Mineralogical and Geochemical Control of Altered Andesitic Tuff upon Debris Slide Occurences at Pelangan Area, Southern Mountain of Lombok Island, Indonesia

Abstract

Debris slides were recently found in the Pelangan area at Southern Mountain of Lombok Island, Indonesia. Pelangan is well known as the gold mineralization and hydrothermal alteration area. This study is aimed to identify the mineralogy and geochemistry of altered andesitic tuff that controlled slope instability and induced debris slides. For this purpose, it is necessary to prepare the field observation and laboratory analyses. Landslide inventory reveals that the Pelangan debris slides mostly occur in altered andesitic tuff. Based on the outcrop observations in the field, andesitic tuff found around the Pelangan debris slides have been altered in general. The strong intensity of alteration developed by hydrothermal alteration in this study area produces large amount of clay minerals especially montmorillonite, kaolinite, and illite. The abundance of those clay minerals reflect the intermediate argillic alteration. Montmorillonite is a type of clay mineral that easily swells at wet condition and easily shrinkages at dry condition. Swelling of clay mineral destroys intersheet and interlayer bonds, and reduces shear strength. The presence of clay minerals in the altered andesitic tuff of intermediate argillic zone can be considered as one of the factors that induced to the Pelangan debris slides. Further studies on geotechnical and slope stability analysis of the landslide area are crucial to be done for better understanding of the characteristics of the altered rocks inducing hazardous landslides.

Clay mineralogy of landslide occurrences in hydrothermally altered area: A case study of Durensari Area, Purworejo, Central Java

“…On the other hand, based on previous research, Smectite mineral group as one of the main clay minerals found in this study also known by its swelling or expanding properties. This expanding property later is expected as the minerals that have strong influence to landslide susceptibility in argillic alteration area because it can break rock bonds [12]. Smectite group minerals are found in all locations in the study area.…”

“…In several cases, the presence of hydrothermal activity is often related to the landslide event [12] have resulted in that clay minerals in the hydrothermally altered area could affects the landslide susceptibility. Ludovic et al (2004) [11] conclude that clay minerals influenced the rocks by increasing it pore pressure of water by increased porosity and low permeability.…”

“…Frolova et al (2014) [9] stated that the low-temperature hydrothermal alteration would weaken the rocks as it commonly produced clay minerals that could increase the porosity and moisture, but lower the permeability, density, and strength of the rocks. Winarti et al (2016) [12] conclude that clay minerals could break the interlayer sheet bonding of the rocks, especially for Smectite group mineral for its swelling properties.…”

Soil Micromorphology of Land Cover in Landslide Susceptibility Area in Kelara Subwatershed, Jeneponto, Indonesia

“…The clay fraction resulted from further weathering of pyroxene [(Mg, Fe) 2 Si 2 O 6 ] minerals sourced from andesitic rocks. The increased content of montmorillonite and illite clay minerals from weathering of andesitic tuff rocks resulted in the occurrence of debris slides in the Pelangan Area, Lombok, Indonesia 29 .…”

The mechanism of landslide-induced debris flow in geothermal area, Bukit Barisan mountains of Sumatra, Indonesia

“…The sliding surface's potential in the geothermal area consists of a clay layer dominated by the mineral of halloysite and metahalloysite [16]. Besides, montmorillonite clay minerals in the geothermal area also contribute to the landslide due to their swelling potential [17]. A significant landslide in the altered hydrothermal area is mostly related to argillic alteration when medium to small landslides mainly occur in prophylitic alteration [18].…”