Lag Sampling: Another lost Jedi art?

Working with a number of clients over the last couple of months, the topic of whether soil sampling is an appropriate technique has been a reasonably hot topic of discussion. Areas where exotic aeolian sand & loess is a major contributor to the "soil" may not be appropriate for fine fraction (yes, -80#) soil sampling. The suggestion of completing a lag sampling survey instead of soil sampling has often met with blank stares and thus the reason for this post.

So what is lag? Mindat.org gives the following definition: a residual accumulation of rock fragments remaining on a surface after the finer material has been blown away by winds. This, by our definition, would be regarded as residual or deflationary lag.

Why do it? Put simply, to enhance the targeted bedrock signature by removal of dilutant fine fractions.

So how do you sample for lag? Summarising from Mazzucchelli's excellent description in the AusIMM Field Geologists Manual - Fifth Edition

Material sampled: sieved or hand-picked lithic fragments from land surface. The unconsolidated stony material is swept from the surface and sieved to retain the -6 +2 mm fraction. The objective is to enhance the bedrock signature by removal of dilutant fine fractions, particularly in aeolian or highly leached (lateritic) environments. If lag is sparse, hand-picking of coarse fragments may be required.

Quantity of sample: sufficient -6 +2 mm sieved lag can be contained in a 125 × 75 mm kraft paper sample bag (up to 200 g) to provide a representative sample that meets all analytical needs. Because hand-picked lag samples are usually made up of coarser particles, a larger sample (up to 1 kg) may be required to achieve a representative sample. Lag samples require pulverisation prior to analysis.

And how do you interpret the data? McQueen (2004) examined the partitioning of ore elements and pathfinder elements in residual and transported lags related to Cu-Au systems in the Cobar Mineral Field in New South Wales, Australia. He found that goethite is an important host for Zn, Cu, As and to a lesser extent Pb, Bi and Sb in the in situ regolith. Hematite is the predominant host for Cu. In goethite-dominant near-surface caps there is a strong correlation of As and Zn with Fe, whereas in hematite-dominant phases there is a strong correlation of Cu with Fe. These results indicated that pathfinder elements should be normalised against Fe for their correct interpretation.

So, complete your regolith map, walk the ground to do your "orientation" and then decide on your sampling method. A discussion the use of residual/deflationary lags versus mature/transported lags might be useful in a future post.

Time to get sweeping! And please no more photos in ASX / TSX releases of aeolian sand being sampled....

References

McQueen, K, 2004. Element fractionation and mineral hosts in the regolith, in Abstract volume – Minerals Exploration Seminar – 2004. (CRC LEME: Perth).