Geological structure control on hydrothermal alteration distribution in Sekotong district, Lombok, Indonesia

In an economic geology point of view, Indonesia has potential economic deposit prospect areas. In example, Cu-Au porphyry deposit prospect area that found in Sekotong District, West Lombok, Indonesia. This prospect is indicated by the presence of various intensity hydrothermal alteration associated with stockwork veinlet. The aim of this study is to determine structural geology that control formation and distribution of hydrothermal alteration. Methods used are lineament analysis, geological and alteration mapping, petrography and XRD analysis. The stratigraphy unit of the area are divided into andesite lava (1 and 2) unit, dacite lava unit, tuff unit, diatreme breccia unit, and alluvial unit. The structural geology that found are northeast-southwest strike slip fault and northwest-southeast strike slip fault, as the result of Middle-Late Miocene deformation. The northeast-southwest strike slip fault is interpreted as a trigger of tonalite intrusion (found in subsurface data). And this fault is also influenced the formation of wall-rock permeability. This intrusion is the source of hydrothermal fluid that control the formation of wall-rock hydrothermal alteration. And the northwest-southeast normal fault be the path for the hydrothermal fluid. It is proved by the direction of vein and veinlet orientation that dominantly northwest-southeast. The hydrothermal alteration that can be determine in research area are argillic(kao-mont-qz), phyllic(ser-qz), inner propylitic(epi-cal) and outer propylitic(chl-cal) alteration, with various intensity (moderate to totally). This research still requires further analysis such as mineragraphy and fluid inclusion, to be able to lead the exploration specially to determine which is economic area prospect and further to calculate the resource and reserves.