Perhaps a bit unconventional for these blog posts, but I want this to revolve front and center around this amazing resource I found from ALS Labs in the depths of my hard drive… a periodic table of the elements that contains information about the types of digestions required to liberate each element (note that photon assay is missing for Au analysis, but otherwise the list is comprehensive).
No matter what the accountants are telling you about cost savings, it is imperative to hold your ground, after all what is the point of collecting chips, grabs, rocks, and even drilling holes if you are not going to get the information that you require out of it?
In setting up a geochemical sampling campaign for rocks it is important to consider the following:
- Type of sample
- Size and character of sample
- Best indicator elements
- Background ranges of indicator elements associated with different rock types and anomalous threshold levels
- Applicability of mineral separates
- Effects of weathering, rock type, hydrothermal alteration, and other geological variables on background and contrast of anomalies
- Shape, extent and homogeneity of anomalies and reproducibility of values for a single site
- Methods of sample decomposition and analysis
- Reproducibility of sampling
- Data interpretation procedures
All this said, we would never recommend anything less in strength than a 4-acid digest in a lithogeochemical sampling campaign, but depending on your objectives you may just need the whole enchilada – a whole rock geochemical analysis.
Remember too that not only are you going to be selecting the digest (e.g., 4-acid, lithometaborate fusion), but also the analytical finish (e.g., ICP-OES, ICP-MS, XRF…) and even the sample preparation. Your orientation survey will prepare you for the answers that you need to make your best geochemical program.
Now let’s talk about drill programs specifically…
Never use anything less than a 4-acid digest. It is a waste of money and will provide meaningless data. Now you may be thinking well why are we still talking? What more is there to think about…
Let’s talk about some additional (and unfortunately common) mistakes:
1. Switching labs,
2. Not sampling the entirety of the drill core, and
3. The underappreciation of a whole rock analysis.
Part of your orientation study very well may have to include which lab is right for your program (we can address Round Robins in a future post). Once you’ve selected a lab (e.g., the best lab with the most consistent -and hopefully precise- results), stick with it. Do not let the accountants sway you towards choosing another lab a few years down the road because of a discount. Your ability to make interpretations and models will be impacted… perhaps you will even have to resubmit samples for consistency in your resource model.
Just because the project geologist (sorry guys!) cannot bear to submit 500m of overburden because “it is a waste of money”… stick up for yourself. Potentially, this data could help during the exploration phase (there can be some indications in there for approaching the base of overburden or some trace elements interesting to follow up!), but it will be great knowledge to have during mine planning and remediation. At some point, the overburden will get sampled. In the end it will be at a higher cost to the company because when it’s resampled, it will likely be done with new drill holes (le sigh).
In a whole rock analysis you will have the nearest to complete digestion of all elements. This alone is exciting! In doing this you may discover some hidden gems that make your lithogeochemical analysis so good that there is no turning back towards 4-acid. In another example, you may find out some crucial information in the C and S data that precludes going back to the more basic 4-acid package because you’re working in a skarn or it’s essential for geometallurgy and the environmental geochemists. Perhaps there’s nothing in the information at all and its no big deal to switch to a 4-acid dataset. You can always reevaluate after your first drill program and recognize that there’s no need to use the Rolls Royce of geochemistry packages, but at least you made this decision from a point of scientific knowledge than accounting.
Especially in the early days of a project, it is essential to consider what is going to happen to samples throughout the mining value chain. Remember geochemical data is multi-client, it should be available throughout the entire mining chain for a variety of purposes. As soon as we start thinking of it as an asset and not an expense, that’s when the accountants will stop governing geoscience decisions.