Summary:
The San Antonio Project mineralized zones are considered to be typical of mesothermal vein-style, or orogenic-style gold deposits.
Mineralization has preferentially developed in zones of stockworking, cataclasite and mylonite within the shear zones.
The proximity of the Triunfo and San Antonio districts, both of which are localized in similar shear zones settings and in similar host rocks, suggests a possible similar age and origin.
However the geochemical signature characterizing the two districts is very different.
San Antonio is a gold-only mineralized district with average silver to gold ratios of 1:1 or less, whereas Triunfo is silver dominated with average silver to gold ratios of around 250:1 and associated lead, zinc, and antimony.
In addition, the orientations of the mineralized zones are different. The Triunfo zone strikes generally N20–30E and dips to the east. The San Antonio shear zones have a persistent north– south strike and the shears generally dip at about 45ºW.
Deposit Descriptions
Mineralization has been identified over approximately 1.8 km of strike length within the Project area, and was subdivided into four fault-bounded zones referred to as Los Planes, Intermediate, Las Colinas, and La Colpa.
Local normal and listric faulting displaces and truncates the mineralized zones (Herdrick, 2009). Drilling suggests that a northwest-trending graben basin has displaced the Los Planes deposit by more than 200 m, over increments of 20-100 m. These blocks are bounded by listric faulting and are dropped en echelon down to the northeast along N40W-trending structures.
Much of the outcrop is covered by pediment that thickens to the east, featuring cobbles and boulders of schist and gneiss.
Los Planes
Deposit Dimensions
Mineralization extends along a north-south strike length of approximately 975 m. Drilling has encountered continuous mineralization to depths of 400 m from the surface. Mineralized lenses average 90 m in thickness. Mineralization thickness is highly variable due to the stockwork-style of mineralization, and zones can locally coalesce into broader intervals >100 m in thickness. Mineralization is interpreted to remain open along strike to the north and at depth.
Structure
The shear zone strikes southeast-northwest and dips at 55° to the northeast. An east-west striking fault dips at 50° to the north and separates the Los Planes deposit from the Intermediate deposit to the south.
Mineralization
Typically, the cataclasite contains a zone where sulphides increase with abundance to as much as 20%. Pyrite appears more commonly throughout the system; however the more intense development of mineralization is accompanied by an increase of arsenopyrite and pyrite in addition to pyrrhotite. Sulphides are present in brittle structures including cracks, faults, joints and micro-fractures. Pyrite lines the walls of many fractures and joints as observed from drill core and outcrop. Quartz veining is also common (Coyan et al. 2007).
Chemically, the system has low silver relative to gold, having silver gold ratios of approximately 0.4. Gold is also accompanied by anomalous arsenic, copper, and bismuth.
Oxidation
Although mineralization is intimately associated with the presence of sulphides, the Los Planes zone also contains well-oxidized areas, which include pure oxide and oxide-transition material. The oxidized zone can reach up to 100 m in depth from surface and appears to be related the Quaternary Los Planes fault. Oxidation at Los Planes is much stronger and deeper than in the Intermediate and Las Colinas zones. When the stockwork sub-zone appears oxidized, it has a physical appearance of hematite-rich rubble.
Las Colinas
Deposit Dimensions
The mineralized zone has a roughly north-south strike and dips at about 50° to the west. The zone is 675 m long, averages 40 m thick, and extends to a depth of 250 m. The zone remains open at depth, but is limited to the north, where it is interpreted to end where a northwest-trending fault intercepts the strike of the mineralization.
Structure
A southwest-northeast striking fault dips at 57° to the southeast and separates the Las Colinas deposit from the Intermediate deposit to the north.
Mineralization
The Las Colinas deposit consists of gold and arsenic occurring with disseminated and veinlet sulphides associated with cataclasite and locally extending into the wall rocks.
Where high-angle structures intersect open flat-lying thrust faults, mineralization is developed over 1 m to 50 m thicknesses. The mineralized zone averages 20-40 m in thickness and grades into strongly fractured and altered gabbro or diorite. The wall rock may locally be extensively sheared.
The gold mineralization is interpreted to consist of two stages, the first being disseminated gold-arsenic deposition during mylonitization, and a second stage gold-silver-bismuth mineralization hosted by crosscutting, high-angle, northeast-trending structures.
The primary alteration assemblage consists of sericite, 2-5% sulphides including pyrite, arsenopyrite, pyrrhotite, and minor quartz and K-feldspar.
Oxidation
The level of oxidation at Las Colinas extends only a few metres vertically from surface.
Intermediate
Deposit Dimensions
The zone is hosted within a shear zone that strikes approximately north-south and dips at about 50° to the west. Mineralization extends along strike for about 900 m in this fault block and zones are 45 m in thickness on average. Drilling has intercepted continuous mineralization to about 325 m depth. Mineralization is truncated to the north and south by west-northwest-trending faults that that separate the zone from the Los Planes and Las Colinas zones, respectively. Despite being limited on strike, the Intermediate zone remains open down dip to the west.
Structure
An east-west striking fault dips at 50° to the north and separates the Intermediate deposit from the Los Planes deposit. A southwest-northeast striking fault dips at 57° to the southeast and separates the Intermediate deposit from the Las Colinas and La Colpa deposits to the south.
Mineralization
Gold mineralization in the Intermediate zone is transitional between Las Colinas and Los Planes. In the southern part of the zone, mineralization tends to occur as more narrow and lower-grade intercepts. Gold grades are higher in the stockwork zones (typically 1+ g/t Au) and decrease away into the cataclasite to <1 g/t Au.
Similarly to both Los Planes and Las Colinas, silver values in the Intermediate zone are very low and Ag/Au ratios range from 0.3 to 1 in low-gold-grade zones, to 0.8 to 1 in zones with higher gold grades.
Between cross sections 39,100mN and 39,200mN, mineralization is in clear, sharp contact with the footwall, and demarcated by a gouge/fault zone that is sub-parallel to the west-dipping mineralized zone.
Oxidation
Oxidation depths are similar to those at Las Colinas; however, very little of the gold mineralization is oxidized in the Intermediate zone.
La Colpa
Deposit Dimensions
The mineralized zone has a roughly north-south strike and dips at about 35º to the west. The zone is 625 m long, averages 60 m thick, and extends to a depth of 250 m. The zone remains open down dip and along strike to the south.
Structure
Mineralization is hosted by sheeted veins and intermediate stockwork zones. Originally, the mineralization was interpreted to be hosted in a shear zone; however, no significant faults are mapped or modeled in the La Colpa area.
Mineralization
Pyrite is the dominant sulphide and arsenopyrite is less common than in Las Colinas.
Mineralization is hosted in less-deformed, subhorizontal shear bands that occur on the footwall zone of the main mineralized zone of Las Colinas and is dominated by pyrite as disseminations and veinlets. Bands of mineralization vary from 8–25 m.