Why it’s a Mistake Not to Use Geochemical Pulp Samples
They are an excellent sample type for spectral analysis!
By Dr. Sasha Pontual, AusSpec International
As users of the TerraSpec® 4 mineral analyzer, you will know that spectral analysis is a tool not only to obtain mineral information on your spectra but also to analyse variations in mineral chemistry, such as for chlorite and white mica. These mineral variations are invaluable to assist you in understanding the alteration: mineralisation relationships in your project area, which in turn will assist you to better target your exploration drilling.
To better understand the relationships between geochemistry and mineralogy, many TerraSpec users are routinely and successfully using geochemical pulp samples for spectral projects in mineral exploration. A misconception about the use of pulps has arisen as the quality of the spectra of pulp samples can vary between different laboratories (see figure below). Typically variations in drying temperatures and grinding times can lead to differences in the level of degradation of samples between different laboratories. These differences can be observed spectrally, commonly as noisy spectra and/or poorly developed absorption features. Note also that pulp samples will generally produce brighter spectra than the equivalent core spectra, but the absorption features are typically weaker due to increased scatter from the powder surface. Therefore, pulp spectra can be more noisy than spectra taken from the core sample.
While the above issues can often be dealt with from experience when interpreting spectra, they can also be overcome by using a split from the jaw-crushed reject samples that can be obtained from the geochemistry lab before the laboratory pulverises the samples. The result would be a better sample for spectral analysis which can also be directly compared with the geochemical data.
This approach provides an opportunity to make direct comparisons between the geochemical data and the spectral data. Because the geochemical samples are usually pulped core intervals, commonly representing 1-2m of core, they typically represent a homogenised interval, This means that in a single reading (taking 5-10 seconds) you will get a close to true representation of the relative mineral proportions in that interval of core. This is very useful in large alteration systems such as porphyry systems, as it is usually the broad alteration patterns which are the most relevant in exploration, not necessarily the fine scale details.
In addition, the crushing process often liberates minerals that may be preferentially developed along fractures, thin veins or as spotting in the sample. This can enhance alteration signatures where these minerals are spectrally responsive (for example carbonate veining).
Perhaps one of the biggest advantages is that measurement of pulped samples is very fast, allowing you to build up large spectral data sets for multiple holes within hours or days, rather than weeks. By using the mug light or by loading up the powdered samples into petri dishes, it can be a very efficient way of measuring samples. From experience, the use of pulps provides a systematic way of measuring large data sets without relying on the operator to choose suitable features on the core to measure. This approach provides a relatively objective way of measuring and therefore makes it possible for the measurement to be carried out by a field assistant, who may not necessarily be a geologist.
All that being said, there are some disadvantages to using geochemical pulp samples that are worth mentioning that relate to sampling procedures. Sometimes geochemical samples are not systematically selected as only the interesting parts of the core are sampled and analysed (i.e. samples which are visibly altered and mineralised). Where this has happened, the sample suite would be too limited for a systematic spectral study as it only represents the proximal alteration signatures. This is important as the spectral data can often be used to identify an alteration envelope beyond where it has been visibly logged and can assist you in identifying the subtleties of a broad alteration halo.
In contrast to this, there are situations where the spectral response of pulp samples can be less responsive to small scale alteration indicators within the sampled interval represented by the pulp sample. This is a potential problem where alteration-related minerals are not pervasive but are restricted to small veins, vein selvages or fracture surfaces. Experience suggests that the best interval to use is 1m or less. Intervals of 2m can be used in some alteration systems such as porphyries, but typically intervals greater than 3m are usually too broad.
If you have any questions or thoughts about this topic, please share a comment.
P.S. Attending IGC in Brisbane in a few weeks? Don’t forget to sign up for the Spectral Geology Workshop presented by Dr. Sasha Pontual, AusSpec International. Sunday, August 5 from 9 am to 5 pm. $850(AUD)/person.
Learn how to pursue a coordinated and consistent way of using spectral data effectively in exploration and mining. This workshop is intended for TerraSpec instrument users and those who are interested in using TerraSpec and TSG Pro™ in their mining exploration program. No prior experience necessary!
Visit the workshop webpage or email email@example.com to learn more.