Summary:
Mineralization in the Filo del Sol area shows affinities with both porphyry copper-gold-molybdenum and high-sulphidation gold-silver epithermal systems. The deposit defined by the mineral resource is best classified as epithermal in the north (Filo del Sol) and coppergold porphyry plus epithermal in the south (Tamberias). The mineralized system in its entirety is thought to represent a telescoped porphyry – epithermal system, with multiple intrusive and breccia centres, and so combines aspects of both these deposit types. The Flamenco fault is an important structure that separates the exposed porphyry Au-Cu system in the south at Cerro Vicuña - Tamberias, from the relatively down-dropped block to the north with the preserved high-sulphidation system at Filo del Sol.
The Filo del Sol deposit encompasses both the Filo del Sol high-sulphidation epithermal system and part of the Tamberias area which is interpreted as a fault-offset part of the same hydrothermal system. The deposit occurs within a porphyry Cu-Au and high-sulphidation epithermal Cu-Au-Ag system that formed during rapid uplift and erosion in the Middle Miocene. Porphyry intrusions and associated Cu-Au mineralization that are associated with an early, and largely eroded, epithermal system have been overprinted by at least one subsequent stage of porphyry intrusion and epithermal system development. The Filo del Sol system is largely associated with this second, later part of the progressive development of the system.
At Filo del Sol, mineralization is hosted within a high-sulphidation lithocap domain that was formed within conglomeratic host rocks above a regionally-significant unconformity. Steam heated and residual silica alteration along the ridge at Filo mark the uppermost part of the system, with gold mineralized domains formed at a receding paleowater table. Alteration extends downward to massive silica, quartz-alunite and argillic alteration. The basal conglomerate contact controls an important high-grade silver horizon (M zone) within the lithocap domain that has a distinctive metal signature (Ag, Sb, Cu) and forms a key part of the deposit. The conglomerate contact is slightly northwest-dipping as is the M zone formed along it. The top of the M zone is irregular but generally coincides with the lowest extent of the silica alteration. The western side of the Filo section of the deposit is bound by the premineral Frontera fault, a high-angle structure that may have controlled fluid upflow within the system. IP geophysics clearly defines this western boundary. To the north of the Filo section of the deposit, post-mineral faults complicate the geology and the extension of the M zone becomes more irregular, although the deposit remains open in this direction.
Leaching of the uppermost parts of the system has resulted in the development of a gold-only oxide zone (AuOx) that is underlain by supergene Cu enrichment in an oxide zone composed largely of Cu- sulphate minerals that form a bright blue blanket across the surface and extend to depth (CuAuOx). The intersection of the upper leached zone and the M zone forms the thickest, highest-grade copper mineralization in the deposit. The M high-grade silver zone marks the base of the highest soluble copper grades, but hypogene copper and gold sulphide mineralization continues to depth and has not been extensively drilled. Recent work indicates that the presence of blind porphyry intrusions in the hypogene zone correspond to areas of higher copper and gold grades.
The Tamberias section of the deposit is offset from the Filo section across the Flamenco fault. In Tamberias, 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 and are largely blind to the surface, and are associated with copper sulphide mineralization and elevated gold values. The western side of Tamberias hosts epithermal alteration that overprints the earlier potassic domain; this is interpreted to be the lateral continuation of the Filo system, with gold, silver and antimony mineralization. Alteration in the epithermal zone is largely composed of residual silica which corresponds to a leached domain with no copper. No high-grade silver domain occurs in Tamberias; the leached zone is underlain by an oxide zone with Cu-sulphates, that progresses down to a copper-gold hypogene sulphide domain that has only been drill tested in only a very limited area.
The distribution of copper, silver and gold reflects several types of hypogene hydrothermal mineralization and at least some supergene remobilization, resulting in a relatively complex zonation. The highest copper grades correspond to the soluble copper in the oxide zone.
In the hypogene zone, copper values tend to decrease, averaging 0.28 % Cu, with maximum values to 2.1% Cu.
The distribution of gold is more irregular and it occurs throughout all zones of the deposit. Maximum values reach 21.7 g/t over a 2 m RC sample length. The gold zones coincide, at least in part, with domains of massive and opaline silica that formed at the paleo water surface during progressive recession of the paleo water table. There is a general correlation of some of the better gold grades in Filo and Tamberias with the lower section of logged silica alteration within the epithermal system. Some values may also occur within steep structures; however, these structures have been difficult to identify conclusively in vertical RC drill holes. Gold values also occur associated with copper in the hypogene sulphide zone.
Silver mineralization occurs within a relatively restricted domain at Filo del Sol between 10 m and 50 m thick that is inferred to correspond to the base of the clastic conglomerate unit. It occurs with the mineral zone defined as ‘M’. Maximum values above 3000 g/t Au over a 2 m sample length are reported in intercepts of the M zone. A lower grade silver domain has also been defined at Tamberias, with similar metal associations (including antimony) as at Filo del Sol, in an area that is being explored as the potential offset lateral equivalent to the Filo epithermal domain.
High-grade Silver Zone (M Zone)
A high-grade silver zone is a key part of the Filo del Sol deposit, occurring as a shallowly north (20°) and west-dipping (10° to 15°) zone 20 m to 50 m thick and extending at least 1,200 m N-S and 400 m to 600 m E-W. It includes gold mineralization, generally increasing from south to north. It usually appears in drill cuttings as unconsolidated grayish to black sandy mud, commonly with associated soluble copper mineralization as Cusulphates. Silver mineralization in this zone is mostly composed of chlorargyrite (AgCl) and Ag and Cu sulfosalts 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 mineral resource for this zone is shown in Table 14-19 and it remains open to the north and east. It is truncated to the south by the Flamenco fault (approximately coincident with its surface outcrop) and to the west by the NS-trending Frontera fault.