Project of the Week: Guanajuato Silver

13th December 2021

 

 

Hundreds of years ago, Spanish explorers (among others) came to the New World in search of ‘El Dorado’ — the city of gold. The legend spoke of an entire city, shining brilliantly gold in the American sun. A land of wealth and riches supposedly lay deep in the jungles, deserts, or mountains of Central America. Unfortunately, the hidden city of El Dorado was never found. No streets of gold, no brilliant yellow reflections of light from the most precious of metals. However, a land of wealth and riches was discovered by the European colonizers, whether they knew it or not. The difference? They were looking for the wrong metal (don’t worry - there is still gold in this story!).

Mexico is the world’s largest silver producer by a large margin. With more than 178 Moz produced in 2020, Mexican production dwarfed the competition (for example, Peru is 2nd with just over 100 Moz produced; https://www.statista.com/statistics/253339/leading-silver-producing-countries/). So what makes Mexico so special as a silver resource? Silver usually occurs at low-temperature magmatic-hydrothermal deposits close to the earchs surface (less than ~2 km depth), and Mexico has just the right geology to accommodate it. Junior mining company Guanajuato Silver certainly knows this, and they are doing their best to take advantage of it!

Figure 1. Map of Mexico highlighting the Sierra Madre Mineral Belt and Guanajuato Silver mine locations. (Guanajuato Silver corporate presentation, Nov 12)

Guanajuato Silver (TSXv: GSVR; OTCQX: GSVRF), formerly VanGold Mining, is currently the largest producer in the state of Guanajuato, located in Central Mexico. They have been involved in the Guanajuato City, the capital (Figure 1). It was originally colonized in the 1520’s for - you guessed it - silver mining! Guanajuato Silver purchased the El Cubo mine for just $15 M from Endeavour Silver, who operated the mine from 2012 -2019. After two years of reworking the operation strategy, the mine saw its first sale in October 2021, selling 75 t of bulk silver-gold concentrate (approx. value. $500,000) — the first concentrate from the mine in more than 2 years (October 28 news release)! Similarly, El Pinguico, an operating mine until the Mexican Revolution (production ceased in 1913), should see it’s first production in more than a century next year.

With two operating mines, Guanajuato Silver is primed for continued exploration in the district. Several targets making up more than 7,800 hectares of claims within the district, are prospective with silver and gold mineralization. Let’s take a deeper look at the formation of the world-famous Mexican silver deposits and Guanajuato Silver’s mining and exploration projects!

Mexican Silver

The Guanajuato Mining district, and many of the mineral deposits in the Sierra Madre Occidental, consist of extrusive igneous (andesites and rhyolites) and continent-derived sedimentary rocks (Guanajuato Fmn.). These “cover rocks” overlie the basement sequences consisting of Mesozoic metasedimentary units. Magmatism in the Guanajuato district occurred from 37-27 Ma, producing the rocks that host the mineralization (Godchaux et a., 2003). These permeable rocks are the hosts of Ag-Au mineralization, though it is also common for Ag-Zn-Pb deposits to be hosted in the sedimentary package below.

The Ag-Au mineralization is observed as epithermal (shallow) vein systems that cut across the geologic sequence along major fault structures. These veins contain banded sequences of quartz, carbonates, and sulfur-bearing minerals called sulfosalts, which host the silver in their crystal structure. Repeating bands represent cyclic episodes of the build-up and release of hydrothermal fluids. 
When intrusive rocks enter the middle and upper crust during subduction and the formation of back arc settings (think ocean plates subducting under the continents), they exsolve magmatic-hydrothermal fluids which mix with meteoric waters and subsurface brines trapped underground. These fluids produce the low- to intermediate- sulfidation epithermal veins when the gold- and silver-bearing fluids travel a long way from the magma. But how do the fluids move so far?

Figure 2. Model for intermediate- and low-sulfidation epithermal deposits. Guanajuato is classified as a "Type B" intermediate sulfidation deposit, formed in fault zones in the southern Sierra Madre Occidental. Figure from Camprubi and Albinson (2007).

Well, that’s where the fault systems come into play! These fluids build up pressure underground before they escape - generally along fractures (what we find as veins) or by percolating slowly through the rocks. Faults play a couple roles in these systems: 1) they allow a lot of fluid to escape, causing pressure drops and 2) they focus all that fluid into relatively narrow zones. When a seismic event occurred, rapid up-flow of these hydrothermal fluids caused boiling and mineral precipitation on the vein walls. This is exactly the process which forms these epithermal Au-Ag deposits in the Mexican crust (Fig. 2).

