INTRODUCTION

Theoretical and, especially practical problems generated by the sampling of materials containing gold and silver have been given enormous attention by many specialists for a long time. Nevertheless, solutions to the sampling of gold and silver are still unsatisfactory: sometimes propositions are too theoretical and useless for the engineer, and sometimes they are too simplistic and based on empirical observations without solid foundations.

In this course, a special effort is made in presenting easy practical examples. This makes it possible to quickly calculate the variance of the Fundamental Sampling Error involved during the sampling of gold and silver.

Peculiarities about the sampling of gold and silver can be divided into three categories:

• Financial.
• Theoretical.
• Practical.

FINANCIAL PECULIARITIES

Relatively small amounts of material can involve very large amounts of money; therefore, problems of precision and accuracy quickly become a primary concern. This is the case during early exploration, for mining evaluations and planning, for milling operations, for recycling smelters, etc.

THEORETICAL PECULIARITIES

One of the main distinctions between gold and silver and other metals is the fact that they are economic at very low levels. Base metals, for example, are often estimated in percent, while gold and silver are often estimated in parts per million.

PRACTICAL PECULIARITIES

Because gold has an important place in the sampling of precious metals, and is known to generate numerous practical difficulties, our discussion will often concern only this metal. However, the extension of these discussions to other precious metals is straightforward.

The gold content of a sample and the gold content of the surrounding ore can be very different. Furthermore, the gold content of a tiny analytical 30-g subsample and the gold content of the 10000-g from which it was selected can also be very different. The density of gold is enormous (ρAu = 19.3), promoting strong segregation phenomena as soon as some gold particles are liberated. Gold particles do not comminute very well; therefore, gold smears and easily coats sampling equipment generating unacceptable losses and cross-contamination problems. As a result, a finely ground analytical 250-g subsample, believed to be 100% minus 100 microns, can still contain a few gold particles that may be 200 microns or even larger. This delayed comminution confuses many sampling experts in their calculations of the variance of the Fundamental Sampling Error, generating endless debates. All these problems are amplified as gold grade becomes lower, as the economics of gold deposits become marginal, and as the distribution of gold in rocks becomes erratic as studied in Part 6 of this book.