Summary:
Silver, gold, lead, zinc, and copper mineralization occurs in the El Tigre District (the “District”) mostly in fissure veins within a narrow, north-trending belt 5.3 km long. The District contains nine known veins. These include the Sooy, El Tigre, Seitz-Kelly and Combination Veins in the southern area and the Aquila, Caleigh, Fundadora, Protectora and Escondida Veins in the northern area of El Tigre. Silver and gold mineralization in the El Tigre area occurs in both the fissure veins and in a low-grade stockwork halo around the veins.
Deposit Type
Epithermal systems may be classified as high, intermediate, and low sulphidation styles. They are characterized by the sulphidation state of the hypogene sulphide mineral assemblage, and show general relations in volcano-tectonic setting, precious and base metal content, igneous rock association, proximal hypogene alteration, and sulphide abundance (Sillitoe and Hedenquist, 2003).
The veins at El Tigre closely resemble those that form quartz-adularia, low sulphidation epithermal deposits. Epithermal deposits, as classically defined, are the products of igneous-related hydrothermal activity at shallow depths and low temperatures, with deposition normally taking place within ~1 km of the surface in the temperature range of 50° to 300°C. Most deposits are in the form of quartz veining and related stockworks and breccias. These open-space fillings are common and, in most deposits, are the dominant mode of mineralization.
There are two types or styles of silver and gold mineralization found in the El Tigre area. The first and best-known are the fissure veins that host silver, lead, zinc, copper, and gold mineralization within a narrow, 5.3 km long, north-trending belt. The second is the undeveloped low-grade stockwork halo near the veins. This mineralization is associated with fractured volcanic rocks and occurs as stockwork veinlets containing minor quartz, pyrite, chalcopyrite, sphalerite and galena. These systems generally have basic to neutral pH fluids enriched in potassium and silica. Very little evidence of boiling has been found in the El Tigre Vein, as it appears that the quartz and sulphides were deposited in a passive, low energy environment.
The veins occur along fissures that generally dip steeply to the west, although steep dip reversals to the east occur in some sections of the veins. Vein mineralization consists of quartz and varying proportions of zinc, iron, lead, copper, and silver sulphides; silicified/adularized or argillized fragments of host rock are generally part of the vein material. Gold in minor amounts is associated with copper-silver sulphides. The mineralization occurs in discontinuous lenses of high-grade sulphides along the veins and as low-grade impregnations in the vein material.
Deposit Geology And Mineralization
The El Tigre silver and gold deposit is related to a series of high-grade epithermal veins controlled by a north-south trending faults, which cut across the andesite and rhyolite tuffs of the Sierra Madre Volcanic Complex within a propylitic alteration zone, as much as 150 m in width, in the El Tigre Formation. The veins dip steeply to the west (although steep dip reversals occur locally) and are typically 0.5 m thick (locally up to 5 m thick). The veins, structures and mineralized zones outcrop on surface and have been traced for 5.3 km along strike.
Exploration work by Silver Tiger at El Tigre has identified four mineralization styles:
1. Epithermal veins;
2. Stockwork Zone;
3. Black Shale Zone; and
4. Sulphide Zone.
Vein mineralization consists of quartz and varying proportions of zinc, iron, lead, copper, and silver sulphides with silicified or argillized fragments of host rock. Gold is associated with coppersilver sulphides. The mineralization occurs in discontinuous lenses of elongated, high-grade sulphides along the veins and as low-grade impregnations in the vein gangue material. A common feature of many of the mineralized bodies in the historical Mine was that they were much more extensive along strike than down-dip. Dilatancy was identified as one of the primary mineralization controls in the Mine and deflections of the vein gave rise to the characteristic horizontal elongation of the higher-grade mineralized bodies (Mishler, 1920). Intense alteration and fracturing of the brittle volcanic units along the veins host oxidized disseminated stockwork mineralization.
Metal zoning data collected during Anaconda’s investigation suggest that the upper portions of the veins, which are at higher elevations on the Property (specifically on Gold Hill, where the original high-grade gold discovery was made), host bonanza-grade gold mineralization in discrete veins and disseminated lower-grade material in the altered stockwork zones.
Mineralization consists, in order of abundance, of pyrite, sphalerite, galena, argentiferous galena, chalcopyrite, tetrahedrite, and covellite. Tetrahedrite occurs as its argentian variety, freibergite. Gold occurs in the native state as µm-sized specks, or as inclusions in galena and chalcopyrite. Sulphides occur in small amounts in the veins, averaging 5 to 8%, although locally may reach 60% in banded lenses. Massive, coarse-grained, sphalerite and galena intergrowths are observed locally in those lenses, with subordinate amounts of coarse-grained chalcopyrite and pyrite. Tetrahedrite is associated mainly with chalcopyrite and, to a smaller extent, with the other sulphide phases.
Alteration
Adularia replacement, minor silicification, argillization and propylitization are alteration styles that affect the wall rocks of the veins of the district. Although there is a general alteration zoning pattern outward from the vein in the order listed above, the distribution and width of alteration types appear to be controlled by the nature of the host rock.
In the level four mine area at the northern end of the southern vein system, the Cliff Formation stands out prominently, due to the intense adularization of the rock. In this area, veins containing quartz and mineralization show pink adularia rims on rock fragments that have also been adularized. In the El Tigre Vein, evidence of some intense silicification is found adjacent to vein.
Farther to the south along the vein system, adularization declines and a broad argillic halo becomes evident. The internal character of the veins also changes as mineralization is found in crushed host rock and minor quartz vein material. Oxidation becomes dominant since the rocks are broken and brecciated.
Fine-grained pyritization is widespread and stronger immediately adjacent to the veins. The complete alteration assemblage is found in silicified rock fragments inside the vein. Some of the fine-grained silicification is due to adularia flooding of a receptive rock type. Argillization occurs as wide, bleached envelopes around the veins, and consists of illite, kaolinite, and montmorillonite.
Propylitization is typically observed outside the argillic zones, although it may occur adjacent to the veins. Propylitic alteration consists of a mixture of quartz, chlorite, calcite, sericite, and illite and gives the rock a characteristic greenish light-grey colour. Medium-grained pyrite, slightly coarser than in the silicified zone, invariably accompanies both argillic and propylitic alteration.