Orientation Sampling – a waste of time & money?

Many years ago, I spent a winter mapping and rock sampling following up 1ppb BLEG (Bulk Leach Extractable Gold) stream sediment gold “anomalies”.?It was a miserable winter for our team – snow, ice, fallen trees blocking roads, obligatory sub zero temperatures - and most of the “anomalous” sites turned out to be either due to very small quartz veins or alluvial gold from under recent basalt flows.?

After that unsuccessful campaign, we took our group geochemist in the field to a new area that we were planning to sample.?We discussed the lack of success over the previous winter and our concerns about this new area – heaps of alluvial gold, perched terraces associated with old river channels half way up hills, old workings disturbing the ground, overlying young basalt and the ubiquitous small quartz veins with minimal but persistent course grained gold.?We didn’t want a repeat of the previous winter and thus a technique different to BLEG was what we thought was required.?Although it felt like to me it was a day of world championship whingeing, I now realise that what we had done was in fact start a process of “orientation without sampling”.

On a recent post, I mentioned a company that spent 50% of their budget on collecting various different mesh sizes in order to determine the optimal sample fraction.?This concept of “orientation sampling” by collecting different mesh sizes might seem like good science but over the last couple of years I’ve become more convinced it’s not good exploration.

Firstly, I have to admit I’m a proponent of “Scumbag Exploration”.?By that I mean I believe that exploration programs must be executed as efficiently and cheaply as possible – and that includes time! ?Determining where “steps” can be omitted or supplanted in the exploration process by more cost-effective solutions will, I believe, enable reduction of discovery costs in an industry where we lament - but don’t seem to doing anything about – escalating exploration expenditure.

But first, let’s look at some examples of orientation sampling:

Example 1

“A soil size fraction orientation survey was carried out with three lines of soil samples over the XYZ prospect previously identified as being anomalous for gold in both soil and outcrop. This involved the collection of 49 samples.?The size fractions included the whole unsieved sample, and three sieved fractions corresponding to >20 mesh (841um), <20>80 mesh and <80 mesh (180um).?All were submitted for analysis for a total of 196 samples”.

OK so let’s break this down.?First, gold appears to be the prime target and one good thing is there is mineralisation present where the orientation is being conducted – that is not a given!?But what’s missing for me is there is no discussion about the “landform” – Is it a hill? What are the soils? Transported or in-situ? Let’s continue…

“The assay results from the different fractions and the whole sample were compared visually and then plotted and contoured. The whole sample gave the strongest and the most consistent results that could be logically contoured. This included two moderately to strongly anomalous areas that were either not picked up as anomalous or were only weakly anomalous in the other size fractions. There is one gold anomaly in the <80 mesh fraction that did not show up in the whole sample analysis but coincident As and base metal anomalism were present in the whole sample at that particular location. This indicates that if gold is occasionally “missed” in the whole sample analysis, the pathfinder elements should still highlight the gold anomaly. The final decision was therefore to use the whole unsieved soil sample for the remainder of the soil survey.?A further 750 samples have been taken and submitted for analysis”.

The decision on what was a good fraction seems to have been based primarily on whether gold was detected but it’s great to see that pathfinders have also been integrated into the decision process.

But, of a total of ~950 samples collected, ~200 samples were “orientation”. So, 21% of total samples were not effectively exploring the ground (i.e. finding new mineralisation). ?I’ve walked the ground (with a spade, a couple of sieves & a hand auger) at this location – call it “half day regolith mapping” if you like -before I was given the above information and concluded from observations of the landform that bulk soils would be an effective technique.

Costs: ? day of geo time versus 200 samples (5 days labour plus 4 weeks at the lab).?Even if the dollar costs were the same, the ? day geo saved a month of inactivity.

Example 2

The plot below 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 provide a relatively very weak response.

The photo below illustrates the terrain where these samples were collected.?Heaps of outcrop/subcrop shedding rock lag into coarse fraction samples while the fine fraction is mainly aeolian silt and sand with negligible insitu soil present.

Again a couple of hours with a spade, a couple of sieves & a hand auger reveals plenty.

Example 3

“Peak to background ratios or Response Ratios were calculated by determining the mean background value from the lowest quartile assay values for each element and then dividing each assay value by the resultant mean background value.

“Technique A produced the best Au Response Ratios for the samples collected. . Techniques B and C also defined the same mineralised zone but recorded lower ratios. The different analytical methods also recorded similar responses for Cu across the mineralised zone. The results of the orientation work indicate the BCL (Bulk Cyanide Leach) analyses were unsuccessful because it did not satisfactorily dissolve Au or there was some other unrecognised laboratory of sampling methodology problem.

“The orientation soil programs confirmed that of the four methods trialled Technique A is the most appropriate for detecting low concentrations of Au and Cu within the overlying regolith.

A discussion about the validity of response ratios is not the purpose of this post so let’s set this aside.?What is of note to me is that three of the four techniques detected the mineralisation.?Sure, Technique A had a better Au ratio but the analytical cost was substantially more than Technique C. ?The Scumbag Explorationist votes Technique C!

?So what do I suggest you do??

·???????First, prepare a regolith map (another post to be written next month).

·???????Second, field check it – walk the ground with a spade, a couple of sieves & a hand auger; look at the landscape – where are the slope breaks? What do they mean? Think about how representative different sampling mediums might be.

·???????Third, select an appropriate sample method and go for it

Last week I was in western NSW.?The rock lag in the photo is a mixture of ferruginous sediment, quartz and ferricrete (top right).?The fine grained material (0.1-0.5mm) is dominantly aeolian sand and silt with minor skeletal soil.?Conclusion: collecting a surface sample of rock lag (without the ferricrete) here would be more useful that sampling the fine fraction.

?In conclusion, although I prefer landscape mapping / orientation without sampling, I can understand there are situations where physical orientation sampling will be of use.

However, all I request is that before you embark on an orientation sampling program ask these questions:

·???????Do I really need to do this?

·???????Do I really need to take four/five different size fractions?

·???????Is there any value in orientation samples from an unmineralised terrain?

·???????Am I collecting enough samples such that the results are statistically meaningful?

·???????Is this technique/analysis method appropriate for the elements I am targeting?

And one other thing, make sure you take someone else into the field to show them how to do it!