Complex Ore bodies

 Refractory generally means that you are getting gold extractions of less than 90%. Refractory is a term that has been misused in the past to describe only a situation where the gold recoveries are less than 90%. However, refractory has also been used as a term to describe gold enclosed or locked in other minerals and uneconomic to extract.

Many high grade, gold bearing ore bodies have been known for some time but have been considered too difficult or uneconomical to extract due to their properties. They have been passed around from junior to major – and back again. They require a realistic, economical process (or price) to extract the gold.

So, if an ore body becomes more refractory, it means that the standard extraction and mining processes are not going to give acceptable economic results.

So overall, refractory ore means that it will not be easy to extract the gold for either one or all three main reasons:

• Economic constraint i.e. deeper and more difficult to access making it uneconomical to mine (depending on grade).
• Ore characteristics (hardness, toughness, abrasiveness) i.e. more enclosed in quartz or similar host rock, harder to mill, more costly to mill.
• Mineralogy (ore characteristics) i.e. more enclosed in sulphide minerals or other minerals. It can also frequently be associated with copper bearing minerals. All these require a more complex or adapted extraction process.

For harder free milling ores, and ore where gold is locked in non-sulphide minerals, it can either be treated with a more complex milling circuit (more energy) or by adapting the circuit to add gravity or ultra-fine grinding of gravity concentrates for example.

Sulphide Locked Gold – Requires pre-oxidation

However, refractory ores where there is gold locked in, or closely associated with, sulphides such as pyrrhotite, pyrite and chalcopyrite (called labile sulphides) and/or associated with other base metals such as copper (and its associated minerals), need to be treated differently to extract the gold economically.

The iron and copper from these sulphide minerals can, when broken down, consume large quantities of reagents which can make the extraction process expensive for example. If the gold is locked in these sulphide minerals it is usually finely disseminated and fine grinding is needed to liberate the gold.

Thus, we must adapt for it by changing the process circuit in various ways depending on the ore body (mineralogy). Flotation of the ore to produce a sulphide concentrate, followed by ultra-fine grinding of the concentrate is one example. Pressure leaching and Albion Process are some others – to name a few.

Free Gold – Recoverable via gravity and cyanide leaching .

The main aim of the adapted design is to only treat the refractory portion of the ore body by liberating the gold, and not to treat the complete ore body with this more expensive advance process. You can still treat the free milling ore by conventional processes economically.

Ore sorting is another process gradually becoming more acceptable. The purpose of this step is to separate and concentrate the ore containing the gold from the waste material so that we treat only a smaller stream.