Summary:
Deposit Type
The primary deposit type of interest at the Valenciana Mines Complex (VMC) is low sulphidation epithermal silvergold mineralization.
The mineral deposits in the Guanajuato area are classic fissure-hosted low sulphidation epithermal gold-silver-bearing quartz veins and stockwork. Low sulphidation epithermal mineralization are vein type deposits that form at shallow depth from dominantly meteoric fluids with neutral to near neutral pH and low temperature. 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 often 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 VMC consists of fine-grained disseminations of acanthite, electrum, aguilarite and naumannite with accessory pyrite, and relatively minor sphalerite, galena, and chalcopyrite. Gangue minerals include quartz, calcite, adularia, and sericite. The veins are accompanied by hydrothermal alteration consisting of argillic, phyllic, silicic and propylitic facies. Mineral textures in this zone are typically fracture-filling, drusy and coliform masses.
Epithermal type precious metal deposits in the Veta Madre vein system are strongly vertically controlled. The mineralization at the VMC is more related to fault filling silica breccias than specific veins. Historically, mineralization was between 2,100 and 1,800 masl, with specific steep plunging shoots going down to 1,600 masl. The Company is focusing exploration attention on the upper portion of the system, specifically on parallel structures both in the hanging and footwall of the Veta Madre.
The low sulphidation epithermal system deposit characteristics encountered in the Rayas, Cata, Valenciana, Guanajuatito, Pozos, Promontorio, and Santa Margarita zones in the Guanajuato operation include a quartz-adularia vein/breccia system; native silver; electrum; sulphides and silver-sulphides; sulphosalts; quartz and calcite; accessory pyrite, galena, sphalerite and chalcopyrite; shear controlled; and a vertical extension of over 700 m.
Mineralization
Mineralization at the VMC is closely associated with the structural history of the region.
The Veta Madre quartz-adularia vein/breccia system is closely associated with the Veta Madre fault and an associated diorite dyke (thickness varying from discontinuous lenses at Guanajuatito to a 50-100 m thick body in the Cata, Los Pozos and Santa Margarita areas), oriented 325-degrees with a 45-degree southwest dip. The Veta Madre forms along the dyke contacts, and in the footwall Esperanza Formation.
The mineralizing event is thought to have taken place during the early Oligocene, a period of intense felsic volcanic activity in the area and comprised three stages. The three stages are summarized as follows: i) the first stage of mineralization consists of trace silver and gold with accessory quartz and adularia; ii) the second stage comprises an early silver-rich phase associated with adularia, as well as a later low-silver variant, which is typified by calcite and quartz; and iii) the final “post-mineralization” stage is precious metal poor, with accessory calcite, dolomite, and fluorite. Zone thickness ranges from centimeter-scale to tens of meters.
The vertical extent of the deposits at the VMC spans over 600 m. Mineralization occurring above the 2,100 m elevation (masl) was termed “upper ore”, between 2,100 m and 1,700 m “lower ore”, and below the 1,700 m elevation “deep ore” (Randall et al., 1994). The use of the term “ore” is presented based on the historical terminology used and does not suggest that there are Mineral Reserves at the VMC. Fluid inclusion data (Moncada et al., 2011) from over 850 samples gathered through the mine and in deep drilling from the Santa Margarita area, indicated boiling zones from the 2,100 m to 1,500 m (deepest drilling at the VMC) elevations. Moncada’s work, along with Barclay and Rhys’s structural observations suggest up to 8 stages of crosscutting brecciation. The variable range of silver to gold ratios indicate that the mineralization along the Veta Madre is associated with multi-phase structural activity and fluid flow.
The best mineralization is often found related to bends in the Veta Madre orientation (Barclay, 2007; Rhys, 2013) such as at San Vicente in the Rayas area, as well as at Cata and Santa Margarita. These structural bends may be due to changes in rock type competencies, and varying thickness of the diorite dyke. There is potential to find further mineralization both laterally and in parallel breccia structures to known precious metal mineralization zones.
The primary commodities of significance are silver and gold, with silver the more important of the two. Base metals do not normally occur in significant concentrations. Average silver grades of the mineralized material are typically in the 100 to 500 g/t Ag range, but locally can be over 1,000 g/t Ag. Gold grades are generally in the 0.5 to 2 g/t Au range, except for Santa Margarita where average grades are in the range of 5 to 7 g/t Au. Relative gold and silver contents at Santa Margarita are quite different from Cata, Los Pozos and Guanajuatito. The average silver to gold ratio in Cata is roughly 225:1, at Pozos 250:1, at Guanajuatito 275:1, while at Santa Margarita 3.5:1. Within the mine, drill core and channel samples are not normally analysed for base metals therefore an average grade for Cu, Pb or Zn is not available.
The mineralized areas of Guanajuato are summarized as follows:
• At Guanajuatito, the main mineralization occurs just into the footwall Esperanza Formation deformed siltstone and shale. Two main zones are present at Guanajuatito, with the Veta Madre and the closely associated footwall (FW) zone being dominant below the 80 level.
• In the Valenciana area there are parallel mineralized structures (Veta Madre) at the Esperanza Formation – Diorite contact, and into the Esperanza Formation.
• In the Cata area, Veta Madre mineralization occurs along the base of the diorite dyke near the footwall contact with the Esperanza Formation, and as seven separate zones within the diorite. Several of these zones are shallow dipping structural splays.
• The Los Pozos area zones, between the Cata and Rayas shafts, are comprised of two vein stockwork to breccia systems (Veta Madre) at the base of the diorite dyke and into the Esperanza Formation, as well as on the upper diorite dyke contact with the Guanajuato Formation conglomerates.
• The Santa Margarita area zones form a complex structural set of five bodies within the diorite dyke and at its upper contact with the Guanajuato Formation conglomerates or basal andesite. These are above the Veta Madre breccia which is at the diorite contact with the footwall Esperanza Formation.
• The six zones of the Promontorio area occur in the hanging-wall Guanajuato Formation conglomerates immediately above the Veta Madre structure at the contact of the Guanajuato Formation and the diorite dike, and as well in the diorite dyke.
The El Borrego vein represents a new target area for precious metal mineralization at the VMC and is situated in the hanging wall of the Veta Madre. The El Borrego vein is hosted within the Esperanza Formation and runs parallel to the Veta Madre, approximately 200 m to the east. The El Borrego structure is potentially syn-genetic with the event associated with the formation of Veta Madre, prior to the hydrothermal activity, and has the potential to host silver and gold mineralization.