Once the drilling program is complete, the next step is the determination of gold reserves in the deposits. It must be indicated that the tonnage of mineralized ore in a gold deposit can be determined if the volume of the deposit is calculated. The calculation procedure involves the widths and depths of the holes and the distance between them. It is common to perform the calculations using especial softwares, which show a three dimension view of the deposit. Then, multiplying the volume by the average density of the mineralized zone results the tonnage of the gold deposit. At the moment of determine the tonnage it is important to consider the variability of density, gold grades, slopes of the drill holes and possible shape of the orebody, which is not necessarily regular.
The yearly statement of gold reserves, if made available in any form to outsiders, therefore does not accurately reflect the possible ultimate production of the mine. This is practiced due to fluctuations of gold prices, which can convert the valuable deposit to one with lower value; other reason is taxes, which may be applied on an inventory basis to a wasting asset; labor negotiations may be difficult when there is a erroneous impression as to longevity of the operation; and legal aspects related to speculative consideration of the deposit. Basically, when a Project Manager explain the gold production and reserves of the deposit, it is common to make some consideration for material that lies far ahead of the current work. This is common for gold companies that have a small quantity of reserves and don´t consider the money required to grow the reserve is a good idea.
The gold grades determined by sampling are plotted, and weighted average of the gold grades is determined indicating a resource form. Essentially, this estimate indicates the gold grade and tonnage of the deposit considering the information obtained in the drilling program. It is common practice to include more tonnage in areas that were not drilled. Reserves are classified in three categories, measured, indicated and inferred. Measured reserves are also known as proven reserves and consider the tonnage computed from information revealed in outcrop, trenches, working and drill holes, and for which the gold grade is considered from adequate sampling. The sites for sampling and measurement are closely spaced considering geological information. Indicated or probable reserves are obtained when a significant number of drilling has been performed, but some areas are not fully tested. Gold ore for which tonnage and grade are computed partly from specific measurement, samples or data and from projection for a reasonable distance on geological evidence is considered in this category. The last category, inferred or possible resources is obtained from widely spaced drill holes. Basically, estimates of possible gold ore are bases on geological information and data from few samples. They are determined on assumed continuity or repetition for which there geological evidence, which may consider comparison with deposits of similar type.
Reserves and resources are updated during the exploration