Pétrologie des péridotites de tello (plateau de l’Adamaoua) : prospection géochimique du platine et du palladium

Abstract

Tello is a locality in the Adamaoua region, some 50 km from Ngaoundéré, the regional capital, where peridotite caves outcrop. These peridotites were studied as part of geochemical prospecting (platinum and palladium). The Tello peridotites are greenish, dense, with a vacuolar structure and protogranular texture, composed of olivine orthopyrosene, clinopyrosene and spinel They are characteristic of ultrabasic rocks, with moderate and slightly variable SiO2 and MgO contents, low Fe2O3,Al2O3. The sum of alkalis and alkaline earths (Na O+K O+CaO+MgO) 22 varies slightly between 45%. They also contain very high levels of Cr, Ni, and Co. The elements Sr, Ba , Zr and Cu are moderate. levels of Nb (1.12- 2.5 ppm) and Ga (1.19-2.36 ppm) are very low. Other trace elements are very low, below 2 ppm. As for rare earths, the sum of their contents varies between 4 and 10 ppm, with high levels of Ce and Nd. The other rare earths have very low contents of less than 1 ppm. The sum of LREE contents is higher than that of HREE (0.38-0.83), with LREE /HREE ratios ranging from 6 and 15 ppm, hence an enrichment in LREE compared to HREE. Ce/Ce* and Eu/Eu* values are roughly equal to 1 ppm, and (La/Yb)N ratios range from 6 and 23ppm. Normalization of the rare earths with respect to the primitive mantle shows that in all samples, the rare earths have a similar behavior in Tello peridotites. Platinum Group elements with high contents in the samples are Ru (4-6 ppm); Pt (>3ppm). This incompatibility between gold and PGE would be due to the fact that they would not concentrate in the same mineral phases. The Rhodium, palladium and osmium are very mobility of PGMs in the Tello peridotites. Thus, geochemical prospecting for gold and platinum-group elements shows that grades are of little interest in the Tello peridotites