Local Topographic Model using Position Index for Analyzing the Characteristics of Unserpentinized Lateritic Zones in Sorowako Nickeliferous Laterite Deposit, Indonesia

An investigation of the relationship between slope position classification and nickeliferous laterite zones over serpentinized ultramafic terrain in Sorowako, South Sulawesi has been conducted using topographic position index from Light Detection and Ranging data for digital elevation model as references with detail resolution of 5m. The index is calculated by comparing the elevation of each cell in the elevation model to the mean of a specified neighborhood around that cell. The classification has six classes, i.e., valleys, lower slopes, gentle slopes, middle slopes, upper slopes and the local ridges. Chemical properties data from 413 drill holes were used for analysis to confirm the laterite zones -limonite and saprolite. By topographic model, slope position class of local ridges, upper slopes and lower slopes with the slope average of 9.82°, 16.05°, and 14.61° respectively, generally indicated the distribution of thick limonite zones, while saprolite zones were in significantly different pattern due to less or no correlation. 2D semivariogram model for spatial thickness distribution also confirmed the corresponding factor between the significant direction of landform reliefs and limonite thickness. Based on the geochemistry profile of limonite zone, the rate of weathering process in laterite formation is longer than the physical process of removal top profile by erosion or accumulation by transported top materials.