Figure 3. Microscope images of mineral textures of banded sulfides (black), sulfosalts (black), and carbonates (brown and gray) growing on top of quartz crystals (white). The ore minerals precipitate during boiling of the hydrothermal fluids. (Figure from Moncada et al., 2012).

The Guanajuato Mining district contains 3 different fault zones, all trending to the NW, separated by roughly 10 km each. The eastern most system, the Sierra fault system, hosts Guanajuato’s El Cubo mine. The central fault system, the world renowned Veta Madre system, contains several modern and historical mines near the city of Guanajuato, as well as the El Pinguico mine. The westernmost La Luz system also hosts a range of epithermal Au-Ag deposits. This “en echelon” pattern of fault systems is represented in the model from Camprubi and Albinson (2007) in the shallow crust along the flanks of the rhyolite dome complex in Figure 2B. At the El Cubo deposit in the Sierra vein system, the average vein mineralization is 1-2 m in width, but breccia zones chock full of silver can be up to 10m wide!

Figure 4. Mining team at El Cubo drills into the high-grade Ag-Au breccia zone. Photo by Andrés Cruz.

The El Cubo mine and Pinguico mine lie along separate fault trends, but are quite close to each other as the bird flies (Fig. 5). A preliminary economic assessment amended in 2021 indicates 508,000 t of ore remain at 194 g/t Ag (3.2 Moz) and 2.44 g/t Au (39,860 oz). Inferred resource numbers are even rosier - with over 10 Moz Ag and 129,900 oz Au still present at the El Cubo deposit. The Pinguico deposit contains indicated resources from its surface and underground stockpiles, which host more than 500,000 oz of Ag and 5,000 oz of Au waiting to be processed. As exploration continues, we will see just how much the resource zones expand. Likewise, the Veta Madre fault system also hosts the exploration targets Patito I and II, just 1.5 km and 3 km south of Pinguico, respectively. Additionally, the company owns additional exploration targets about 100 km to the east of Guanajuato to try to expand their silver-rich portfolio.

Figure 5. South-looking projection of the El Cubo and Pinguico mineral systems with current workings and regional veins. Guanajuato Silver's Patito 1 and II exploration targets also lie in the Veta Madre vein system.

While exploration is important, Guanajuato will currently look to expand production at its two active mine sites, looking for step out and resource expansion along these established deposit trends. The Guanajuato Mining District is a mining friendly jurisdiction, with tons of infrastructure supporting other operations by Fresnillo PLC - Mexico’s largest silver producer, among others. As Guanajuato Silver modernizes the El Cubo operation, funds will continue to pour in for the development of Pinguico and other projects. Make sure to stay tuned to Spotlight and the latest updates!


References:


Camprubí, A. and Albinson, T., 2007. Epithermal deposits in México—Update of current knowledge, and an empirical reclassification. Geology of Mexico: Celebrating the centenary of the Geological Society of Mexico422, p.377.

Moncada, D., Mutchler, S., Nieto, A., Reynolds, T.J., Rimstidt, J.D. and Bodnar, R.J., 2012. Mineral textures and fluid inclusion petrography of the epithermal Ag–Au deposits at Guanajuato, Mexico: Application to exploration. Journal of Geochemical Exploration114, pp.20-35.

Guanajuato Silver Corporate Presentation: https://gsilver.com/images/pdf/corporate_presentation/2021/GSilver_Corporate_Presentation_Nov_12_1.pdf

Preliminary Economic Assessment: https://www.gsilver.com/images/pdf/20-076_VanGold_El_Cubo_PEA_FINAL_Amended_06_May_2021_Signed_1.pdf

Aaron Hantsche

View posts by Aaron Hantsche
Aaron Hantsche recently obtained his PhD in Geology from the University of Geneva in Switzerland, where he studied distal Pb-Zn skarn deposits in southern Bulgaria. Magmatic-hydrothermal deposits are his primary focus, with a bent towards geochemical vectoring tools and field geology. In addition to his work as a geologist, Aaron is a director for the science communication platform, Ore Deposits Hub. His passion for mineral deposits and geo-communication led him to join the Spotlight Mining writing team in October 2021.