Summary:
Mineral Deposit Types
The San Jose de Gracia and Alta Gracia gold-silver sub-districts of the Don David Mine are characterized by classic, high-grade silver-gold, epithermal vein deposits with low to intermediate-sulfidation mineralization and quartz-adularia-sericite alteration. The veins are typical of most epithermal silver-gold vein deposits in Mexico with respect to the volcanic or sedimentary host rocks and the paragenesis and tenor of mineralization. The Alta Gracia mineralization is typical of a low-sulfidation deposit. The Arista mine vein systems are intermediate-sulfidation in nature, which is standard of many Mexican deposits, characterized by Camprubí & Albinson, 2007, and described as polymetallic Ag-Au by Corbett.
The structural geology of the Alta Gracia area is somewhat masked at the surface by the presence of expansive soils and vegetation. However, numerous quartz veins are in accessible underground workings and prospect pits at Alta Gracia. Veins are mainly hosted in rhyolite at the surface, and andesite was developed in deeper underground mine workings. Two dominant vein trends have been mapped: N30°E dipping 65° - 85°NW and N50°E dipping 65° - 85°NW. Vein widths generally average from 0.2 to just over 2 m (true width). At least nine significant veins/vein systems have been identified at Alta Gracia that include the following:
1. Mirador Vein - The Mirador vein is a fissure filling vein hosted in andesite with a bearing 240°-250°, a dip of 60°-80°NW and a variable thickness of 0.80 m to 1.80 m. The Mirador vein is offset by a system of transverse faults bearing 340°-350°, dipping 45°-60°NE, with displacements of 1 to 11 m.
2. Huaje Veins - Two principal parallel veins, separated by 25 to 75 m, comprise the Huaje vein system. These veins strike from 230°-240° with a dip of 65°-70°NW and variable thicknesses from 0.80 to 0.90 m. The Huaje veins occur along faults hosted in andesite.
3. San Juan Veins - The San Juan and at least five subparallel ancillary veins, strike 200°-210° with a dip of 60°-80° NW and a variable thickness of 0.30 m to 1.20 m. The veins are hosted in rhyolitic flows.
4. Victoria Vein - The Victoria vein strikes 210°-225°, dipping 70°-80°NW, and has a variable width from 0.15 m to 0.60 m. It is hosted in rhyolite flows.
5. Independencia Vein - The Independencia vein has a bearing of 240°-250°, a dip of 60°-80°NW and average thickness of 0.40 m to 1.20 m with intervals of up to 10 m (pinch and swell). It is mainly hosted in rhyolitic flows.
6. Aguacatillo Veins - The Aguacatillo area is comprised of two vein systems with very similar strikes of 040°-050°, dipping 80°-85°SE, and thicknesses varying between 0.25 m to 0.50 m. Rhyolitic flows host veins to the west and the east. They occur in andesitic tuff.
7. Chamizo Vein - The Chamizo vein has a bearing of 260°-280° and dips 45°- 70°NW. The vein contains good base metal values over very narrow widths (0.10 - 0.30 m). The Chamizo vein is hosted in andesitic tuff.
8. Navajas Veins - Navajas veins consist of a system of subparallel veins of variable thickness (0.20 - 0.30 m) with a bearing of 030°-040°, a dip of 70°-80°SE and contain significant levels of gold and silver. The veins are hosted in rhyolitic flows.
9. Base Metal Prospect - A prospective area with significant base metal showings in the southwest part of Alta Gracia. Mineralization is hosted at the rhyolite tuff and andesite contact with abundant carbonate flooding and local fault gouge. Geologic mapping indicates that the zone lies at the intersection of three structures. The intersection coincides roughly with an N45°E trending fault/contact between andesite and rhyolite.
Alta Gracia Veins
The Alta Gracia Project hosts multiple sub-parallel veins and splays of varying length and width. Visible silver mineralization observed in the Alta Gracia veins includes accessory sulfide and sulfosalts such as pyrargyrite-proustite, arsenopyrite, abundant (3-5%) high color euhedral, and disseminated pyrite, sphalerite, traces of covelite, jamesonite, tetrahedrite, stibnite, and galena. Vein textures include carbonate coatings on quartz, bladed carbonate replacement by silica, banding/cockade white to grey quartz, drusy quartz coatings, massive amethyst, and open space voids and clay fillings. Other evidence for the presence of mineralization includes minerals such as malachite-azurite, limonite-hematite, and other oxides; argillization alteration as pervasive kaolinite, sericite, and illite, as well as zones with vuggy silica flooding. Locally abundant pervasive silicification is noted, which often hosts disseminated pyrite.