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.

Dredging is the major mining method for the extraction of Hellyer tailings. Hydraulic mining with slurry pumps mounted on a platform and water cannon wash are the other mining methods to be used.

The planned reprocessing of Hellyer Tailings is designed to produce three saleable products:
• Lead concentrate
• Zinc concentrate
• Gold/silver/pyrite concentrate.

The processing plant is expected to operate at an average tailings feed rate of 154 dmt/hr with availability of 92%, which was achieved during the previous dredging operations in 2006 to 2008. The plant is already equipped with a high degree of instrumentation and process control equipment with a high level of automation and on line, real-time
measurement and recording.

The rate of 154 dmt/hr has also to be achieved by the feed pumps of the surge tanks near the Main Dam via the 2,000 m pipeline to the mill.

There are two mill feed tanks outside the mill. Each of them has 12,000–13,000 m3 capacity or 26,000 m3 in total. Both filled can allow a 12 hours work of the processing plant when the dredge moves to another location.

The metallurgical processing characteristics of the Hellyer ore have been extensively tested and are well known. Recent tests have been conducted by ALS in Burnie and reviewed by Pitt & Sherry Group (2016) with the objective of defining a new flowsheet for the reprocessing of Hellyer tailings and optimising metal recoveries. The testwork was relatively conventional and replicated the original Hellyer flowsheet albeit at a finer primary and regrind size target, as well as with the inclusion of a pyrite float with payable precious metals (gold and silver). Forty batch tests were also conducted by ALS as well as three locked cycle tests to simulate the recirculating loads under balanced circuit conditions. The results obtained confirmed that a combined lead/silver concentrate can be floated separately from a subsequent zinc concentrate and thus the opportunity to produce the following three saleable concentrates:

• Combined lead/silver/copper concentrate grading 37% lead and maximising silver content.

• Zinc concentrate grading 45% zinc.

• Gold/silver/pyrite concentrate with a 46% sulphide sulphur content.

To achieve this, in both cases the base metal rougher concentrates were reground prior to cleaning and following base metal flotation, a high grade (S) pyrite was floated from the residual out of the zinc scavenger circuit tailings. The final tails from the pyrite flotation circuit comprised a low sulphur grade and other gangue material.