Overview
The Fundadora mine, a critical component of the broader El Tigre Silver-Gold Project, represents one of the most promising brownfield development opportunities in the historic mining regions of Sonora, Mexico. Situated within the municipality of Nacozari de García, the Fundadora vein is part of a prolific epithermal system that has seen renewed industrial interest following the release of a comprehensive Preliminary Economic Assessment (PEA) by Silver Tiger Metals Inc. in late 2023. Historically, the El Tigre district was a titan of production, yielding approximately 100 million ounces of silver equivalent between 1903 and 1938. The Fundadora vein itself, along with neighboring structures like the Sooy, El Tigre, and Seitz-Kelly veins, forms the backbone of a resource base that is being revitalized through modern mineral processing techniques and large-scale heap leach operations.
The significance of the Fundadora project lies in its high-grade mineralization and its strategic location within the Sierra Madre Occidental, a region globally recognized for its precious metal endowments. The 2023 PEA outlines a robust production profile that leverages the oxidized and transitional nature of the near-surface mineralization. By focusing on a high-throughput, low-cost heap leach recovery method, the project aims to capitalize on the favorable metallurgical characteristics of the Fundadora domain. With a projected mine life that scales from an initial 7,500 tonnes per day (tpd) to an expanded 15,000 tpd, the technical planning for Fundadora reflects a sophisticated balance between capital efficiency and maximum metal recovery. This article explores the technical nuances of the processing circuit, the metallurgical benchmarks established through rigorous testing, and the environmental framework designed to ensure sustainable extraction in this historic Mexican district.
Key Process Stages
The processing circuit for the Fundadora mineralized material is engineered for simplicity and scale, utilizing a conventional heap leach flowsheet coupled with a Merrill-Crowe recovery plant. This selection is driven by the amenability of the El Tigre ores to cyanide leaching at relatively coarse crush sizes. The process is divided into several critical stages:
- Primary Crushing: Run-of-Mine (ROM) ore with a nominal top size of 600 mm is delivered to a primary jaw crusher. This initial stage reduces the material to a size suitable for secondary processing, operating at a nominal rate of 420 tonnes per hour (tph) in Stage 1, which doubles to 830 tph after the Year 3 expansion.
- Secondary and Tertiary Crushing: A multi-stage closed-circuit crushing system further refines the ore. The goal of this circuit is to achieve a P80 (80% passing) size of 9.0 mm (3/8 inch). Technical analysis by SGS Lakefield has identified this specific size as the “sweet spot” for balancing throughput with optimal leaching kinetics.
- Heap Loading and Irrigation: The crushed material is transported via a series of “grasshopper” conveyors and stacked systemically onto a lined leach pad. The heaps are then irrigated with a dilute sodium cyanide (NaCN) solution using a network of drippers and sprinklers to initiate the dissolution of gold and silver.
- Leach Cycle: The project utilizes a 77-day leach cycle. During this period, the solution percolates through the heap, collecting precious metals and forming a “pregnant” solution that is collected in a specialized pond.
- Merrill-Crowe Recovery: The pregnant solution is pumped to a Merrill-Crowe plant. The process involves clarification to remove solids, de-aeration to remove dissolved oxygen, and the addition of zinc dust to precipitate the gold and silver out of the solution.
- Smelting: The resulting zinc precipitate is filtered and then smelted in an on-site furnace to produce dore bars, which are then shipped to external refineries for final purification.
Critical Data
The following table summarizes the preliminary process design criteria and metallurgical expectations for the Fundadora and El Tigre integrated circuit based on the 2023 technical reports.
| Parameter | Value | Unit |
|---|---|---|
| Initial Throughput (Years 1-3) | 7,500 | t/d |
| Expanded Throughput (Year 3+) | 15,000 | t/d |
| Gold Head Grade (LOM Average) | 0.39 | g/t |
| Silver Head Grade (LOM Average) | 14.0 | g/t |
| Target Crush Size (P80) | 9.0 (3/8) | mm (in) |
| Anticipated Gold Recovery | 80.0 | % |
| Anticipated Silver Recovery | 61.0 | % |
| Merrill-Crowe Efficiency | 99.0 | % |
| Sodium Cyanide Consumption | 0.22 | kg/t |
| Lime Addition Rate | 2.5 – 2.73 | kg/t |
| Leach Cycle Duration | 77 | days |
| Specific Gravity | 2.6 | g/cm³ |
Technical Details and Sustainability
The technical success of the Fundadora project is heavily contingent upon the metallurgical optimization performed during the scoping and pre-feasibility stages. Extensive column leach tests conducted by SGS Lakefield provided the foundational data for the current flowsheet. These tests indicated that while laboratory recoveries reached as high as 83% for gold and 64% for silver at a 9.0 mm crush size, a 3% discount was applied to the process design to account for real-world field conditions, such as temperature fluctuations and non-ideal solution distribution. This conservative approach ensures that the financial modeling remains grounded in attainable operational metrics.
A significant technical challenge addressed in the design is the management of silver-rich mineralization. Epithermal systems like Fundadora often present silver-to-gold ratios that can impact the capacity of the recovery circuit. The Merrill-Crowe process was specifically selected over Carbon-in-Column (CIC) methods because it is more efficient at handling high silver concentrations in the pregnant solution. The phase 1 Merrill-Crowe plant is designed with a capacity of 770 m³/hr, providing ample headroom to process the anticipated metal volumes during peak production years.
Sustainability and environmental stewardship are integrated into the Fundadora project’s core design. Given the location in Sonora, water management is a primary concern. The project incorporates a closed-loop system for its process water, significantly reducing the requirement for fresh water makeup. Make-up water is sourced from nearby surface wells and is recycled through the pregnant and barren solution ponds. Furthermore, the use of a lined heap leach pad ensures that no cyanide-bearing solutions migrate into the local groundwater. The historic nature of the district means that Silver Tiger Metals is also tasked with managing existing environmental legacies, such as old tailings from the early 20th century. By modernizing the site and implementing strict international standards for tailings and waste rock management, the project seeks to leave a positive environmental footprint compared to the historical lack of regulation.
Looking forward, the transition from the PEA to a Pre-Feasibility Study (PFS) in 2024 will involve further refinement of the Fundadora vein’s contribution to the mill feed. Recent drilling has identified high-grade “halos” and sulfide-rich zones at depth, which may eventually require a flotation or CIL (Carbon-in-Leach) circuit addition as the mine moves from the oxidized cap into the primary ore. For the immediate future, however, the focus remains on the rapid execution of the heap leach strategy, providing a streamlined path to production for this legendary Sonoran asset.
Source: Fundadora | Silver Tiger Metals Inc. PEA 2023
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
