Summary:
Deposit types
The Juanicipio Property is located within the Fresnillo silver mining district, a northwest-southeasttrending corridor that extends 500 km from the Guanajuato District in the south-east through to the Sombrerete-San Martin-Sabinas silver district in the north-west. Precious metal deposits within the area consist of several low-sulphidation epithermal quartz-carbonate veining systems that crosscuts Middle Jurassic to Late Cretaceous volcanic and sedimentary rocks of the Guerrero Terrane. Structures hosting epithermal veins in the Fresnillo area are associated with a regional north-west-trending sinistral shear zone that is locally crosscut by younger northeast-southwest trending faults. Most of the low-sulfidation epithermal vein deposits are located at the intersection of the two structural trends and many occur in dilatational zones associated with the north-westerly structural trend (Megaw, 2010).
In the Fresnillo area, epithermal veins are laterally extensive, with continuous mineralization over strike lengths between 1.3 and 8 km (Ruvalcaba-Ruiz and Thompson, 1988). The veins typically dip steeply to the south and reflect the orientation of their host structure, though the veins may branch and flatten to form stockworks as they approach the upper extents of the mineralizing system. Although the structures hosting the deposits are vertically continuous, precious (Ag and Au) and base (Pb, Zn and Cu) metal mineralization is typically vertically zoned within a restricted window of the structure where mineralizing fluids range in temperature between ~180 to 280°C. Vertical metal zonation within the veins reflects the relationship between ascending hydrothermal fluids, boiling of these fluids, and depth below the syn-mineralization paleowater table (Hedenquist and Henley, 1985; Albinson, 1988). Horizontal metal zoning may also occur with higher metal content typically associated with the core(s) of fluid up-welling zone(s).
Mineralization
The Fresnillo silver district contains low-sulphidation epithermal quartz-carbonate veins which form an extensive array of stacked, steeply dipping, west- to west-northwest-trending veins. The veins are laterally extensive and although the structures are persistent with depth, the silver-gold rich section is typically limited to a 300 m vertical interval corresponding to the boiling zone of the fossil hydrothermal system. Metal distributions show a sub vertical zoning with base metal abundance increasing with depth. Precious metal mineralization within the project area is hosted by two significant epithermal structures discovered to date: the Valdecañas vein system and the Juanicipio vein. Two plan views are located in the north-east corner of the claim and dip 35° to 70° to the south-west, with an average dip of 58°. The Valdecañas vein system extends beyond both the north and east property boundaries. The Juanicipio vein extends to beyond the east boundary and is open to the west.
Valdecañas vein system
The Valdecañas vein system displays a metal zonation typical of the Fresnillo District and epithermal systems in general. The zonation comprises of an upper precious metal zone, the so-called “Bonanza Zone,” grading downwards into the “Deep Zone”, a deeper, base metal-dominant zone. Significant copper mineralization has recently been discovered within the deeper levels of the Valdecañas vein system. The Valdecañas structure hosts all the Indicated Mineral Resources and approximately 46% of the Inferred Mineral Resource tonnage currently estimated for the project.
The Valdecañas vein system was previously interpreted as an en echelon vein system and referred to as the V1E and V1W veins. Subsequent diamond drilling has shown the Valdecañas vein to be a continuous, moderate (50°) south-west dipping vein with a strike length of 1,100 m at the top of the vein and down dip continuity up to 2,000 m amsl. The average true thickness of the is 6.2 m but up to 29 m wide. The Valdecañas vein system consists of the Valdecañas itself, and the Ramal 1, Venadas, Pre-Anticipada and Anticipada veins.
A paragenetic sequence for mineralization in the Valdecañas vein system is separated into five stages:
1 Sphalerite and galena.
2 Quartz and calcite with minor sulphides.
3 Alternating bands of chalcedonic quartz- calcite – epidote-sphalerite – galena – and silver-bearing minerals.
4 Quartz – calcite – dolomite – ankerite with coarse-grained pyragyrite.
5 Barren quartz – calcite and fluorite. The main ore minerals are sphalerite, galena, pyragyrite, polybasite, and acanthite, with gangue minerals consisting of pyrite, arsenopyrite, quartz, and calcite .
The Ramal 1 Vein, previously referred to as the Desprendido and V2W (Ross 2012; Ross et al., 2017), is located in the footwall to the Valdecañas vein. It is moderate (53°) south-west dipping with a strike length of up to 850 m and down dip continuity between 940 and 1,930 m amsl. The average true thickness of the vein is 2.0 m and reaches up to 13.8 m. Vein mineralogy comprises white quartz with trace sulphides in the upper part of the system. Sulphide content and chlorite and epidote alteration with pervasive silicification increases with depth.
The Anticipada Vein, previously referred to as VANT (2017 AMC Technical Report), is located in the hanging wall to the Valdecañas vein. It is moderate (60°) south-west dipping with a strike length of 800 m and down dip continuity between 1,900 and 1,185 m amsl. The average true thickness of the vein is 1.9 m and reaches up to 17.8 m. Vein mineralogy comprises thin quartz, crustiform and banded layers of sphalerite and galena, and disseminated pyrite with local brecciated massive sulphide sections.
Juanicipio vein
The Juanicipio vein is located approximately 1,100 m south of the Valdecañas vein system. It is moderate (45°-55°) south-west dipping with a strike length of 1,100 m and down dip continuity between 1,360 and 2,100 m amsl. The average true thickness of the vein is 0.9 m and reaches 3.0 m. Vein mineralogy comprises white quartz and calcite with disseminated sphalerite and galena.
Venadas vein
The Venadas vein is atypical of known vein types on the Juanicipio project. Located in the hanging wall of the Valdecañas vein and north-west of the current known extent of the Juanicipio vein, the Venadas vein is steep (78°) north-west dipping with a strike length of 830 m and down dip continuity between 1,510 and 2,070 m amsl. The average true thickness of the vein is 1.0 m and up to 3.4 m. Vein mineralogy comprises banded quartz, sphalerite, galena, and disseminated pyrite.
Styles of mineralization
The veins are interpreted to have undergone multiple mineralizing events as suggested by repeated stages of brecciation and quartz sealing, local rhythmic microcrystalline quartz-pyrargyrite banding, and open-space cocks-comb textures. The veins exhibit vertical metal zoning, characteristic of other epithermal veins in the Fresnillo district. This vertical zoning is characterized by a change from silver and gold rich zones at the top, the so-called “Bonanza Zone”, to increased lead and zinc in the deeper parts of the known system termed Deep Zone, with copper values being seen in at the deepest levels. Notably, gold rich mineralization crosscuts the silver-dominant zones, which in turn cut earlier base-metal dominant mineralization, indicating complex multi-stage mineralization as is seen separately in other parts of the district. Gold appears to be present with silver as electrum.
Mineralization in the so-called “Bonanza Zone” consists of precious metal-rich, banded, or brecciated quartz-pyrargyrite-acanthite-polybasite-galena-sphalerite veins. Within 10 to 20 m of the upper extent of the veins, the wall rocks are progressively and pervasively silicified and cut by quartz veinlets carrying pyrite-sphalerite-galena.