Red Dog’s ore-bodies formed 338 million years ago at the bottom of an ocean basin. Water flowing through faults and fractures formed hot mineral rich springs on the seafloor from which zinc and lead precipitated forming the ore-body. The ore minerals filled cracks, replaced some of the sediment and formed layers on the seabed. Faulting that formed the Brooks Range 126 million years broke the mineralized zone into three ore-bodies (Main, Aqqaluk and Qanaiyaq). Modern day examples of this process occur in a number of ocean settings.
The original ore zone, Main Deposit, contained 77 million tonnes (metric ton = 2,200 pounds) of rock containing 17% zinc, 5% lead and 2.4 ounces per ton of silver. Drilling in the 1990s added the Aqqaluk and Qanaiyaq Deposits bringing the total for all three ore zones to 119 million tonnes. Beginning with the start of mining in 1989, ore came from the Main Deposit and averaged 20% zinc. At that grade, each tonne of ore mined contains 440 pounds of zinc metal. The Aqqaluk Deposit is currently being mined. The ore-body is close to the surface allowing it to be mined using "open-pit" mining methods. “Waste" rocks (those containing insufficient zinc and lead to economic) cover the ore-body and must be removed. On average, one tonne of waste must be mined to expose each tonne of ore. Surveyors mark the layout of holes that are then drilled and loaded with explosives. Once broken, loaders and trucks are used to move waste rock and ore to the appropriate stockpiles.
Red Dog’s ore is in a hard sedimentary rock. The ore occurs as massive layers, disseminated grains or veins. Zinc occurs in a glassy, reddish to yellow brown mineral called sphalerite. Sphalerite is a zinc sulfide and it contains 66% zinc and 34% sulfur. The ore also contains galena, a steel gray metallic lead sulfide that contains 86% lead, 14% sulfur and traces of silver. The ore-body also has pyrite (fool's gold) which is made of iron and sulfur. Two minerals are common in Red Dog’s ore, quartz and barite. Neither is of any value and both are discarded along with the pyrite as tailings during the milling process.
Milling and flotation processes remove the zinc and lead minerals from the rock and then concentrates them. First, the rock is ground to a fine powder (almost the consistency of talcum powder) to separate the sphalerite and galena from other minerals that comprise the rock. Flotation is then used to separate and concentrate these minerals. This is done by mixing the finely ground rock with water and chemicals in large tanks and injected air into the bottom of the tank. In a two-stage process, the grains of galena, followed by sphalerite, attach to bubbles and float to the surface where they are skimmed off. The resulting product is concentrate, two types are made, zinc concentrate (concentrated sphalerite) and lead concentrate (concentrated galena). The concentrate is shipped to smelters throughout the world. Smelting removes the sulfur producing the pure zinc and lead metals.