So…do you drill it?

It’s not a trivial question.?“Anomalous” results are reported all the time but it seems too many of them don’t have an orebody associated with them. Maybe we need to ask - Is “anomalous” actually “significant”?

As a number of people noted in response to the previous post, there was not much information supplied with the intersection.?Was this really a surprise??Doesn't exploration thrive on using limited data to make major predictions? Realistically, this doesn’t seem to matter to a number of explorers– a good copper hit is surely worth follow up…or is it??How much do we really know unless we drill at depth?

The diagram below shows the copper values from the drillhole plotted against iron.?Fortunately, iron was one of the few other elements analysed!?The strong correlation of Cu with Fe should raise alarm bells.?It is not unusual for Copper to be strongly mobilised in the weathering environment.?A linear relationship such as this in the weathering environment would indicate possible depletion & enrichment due to processes such as hydromorphic dispersion.

So a request was made to the client for a bit more information.?Could we have a look at the drill chips or, at the very least, are there high quality photos of the chip trays available? Note: the hole was drilled RC percussion.?Access to the chip trays was provided and the following log was made.

The drillhole intersected 6m of transported cover before passing into a thick (34m) bleached upper saprolite.?A distinct red-pink interval at 18-20m was identified as a potential palaeowater table and noted as “Fe stillstand”.?A zone of brown to red-brown saprolite / saprock was present between 40-50m before the hole passed into primary chlorite altered mafic volcanic / volcaniclastic between 50-60m.?The “anomalous” Cu intersected correlated with the Lower Saprolite zone.

Applying the logging to the Cu/Fe XY scatter plot was illustrative (see below).?The transported material (light blue) has low and somewhat erratic values of Cu & Fe which would be expected given its non homogeneous composition.?Upper saprolite (brown) shows a linear decrease in values from the bedrock (green) values and the lower saprolite (purple) shows a linear increase from the bedrock values.?The north-east linear nature of the data is what would be expected from copper being scavenged by iron (Lower Saprolite) and depletion of copper in the Upper Saprolite.?A rough calculation of the amount of copper removed from the Upper Saprolite compared with bedrock values pretty much equals the amount of enrichment in the Lower Saprolite in this case.?Thus it can be concluded that the elevated copper in the Lower Saprolite is due to removal of copper from the Upper Saprolite and deposition of this material in the Lower Saprolite.

If this example represented a mineralised signature, there would be a “northwest” trend on the XY plot in part of the data breaking away from the northeast trend.?In other words, there would be copper values too high for an expected value of iron.

So…would you drill it??

The client did and didn’t intersect any mineralisation.?In fact, they “depth tested” five similar “anomalies” with the same disappointing result.

Part of the reason they tested these “anomalies” was that they used maps (univariant plan view) to identify targets.?Max in hole, bottom of hole, interface, base of transported, top of bedrock values were diligently plotted– one plan for copper and one plan for gold.? Unfortunately, maps of copper values alone do not necessarily account for or reflect hydromorphic dispersion or scavenging / depletion of copper.

The data below comes from an excellent book focused on how to explore for ore bodies authored by Reedman and published in the late 1970’s.?It would be great to see these copper soil values plotted against iron but it’s unlikely iron was analysed.?So which one would you have drilled??The one that had big copper numbers or the one that delineated a copper resource?