During the early stages of the project, is common to estimate the gold reserves of the deposit and this calculation has a significant importance for the viability of the project. It is well know that a resource estimate indicates the quantity of gold mineralized zones, and this number is not a preliminary idea of the deposit. A reserve estimate tends to refine the resource estimate by considering economic constraints on the size and grade of the ore that is considered in the calculation procedure. A part very important of this process is the cutoff grade, which indicates the gold grade below the ore is not economical for the project. Geologists consider that the cutoff is influenced by sacral factors such as mining costs, gold recovery, credits form associated minerals considered as by products. It is common that in a reserve calculation that some zones that have lower grades than the cutoff grade are separated of the calculation procedure.
It is important to mention that during the calculation procedure, it is essential to consider mineralized areas that are wide enough to mine due to small areas of waste between and along the sides of mineralized areas will be considered as dilutive material. When the design of the mine considers an open pit operation, the design must consider the limits of mineralization and the waste material that it is necessary to remove. Other factor to consider in the economical part of this calculation is the stripping ratio, which is considered as the material that it is necessary to remove to mine a unit mass of gold ore. For instance, if a 30 million tonnes deposit is mined from a pit with a total of 100 million tonnes of rock, the 70 million t of wastes material indicate a stripping ratio of 2.3. It is common to perform several reserves calculations based on the variability of the cutoff grade in order to generate several mine designs and select the economical option.
It is not unusual for some gold projects to go through two or more stages of development and redevelopment. A relatively modest facility might be entirely adequate to develop and mine 50 tons of ore per day in relatively rich gold material near the surface. After several years, long term plans may indicate the need for a much larger head frame and hoist, when development of large tonnages of low gold grade deeper in the mine makes possible a production rate of more tonnages per day with a new larger processing plant. The smaller operation may have financed the main development work, and proven the larger ore reserve far more thoroughly than exploration work that might have originally been justified. These developments result of high gold prices, unexpected good results in initial development, new milling methods and the amenability of the material to the recovery process. All this development is based on the appropriate determination of gold reserves.