Comparison of Precision in Assay Values

Gold is scarce in nature. Even in gold deposits with economically viable grades, the abundance of gold is a small quantity compared to the rock mass. Mineral resource estimates are based on analytical data from drillhole samples, called assays. The units of the assay data and the precision of the values impact the resource estimation in several ways. In this article, we will illustrate these impacts, starting with the inherent difference in the precision of units. In most of the world, the metric system is prevalent, and there is no question about the units to use for resource estimation: grams per metric ton, equivalent to parts per million. However, in the United States, mines are planned and built in length and weight units derived from the Imperial system. To add to the unit complexity, the troy ounce is the common weight unit for precious metals, and is slightly heavier than the avoirdupois (avdp) ounce, which is the common definition of “ounce” in the U.S. (i.e. one pound is 16 avdp ounces).