ALTERASI DAN MINERALISASI DI SAYAP BARAT PEGUNUNGAN BUKIT BARISAN: KASUS DAERAH KOTA AGUNG DAN SEKITARNYA KABUPATEN TANGGAMUS - PROPINSI LAMPUNG

Abstract

Petrographic and mineragraphic analyses have been carried out over thirty rock samples that were collected from Kota Agung area in western flank of the Pegunungan Bukit Barisan, Sumatera. Most of studied area is covered by Hulusimpang Formation that is well known to be associated with alteration and mineralization. Petrographic analysis of rocks shows various mineralogical compositions from acidic to basic. Alteration process occured in study area are indicated by the existence of chlorite, carbonate, epidote, silica and clay minerals. These altered minerals are associated with metallic minerals such as pyrite, chalcopyrite, sphalerite, magnetite, galena, tetrahydrite and gold. These assemblages are considered being equal to prophylitic and argillic alteration zone with temperatures ranges from about 200°C to 250°C or equal to epithermal system of low sulphidation types. Mineralization within most of study area associated with prophylitic alteration types which were replaced by argillic, and argillic types. These phenomena can be observed by the occurence of overprinted crystals or veins and shearing or opening of quartz veinlets. Moreover, deposition of pyrite, magnetite, and sphalerite show replacement texture; while pyrite and galena show simple locking texture as observed from Way Kerap samples. Both assemblages have different range of crystallization temperatures. These facts indicate that more than twice hydrothermal process have been occured in the study area.