Bedrock characterization and geochemical evolution of whole-rock weathering profile at GAG Ni laterite deposit, Southwest Papua, Indonesia

Abstract A series of laboratory analyses were conducted to investigate the correlation of petrography analyses of a bedrock coupled with microscope observation and X-Ray Diffraction (XRD) then compared with distribution of geochemical elements of whole-rock weathering profile by X-Ray Fluorescence (XRF). The microscope identification revealed that the bedrock was marked by the appearance of pyroxene predominantly surrounding olivine and serpentine, then magnetite, while XRD clarified that the bedrock was predominantly contained by a number of serpentine (lizardite), then pyroxene (enstatite), and magnetite. The geochemical evolution by bulk major-minor elements geochemistry as well as gain and losses of elements (τ) and compared with the equation from the ultramafic index alteration (UMIA) revealed that Fe 2 O 3 , NiO, Al 2 O 3 , CoO, Cr 2 O 3 and MnO contents in the bedrock were 7.18wt%, 0.23 wt%, 1.23wt%, 0.01wt%, 0.35wt%, and 0.11wt%, respectively, then strongly increased towards the transition zone to 35.89wt%, 2.04wt%, 9.01wt%, 0.10wt%, 1.87wt%, and 0.51wt%, respectively. SiO 2 and MgO revealed an inverted trend where it enriched in the bedrock then extremely decreased limonite layer with SiO 2 (τ = ~ -0.92) and MgO (τ = ~ -0.99). The highest value of UMIA as well as consequently the highest degree of chemical weathering in this study was located in the upper limonite with a value of 63.87.