Rare Earth Elements Distribution in Weathered Volcanic Rock from Mamuju Area, West Sulawesi

Abstract

The study on the distribution of rare earth elements (REE) was carried out from bedrock and its weathering profile at North Botteng Village, Simboro District and Kelapa Tujuh Village, Mamuju District, Mamuju Regency, West Sulawesi. This study aims to determine the potential and distribution of REE in the bedrock and its weathering profile and to identify the REE-bearing bedrock type. In addition, the relationship between REE and Zr element as one of REE associated elements is also discussed. The analytical method used include petrographic observations to identify the REE-bearing host rock and the ICP-MS (Inductively Coupled Plasma Mass Spectrometry) method to determine the REE content. Samples were taken from drilled hole samples in three different locations and classified into three groups, namely Group 1 (K-01 drill hole sample) and Group 2 (K-62 drill hole sample) from Kelapa Tujuh while Group 3 taken from the P-05 drill hole sample from North Botteng Village. Petrographic observations on the bedrock show that the REE are hosted by phonolitic leucitite composed of mainly leucite as phenocryst set in K-feldspar, plagioclase, pyroxene and opaque groundmass. REE content (expressed as TRE2O3 or total rare earth oxides) show a high concentration values ranging from 2000 - 6400 ppm and display variations enrichment in depth. The highest total REE content in Group 1 found in samples from 9 - 11 meters depth, which is 4600 ppm, while in Group 2 the highest concentration is from 4 - 5 meters depth with a total REE value of 2380 ppm and in Group 3 the highest value of REE content encountered at a depth of 6 - 7 meters, which is 6400 ppm. Zr content in Group 1 samples shows value range between 1780 - 2870 ppm whereas in Group 2 ranges from 1670 - 2380 ppm with the highest Zr concentration at a depth of 4 meters while in Group 3 the Zr content show values ranging from 2100 ppm - 4480 ppm with the highest concentration encountered a depth of 11 meters. The study results suggest that the relationship between REE and Zr element in Group 2 samples showed a positive relationship. Meanwhile, the Group 1 and 3 samples show a varied relationship. The REE concentration is controlled by elevation, in which areas with higher REE concentrations are encountered at elevations above 359 meter above sea level (masl) while low REE content concentrates in areas with elevations below 365 masl. REE enrichment is caused by weathering process that occurs on phonolitic leucite in the study area.