Sampling and assaying of coarse gold at La Herradura - Minera Penmont S. de R.L. de C.V., Mexico
Minera Penmont’s La Herradura, which began in July 1998, is a joint venture between Industrias Peñoles, S.A. de C.V. and Newmont Mining Corporation. The mine is located in the northern Mexican state of Sonora. La Herradura consists of an open pit mine with a two-stage crushing plant and a heap leaching facility. Gold is recovered from the pregnant solution using the Merrill-Crowe process. La Herradura produces about 130 000 oz gold per year and its treatment capacity is approximately 22 000 t/d. Ore mineralization occurs in a shear zone with quartz and native gold. Gold distribution is quite heterogeneous due to coarse visible gold, which in some cases ranges from 100 to 600 microns.
Sampling and assaying protocols were established during exploration and prior to start-up of operations, based on nomographs developed from early heterogeneity tests. The coarse gold problem became noticeable in 2000 as the amount of gold produced was consistently higher than budgeted figures, and this extra gold could not only be attributed to better gold recoveries. The most reasonable explanation for the extra gold production was that the gold head grade had been underestimated by ore control sampling and/or assaying procedures. Due to this inconsistency, a project was launched to review the sampling protocols and to develop new sampling protocols to achieve an acceptable relative standard deviation.
An experimental program was developed and conducted on a selected ore block consisting of 195 blastholes; regular ore grade control procedures were applied to both sets of data to evaluate the difference in results between the protocol established prior to start-up of operations (existing) and the new protocol (leaching). In the existing protocol, sub-sampling is done through riffle splitters, and coning and quartering. Fire assaying was used to estimate the gold content on a 50 g sample. In the leaching protocol, a 12 000 g sample of bottle-rolled cyanide was leached, and the leach residue was assayed on a 50 g sample using fire assaying, and a weighted average total gold content was calculated. Currently, a new alternative method is being evaluated, where sub-sampling is done using rotary splitters and fire assay on a 50 g sample; in this rotary sampling protocol and for comparison purposes, gold assays were also repeated using the leaching protocol.
Test results were compared in three groups: 0.8 g/t. The results of the testing program can be summarized as follows:
For a gold assay less than 0.3 g/t, and from 0.3 to 0.8 g/t, the most accurate estimation of the gold was found using the leaching protocol, as more sample was used, ~12 000 g vs ~50 g.
The existing protocol underestimates the gold content in the gold assay ranges less than 0.3 g/t, and from 0.3 to 0.8 g/t. This was due to the nature of the gold present in the ore (a large portion is coarse gold).
The rotary sampling protocol is a good practical alternative to the leaching protocol, as rotary sampling can reduce the error caused by the presence of coarse gold to acceptable levels (currently, all blasthole samples are processed using this protocol).
The overall impact caused by using the gold assays obtained from the existing method is actually reduced during mining due to other geological controls in place. However, overall, it is better to use the most accurate assay values available. In this manner, the correct cut-off will be applied during ore control, and the calculation of gold ounces sent to the process will be more accurately estimated.