GEOLOGY 101 — Epithermal deposits, Part 2

Epithermal gold deposits, which contribute significantly to the world’s gold supply, are an important exploration target which must be evaluated carefully based on the amount of metal they might provide, and at what cost.

The amount of gold in any type of deposit is calculated based on the ore’s grade (the amount of gold per tonne of rock) and tonnage (total number of tonnes) available at that grade. The higher the grade of the material, the lower the tonnage required to make recovery economical.

A high-grade deposit could have gold values ranging from 10 to more than 150 grams per tonne, whereas a low-grade deposit grades in the range of 1 to 5 grams. Low-grade deposits may have up to, and possibly more than, 200 million tonnes of rock, whereas a high-grade deposit is frequently smaller.

Assay results acquired through drilling are important indicators of a deposit’s grade and tonnage. High grades over short distances can be as significant as low grades over longer distances, and both types of deposit can be mined profitably.

Drill results, however, offer only a limited view of a deposit and may be difficult to reproduce. For instance, a single drill hole may intersect a high-grade zone in an otherwise low-grade (high sulphidation-Type epithermal) deposit, giving the appearance of a higher grade than actually exists.

Factors other than tonnage and grade come into play in calculating the economic significance of an epithermal deposit. For instance, the presence of other metals in the ore can increase the value of a deposit, and many epithermal deposits contain a significant silver and/or copper content.

The price of gold (and other metals) is also an important condition in economic evaluation, as low prices may render small or low-grade deposits uneconomic.

Many epithermal deposits occur in remote regions of under-developed countries, and the construction of infrastructure, such as roads and mills, may be necessary before deposits can be mined. These expenses increase the cost of a mining operation and must be taken into consideration when calculating the economics of a deposit.

Mining and processing methods are also important in determining economics.

Since epithermal deposits are often formed at depths of less than 2 km (closer if erosion of overlying material has resulted), many are amenable to relatively less expensive open-pit mining methods. Deeper deposits that can be exploited only through underground methods are more expensive.

Finally, recovery methods for epithermal gold deposits can entail either conventional milling or cyanide leaching. The cost of both procedures can increase if gold is contained in minerals that are difficult to process, such as arsenopyrite.