Geoff Sachtler, of King's High School, asks :-

How do you get gold out of rock?

Dave Craw, an economic geologist at Otago University, responded.

Gold occurs in rocks in two main ways, as quartz vein gold and as alluvial gold. They have different techniques for separation.

Quartz vein gold. This gold occurs as small (0.01 - 1mm) grains distributed through quartz veins in bedrock such as the schist of Otago, including the Macraes mine. This gold occurs with minerals such as pyrite (fool's gold, FeS2). Gold-bearing rock is crushed and mixed with water to make a slurry. A frothing agent (similar to soap bubbles) is added to separate rock particles rich in pyrite and gold. This concentrate is then stirred in a cyanide solution which dissolves the gold. The gold then precipitates on to carbon fragments (baked coconut shells) in the slurry. Gold is extracted from the carbon via an electrical process and then melted and poured into bars.

Alluvial gold. This gold (0.1mm - 1cm grains) is found in river gravels (such as the Clutha River) which have been eroded from gold-bearing hills. The gold is naturally concentrated by the river with other dense minerals such as magnetite (black sand, Fe3O4). Because gold is about eight times denser (eight times heavier for same sized grain) than normal gravel material, separating is achieved with gravity-drawn methods in a slurry. The gravels are sieved, vibrated, and spun to separate the dense minerals, with final concentration by hand with a gold pan.