Summary:
Deposit Types
Epithermal systems may be classified as high, intermediate, and low sulfidation styles. They are characterized by the sulfidation state of the hypogene sulfide mineral assemblage, and show general relations in volcano- tectonic setting, precious and base metal content, igneous rock association, proximal hypogene alteration, and sulfide abundance (Sillitoe and Hedenquist, 2003).
The veins at El Tigre closely resemble those that form quartz-adularia, low sulfidation 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.
Drusy cavities, cockade structures, crustifications, and symmetrical banding are generally conspicuous. Colloform textures characteristic of epithermal environments presumably reflect relatively low temperatures (e.g., shallow depths) and hydrothermal fluid circulation through open spaces formed by mechanical anisotropies such as networks of fractures, contacts between units with dissimilar mechanical properties, and/or cross-cutting structures, intrusive bodies and shears. (summarized from Guilbert and Park, 1986).
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 usually 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 sulfides 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 sulfides; silicified/adularized or argillized fragments of host rock are usually part of the vein material. Gold in minor amounts is associated with copper-silver sulfides. The mineralization occurs in discontinuous lenses of high-grade sulfides along the veins and as low-grade impregnations in the vein material.
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 to the east occur locally, and are typically 0.5 m wide, and locally can be up to 5 m in width. The veins, structures and mineralized zones outcrop on surface and have been traced for 5.3 km along strike. Historical mining and exploration activities focused on the 1.5 km portion at the southern end of the Deposit, principally on the El Tigre, Seitz Kelly and Sooy Veins, whereas the Caleigh, Benjamin, Protectora and Fundadora Veins to the north remain under explored. 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) sulfide zone.
Vein mineralization consists of quartz and varying proportions of zinc, iron, lead, copper, and silver sulfides with silicified or argillized fragments of host rock. Gold is associated with coppersilver sulfides. The mineralization occurs in discontinuous lenses of elongated, high-grade sulfides 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 hosts 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.
The principal veins consist predominantly up to 80 to 90% gangue material, including silicified rock fragments, quartz, gouge, rock flour, clays and minor calcite, in order of abundance. The silicified fragments are angular to subangular and range in size from a few mm to 15 cm to 20 cm across. Larger blocks or slabs detached from the walls by faulting, occur in places and are criss-crossed by hairline fractures, with or without quartz or sulfide filling. Quartz occurs in lenses, bands, fragments, dissemination, and breccia matrix, and is the major gangue mineral in the vein. Rock flour, partially indurated, gouge, and clays occur throughout the vein in minor amounts as breccia matrix and fault linings. Minor calcite occurs in irregular veinlets and is locally associated with mineralized sulfides.
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. Sulfides occur in small amounts in the veins, averaging 5 to 8%, although locally may reach 60% in lenses with banded structure. 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 sulfide phases.
Fine-grained argentiferous galena occurs associated with pyrite and quartz with little or no sphalerite. Pyrite occurs with quartz and hematite, or with other sulfides in lenses and in clusters or in strongly disseminated patches. It also fills numerous irregular veinlets in large rock fragments and slabs in the vein and in the wall rock. Quartz occurs in substantial amounts in all the occurrences noted above. A significant amount of sulfides occur as vein fragments and crushed material. Grain size varies from virtually pulverized to fragments ranging in size from a few mm to a few cm. Larger fragments preserve their textures, however, are subordinate in volume to crushed sulfides. Pulverized sulfides, mostly pyrite, occur along the walls of the vein. Sulfide dissemination is, except for pyrite, restricted to rock fragments or massive quartz in the vein. Minor drusy structures near the centre of the vein are typically lined with pyrite.
The mineralization in 16 drill core samples consisted mainly of sphalerite, galena, chalcopyrite, pyrite, and tetrahedrite-tennantite. EDS analyses confirmed that tetrahedrite-tennantite and galena are the main silver-bearing minerals. In some samples, tetrahedrite-tennantite occurs including and cutting the other sulfide phases, which suggest that it was a relatively late-forming phase during mineralization.