Hard Rock Mining: The process

Gold mining is the science, technology, and business of discovering gold, removing it and selling it on the market. Gold can be found in many places. It can even be found in very small quantities in sea water. More often, though, it is found in greater quantities in veins associated with igneous rocks like quartzite, which are rocks created by heat. The gold originates deep within the earth in places called pockets. These pockets are filled with gold, heavy ore, and quartz.

Since the costs of exploration and removal of gold from rock can be high, large companies are created in order to raise the money necessary for the development of hard rock mines. This is different from the solitary individual or small group associated with placer mining (see “Lode vs. Placer Mining” for a discussion of the different mining methods).

Mining for gold is only worthwhile financially where there is a significant concentration found in the ore. The fixed price of gold in 1934 increased from $20.67 U.S. to $35 U.S. per troy ounce and remained fixed at $35 USD until 1968. Initially the jump made gold mining a great venture in the 1930s, but the fixed price eventually discouraged mining because increasing inflation over the same period decreased the profits to the companies.

Before hard rock mining operations even begin, companies explore areas where gold may be found and analyse the rock. If enough gold is discovered in the ore, the technological process of hard rock mining begins.

Underground mining

In the 1930s, mining was a very labour-intensive, dangerous undertaking. Miners dug tunnels into solid rock by hand. Miners often risked their health, digging with picks and shovels during long shifts in these dark, damp tunnels, building the shafts and carting out the ore.

Most injuries underground involved falling rock, slips, and explosions, but the workers also had to inhale dust into their lungs in this era before safety regulations, safety equipment, and improved ventilation. The miners were willing to take the risks in order to provide for their families.

The miners would cart the ore out of the mine in wheeled carts pushed on rails and take it to the mill.

The gold milling process consisted of three general steps:

(1) Sorting the ore by size
(2) Crushing the rock
(3) Extracting the gold

The rock fragments were sorted according to size in a grizzly, which was device consisting of a series of spaced bars, rails, or pipes, above a forward moving conveyer belt to a crusher machine.

After secondary washing, a shaker screen filtered out fragments of less than 1/2 inch diameter into a fine ore bin, or box. Larger ore fragments were pulverized or crushed in the crusher. The fine ore was fed by conveyer belt to a ball mill, which was a rotating steel cylinder filled with tumbling steel balls that further crushed the fragments to a consistency of fine sand or talcum powder. This powder was fed into a thickener with a cyanide and water solution to create a sludge (a sticky, mud-like material). The liquid sludge was diverted into holding tanks and referred to as the pregnant solution – a liquid sludge containing 70 percent of the gold.

Gold ore must go through an intensive process to separate it from the rock

The sludge was drawn from holding tanks through a clarifier, a device that removed all the remaining rock or clay from a pregnant solution. In the next step, the material was taken to a de-aerator tank that removed bubbles of air and further clarified the solution.

Zinc was added in dust form to the de-aerated solution, which was drawn under pressure through a filter press that caused the gold and zinc to precipitate onto canvas (heavy cloth) filter leaves. This zinc-gold precipitate (condensed into a solid) was then cleaned from the filters while extreme heat burned off the zinc.

Water passing through the filters was chemically tested for gold residue before being discharged into tailings ponds. Gold-bearing water may be passed through the filtering process several times to remove all of the gold and separate it from impure substances.

Gold recovered from the ore through the milling process was poured into bricks that were shipped to be assayed and sent to the Royal Canadian Mint in Ottawa, where coins were struck (made).

Bars of gold

The result of the milling process were three types of tailings rocks which cover the landscape of Wells. Hydraulic mine tailings are the resulting rocks washed down the mountainside with water monitors. Another type of tailings is taken out from the mountainside during tunneling.

The rocks left over after being processed through a mill are referred to as mill tailings. Tailings have altered the shoreline around Jack of Clubs Lake and have the appearance of pale yellow, layered mineral sands.

Posted in .