The Hellyer deposit is a volcanic hosted polymetallic massive sulphide deposit located within the Mount Read volcanic arc of western Tasmania. Mineralisation is sulphide hosted and comprised predominantly of pyrite, with lesser sphalerite, galena and arsenopyrite. The pyrite must be carefully managed when exposed to air to prevent its oxidation and potential to form acid mine drainage issues. The economic metals mined at Hellyer were lead, zinc, copper, gold and silver. The Hellyer deposit was mined by underground methods during the period 1989 to 2000. The Fossey deposit extends down plunge from the Hellyer deposit and was mined by Bass Metals between 2010 and 2012.

Tailings from the mill were deposited in a depression approximately 1 km to the west of the Hellyer mill. The tails were inundated with water to prevent oxidation of the sulphide species present in the tails. The tails are typically stratified, having been deposited from discharge pipes set around the dam. The sedimentary stratification is also reflected in a mild gradient in grades of Zn and Pb, with grades typically higher in the deeper parts of the tails and decreasing as the depth of the tails sediments shallow. The tails sediments are unconsolidated.

The mining method of using a cutter suction dredge to excavate tailings material and transport it to shore via a floating pipeline, was the method used from September 2018 and remains the method used today and forecast for the remaining LOM at Hellyer. The dredged material is delivered to surge tanks via trommel screens to remove debris in the form of oversize rocks and timber and other vegetable matter. From these surge tanks the material is pumped via 300 mm diameter high-density polyethylene (HDPE) pipelines to the processing plant.

Mining is now exclusively by suction cutter dredge; there is no hydraulic mining.

To maintain a feed rate of 100,000 tpm and more, given the potential of greater throughput in the processing plant following alterations to the flotation circuit, a second dredge is to be utilised. This second unit will not only ensure steady feed rates with less interruptions for maintenance and unforeseen downtime events, the second dredge unit permits blending of feed material to the process plant.

The use of this second dredging unit will enhance the project in two ways: it will cover any extended production losses from any major dredge downtime and it provides the ability to blend grades from different mining blocks to smooth out the grade of material delivered to the process plant. Feed of consistent grade material, without peaks and troughs, allows finer tuning of the process plant and improved metal recovery. It is proposed to increase the plant throughput in increments to 180 tph (tonnes per operating hour) by March 2021. The second dredge unit, coupled with the existing primary surge tanks, will ensure that this schedule is met.

Currently in use:
1 x Ball mill – Allis Chalmers 4.6 m x 6.1 m, 2,100 kW;
3 x Vertical tower mills – Kubota, 335 kW each.

In addition, the following process equipment was on site and could be used in future operations:
- 1 x 60x48 Kemco primary jaw crushing plant.
- 1 x SAG mill – Allis Chalmers 6.7 m x 2.4 m, 2,100 kW

The processing plant was in reasonable condition and under a maintenance program until February 2018, when refurbishment was begun in preparation for the proposed retreatment project. Refurbishment focused on milling, flotation and regrind circuits, reagents preparation areas, dewatering filters, and tailings.

The major process equipment then available and being used for the tailings included:
• 1 x Ball mill – Allis Chalmers 4.6 m x 6.1 m, 2,100 kW
• 3 x Vertical tower mills – Kubota, 335 kW each
• 3 x 32 m³ High-intensity conditioners, 810 kW each
• 25 x 54 m³ Maxwell flotation tank cells
• 15 x 38 m³ Maxwell flotation tank cells
• 8 x 17 m³ Dorr Oliver trough flotation cells
• 3 x 8.5 m³ Dorr Oliver trough flotation cells
• 10 x 2.5 m³ Agitair trough flotation cells
• 4 x 1 m³ Agitair trough flotation cells
• 2 x Ingersoll Rand, Lasta 60 plate pressure filter presses
• 2 x 20 m diameter thickeners
• 1 x Lime slak ........