Summary:
Deposit Type
The Guanajuato Mining District in general is a high-grade, silver-gold, epithermal vein system with low sulphidation and adularia-sericite alteration. The primary deposit type of interest at the El Cubo and El Pingüico properties is low sulphidation epithermal silver-gold mineralization.
The mineral deposits in the Guanajuato region are classic fissure-hosted low sulphidation epithermal goldsilver-bearing quartz veins and stockwork. Low sulphidation epithermal mineralization are vein type deposits that form at shallow depth from dominantly meteoric fluids at low temperature with neutral to near neutral pH; thus, there is very little acidic alteration within the host rocks and no widespread pyritic haloes. Banded veins, drusy veins, crustiform veins, and lattice textures are common. Low sulphidation deposits typically have gold-silver mineralization, occasionally with banded adularia, sericite, rhodonite and rhodochrosite. Alteration in these systems is commonly sericite-illite proximal to mineralization, grading to illite-smectite and to chlorite ± epidote ± calcite alteration on the outer margins of the system. Mineralization in low sulphidation systems generally consists of Au ± Ag with minor Zn, Pb, Cu, Mo, As, Ab and Hg (Sillitoe and Hedenquist, 2003; Cooke and Hollings, 2017).
Mineralization of significance at the El Cubo and El Pingüico properties consists of veins containing significant silver- and gold-bearing metallic minerals including native silver, native gold, argentite or acanthite, electrum, pyrargyrite, polybasite, naumannite, and aguilarite. Accessory metallic minerals include pyrite, galena, sphalerite, and chalcopyrite. There is only little mineralization disseminated in the surrounding wall rocks. Gangue minerals include quartz (locally amethyst), calcite, adularia, and sericite. The veins are accompanied by hydrothermal alteration consisting of silicic, potassic (adularia+quartz), argillic, and phyllic alteration, and peripheral propylitic alteration. Mineral textures in this zone are typically fracture-filling, drusy and coliform masses.
Epithermal type precious metal deposits at El Cubo and El Pingüico are strongly vertically controlled. In the Guanajuato region there is commonly a well-defined, sub-horizontal zone where the hydrothermal fluids deposited gold and silver mineralization. Regionally, mineralized material horizon thickness ranges from at least 300 m to greater than 500 m. High-grade material occurs where the hydrothermal fluids boiled, and below the higher-grade silver-gold mineralization zones, the silver and gold grades tend to decrease but the base metal grades tend to increase. This creates a significant mineralogical zonation in the vein systems: the upper levels are argentite/acanthite + adularia + pyrite + electrum + calcite + quartz and the lower levels are chalcopyrite + galena + sphalerite + adularia + quartz + argentite/acanthite.
The low sulphidation epithermal system deposit characteristics encountered at the El Cubo and El Pingüico properties include: a quartz-adularia vein/breccia system; native silver; native gold; electrum; sulphides and silver-sulphides; sulfosalts; quartz and calcite; accessory pyrite, galena, sphalerite and chalcopyrite; fault and vein control; and a vertical extension of over 300 m, and up to ~825 m at El Cubo.
Mineralization
Mineralization at El Cubo consists of silver and gold occurring in several stratigraphic formations, with the La Bufa Formation, the Guanajuato Formation, and the Calderones Formation being the most important hosts.
Mineralization at El Cubo is typical of the classic high-grade silver-gold, banded epithermal vein deposits with alteration characterized by silica-adularia-sericite. Mineralization occurs as open-space fillings in fracture/fault zones or impregnations in locally porous wall rock. Weak stockwork style mineralization occurs in an historic open pit on the Dolores Vein in the vicinity of the El Tajo mill. The most productive veins are sub-parallel to the Veta Madre system as north-northwest striking veins and local stockwork style mineralization. Several transverse, northeast striking veins with high-grade gold mineralization such as the Marmajas, La Reina, and San Juan de Dios also occur. Mineralization is open-ended due to a lack of exploration drilling and development.
Silver occurs in dark sulphide-rich bands within the banded veins with significant alteration minerals in the surrounding wall rocks. Significant silver and gold bearing metallic minerals include argentite or acanthite (Ag2S), electrum (native Au/Ag), ruby silver sulfosalt minerals such as pyrargyrite (Ag3SbS3) and polybasite [(Ag/Cu)6(Sb,As)2S7][Ag9CuS4]), naumannite (Ag2S), native silver (Ag), native gold (Au), and aguilarite (Ag4SeS). Other metallic minerals include pyrite (FeS2), galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS2). The silver sulfosalts are commonly found at depth while native silver is generally supergene and found in oxidized areas. As is typical of this type of systems, galena, sphalerite, and chalcopyrite are found deeper in the vein zones.
The silver-rich veins, such as Villalpando, contain quartz, adularia, pyrite, argentite (acanthite), naumannite, and native gold. Gold-rich veins, such as San Nicolas, contain quartz, pyrite, minor chalcopyrite and sphalerite, electrum, and aguilarite.
There is significant mineralogical zonation in the vein system. The upper levels are argentite (acanthite) + adularia + pyrite + electrum + calcite + quartz, and the lower levels are chalcopyrite + galena + sphalerite + adularia + quartz + argentite (acanthite). Boiling of the hydrothermal fluids in the upper levels locally produced bonanza silver and gold grade mineralization.
The gold:silver ratio in the more gold-rich veins typically ranges from 1:15 to 1:30. The gold:silver ratio in the silver-rich veins typically ranges from 1:60 to 1:150, and locally higher. The overall gold:silver ratio to date is 1:64. Metal zoning appears to be related, at least in part, to elevation. Ranges for gold:silver ratios at El Cubo vary from 1:10 to 1:20 in the upper mine levels, from 1:40 to 1:50 in the middle mine levels, and 1:100 to 1:150 at depth. These ratios could be of some importance in evaluating outcropping vein occurrences.
Low sulphidation epithermal deposits in Mexico, such as El Cubo and El Pingüico, commonly have a welldefined, sub-horizontal zone where the hydrothermal fluids deposited gold and silver mineralization. Regionally, mineralized material horizon thicknesses range from at least 300 m to greater than 500 m. High-grade material occurs where the hydrothermal fluids boiled. Below the higher-grade silver and gold mineralization zones, the silver and gold grades tend to decrease but the base metal grades tend to increase. Above the boiling zone, veins locally disappear or can be deflected into something as simple as a calcite vein with barely anomalous silver values or a fracture with argillic to phyllic alteration.
Typical of this style of mineralization, economic concentrations of silver and gold occur in mineralization shoots distributed vertically and laterally between barren or weakly mineralized portions of the veins. Bonanza grades may occur at the site of vein intersections, such as the nearly perpendicular San NicolasVillalpando vein intersection.
At the El Pingüico Mine, the major vein consists of both silver and gold in crumbling sugary to white crystalline quartz and calcite veins, within brecciated rhyolitic rock, and as a replacement in the altered rhyolite. Mineralization consists of native gold and silver, polybasite, pyrargyrite, tetrahedrite, marcasite, sphalerite, galena, pyrite, and chalcopyrite.