Geochemistry of peridotite and granite xenoliths during the early stage of weathering in the Nyos volcanic region (NW Cameroon): Implications for PGE exploration

Peridotite and granite xenoliths, in the early stage of weathering, occur in the Nyos volcanic region (NW Cameroon). Geochemical data shows that peridotites are marked by high concentrations of MgO (42.30 wt.%, with SiO2/MgO ∼ 1), chromium (2100 ppm), nickel (2100 ppm) and cobalt (104 ppm), as well as by low lanthanide contents (ΣREE: 7.41 ppm). Granites display SiO2 contents (70–73 wt.%), and are mostly peraluminous (1.40 > A/CNK < 1.6). They are also characterized by low contents in chromium (<24 ppm), nickel (ranging from 6 to 15 ppm) and cobalt (ranging from 3 to 6 ppm). Granites possess high lanthanide contents (ΣREE varying between 248.00 and 463.00 ppm), particularly in light lanthanides (LREE/HREE ratios ranging from 21 to 32). The chondrite-normalized patterns of the studied xenoliths are characterized by: (i) LREE enrichments in both rock types; (ii) negative Eu anomalies ([Eu/Eu*] ranging from 0.45 to 0.64) and weak positive Ce anomalies ([Ce/Ce*] ranging from 1.06 to 1.46) in granites. The weathering process provokes a remobilization of several trace elements notably light lanthanides.The geochemical survey of Platinum-Group Elements (PGE) done in these rocks in the early stage of weathering shows that PGE contents are less than 7 ppb in the peridotites. The highly concentrated elements are ruthenium (6.26 ppb) and platinum (5.53 ppb). The total PGE content is 14.57 ppb. These concentrations normalized with respect to chondrites display a flat spectrum from iridium to platinum. PGE contents in the granites are below detection limit except for two samples (LNY01 and LNY02) whose platinum content is close to 0.23 ppb.