Dear Mr. McCaffrey:

It strongly depends upon the size and zonation and on the state of exposure by mining operation. Small-sized (up to 5 to 10 m in thickness) pegmatites and aplites well exposed at outcrop are not a difficult task for the sampling campaign. You can carry out sampling perpendicular to the zonation making use of grasp sampling or statistically more secure channel sampling given you have the technical facilities at hands in the field. In a large pegmatite body exploited by underground or opencast mining operations I would try and get in contact with the mining company to obtain either DD cores or cuttings. Normally, the core shed is full of material suitable for sampling. Sections should be sampled and the statistically reliable aliquots created by downsizing the samples using a technical device to work in accordance with the requirements set by the sampling statistics. In all these cases bulk or whole-rock samples are fine and XRF, ICPMS, etc. are the measurements of choice.

In areas where the setting depicted above does not exist or you cannot avail yourself of the devices in the field I suggest to shift your attention to mineral chemistry. You try and focus on one of the minerals ubiquitous in the pegmatite or aplite, e.g., feldspar, quartz, or mica and use it through the entire sampling as the reference or basic mineral. The chemical analyses obtained by the above methods you can refer or normalize to the minerals in question, e.g., Li in mica, P in feldspar, Ti in quartz (also useful for temperature determination). You should get now a good overview of the way of fractionation. It is a rough scheme you can tailor-made for your case histories. For further reading I recommend my recent trilogy of studies on pegmatites published in Ore Geology Reviews.

I wish you much success

H.G.Dill

You might want to use chip channel sampling in order to obtain representative samples.

Thank you all for your input. It has been very helpful.

I may just be unfamiliar with this subject, but when determining K/Rb, K/Cs, and other trends from specific minerals, such as K-feldpsar, is it protocol to take samples containing mostly or all K-feldpsar, crush it, and pick it for a clean sample? I'm wondering specific about if the K-feldspars did not compose the entire sample, such as included quartz, but you wanted to measure the chemical trends of the pegmatite using specific minerals like K-feldspar. Additionally, what if it was perthite, would that sample be picked for individual K-feldspar or avoid altogether?

Leonid, this may seem like a simple question, but why would all of the marginal zones be different between pegmatites? If the pegmatites in a specific field were injected from the same late-stage batch of magma/fluid, why would they not have similar marginal compositions? Unless highly influenced by host rock, which I did not think was the case.

Hi Dalton,

I have worked quite a lot with whole-rock and mineral geochemistry of different zones of pegmatites, so I'll jump into this discussion.

About your last question "...when determining K/Rb, K/Cs, and other trends from specific minerals, such as K-feldpsar, is it protocol to take samples containing mostly or all K-feldpsar, crush it, and pick it for a clean sample?", I would say it is common to calculate these ratios from electron microprobe analyses because (1) Rb is high enough so results are very good for K-feldspar and muscovite, (2) you have great spatial control to avoid inclusions, exsolutions and alteration (all very common), and (3) you can pick up different zones within crystals when present. Crushing and dissolving minerals is never ideal when dealing with pegmatites, because you want to have a good textural control of what you are analysing.

I would be very cautious to use whole-rock analyses for pegmatites because they are usually just too heterogeneous, and it is tricky to get to the volume that would make a representative sample as mentioned in previous answers. So getting normal-sized samples with K-feldspar and muscovite from different zones, understanding well their textural relationships with other minerals, and analysing them with electron microprobe (and ideally other trace elements as Cs, Ta and Li with LA-ICP-MS) would give you plenty of data to investigate fractionation trends.

Hope that is helpful!

Thank you for this questions but let me attempt an answer for the B part of your question. I think the petrographic and microprobe analysis could go for the whole rock analysis since petrographic methods go more for the modal classification of the mineral constituents in the rock while mineral extracts from whole rock could be analyzed using the XRF method.