Summary:
The principal mineral deposits in the Platosa area are high-temperature epigenetic silverlead-zinc carbonate-replacement deposits (CRD).
The Platosa Mine lies in the Sierra Bermejillo, a northwest-trending anticline-syncline pair developed in Mesozoic sedimentary rocks. Platosa is underlain by folded and faulted Mesozoic sedimentary rocks, locally intruded by dykes and sills of Laramide age. The principal fault system in the property area is the Platosa Structural Zone (PSZ), a 250- to 1,500-metre wide northwest-trending zone of fracturing and shearing that traverses the eastern margin of the Sierra Bermejillo. The PSZ includes a series of fault planes that strike north–northwesterly and dip steeply east; it has been mapped along strike for 5 kilometres northwest and southeast of the Platosa Mine (Megaw 2002).
The Platosa Mine targets polymetallic (silver, lead, and zinc) mineralization occurring as shallow to steeply dipping bodies of massive carbonate replacement deposits or mantos. The main silver-, lead- and zinc-bearing minerals are galena, sphalerite, acanthite and lesser proustite. The mantos dip in accordance with the stratigraphy towards the east where a series of late extensional features extend the mineralization to depths ranging from 60 metres on the west side of the mine, and approximately 320 metres on the east.
Mineralization
The bulk of mineralization currently defined on the Platosa Mine occurs as shallow to steeply dipping bodies of massive carbonate-replacement deposits. These bodies have been identified and categorized as discrete pods or mantos based on structural setting and concentration of sulphides. The main manto bodies currently defined at Platosa are listed below from west to east:
• Mantos 6A/6B
• Mantos 4A-C
• Manto 5 • Guadalupe
• Guadalupe South
• Pierna • Rodilla
• Manto 623
• Manto 674
• NE-1
• NE-1 South
The footprint of the Platosa manto system currently measures approximately 400 by 700 metres. The mantos dip in accordance with the stratigraphy towards the east where a series of late extensional features extend the mineralization to depths ranging from 60 metres on the west side of the mine, and approximately 320 metres on the east.
The main lead-, zinc-, and silver-bearing minerals are: • Galena (main lead-bearing mineral)
• Sphalerite (main zinc-bearing mineral) • Ac
anthite and lesser proustite (main silver-bearing minerals). Acanthite is predominant; proustite is visible where grades typically exceed the average grade of the mineralized body.
Drilling has also occasionally intersected anomalous gold and copper mineralization, which is believed to indicate a hotter source or hotter mineralizing fluid pathways within the system. These anomalous values are seldom of economic value to the project; however, they are important vectors for exploration.
The most significant mantos at Platosa are the 623, NE-1 and NE-1 south mantos, which are currently being mined and that are open for expansion. These mantos have been defined in drilling from surface with an average drill spacing of 15 to 20 metres. In 2020, Excellon commenced a program of infill and expansion drilling ahead of production. This drilling was conducted on 10-metre spacing in production areas and has been successful in discovering and defining additional mineralization near existing mine infrastructure, most notably in the 623, Pierna, NE-1 and NE-1 South mantos.
Mineralization is also seen in the form of vein, breccia, and skarn systems in the following propertyscale exploration prospects:
• San Gilberto: Anomalous mineralization of 10 g/t silver has been intersected in drilling of north-south-oriented structures in 2006 (hole EX06STS01). Other structures, believed to be parallel to this one, have been exploited by small-scale miners at San Gilberto where the target appears to be a black siliceous vein. Prospecting results in this area by Excellon geologists have returned samples with elevated silver and base metal values.
• Jaboncillo: A trend of anomalous silver and base metal values measuring more than 1.0 kilometre was identified by company geologists in 2017. This trend is believed to indicate the presence of hotter mineralized fluids venting off a sulphide bearing fluid system that could be analogous to the mineralization at Platosa. Recent drilling has intersected zones of strong dolomitization, and jasper-gossan horizon with anomalous zinc and lead values and high lead/zinc ratios indicating proximity of the source of mineralization.
• Saltillera North and South: Skarn mineralization including epidote, pyrite, and chalcopyrite as well as silver values in excess of 600 g/t have been intersected in a hornfelsed unit (EX08ST78). These targets are also the site of historical small-scale silver and base metal mining.
• Halcón: Multiple structural intersections have been mapped here which are believed to be related to the emplacement of deeper intrusions into this sequence. Surface samples were anomalous in silver, lead, zinc, and arsenic.
• PDN: This geophysical target is defined by a large gravity anomaly coincident with resistivity anomalies. Structures and veins (calcite veining, silica veining and jasperoids) have been mapped on surface. Grab and surface samples have returned anomalous silver, lead and zinc and elevated arsenic values.
• Refugio: Located 1.4 kilometres southwest of the Platosa Mine on the western flank of the Sierra de Bermejillo where several small-scale historical mine workings are present; this prospect consists of outcrops of irregular silica breccias and jasperoid. The silica breccias cut beds of the Cuesta del Cura Formation and do not appear to be controlled by any obvious structure. Several historical samples of silica material returned gold values up to 7.79 g/t. Lead, zinc, and silver values have been reported in both the jasperoids and silica breccia boulders.
• Rincón Del Caído: Skarn mineralization was intersected in drilling at a depth of approximately 500 metres, most notably in diamond drillholes EX12LP1019, EX12LP1023A, EX12LP1024, EXLP1025 and EX12LP1030. The mineralized zone consists of green garnet with dispersed pyrite and subsidiary sphalerite and galena; it measures approximately 120 by 140 metres and extends down plunge for approximately 80 metres; it may indicate a larger skarn system in the area and the potential for additional mineralization at depth.
• La Zorra: A set of old artisanal workings mined for silver, lead, and zinc/
• Cañón Colorado: Excellon drilled holes into what is believed to be a rhyolite tuff that was enriched in rare earth elements. This area, located at the intersection of two regional faults, is believed to have potential to be a high-level portion of a mineralizing system.
• Cañón de la Güera: Prospecting and mapping led to initial drilling in this area. No significant mineralization has yet been intersected, although it is located in a structurally complex part of the northern PSZ.