Rock preparation

The importance of quality rock preparation

Consistency of particle size before sub-sampling is a vital part of any rigorous sampling procedure. ALS has several quality control measures in place during sample preparation to ensure samples meet the requested particle size specification and to mitigate any carryover between samples. A sub-sample from the first sample of every batch is screened after both crushing and pulverising so that equipment and time processed can be adjusted to produce the desired particle size. A minimum of one in every 50 samples after that is checked to be sure samples continue to pass the criteria for the method selected. This information is available through Webtrieve™. Multiple cleaning procedures are in place during the sample preparation process, including the use of clean rock and silica to clean equipment between each batch and use of compress air to clean between each sample as standard practice. A selection of commonly used packages is listed in our fee schedule, but many other options are available.

Coarse crushing

This preliminary step may be required when the dimensions of samples are too large to be passed through routine crushing equipment. An example where this step will be required is for whole core or rock samples too large to enter a standard jaw crusher. This preliminary step crushes samples down to nominal 6mm, and then the whole sample is crushed to a finer grain size with standard crushing equipment. Generally, as the full sample is added to the standard crusher, there is no quality control verification on the particle size performed. If size verification is an important requirement for the preparation of your samples, quality control checks may be requested by adding code CRU-21q to the sample submittal to confirm greater than 70% of the sample passes 6mm.

Preparation packages

Preparation of drill core, rock, and chip samples generally requires drying, crushing, splitting, and a pulverisation step to fully homogenise samples and produce the grain size necessary for geochemical analysis. Commonly used combinations of crushing and pulverisation options are offered as packages. These include splitting utilising either a riffle splitter or rotary splitter for crushed samples. Options are available for crushing and pulverising to a variety of finenesses. Pulverisation on a variety of sub-split sizes from the crushed sample is also offered.

Preparation of high-grade ore

Samples that contain high-grade ore or coarse precious metals are challenging to prepare. These samples can benefit from crushing and/or pulverising to a finer size and/or pulverising a larger sub-split to improve sample representivity and homogeneity. However, any preparation of these types of samples is best defined by initial test work.

Nugget effect

Nugget effect is where the distribution of an element of interest is inconsistent within the sample. This results in concentrations of the element into parts of the sample i.e., nuggets. The difficulty with sampling this material is that two samples taken in proximity can have very different concentrations if a nugget is included in one of the samples and not the other. Or even worse if neither sample selected includes a nugget present in the original material. Gold is the element that is most commonly affected by this phenomenon.