Overview
The “Halo” mine project, fundamentally centered on the expansion of the El Tigre Gold-Silver Project in Sonora, Mexico, represents a significant evolution in mineral extraction strategy within the prolific Sierra Madre Occidental gold-silver belt. Operated by Silver Tiger Metals Inc., the project focuses on the high-tonnage “Halo” mineralized domains, including the Halo Sooy, East Sooy, and Protectora Halo zones. These domains represent a shift from historically mined high-grade narrow veins to a bulk-tonnage approach that leverages modern mineral processing technologies to extract value from lower-grade, widely disseminated mineralization.
Located in the Municipe of Sahuaripa, the El Tigre project covers a vast area that has seen mining activity dating back to the early 20th century. However, the recent technical focus—formalized in the 2023 Preliminary Economic Assessment (PEA)—highlights the processing of “Halo” material through a sophisticated heap leach and Merrill Crowe recovery circuit. This strategic direction is designed to maximize the economic viability of the project by minimizing initial capital expenditure (CAPEX) while maintaining high operational efficiency through a multi-stage expansion plan. The project is situated approximately 200 kilometers east-northeast of Hermosillo, benefiting from a region with a deep-rooted mining history and established infrastructure, although its high-altitude terrain requires rigorous engineering for processing facilities and solution management.
The significance of the Halo zones lies in their ability to support a large-scale operation. The Stage 1 development targets a nominal throughput of 7,500 tonnes per day (t/d), which is planned to double to 15,000 t/d in Stage 2. By focusing on the Halo domains, Silver Tiger Metals aims to produce a life-of-mine (LOM) average of nearly 10 million ounces of silver equivalent annually, positioning the Halo mine as a cornerstone of precious metals production in the region. The technical architecture of the processing circuit is specifically tailored to the oxidized and transitional nature of the Halo mineralization, ensuring that the recovery of gold and silver remains consistent even as the operation scales up to handle higher volumes of material.
Key Process Stages
The mineral processing circuit for the Halo domains at El Tigre is designed as a conventional heap leach operation, emphasizing high-availability crushing and efficient solution chemistry. The circuit is divided into several critical stages:
- Primary Crushing: Run-of-Mine (ROM) material is delivered to a primary jaw crusher. This stage reduces the large-scale ore blocks (nominal top size of 600 mm) into a manageable feed for the secondary and tertiary circuits. The jaw crusher system is engineered for a nominal capacity of 420 t/h in Stage 1, expanding to 830 t/h in Stage 2.
- Multi-Stage Conventional Crushing: To ensure optimal liberation of the gold and silver within the heap, the ore undergoes secondary and tertiary cone crushing. The final product is targeted at 100% passing 3/8-inch (9.0 mm), which is the calibrated size (P80) determined through metallurgical column testing to maximize permeability and recovery.
- Conveyor Stacking and Heap Leaching: The crushed ore is transported via a series of conveyors to lined heap leach pads. Here, the material is stacked in lifts and irrigated with a dilute sodium cyanide solution. The solution percolates through the ore, dissolving the precious metals as it moves toward the collection system.
- Solution Management: Pregnant Leach Solution (PLS) is collected in specialized ponds. The solution management system includes barren and pregnant solution ponds equipped with high-capacity pumps to ensure a continuous irrigation and recovery cycle.
- Merrill Crowe Recovery: The PLS is processed through a Merrill Crowe facility. This involves clarification of the solution to remove suspended solids, de-aeration to remove dissolved oxygen, and the addition of zinc dust to precipitate the gold and silver. This method is preferred for the high silver-to-gold ratios typical of the Halo mineralization.
- Refining and Smelting: The resulting precipitate is filtered and then smelted in an on-site refinery. This final stage produces doré bars, which are then shipped for final refining to bullion-grade metals.
Critical Data
The following table summarizes the primary design criteria and technical parameters for the Halo mineral processing circuit at the El Tigre Project.
| Parameter | Value | Unit |
|---|---|---|
| Nominal Throughput (Stage 1) | 7,500 | t/d |
| Nominal Throughput (Stage 2) | 15,000 | t/d |
| Annual Plant Feed (LOM) | 2,737,500 to 5,475,000 | t/y |
| Primary Crusher Capacity (Stage 1) | 420 | t/h |
| Crushed Product Size (P80) | 9.0 (3/8) | mm |
| Gold Recovery (Anticipated) | 80.0 | % |
| Silver Recovery (Anticipated) | 61.0 | % |
| Merrill Crowe Capacity (Phase 1) | 770 | m³/hr |
| Cyanide Consumption Rate | 0.22 | kg/t |
| Lime Addition Rate | 2.5 | kg/t |
| Crushing Circuit Availability | 75.0 | % |
| Process Plant Availability | 95.0 | % |
Technical Details and Sustainability
The technical success of the Halo mine project is deeply intertwined with its metallurgical characteristics and the environmental considerations inherent in the Sierra Madre region. The choice of heap leaching over conventional milling and flotation was a strategic decision based on the amenable nature of the Halo mineralization to cyanide leaching and the significantly lower operational costs associated with this method. Extensive column leach tests performed on various Halo domains (Halo Sooy, Protectora, etc.) indicated that while the ore is relatively hard, it exhibits excellent recovery profiles when crushed to the 9.0 mm target size. The Merrill Crowe process was specifically selected because it is more efficient than Carbon-in-Column (CIC) circuits when dealing with high-silver-grade ores, as the silver concentrations in the pregnant solution would quickly load and overwhelm a standard carbon circuit.
From a sustainability and environmental perspective, the Halo project incorporates several “green” mining initiatives and rigorous management systems. Water management is a critical factor in the arid to semi-arid climate of Sonora. The project utilizes a closed-loop solution system where the barren solution—remaining after the gold and silver have been precipitated—is recirculated back to the leach pads after being recharged with cyanide. This minimizes water consumption and prevents the discharge of process solutions into the environment. Furthermore, make-up water is sourced from local surface wells and carefully monitored to ensure minimal impact on the local water table. The project’s design also accounts for the high-intensity seasonal rainfall common in the region, featuring robust storm-water diversion channels and containment ponds to prevent solution overflow.
Environmental management at the Halo mine also extends to the reclamation plan. The heap leach pads are designed to be neutralized and reclaimed at the end of the mine life. During operations, lime is added at a rate of 2.5 kg/t to maintain a high pH in the leach solution, which not only optimizes the leaching process but also prevents the formation of hazardous hydrogen cyanide gas. The use of zinc precipitation in the Merrill Crowe circuit is a well-understood process that allows for high-purity doré production while maintaining a manageable environmental footprint.
Looking to the future, the Halo mine’s expansion from 7,500 t/d to 15,000 t/d underscores the scalability of the current processing architecture. The crushing circuit is designed with a footprint and primary infrastructure capable of supporting this growth with minimal disruption. The modular nature of the Merrill Crowe plant also allows for additional capacity to be added as throughput increases. This phased approach ensures that the project remains economically resilient while continuing to provide significant employment and development opportunities in Sonora. The Halo mine stands as a benchmark for modern, efficient mineral processing in Mexico, balancing technical rigor with environmental stewardship.
Source: Halo | Silver Tiger Metals Inc. Reports_276
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
