinting of the Cu-Au mineralized porphyry domain by high-sulphidation Cu-Au-Ag epithermal mineralization within a large area of advanced argillic alteration.
In the Filo del Sol-Aurora zone, a suite of north-northeast trending, pre-mineral mafic dykes were guided by pre-existing faults through the host rhyolite and overlying clastic rocks. At higher levels in the host rock sequence, mafic sills were guided by layering within the clastic rocks. Porphyry intrusions (~15.4 Ma) and related intermineral magmatichydrothermal breccia were subsequently emplaced along the same structural trend, with associated Cu-Au mineralization in potassic alteration. A high rate of uplift and syn-mineral erosional unroofing of the system is inferred as the potassic porphyry Cu-Au mineralization is largely overprinted by advanced argillic alteration with associated high-sulphidation CuAu-Ag mineralization as pyrite, enargite, bornite, chalcocite, covellite and Ag-bearing sulphosalts in a core zone with vuggy residual silica, silicification and surrounding quartz-alunite alteration. Metal distribution within the hypogene part of the deposit is controlled by these two types of alteration and mineralization, with a relatively sharp boundary between the two at depth. A later style of pyrite mineralization with high silver grades is related to late, higher-level phreatic breccias along the north-to-northeast mineralized corridor. Steam heated alteration is preserved as the uppermost part of the lithocap domain, forming the ridgetop at Filo del Sol. Drilling is currently ongoing in the Aurora zone at depth beneath, and north of, the currently defined resource, and has so far demonstrated that the Filo del Sol deposit is at least 3 km N-S, 400 m EW and extends at least 1.5 km below surface. The deposit remains open to the north, south, east, west and at depth, although mineralization appears to be weakening in the deepest holes drilled to date.
A high-grade Ag zone is a key part of the Filo del Sol deposit, occurring as a shallowly north (20°) and west-dipping (10° to 15°) zone 10 m to 130 m thick and extending at least 1,200 m N-S and 400 m to 600 m E-W within the resource area, overprinted by the oxide zone. However, to the north of the currently defined resource, the Ag geochemical zone extends another 1800 m along the same trend. Ag grades in this zone are above 60 g/t, averaging 200 g/t with a maximum value of 6980 g/t Ag over a 2-m sample length. It includes Au mineralization, generally increasing from south to north (Figure 7-7. In the southern part of the deposit, it commonly appears in drill cuttings as unconsolidated grayish to black sandy mud, commonly with associated soluble Cu mineralization as Cu-sulphates. Ag mineralization in this zone is mostly composed of chlorargyrite (AgCl) and Ag and Cu sulphosalts of proustite - pyrargyrite [Ag, (As, Sb), S] (Di Prisco, 2014). It has a distinct geochemical anomaly pattern characterized by anomalous values of metals such as Cu, Ag, Mo, Sb, (±Au), As, Hg, W, (±Bi, Sn) and low values of Al, Ca, Sr, V, (±Th).
The epithermal system at Filo del Sol-Aurora extends southward into the Tamberías area, where zones of quartz-alunite and quartz-white mica-clay alteration with Au and Ag mineralization overprint the older (~16 Ma) host dacitic Tamberías porphyry intrusions. Telescoping of the younger Filo del Sol-Aurora system over the older Tamberías porphyry is inferred to largely be responsible for the juxtaposition, although normal faulting along north- and northeast-trending faults through the system also break geologically different domains. Alteration in the epithermal zone is composed of residual silica, which corresponds to a leached domain with no Cu. No high-grade Ag domain occurs in Tamberías; the leached zone is underlain by an oxide zone with Cu-sulphates that progresses down to a Cu-Au hypogene sulphide domain that has only been drill tested in a limited area. In Tamberías, dacitic plagioclase-hornblende-biotite porphyry intrusions intrude the rhyolite basement and have associated biotite-magnetite (potassic) alteration. These porphyries are intruded by younger feldspar-phyric porphyry phases that are only partly exposed, are largely blind to the surface, and are associated with Cu sulphide mineralization and elevated Au values.
The defined resource at Filo del Sol predominantly includes the uppermost oxidized part of the deposit at Filo del Sol and Tamberías. Leaching of the uppermost parts of the system (LIX) has resulted in the development of an Au-only oxide zone (AuOx). This zone includes drill intersections in holes VRC097 (84 m @ 1.36 g/t Au) and VRC099 (78 m @ 1.02 g/t Au). The leached zone is underlain by supergene Cu enrichment at the top of an oxide zone (OX) that ranges from 40 m to 300 m in thickness, deeper in the north. The oxide zone is characterized by the presence of Fe, FeCu and Cu oxides and hydroxides. It hosts the important soluble Cu mineralization comprising hydrated sulphate minerals (chalcanthite, copiapite, cuprocopiapite) that form a bright blue blanket across the surface and extend to depth (CuAuOx Zone). This zone formed as a result of the combination of the highly acidic environment generated by oxidation of abundant marcasite and pyrite and the arid climatic conditions. The M zone (M) corresponds to the high-grade silver zone that occurs as a moderately north-northwest dipping tabular zone ranging from 10 to 130 m thick (Figure 7-8). It is overprinted by the oxide zone in the resource area. The hypogene zone (HYPO) is characterized by the presence of sulphides and the absence of oxide minerals. It includes both porphyry Cu-Au and high-sulphidation Cu-Au-Ag epithermal mineralization. The resource is bound to the west by a sharp cutoff in grade, possibly a fault. Mineralization extends to the eastern limit of the pit as the current resource does not include the most recent drilling into hypogene Cu and Au mineralization to depth.