ni:cr ratios

If any geos or anyone else for that matter answer or help with the following. If ni:cr ratios can be proxies of Mgo levels what is the significance of descending ni:cr ratio while Mgo increased.

"A clear correlation was demonstrated between areas of stronger near-surface secondary Ni enrichment and areas in fresh rock with higher Ni levels. This was highlighted in CAMRC-011, which is in the area with the highest geochemical soil value for Ni. CAMRC-011 also showed a descending Ni:Cr ratio at depth while the MgO values increased at depth.
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Thanks