Overview
The Pluto deposit represents a high-value geological asset within the Higginsville Gold Operation (HGO), a cornerstone of Westgold Resources Limited’s production portfolio in the Eastern Goldfields of Western Australia. Located approximately 8 kilometers south-southwest of the central Higginsville processing plant, the Pluto mine area is strategically positioned at the southern terminus of the renowned Challenger-Swordsman Paleochannel system. This complex network of ancient riverbeds includes other prolific deposits such as Neptune, Saturn, Jupiter, Mars, Mercury, Venus, and Bullseye. The HGO complex itself sits in a Tier-1 mining jurisdiction, situated between the major logistical and mining hubs of Kalgoorlie and Norseman, benefiting from proximity to the Coolgardie-Esperance Highway and established utility corridors.
The Pluto deposit’s history is characterized by successive phases of rigorous technical modeling and resource definition. Originally modeled by Avoca Resources in 2006, the deposit underwent a comprehensive revision by Metals X in 2014 to better align with contemporary mining standards. Historical mining in the broader Challenge area, which includes the northern portions of the Pluto system, was remarkably successful in the 1990s. Under the management of Resolute, the area produced approximately 2.06 million tonnes of ore at an impressive grade of 3.4 g/t, yielding over 228,000 ounces of gold. Today, Pluto is managed as part of an integrated hub-and-spoke production model. High-grade ore from Pluto and surrounding paleochannel targets is hauled to the central 1.6 Mtpa Higginsville Mill, which serves as a regional processing hub for both open-pit and underground sources, including the Beta Hunt mine. This centralized strategy allows Westgold to optimize the economic margins of the Pluto deposit by leveraging the existing 8 MW grinding capacity and quaternary crushing infrastructure of the Higginsville plant.
The geological significance of Pluto lies in its placement within the south-western lower reaches of the paleochannel. Here, the channel features shallow gradient margins and a slightly undulating base that extends over 200 meters in width. This structural setting has allowed for the accumulation of significant gold-bearing sediments over millions of years. For mining engineers and geologists, Pluto represents a classic example of an Eocene sedimentary gold system, where the interplay between basement Archaean rocks and transported sequences creates a unique set of challenges and opportunities for mineral processing. The ongoing development of the Pluto deposit is a testament to the longevity of the Higginsville region and its capacity to provide consistent mill feed for the Westgold production pipeline.
Key Process Stages
Ore extracted from the Pluto deposit is processed through the Higginsville Mill, a facility renowned for its robust quaternary crushing circuit and efficient gold recovery system. The circuit was originally constructed by GR Engineering Services in 2007 and has undergone several upgrades to enhance its ability to treat harder primary ores and finer transported materials.
- Multi-Stage Crushing and Conveying: The crushing circuit is exceptionally thorough, utilizing four distinct stages to ensure a finely sized feed for the grinding mills. The primary stage employs a 36 x 48 Trio single-toggle jaw crusher to reduce ROM ore size. This is followed by a secondary 1.68 m Trio Turbocone TC66 crusher in a standard configuration. The tertiary stage utilizes an identical TC66 unit but in a “short head” configuration to maximize size reduction. Finally, a quaternary 1.29 m Trio Turbocone T51 crusher—integrated into the circuit in 2010—finishes the product to a nominal P80 of 10 mm. This fine-crushing strategy significantly reduces the energy requirements in the subsequent grinding stage.
- Grinding and Classification: The crushed ore is reclaimed from a 1,500-tonne live capacity fine ore bin and fed into the 8 MW single-stage ball mill. This mill operates in a closed circuit with a cluster of high-efficiency classification cyclones. The target grind size is optimized for gold liberation, typically ranging between 75 and 106 microns depending on the blend of oxide and fresh ore being treated. The 8 MW power rating allows the mill to maintain a nominal throughput of 1.6 million tonnes per annum even when processing harder rock types.
- Gravity Concentration: A critical component of the HGO circuit is the gravity separation stage. Approximately 30-50% of the cyclone underflow is diverted to centrifugal concentrators to capture coarse “free” gold. This prevents the buildup of heavy gold particles in the grinding circuit and reduces the residence time required in the leaching tanks. The gravity concentrate is further refined using an intensive leaching process, such as an Acacia reactor, to produce a high-grade pregnant solution.
