Optimizing Gold and Silver Recovery at the El Tigre Mine: A Technical Overview of Silver Tiger Metals’ 2023 PEA

Overview

The El Tigre Mine, located in the historic mining district of the Sierra Madre Occidental in Sonora, Mexico, represents a significant revitalization of one of the region’s most prolific silver and gold producers. Managed by Silver Tiger Metals Inc. (formerly Oceanus Resources Corp.), the project is situated approximately 90 kilometers south-southeast of the border town of Douglas, Arizona, within the municipality of Nacozari de García. This area is characterized by the rugged terrain of the Basin and Range province, where the eastern flank of the Sierra El Tigre hosts a complex system of low-sulfidation epithermal veins and disseminated mineralization. The site’s historical significance is profound; between 1903 and 1938, the Lucky Tiger Combination Gold Mining Company operated the mine, producing an estimated 67 million ounces of silver and 353,000 ounces of gold from high-grade veins. However, the mine closed in 1938 due to a combination of low silver prices, labor union demands, and newly imposed production royalties.

The modern El Tigre Project, as outlined in the December 2023 Preliminary Economic Assessment (PEA), focuses on a large-scale open-pit mining operation coupled with a phased heap leach processing circuit. Unlike the high-grade underground operations of the early 20th century, Silver Tiger Metals is targeting the lower-grade, bulk-tonnage mineralization that surrounds the historical workings. The project is strategically important for the Sonora mining sector, as it leverages modern recovery technologies to extract value from a resource that was previously considered uneconomic. With a scheduled mine life of over 13 years and a robust expansion plan that doubles throughput by Year 4, El Tigre is poised to become a cornerstone asset for Silver Tiger Metals. The project benefits from its location in a tier-one mining jurisdiction, accessible via existing infrastructure from Nacozari and surrounded by other world-class copper and precious metal deposits. This technical article delves into the mineral processing parameters, the phased expansion strategy, and the environmental considerations that define the current trajectory of the El Tigre Mine.

Key Process Stages

The mineral processing circuit at El Tigre is designed to maximize the recovery of precious metals while maintaining a cost-effective operational profile. The flowsheet follows a conventional heap leach path with a focus on fine crushing to enhance liberation. Key stages include:

• Three-Stage Conventional Crushing: The Run-of-Mine (ROM) material is processed through primary, secondary, and tertiary crushing circuits to achieve a final product size of 100% passing 3/8-inch (approximately 9.0 mm). This fine crush size is critical for optimizing the permeability of the heap and the exposure of gold and silver particles to the cyanide solution.
• Material Handling and Stacking: The crushed mineralized material is transported via a series of conveyors to the lined heap leach pads. Stacking is performed in lifts to ensure uniform solution distribution and to prevent the compaction that can lead to “channeling” or “pooling” within the heap.
• Cyanide Heap Leaching: A dilute alkaline cyanide solution is applied to the top of the heaps using a network of emitters or sprinklers. The solution percolates through the material, dissolving the gold and silver as it migrates toward the bottom of the heap.
• Pregnant Solution Collection: The metal-laden “pregnant” solution is collected in a series of solution ponds. From here, it is pumped to the recovery facility. The system is designed to be closed-loop, minimizing water loss and reagent waste.
• Merrill Crowe Recovery: Given the relatively high silver-to-gold ratio at El Tigre, the Merrill Crowe process (zinc precipitation) was selected over Carbon-in-Leach (CIL). The solution is clarified, de-aerated, and then mixed with zinc dust to precipitate the precious metals out of the liquid phase.
• On-Site Smelting: The resulting precipitate is filtered and then smelted in an on-site refinery furnace. The final product is doré bars, which are then shipped to third-party refineries for final purification.
• Barren Solution Recirculation: After the metals are removed, the “barren” solution is treated with fresh cyanide and lime to maintain the necessary pH and reagent concentration before being pumped back to the heaps for another leaching cycle.

Critical Data

The following table summarizes the primary technical and design criteria established in the 2023 PEA by D.E.N.M. Engineering and other technical consultants.

Technical Details and Sustainability

The technical success of the El Tigre project hinges on its geological setting and the refined application of heap leach chemistry. The deposit is situated within the Sierra Madre Occidental belt, a vast volcanic plateau composed of andesitic to rhyolitic rocks. The mineralization at El Tigre is predominantly hosted in the “El Tigre Formation,” which consists of rhyolitic tuffs and ignimbrites. The primary target for the open pit is a broad halo of stockwork and disseminated mineralization surrounding the historically mined high-grade veins. This disseminated nature is what allows for the bulk-tonnage approach outlined in the 2023 PEA. The specific gravity of 2.6 g/cm³ and a relatively low strip ratio of 2.0 indicate a favorable mining geometry, which helps keep operating costs (OPEX) low during the initial years of production.

One of the most critical technical decisions for the project was the selection of a 9.0 mm crush size. Column leach tests indicated that gold and silver recoveries were highly sensitive to particle size. While laboratory tests showed potential recoveries of 83% for gold and 64% for silver, the engineering team applied a conservative 3% discount to these figures to account for real-world field conditions, resulting in the design recoveries of 80% and 61%, respectively. The Merrill Crowe facility is also a strategic choice; because the silver grades are high (14.0 g/t), a standard carbon-based recovery circuit would require massive amounts of activated carbon and a very large stripping circuit. The Merrill Crowe process, which uses zinc dust to replace gold and silver in solution, is more efficient and cost-effective for high-silver ores.

Sustainability and environmental stewardship are integrated into the modern mine plan. Because El Tigre is a “brownfield” site with over a century of mining history, the project area already contains historical tailings, ruins of old processing plants, and legacy mine shafts. Silver Tiger Metals’ approach involves utilizing these previously disturbed areas for new infrastructure where possible, thereby minimizing the project’s additional environmental footprint. In 2013, a Unified Technical Document (DTU Particular-B) was authorized for the project, covering environmental impact and land-use changes. This authorization is a critical milestone for the reactivation of the district. Furthermore, the use of a closed-loop water system for the heap leach circuit is essential in the semi-arid climate of Sonora. All solution is recirculated, and the only water “loss” occurs through evaporation or retention within the heap’s moisture content, which is carefully monitored to prevent any discharge into the surrounding watershed.

Looking forward, the El Tigre project is moving toward a Pre-Feasibility Study (PFS) scheduled for completion in 2024. This study will further refine the process design and incorporate more detailed metallurgical testing. The phased expansion from 7,500 t/d to 15,000 t/d provides a pathway for the company to fund its growth through initial cash flow, reducing the need for external financing in the later stages of the mine life. As the project progresses, Silver Tiger Metals continues to explore the underground potential of the site, which may eventually complement the open-pit production with high-grade feed, further extending the economic viability of this historic Mexican mining district.

Source: El Tigre

Source: NI 43-101 Technical Report

This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.