Metamorphic Mexico: The Sierra Madre Occidental

June 7, 2021

by Jane Lockwood, Spotlight Mining

Sierra Madre

Abundant gold and silver in the Sierra Madre Occidental have driven colossal enterprise over the last two centuries, with Mexico standing in 2017 as the world’s largest silver producer (200 Moz/year) and eighth largest in gold (125 t/ year), chiefly sourced from this mountain range. This is in the face of significant difficulties in obtaining geological and other data, especially until the turn of the 21st century, thanks in large part to the presence of the Sinaloa and Los Zetas drug cartels in the region causing high levels of risk for greenfield exploration. Nonetheless, the lure of precious metals has been strong enough to establish both large and small mining operations in the region, with the mountains hosting several world-class Au-Ag deposits.

The Sierra Madre Occidental range overlies a complex basement, and the major formations of economic interest are in the two Cenozoic volcanic sequences. The first of these is dated to the Cretaceous and associated with the Laramide Orogeny. This Lower Volcanic Supergroup (LVS) is at least 2000 m (6652 ft) thick, and it contains the majority of the gold and silver mineralization in the area, with abundant sulfides in most outcrops. The LVS consists of lava flows, ranging from basaltic andesites hosting many of the gold-silver veins, to silicic species, interlayered with ignimbrites and tuffs, pointing strongly to an island arc origin.

It is mildly to moderately brittly deformed and forms a close association with a series of batholiths thought to be a continuation of the Sierra Nevada and Peninsula Ranges batholiths in the US. The LVS is unconformably overlain by the heavily ignimbritic Upper Volcanic Supergroup, which forms the spectacular high plateau of the Sierra Madre Occidental. Many mines from the early and mid-20th century exploited deposits in the LVS, but over the last decades more and more have been identified in the upper volcanics as well.

Sierra Madre
Sierra Madre

The classification of the Sierra Madre Occidental Au-Ag deposits has undergone something of a reshuffling over the last decade. While many deposits in the area, including those in the important San Dimas district, were originally thought to have been emplaced by a single magmatic pulse associated with batholith formation around 10 million years ago, new data suggests a much more complex history, with three metallogenetic events. The first of these is the Late Cretaceous gold-copper porphyry event associated with the subduction of the vanished Laramide slab beneath the continent, which is the source of the generally uneconomic copper deposits in the area. Next came the Ag-dominant deposits at around 40-41 million years ago, likely associated with the Piaxtla batholith, since the silver is concentrated in East – West oriented tension structures in its proximity. The Au-dominant event came last, with gold clustered in North – West striking systems associated with Oligocene bathloliths. The Ag and Au events also display highly distinct mineralization and textures in host veins, with trace element studies revealing different metal sources, or at least mineralization conditions, for the two events. In this conception, an originally deep, intermediate-sulphidation silver province was uplifted and overprinted by a shallower, classically epithermal gold deposition event. Nevertheless, the structures infiltrated by the two temporally distinct fluids are generally the same, leaving most veins with a mix of Au and Ag enrichment.

It was evident to early European visitors to the Sierra Madre Occidental that the region could be a major source of gold and silver. For example, the area around Alamos Gold’s Mulatos mine (TSX:AGI, NYSE:AGI) in the eastern part of the state of Sonora was reported on by Jesuit priests in 1635, and throughout colonial history, including during the Mexican revolution, mining continued in the area. Today, major and mid-tier miners scramble for claims in the region. Notable producers include Newmont’s (NYSE:NEM, TSX:NGT) giant Peñasquito mine, which the company gained during its acquisition of Goldcorp, and which is the largest gold and second largest silver producer in Mexico, running two open pits and with mine life expected to run at least another ten years. Another major player is Agnico Eagle (NYSE:AEM, TSX:AEM) with La India, which has reserves of 490 000 Oz of gold and 1.2 million Oz of silver, and several exploration targets on site. Mining operations in the area commonly use the heap leach method to extract precious metals, and mines tend to have large output but relatively short lifespans – for example, La India is expected to operate for another four years, having started commercial production in 2014.

Mammoth Resources
Mammoth Resources Drilling

The Sierra Madre Occidental is not exclusively the domain of big mining business, however. Juniors such as Mammoth Resources (TSX:MTH) have understood the potential in the area, and Mammoth has a 100% owned Au-Ag prospect, Tenoriba, which includes a 6 km (3.72 mi) mineralization trend. Centerra Gold (TSX:CG) previously drilled the property, and the exploration data is extensive, with mapping conducted and over 3000 surface samples collected. Drilling was performed over 26 holes totaling around 5000 m (16 404 ft) along the mineralized trend, with intersections of potentially economic zones along much of it. One such instance was a 58.2 m (191 ft) section grading 0.81 g/t of gold equivalent at a 65:1 gold to silver ratio. The company is currently undertaking IP/Mag geophysical surveying at the site to fill in gaps along the trend, and new 3D models are under development, which already show attractive features. In addition, old geophysical data is being reprocessed with a technique more sensitive to the fine-grained mineralization style specific to Tenoriba. Field data is still being collected, with current goals including the establishment of volcanic flow vectors to direct further exploration, and characterization of the breccias that host so much of the mineralization in the region. Further soil sampling is also expected to considerably expand the sampling grid.

Despite the long history of gold and silver mining in the Sierra Madre Occidental, there is certainly scope for exciting new discoveries throughout this spectacularly rich region. With the big and mid-tier miners focusing much of their exploration on brownfields near proven resources, it may be up to smaller players, such as Mammoth, to bring truly novel deposits to light.

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