Summary:
Although detailed petrologic studies of veins in the Velardeña property have not been completed, individual deposits within the nearby Santa María dome have been studied in some detail and found to correspond to both shallow epithermal and deeper-seated mesothermal styles of mineralization. Epithermal veins, often displaying banded and open-space-filling quartz, occur at La Industria Mine where they are clearly distal to the main intrusive mass (Gilmer et al., 1988). The higher-level veins at Velardeña appear to be of this type.
True epithermal veins occur at Velardeña, but at depth most veins, breccias, and massive sulfide replacements are mesothermal in character. The veins commonly contain arsenopyrite, which may be related to a deeper intrusive source. Exploration strategies at the Project are informed by the above model concepts. Current exploration models explore the deposit in the context of vein controls. It has been recently discovered that eastwest trending vein sets, in general, tend to have lower concentrations of deleterious elements (As and Sb) and slightly thicker true widths.
Velardeña Property
Medium to thick-bedded limestone of the Cretaceous Aurora Formation comprises basement rocks in the Project area. Limestone was first folded then intruded by the multiphase diorite/monzo-diorite Terneras stock and related dikes of Tertiary age that outcrop over a strike length of approximately 2.5 km. In detail, intrusive contacts range from sharp to broad zones characterized by the presence of numerous large, partially metamorphosed blocks of limestone. Alteration of host carbonates consists of a broad front of bleaching and marble formation, with more localized calc-silicate and hornfels. Although intrusive rocks appear fresh in general, alteration and local endoskarn development occurs near contacts. The diorite stock and the contact zone between limestone and intrusive rock primarily host the veins of the Santa Juana, Terneras, San Juanes, and San Mateo deposits. Veins extend into relatively unaltered limestone especially in the northwestern portion of the Santa Juana veins and eastern portion of the San Juanes vein.
The Velardeña property is transected by a series of northeast to northwest striking, west dipping, post-mineral normal faults. From east to west these are the Tres Aguilas, Los Bancos, Buenaventura, and Ordenanza faults which are generally characterized by meters-thick banded calcite vein filling. These normal faults demonstrate west-side-down displacements with the result that the western blocks expose higher portions of the hydrothermal system, have a higher calcite content and generally lower precious metal contents.
Two main vein systems are present on the Velardeña property. The first is the northwest striking system found in the Santa Juana deposit, while the second is the east-west trending vein array which includes the Terneras, San Juanes, Roca Negra, and San Mateo deposits.
Chicago Property
The geologic setting of the Chicago property is very similar to that at Velardeña . The oldest rocks outcropping at Chicago are folded limestone of the Aurora Formation which were intruded by Tertiary diorite stocks and dikes. Intrusive rocks occupy the western portion of the property with a northeast orientation. The limestone-diorite contact exhibits widespread recrystallization and marble formation overprinted by a distinctive green calc-silicate alteration dominated by grossular garnet and lesser wollastonite.
As at Velardeña, a system of post-mineralization faults striking northwest-southeast cuts and locally displaces mineralized structures. These faults are normally filled with calcite and can have widths up to 10 m near surface.
In the Chicago mine, rhyolitic volcanic rocks and calcareous conglomerate of the Ahuichila Formation unconformably overlie the mineralized sequence across the eastern half of the area. Mineralization is similar to that encountered at Santa Juana mine in terms of mineralogy, host rocks, geometry of the structures and vein continuity. The difference between the two is orientation - northwest strike, dipping to the northeast for the Santa Juana system; instead of northeast strike, dipping to the southeast for the Chicago system.
Mineralization
Mineralization consists primarily of calcite-quartz veins with minor calc-silicate hosted skarn and massive sulfide replacement bodies. All mineralization is essentially polymetallic, Ag, Au, Pb, Zn, plus or minus Cu. Individual veins are usually thin (0.2 m to 0.5 m) but remarkably consistent along strike and down dip. Coxcomb and rhythmically banded textures are common in some vein exposures. Historical production in the district has been primarily from the oxide portions of the veins that can extend to depths of several hundred meters. Previous workers have suggested a vertical zonation with increasing Au:Ag and Cu:Pb with depth (Pinet, 2009).
The mineralization at the Chicago property is similar to that encountered at the Santa Juana mine in terms of mineralogy, host rocks, geometry of the structures and continuity at depth and laterally. The difference between the two is geometric - northwest dipping to the northeast instead of northeast dipping to the southeast.
Characteristics of the mineralization are summarized below based on previous reports and Tetra Tech’s observations:
- Veins occur in limestone, marble, calc-silicate, and intrusive host rocks. The geometry of the veins is typically wider but more irregular in the limestone. In addition to being more consistent in width, veins within skarn and intrusive rocks tend to be narrower but higher grade with respect to precious metals. Skarn is the least favorable vein host.
- Although individual veins are typically narrow, zones of vein intersections and certain contacts between intrusions and limestone have focused brecciation and silicification, yielding mineralized chimneys which can reach 7 m in width and extend for tens of meters vertically.
- Within the Santa Juana sector, a zone of sheeted veins has been discovered near the intersection of northwest and east west trending veins. The overall dimensions of this corridor are approximately 500 m along strike and 250 m vertically (level 12 to level 18), with widths up to 100 m.
- Gangue minerals consist of calcite and quartz, which generally represent less than 20% of the volume of individual veins. Higher grade segments of veins generally conform to areas dominated by quartz or quartz-calcite mixtures; calcite rich zones are generally low grade. There is a distinct tendency for the upper portions of many of the veins to be calcite dominant, hence lower grades. Lateral changes in the gangue mineral composition have been observed, suggesting controls other than elevation are at work.
- Depth of oxidation is quite variable and the distribution of oxide and mixed mineral types of complex. Within limestone host rocks, the veins are oxidized down to depths of up to 450 m. Oxides are rare in intrusive and calc-silicate host rocks, reportedly encountered only near the Tres Aguilas and Los Bancos faults, due to increased fracture-controlled permeability and fluid flow.
- The alteration zone along vein margins is generally less than 10 cm and is comprised of argillic alteration and silicification of the intrusive and skarn host rocks, and localized silicification and recrystallization of limestone. While precious and base metal mineralization is generally confined to the veins, sulfide stringers were observed extending outwards along bedding planes within altered limestone.
- Underground drifting and drilling suggest many of the veins are open at depth below the 19th level.