i think some of the confusion regarding ppm vs % results for Ni and Cu may be due to not understanding calcrete sampling. i am trying to find more information and hopefully others can help. Yilgarn Craton which MEU made a reference to is mentioned in the article below:
According to McGillis (1967), the use of calcrete as a geochemical guide to metal deposits may have begun in Russia during the late 1950s. However, its potential may have been recognised earlier by Cuyler (1930) who suggested Ca-rich (and other metals?) waters, rising under hydrostatic pressure, deposit calcrete in faulted areas. Outside of Russia, the value of calcrete as a specific geochemical sampling medium was not fully recognised until the early 1970s. One of the early records of calcrete as a geochemical sampling medium appears to have been in the Yilgarn Craton, WA (Western Australia) when it was investigated as a means to explore for Ni deposits. Initially, it was considered as a geochemical diluent (Mazzuchelli 1972) and other media such as the residual soils themselves were sampled instead or efforts were made to upgrade the metal content of samples by dissolving the calcrete away and analysing the residue, e.g. Garnett et al. (1982). However, Cox (1975) gave further consideration to its specific use when looking for Ni in the Kambalda area (Yilgarn Craton, WA). Other base metal studies in South Africa and Australia followed during the next decade, some of which are documented in the case studies that follow. Exploration companies, however, never systematically used calcrete as a sample medium of choice during these early years. As with many other geochemical techniques such as the collection of soil, rock chip or stream sediments, calcrete sampling has recently benefited from analytical laboratories providing low-cost, rapid chemical analyses with low detection limits. Mineral deposits have been discovered directly as a result of their association with calcrete for centuries. Early mineral discoveries were often made by untrained people such as shepherds or farmers tending their herds and crops and finding evidence of mineralisation at the surface or in animal burrows and scrapes. Copper was one of the first metals found in this manner. Bright green Cu carbonate minerals, which visually contrast with the paler colours of the calcrete, were often the first indications of Cu mineralisation.
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