- Types of Ore

Gold originates in primary orebodies where it is normally deposited by mineralizing fluids in chemically or physically amenable rocks. Many primary gold deposits are in "greenstones". These are believed to be ancient volcanic rocks that were laid down when the earth was still young. Another important group for gold production, although better represented amongst copper miners, is the gold copper porphyries.

These volcanic ore bodies are typically very big, containing from a few hundred million tons to billions of tons, and contain variable grades of copper and gold. Over time, some of these primary deposits are eroded, and gold because of its chemical and physical resistance is redeposited elsewhere in secondary deposits, with the best known being placer deposits.

However, after many years of mining, placer deposits have little economic impact today. In recent years, helped by the heap leach technology for gold and the related Solvent Extraction Electro Winning (SXEW) methods used on copper the oxidized surface expressions of primary deposits have become much more important.

Oxide vs. sulphides

Many primary orebodies have an oxidized skin which can be a few inches to hundreds of feet thick. Oxidization normally breaks down the minerals that may have originally contained or even encapsulated the gold. This normally makes gold extraction easier and facilitates the simple heap leaching methods followed by a simple carbon-based extraction. Carbon-in-leach (CIL) and heap Carbon-in-pulp (CIP) methods can be used to cheaply extract gold from ores with simple metallurgy.

If oxidized ore can be found in quantity, especially if little waste rock has to be moved to get at the ore, then highly profitable operations can be built. . Gold is often found with iron and other metals in sulphides. If the metals were deposited at the same time they can be intimately mixed and the gold can be difficult and more expensive to extract without special treatment.

In the worst case, the gold is intimately (chemically) mixed with iron or sometimes arsenic and the ore is classified as refractory and requires special treatment, which can require that the mine add fine grinding, an autoclave, roaster or BIOX step to the normal gold extraction process to oxidize the ore before economic levels of gold recovery can be achieved.