When the samples arrive to the laboratory for assays, they are codified according to the laboratory system and weighed. Each core sample is entirely crushed to almost 100% passing 1.7 mm (10 mesh). Samples are homogenized and one kilo is split and pulverized to approximately 90% passing 0.075 mm (200 mesh). Then, 200 to 300 grams are split. If the samples are wet, it will necessary to dry the samples without using any heating system in order to avoid any physiochemical change in the samples.
In order to control the sample preparation procedure, approximately 5 to 10% of the crushed and pulverized samples are submitted for a regular particle size analysis by using lab screens. Blank samples are included into this control so that the sample preparation procedure can be done properly. These samples can be added into the core samples at a frequency of 1 in 30.
In order to perform assays for gold and silver, 40-60 grams aliquots are taken for fire assay. This assay method employs fluxes according to the minerals present in the sample due to the fluxes are variable and there is not a unique recipe. Some assays for other elements are performed by atomic absorption. For example, we can have 1,000 to 50,000 samples to be submitted for 20-30 elements. Sometimes, assays for almost all the elements can be done by Inductively Coupled Plasma (ICP). Gold is always assayed by fire assay using 25-35 g charges.
Certificated laboratories have a quality control procedure that includes the sample to be assayed, standard, blanks and duplicate samples into every assay. For example this procedure can consider three standards, two blanks and three repeats in every batch of 80 samples. If any discrepancy is found, the whole batch has to be re-assayed.