Summary:
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 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 Arista mine vein systems are intermediate-sulfidation in nature and standard of many Mexican deposits, characterized by Camprubí & Albinson, 2007, and described as Polymetallic Ag-Au by Corbett.
Geology
The Don David Mine Arista Project is underlain by a Cretaceous sedimentary lithic sequence, composed of fine-grained sandstones intercalated with shale, siltstone, and calcareous rocks. Younger andesitic to rhyolitic volcanic and volcaniclastic units, intrusive dikes, and small stocks of granitic to granodiorite composition crop out within the area. The intrusive rocks may be associated with structural conditions favorable for subsequent deposition of mineralization along dikes, faults, and breccia zones and may be related to possible replacement and skarn deposits in good contact zones with the sedimentary sequence.
The mineralized structures appear to be associated with a trans-tensional structural system intersecting an interpreted Cenozoic-aged volcanic “caldera.”
Mineralized Zones
The historic Don David Mine production at the Arista underground mine (part of the Arista Project) was mainly extracted from two principal veins, the Arista and Baja, and their related splays within the Arista vein system. In recent years, production has mainly focused on the Switchback vein system with extraction principally from the Soledad vein and associated splays and faulted offset sections, including the Selene and Susana veins. Other significant veins and deposits at Arista include the historically exploited Aire and Aguila Manto veins as well as the recently discovered Three Sisters vein system (including the Sandy 1, Sandy 2, Sasha 1, Sasha 2, Sadie 1, and Sadie 2 veins), located between Arista and Switchback.
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 adesitic 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 as occurring 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 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 submicroscopic 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.
Soledad Vein (Switchback Vein System)
Like the Arista vein system, the Switchback vein system consists of subparallel veins, faulted offsets and splays of varying length and width. The principal vein for economic exploitation is the Soledad vein along with associated splays, and faulted offsets, which have been locally named separate veins (e.g., Selene and Silvia). To date, several significant veins have been identified: Soledad, Selene, Silvia, Sofia, Sagrario and Susana.
The Soledad vein is generally NW to NNW striking, and 55-70°NE dipping. However, notable flexures occur along strike and along dip, with sectors of the vein being almost EW or NS striking, while vein dips can be vertical or locally up to 70°SW dipping (in the central NW section of the vein). In the SE the vein is more regularly NE dipping with azimuths of 320°-350° predominating. In the NW sector azimuths of the vein vary from 270°-360° and dips from 70°SW to 60°NE. The vein appears to have developed along several sigmoidal structural zones, with the principal sigmoid associated with the strongest mineralization. The Soledad vein is typically brecciated with fragments of quartz and variably bleached, silicified andesite wallrock fragments with a multi-phase quartz matrix. Carbonates are generally a minor component of the veins. Colloform and/or crustiform textures are common with bands of quartz, sphalerite and galena in places encrusting breccia fragments. Bands of white or amethyst quartz are also present, the latter being both an early and late feature. The breccia matrix mainly consists of finegrained, dark grey quartz; the dark color due to the presence of very fine-grained pyrite. Small, drusy quartz crystals filling vugs are observed locally.