Lead-zinc-silver mineralisation occurs in galena and sphalerite-rich bedding parallel horizons in dolomitic and variably carbonaceous pyritic shales and siltstones.

The Black Star Open Cut resource is additional to the Mount Isa Open Pit resource.

The resource mineralisation occurs inside a designed Stage 7 pit based on a pit optimisation utilising Measured and Indicated Resources.

A total of 3.6Mt at 5.1% Zn, 2.6% Pb and 64g/t Ag was depleted from the Ore Reserves in the 12 months to 31 December 2016 due to mining operations, completing the Stage 5 Reserve.

Drilling and blasting practices are conducted at open cut operations to fragment the rock surrounding the ore. Ore from the open cut mines is loaded into dump trucks.

Ore mined is roughly crushed on site, then trucked to the Mount Isa mills for processing. First, the ore is ground into a fine dust via a series of ball mills. This dust is then mixed with water and reagents in our zinc-lead concentrator’s flotation circuit. The reagents help us to separate the valuable mineral content from waste rock. Air is forced through the slurry creating bubbles, which float to the surface and attract and bind to the zinc or lead particles. The glossy, mineral-laden “froth” is then skimmed off the top and dried. What remains once the water content of the froth has evaporated is zinc or lead concentrate. Our zinc concentrates are sold in the domestic market and exported overseas, while our lead concentrate undergoes further processing and smelting on site to become crude lead, or lead bullion.

Lead concentrate is mixed with high grade silica, limestone and copper slag, a by product of our copper smelting operations. These ingredients are known as fluxes, and help us to maintain the heat and chemical reactivity required to separate the valuable lead content from the waste material within the lead concentrate.

The concentrate mixture is fed into our sinter plant, then a blast furnace heats the concentrate to 1,450 degrees Celsius. This process melts the concentrate into liquid, separating and drawing out impurities leaving only crude lead.

Liquid crude lead is poured and left to settle in one of six, 100 tonne kettles. As it cools, the heavy molten lead with its relatively low melting point sinks to the bottom of the kettle. The lighter impurities float to the surface and begin to solidify, where they’re scooped off. What remains is lead bullion. Lead bullion is railed to Townsville then shipped to Britannia Refined Metals in England for further refining and sale on the London Metal Exchange.

A by-product of our mining and processing operations, silver is extracted from our lead bullion blocks after export to our Britannia Refined Metals Facility in England for further refining.