Did you test for anomalous Ni? This would perhaps be more easy to analyse
in the laterite than Zn in the very refractory chromite. Kimberlitic
zircons and baddeleyites should also be found as resistates in tropical
terrains.
High Zn-magnetite (ZnO up to 33 wt.%) has been found in connection with EPR
hydrothermal vents (Jedwab, J. et Boulègue, J.: Les spinelles de fer et de
fer-zinc trouvés dans des trappes à sédiments amarrées près d'évents
hydrothermaux océaniques. Bull.de Minér.,1986, 109/6, 635-642.)
The significance of Zn is not always fully understood, and high
concentrations may be found in tourmalines from granitic pegmatites, and in
metamorphic terrains (gahnite in the Belgian Ardennes).
J.J.
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