The Distribution of Major Elements By Geochemical Perspective of Whole-Rock Weathering Profile at Ulie Nickel Laterite Deposit in Central Halmahera Island
Authors: Vidya Sari Dwi , Nurdiyansyah Aftri 
Published: December 2024

As one of the world’s largest nickel (Ni) producers, Indonesia should become one of the centers determining the direction of global nickel policy and is expected to be an important source of Ni in the future. In general, Ni laterite deposits evolved with three layers from the bottommost bedrock, saprolite layers, and uppermost limonite layers, as a consequence of intense weathering processes. A common trend of Ni enrichments concentrated in the saprolite layers must be understandable for efficient exploration. This study is to investigate the geochemical characteristics of weathering profiles at Ni laterite deposits from Central Halmahera island, and to clarify the Ni enrichment processes by geochemical perspective through qualitative analyses. The complexity of Ni raw ores may challenging for ore processing in the metallurgical processes, as the reason to identify comprehensive through the different series of analytical approaches methods.

The bedrock type through mineralogical analyses by XRD 2Ɵ Cu-Kα revealed that bedrock was composed of serpentine predominantly, followed by pyroxene, magnetite, and small numbers of goethite. The dominance presence of serpentine was notified with the sharp peaks (e.g., 7.4 Å) and high intensity may indicate that serpentine is the major mineral in the bedrock with high crystallinity. Furthermore, the presence of goethite may assume that bedrock already weathered in advance. Similarly, microscope observation portrayed that the bedrock predominantly evolved from serpentine, pyroxene, and magnetite, where pyroxene appeared as a single large crystal and magnetite identified with dark freckles and usually surrounding serpentine.

The whole-rock geochemical characteristic through XRF leads to analyze the major elements (e.g., SiO2, MgO, Al2O3, Fe2O3, and NiO) and minor elements (TiO2, MnO, CoO, Cr2O3, Sc2O3, and V2O3) in which examined from the bedrock, rocky saprolite, earthy saprolite, transition, lower limonite, and upper limonite layers. The whole rock trends show that there are inverted trends where SiO2 and MgO typically show downward trends and depleted from the bedrock towards the upper limonite layers, compared to Fe2O3 which is enriched towards the top of profile, while Al2O3 which less characterized since in the bedrock but significantly increased particularly in the transition layer. NiO shows fluctuating trends throughout the profile where started less enriched in the bedrock but dramatically enriched further up the rocky saprolite and earthy saprolite layers with NiO 2.4 wt% and reach the peaks of Ni concentration between earthy saprolite and transition layers with NiO 3.5 wt%. Consistently, the gain and losses (τ value) from the bedrock to further up the limonite layers indicate where Fe2O3 and NiO show the positive gain correlation with (τ = 0 - 9) and (τ = 0 - 12), respectively.

The major elements from the XRF result, furthermore, be able to indicate the degree of chemical weathering in which visualized by the equation of the ultramafic index alteration (UMIA). The UMIA value of the bedrock was low (3,3) and extremely increased from the saprolite layers (7.0-18.5) to limonite layers (44.4 – 83.7), assumed that the highest degree of chemical weathering was located in the limonite layers and indicate strongly altered.

The geochemical trends ultimately can provide the information of the center Ni concentration processes and the highest degree of chemical weathering which is located in the saprolite layers and limonite layers, respectively, as a consequence of the mobilization major and minor elements. Keywords: Nickel; geochemical; mining; earth resources; weathering