EDTA (weak acid) digest for "non-sulphide" base metals (Cu, Pb, Zn, Fe), followed by regular 4 acid digest for "total". This was used to model metallurgical zones (not related to supergene) at FS stage. The issue was non-sulphide Pb interfering with Cu flotation. Cost was an issue with this method, so ended up compositing intervals. The results were geologically coherent, but I can’t comment on efficacy of reconciling to processing as the project is still in the ground.

The trick is to make a conscious decision on whether to model the "non-sulphide" as a percentage of "total", or model the actual "non-sulphide" value. The issue with modeling a percentage is that assay precision near the lower detection limits start making the values unstable and nonsense, i.e., pushing data too hard. The other issue was to remember when compositing the percentage "non-sulphide" assay values need to be weighted both by "total" value and length (and potentially also a proxy for density).

Another consideration was aging of sample material. We used refrigeration and nitrogen purging to store the samples.

Typically copper analysis is dependent on whether the requirement is for total copper content or acid soluble copper. Some labs will digest using mixed acids but excluding HF. The final determination will be by short iodide titration or AAS/ICP-OES. Other operations will add HF and then complete with titration or AAS/ICP-OES. I consult to one organization where their copper is determined exclusively by electro gravimetric.

I have been involved in a copper project with oxide-supergene-hypogene zones in the general Mediterranean area. For the characterization of the three zones, we have used the following sequential copper analyses (all solutions measured with AAS):

Oxide copper 5% sulphuric acid
Secondary sulphide copper 10% NaCN
Primary sulphide copper 25% nitric acid

Other people have slightly different recipes, but this worked for us. We also had a total copper value for each assay interval with a three-acid digestion followed by AAS. The total Cu tended to be a little higher than the sum of the sequential assays because the nitric acid digestion did not dissolve all of the chalcopyrite.

This data proved useful, mainly with respect to metallurgical predictions, because:It turned out that the "oxide" mineralization contained a component of primary chalcopyrite (an important consideration when contemplating leaching performance); the supergene zone, while exhibiting a sharp upper contact, showed renewed local oxidation in its upper part, and the sequential copper assays told us how much of the total copper in the supergene zone would not be available for recovery by flotation; and the lower boundary of the supergene zone against the hypogene mineralization was gradual and somewhat diffuse, and the sequential assays allowed the placement of that boundary with greater confidence than the mineralogical observations during core logging - again important, because the flotation response of the two sulphide zones was different (both concentrate grade and recovery).

The metallurgical predictions were thus more detailed and allowed different NSR values per percent copper in the ore for the three ore types to be taken into account for pit optimization and mine planning. There are also obvious environmental ramifications for the deposition of materials below cut-off grade from the three different types of mineralization.

The analyses are used like prediction with the goal measure the potential of copper recovered in the process directly without adjustment or scaling, and then in general it assumes that CuS/CuT ratio will be similar to recovery in plant.
The different analyses always should be related with the mineralisation.

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