Summary:
Deposit Type
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, but locally can be up to five metres 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.
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 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.
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 or 20 cm across. Larger blocks or slabs detached from the walls by faulting, occur in places and are crisscrossed by hairline fractures, with or without quartz or sulphide 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 sulphides.
The El Tigre deposit contains several mineral veins for the open pit mining, they are: El Tigre - 100, Halo - 150, Sooy East - 205, SK - 300, and East SK1 – 305.
Mineralization
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 copper-silver 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 hosts oxidized disseminated stockwork mineralization.
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 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 sulphide 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 sulphides 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.
The sulphide mineralization was studied in reflected light and analyzed with a scanning electron microscope (SEM) by Landin (2022). The mineralization in 16 drill core samples consisted mainly of sphalerite, galena, chalcopyrite, pyrite, and tetrahedrite-tennantite. EDS analyzes confirmed that tetrahedrite-tennantite and galena are the main silver-bearing minerals. In some samples, tetrahedritetennantite occurs including and cutting the other sulphide phases, which suggest that it was a relatively late-forming phase during mineralization.