- Leaching and Carbon-In-Leach (CIL): The cyclone overflow slurry is directed to the leaching circuit, which consists of one dedicated primary leach tank followed by a series of six carbon adsorption tanks. The CIL process allows for the simultaneous dissolution of gold by cyanide and its adsorption onto activated carbon. This is particularly effective for Pluto ore, as the CIL configuration helps mitigate the effects of any naturally occurring carbonaceous material (preg-robbing) that may be present in the Werillup Formation clays.
- Elution and Gold Room Operations: Loaded carbon is stripped of its gold content through a high-temperature, high-pressure elution process. The resulting gold-rich solution is then sent to electrowinning cells where the gold is deposited onto cathodes. The final product is smelted in a furnace to produce gold doré bars for shipment to the refinery. The stripped carbon is regenerated in a rotary kiln at approximately 700°C to restore its adsorptive capacity before being returned to the CIL circuit.
Critical Data
The following table provides a detailed breakdown of the technical specifications for the Higginsville Processing Plant, which currently manages the ore from the Pluto deposit and broader Higginsville Gold Operation.
| Parameter | Value | Unit |
|---|---|---|
| Design Annual Throughput | 1.6 | Mtpa |
| Total Mill Power | 8.0 | MW |
| Crushing Circuit Stages | 4 | Stages |
| Primary Jaw Crusher | Trio 36 x 48 (Single Toggle) | Model |
| Secondary Crusher | Trio TC66 (1.68m Standard) | Model |
| Tertiary Crusher | Trio TC66 (1.68m Short Head) | Model |
| Quaternary Crusher | Trio T51 (1.29m Turbocone) | Model |
| Fine Ore Bin (Live Storage) | 1,500 | Tonnes |
| Nominal Grind Size (P80) | 106 | µm |
| Final Crushed Product (P80) | 10 | mm |
| Gravity Recovery Circuit | Yes (Cyclone Underflow Feed) | N/A |
| Number of CIL Tanks | 7 (1 Leach + 6 Adsorption) | N/A |
| Power Plant Type | On-site Diesel Station | 11 kV |
Technical Details and Sustainability
A deep technical understanding of the Pluto deposit requires an analysis of its stratigraphy and metallurgical behavior. The deposit is hosted within two primary transported sequences. The upper sequence, the Revenge Formation, consists of 10 to 15 meters of sandy and pisolitic clays. Below this lies the critical Werillup Formation, which contains the bulk of the mineralization. This formation is composed of carbonaceous clays overlying a basal sand channel facies. The base of the channel is marked by a quartzitic gravel lag, varying from 0.3 to 3.0 meters in thickness. This complexity necessitates sophisticated grade control and blending strategies. At the ROM pad, Pluto ore is blended with other sources to maintain a consistent head grade and to manage the rheology of the slurry in the CIL tanks, as the high clay content of the paleochannel material can impact settling rates and viscosity.
From a performance standpoint, the Higginsville plant has demonstrated remarkable consistency. Recent technical reports indicate that the variance between the reconciled (calculated) and assayed head grades has generally remained below 1%, reflecting a high degree of confidence in the plant’s sampling and accounting systems. Gold recovery is highly sensitive to the head grade, but the plant maintains a steady recovery curve by optimizing reagent addition and managing the carbon-to-solution ratios in the CIL circuit. The presence of the quaternary crushing stage is a major technical advantage, as it allows the ball mill to operate at peak efficiency by providing a uniform and fine feed, which is especially important when transitioning between the softer oxide clays of the upper Pluto sequences and the harder primary ore from deeper underground sources.
Sustainability is integrated into the operation through efficient resource management. Water is a precious commodity in the Eastern Goldfields, and Westgold secures its supply via the WA Water Corporation pipeline, which delivers water from Kalgoorlie. This minimizes the reliance on local hypersaline groundwater and ensures the longevity of the processing operations. Power for the 8 MW ball mill and the crushing circuit is provided by a localized 11 kV diesel power station. To improve sustainability, Westgold constantly evaluates plant reliability factors, currently targeting an operational utilization rate of approximately 90%. Future plans involve investigating power stability improvements and potentially incorporating renewable energy sources to reduce the carbon footprint of the diesel generators. The Tailings Storage Facility (TSF) is managed with a focus on maximum water recovery and environmental safety, ensuring that the legacy of the Pluto mine is one of responsible and technically sound mineral development.
Source: Pluto
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
