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.
Description of Mineralized Zones
The Don David Mine mineralization occurs as structurally controlled epithermal deposits in veins and stockwork zones consisting of concentrations of sulfides containing gold, silver, lead, copper, and zinc, associated with gangue minerals such as quartz calcite and other minor minerals. The economic mineralization at the Arista mine is gold, silver, copper, lead, and zinc. Structurally controlled epithermal veins and stockwork zones at the Alta Gracia Project (Mirador Mine) contain mainly silver-gold bearing sulfides. The economic mineralization currently exploited at the Alta Gracia Project is only gold and silver.
Primary sulfide mineralization within the mineralized structures contains pyrite, galena, sphalerite, argentite, chalcopyrite, and other silver sulfosalts associated with quartz and calcite as gangue minerals, which are found at depth.
Arista Mine
Arista Vein
The Arista vein consists of multiple parallel veins and splays of varying lengths and widths associated with the predominant fault bounded structure. The vein is partially enclosed in strongly silicified rhyolite breccia, including stockwork zones related to the vein. The veining is also associated with the structural contact between hypabyssal andesitic rocks and Cretaceous sediments (Black Breccia). Mineralization is multi-phase (related to discrete structural and leaching events within the bounding fault), and restricted mineralogy is associated with variable grades and textures from fault contacts inward. Mineralization occurs within a range/mix of breccia, colloform banded quartz, crustiform quartz, and multi-phase banded sulfides with coarse-grained quartz intergrowths. Base-metal sulfides include massive galena, sphalerite, and chalcopyrite; ± disseminated remnants of pyrite; ± trace rhodochrosite; later quartz veins cut through sulfides; other trace sulfides include euhedral arsenopyrite overgrowths on dendritic native silver, magnetite, pyrrhotite, pyrite, acanthite, bornite, and tetrahedrite-tennanite. Areas of secondary sericite, clay, and microcrystalline quartz are often observed in petrographic analysis with complex intermixtures of hydrothermal, metasomatic, and retrograde minerals, including cordierite diopside, albite, calcite, epidote, adularia, chlorite, and clay. EM-EDX analyses confirmed the presence of argentite and freibergite associated with the leaching of base metals. Gold and silver are suggested to occur late in the paragenetic sequence (after base metal sulfides and after a leaching/fracturing event). Gold occurs as micron-size “inclusions” in “recrystallized” arsenopyrite around vugs; antimony also appears related to gold based on petrographic evidence (Hansley, 2012).
Baja Vein
The Baja vein was discovered during an exploration drilling program on the Arista vein and generally hosts high-grade silver mineralization. The Baja vein occurs as a 1.0 to 1.5+ m wide mineralized structure with mineralization hosted within fractures and associated with crustiform features filling opened spaces. The vein comprises several splays and parallel veins of varying lengths and widths, including Splay 66. The general orientation is 310°-320°, dipping 70°SW to vertical, and has been developed, to date, by underground workings in the Arista mine between 460-800 masl elevations. It has a strike length (defined through drilling) of at least 500 m. The vein typically consists of multi-phase vuggy textured, crustiform banded, coarse-grained quartz, with some quartz replacement of carbonate; in addition, adularia replaced by carbonate has also been noted. Sulfides include fine to very fine-grained and banded occurrences, often disseminated at vein contacts. These are characteristics: bladed galena (possibly replacing carbonate), massive sphalerite, coarse stibnite, fine-grained and disseminated chalcopyrite, and pyrite. Other significant sulfides include proustite (Ag3AsS3), pyrargyrite (Ag3SbS3), and other silver minerals. Petrography has identified sub microscopic gold and argentite (after base metal sulfides), antimony associated with gold, and trace amounts of kyanite, corundum, and garnet.
Aire Vein
The Aire vein is located about 100 m west of the Arista vein and is oriented 345°, dipping 70°SW to vertical. It is hosted mainly by andesite, with some rhyolite occurring to the east of the vein. The Aire vein has been traced for over 400 m along the strike. Mineralization styles are similar to those veins previously described with abundant vuggy, replacement (after carbonate), coarse and cruciform quartz (locally recrystallized). Sulfides often occur as massive masses and include sphalerite, galena, proustite (microveinlets in sphalerite), disseminated arsenopyrite, and native silver. Accessory minerals include abundant corundum (inclusions in quartz), adularia (as microveinlets) replaced by alunite, rhodochrosite rhombs (suggested as late-stage or post-event), calcite, sillimanite and kaolinite, fine-grained K-spar and rounded zircon. While not economically significant, its exploration led to the discovery of the Arista mine.
Soledad Vein (Switchback Vein System)
Surface mapping in the ‘’Switchback’’ Hill area, approximately 500 m northeast of the Arista underground mine, indicated the presence of an NNW-SSE trending porphyritic felsic dike with associated intense sulfate (gypsum) alteration and minor quartz-amethyst veining, sub-parallel to the Arista vein system. Geochemical rock chip samples taken by DDGM geologists from this altered zone returned base metal anomalies with weakly elevated silver values. Subsequent holes drilled from the Arista mine underground workings along the strike to the south of this area intercepted multiple zones of well-mineralized vein material associated with a strongly porphyritic felsic dike.
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.