Is it time to move on from minus 80 mesh?

Garrett wrote an excellent article a couple of year ago about the history of -80 mesh sampling (1).?He concludes with the following “Over the last 70 years the use of the minus 80 mesh fraction for soil and stream sediment analysis has proven effective and led to the successful discovery of primarily base metal mineral resources. However, that does not mean the use of the minus 80 mesh should be taken for granted, as directed orientation work may indicate a more suitable size fraction for specific trace elements and mineralogical conditions, in particular precious metals”.

There is little doubt that -80# sampling has had an enormous impact on surface exploration over the last 75 years and led to a number of base metal discoveries globally.?However, as Garrett notes, it may not be the “best” technique for some trace and precious metals.

Effective exploration is to me a cost benefit exercise.?Obtaining a quality sample for the primary element targeted as cheaply and simply as possible is the goal.?Integrating geology and regolith into a sampling plan before the first sample is taken is the cornerstone of this effective exploration and these be explored in another post in the future.?

Back to -80 mesh.

Recently, I’ve had the opportunity to review a number of sampling programs from Australia and overseas and been able to compare -80 mesh data with other mesh fractions and sampling techniques.?The following is presented as a Cautionary Tale (with apologies to Tim Harford) – IMEx’s mantra is to promote efficient & effective exploration to minimise the cost and time of discovery.

The examples below are just a smattering of what could be regarded as “taking the use of -80 mesh for granted” as Garrett warned or perhaps where applying “industry best practice” or “industry standard” sampling equates to sampling without thinking.

The data above are from a sampling program exploring for copper gold where the soils are developed over mafic volcanics and intrusions – i.e. dominantly clay fraction.?Samples have been replicated – i.e. where a -80 mesh (180um) sample was obtained, a -16 mesh (-1mm) sample was also obtained.?Cu, Pb, Zn values between the two size fractions show good agreement.?However, values for Au, Sb & As are lower in the -80 mesh samples by, on average, 45%, 50% and 15% respectively.?But why?

What was attractive for the USGS (and others) in the 1950’s was that with -80 mesh sampling “the metal content of the fine fraction was somewhat, but not greatly, higher than the coarse fraction” and it “..avoids the need of grinding the sample”(1).?Given that the elements targeted in these 1950’s studies were Cu, Co, Pb & Zn, the validity of -80 mesh for these elements is basically confirmed by the results in the example below where the samples were not pulverised.?However, that is not the case for Au, Sb & As and pulverising the -80 mesh sample may have negated some (all?) of the under reporting. ?It would seem the base metal cations don’t have a preference for mesh size but the oxyanion species (As, Sb) are not as well represented in the -80 mesh fraction.

Fortunately, gold was not the prime commodity at this prospect but had it, and the pathfinders As & Sb, been the only elements analysed, the prospect would have been poorly evaluated with -80 mesh sampling.?Sieving the same to a finer mesh size to negate pulverising at the laboratory was in effect a false cost saving measure.

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The second example (above) illustrates gold values in various mesh fractions from a prospect in Western Australia.?The red rectangles highlight where follow up drilling encountered significant mineralisation.?The rock lag samples (+6mm & -6+1mm) appear to offer a better signal to background response compared with the finer fraction samples. The -80 mesh samples (pulverised in this example) provide a very weak response mainly due to dilution by aeolian silt and sand.?

The third example (above) is from a project where a large 80 mesh soil sampling campaign (over 2,000 samples) had been collected.?Interpretation of the data (before the field visit) failed to identify any coherent anomalies – data appeared to be “spotty”.?The course fraction material (“lag’’) is dominantly composed of quartz and maghemite – very likely transported - and the fine fraction is mostly red sand and silt – also, as it turns out, transported.

The photograph below is from a location proximal to that taken above.?The vertical gravel scrape reveals a thin veneer of transported material that mantles shallow basement.?The veneer is thin (<1m) and its geochemistry does not reflect the underlying geology and it covers the entire area of the sampling programme.?The surface sampling program may not have been undertaken if “visual orientation” had been completed.?The 2,000 samples ($80,000 analytical cost) were basically a waste of time and money.

So, is -80 mesh a “bad” sampling technique and is it time to move on??Not at all.?It just may not be the appropriate sampling technique for the targeted element in the terrain you’re exploring.?

The terms “industry best practice” and “industry standard” are often applied to -80 mesh sampling – maybe the exploration industry needs to move on from these sort of comments.

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(1). Garrett, R. 2019, “Why minus 80 mesh?”, Explore, No. 185, December 2019, Association of Applied Geochemists