Petrography and geochemistry of sulfurous volcanic scoria from mount Cameroon area, Central Africa: Implications for Au-PGE exploration

Abstract

This study was initiated to discuss the mineralogical and chemical features of volcanic scoria from Mount Cameroon along the Cameroon Volcanic Line (CVL) for gold and platinum-group elements (Au-PGEs) exploration. The mineral composition has been obtained through thin sections observation and XRD instrument. X-ray fluorescence (XRF) and ICP-MS analytical methods have been used to determine major and trace element composition, respectively. NiS fire Assay using ICP-MS was used to determine Au-PGE contents.Sulfurous rocks are characteristic of strombolian dynamism with dark gray color, vacuolar structure and porphyritic microlithic texture. They are mainly made up of pyroxene, olivine, plagioclase, opaque minerals, quartz and hematite/goethite. The main chemical components of the sulfurous scoria are silicon, aluminum, iron and calcium oxides. They show basanitic composition. The major and trace element distribution reveals that the scoria of Mount Cameroon derived from a common origin. Some samples have negative LOI due to iron oxidation during heating to high temperature that produces weight gains. The high contents in Fe2O3, CaO, Cr, V, Zn, Ni, Cu, Co and light REE in addition to low SiO2 concentration confirm the basic nature of the magma. In these rocks, CaO is associated with Zn, Ba, Ga, Zr, Nb and Rb in plagioclase and/or pyroxene. The (La/Yb)N values and LREE/HREE ratios reveal a REE-fractionation. Some element ratios (Zr/Th, Th/U, Nb/Y and Zr/Nb) indicate crustal contamination.Platinum-group elements (PGEs), especially iridium and platinum-group elements (IPGE: Os, Ir, Ru), have low contents. Au-PGE budget was controlled by a single or a combination of petrological processes which affected the rocks in a similar manner at Mount Cameroon. The sulfurous nature that favors Au enrichment is not responsible for the contents in PGEs. The IPGE depletion, relative to palladium and platinum-group elements (PPGE: Pd, Pt, Rh) enrichment as well as negative Ru-anomalies could be attributed to the nature of the mantle. Contrary to REE, PGEs are associated with CaO (positive correlation) but not with K2O and P2O5 (negative correlation).