by Esme Whitehouse, Junior Mining Researcher at Spotlight Mining
Excellon Resources (TSX:EXN) are a mid-tier mining and exploration company with projects in Mexico, the US and Germany. Their portfolio includes both precious and base metals, and they own and operate 100% of Mexico’s highest-grade silver mine, Platosa.
The Platosa Mine is located 5 km (3.1 miles) north of Bermejillo in Durango State, Mexico. While it remains a very high-grade silver deposit, Platosa is also mined for lead and zinc. The Mineral Resource statement (SRK Consulting, NI 43-101) published in 2018 describes 485 000 t Indicated resources at 549 g/t Ag, 5.6% Pb and 5.9% Zn and a further 13 000 t inferred resources at 516 g/t Ag, 4.7% Pb and 6.5% Zn.
Production at Platosa began in 2005 and extraction targets a number of high-temperature epigenetic carbonate-replacement zinc-lead-silver ‘Mantos’ (Figure 1). The Mantos, or pods, are hosted in a stratigraphy of Mesozoic sedimentary rocks including an evaporite sequence, sandstones, and dominant limestones. There is variable but generally low-grade metamorphism. Mantos are shallow to steeply dipping, irregularly shaped and are concordant with the eastward-dipping trend of the area’s stratigraphy. The mineralized system covers an area of approximately 400-700 m (1312-2297 ft) and ranges from depths of 60 to 320 m (196 to 1049 ft) eastwards in accordance with the dip of the stratigraphy. Primary ore minerals include galena, sphalerite and acanthite, with proustite occurring in the highest grade samples (Figure 2).
During the mid-tertiary extension of the Rio Bravo Rift, widespread magmatism exploited newly reopened pervasive faults, resulting in the pervasive emplacement of shallow level intrusions into Mesozoic carbonates. Thus, the regional-scale hydrothermal system responsible for the Mexican carbonate replacement deposits was created. The largest formed over deep-seated, large-scale faults and Platosa is adjacent to a major NW-trending structure within a larger northwest trend of significant carbonate replacement deposits. In Mexico, the CRD belt is estimated to host more than 80 million tonnes in deposits.
Carbonate replacement deposits commonly grade into structurally controlled metasomatic skarns the closer you get to the source intrusions (Cox et al., 2015) and can have extents of thousands of meters. The Platosa property has both CRD and skarns, and thus represents the transitional ‘middle’ area between the proximal and distal parts of the hydrothermal system. When comparing this character to other CRD systems, we can see that Platosa demonstrates not only a potential prospectivity of genetically related carbonate replacement deposits, but that there is also potential for extension and nearby discovery.
On that latter point, earlier exploration drilling for Platosa revealed slightly anomalous gold and silver grades, and while these were not economic, they were considered useful exploration vectors to another economic area in the same system. Covering 11 000 ha, the Platosa property remains an exploration terrain and as of 2018, there were a number of veins, breccia, and skarn systems all noted for possible further investigation.
Platosa is looking good post-COVID-19. Production recommenced in June, 2020 following a two-month hiatus, with strong results continuing through July and into August (Table 1).
|Ag (oz)||296 281||18 919||116 324||106 662|
|Pb (lb)||1 890 456||129 204||972 066||729 081|
|Zn (lb)||2 131 034||158 735||818 730||997 157|
|AgEq (oz)||523 742||34 924||198 458||169 473|
Table 1 – Production figures from the Platosa mine for 2020.
Despite the delay to operations, Excellon have made up for lost time by producing nearly as much in July and August as they did in the whole of Q1 this year. As of September, Excellon were also seeing a positive feedback from the cost reduction strategy they implemented in Q2. Despite having to reduce their workforce, increasing the efficiency of working schedules has maintained and even increased mine productivity. Other cost reduction measures included negotiating lower treatment charges, general optimizations and changing electricity supplier.
Excellon commenced surface and underground drilling at Platosa and two other targets in September with the express intention of expanding and better defining known mineralization. Exploration activity in another two targets, PDN and Jaboncillo, thus far has revealed that they may be prospective for skarn mineralization and a Platosa-style target, respectively. No definite results have been released from this activity yet, but we await them with excitement to see what the wider Platosa area holds for Excellon Resources.
Cox JJ, Ross D, Michaud RL. (2015). Technical Report on the Preliminary Economic Assessment of the Platosa Mine, Durango State, Mexico. Roscoe Postle Associates. Prepared for Excellon Resources, July 2015
All information in the above article comes from Excellon Resources’ website and materials contained within.
For more information visit: www.excellonresources.com