Abstract
Hydrothermal gold mineralization in the northern part of the Menorehmountain has been studied. It covers Kalisat and Gupit prospect areas and their vicinity, Salaman sub-district, Magelang regency, Central Java. This study is aimed to understand the geological framework and characteristics of the gold mineralization. In Kalisat area, we identified the occurrence of low sulfidation (LS)-epithermal gold deposit. Lithologically the study area is occupied by andesitic lava and breccias of Kebobutak formation and crystalline limestone of Jonggrangan formation. NE-SW-trending banded-comb, cockade texture quartz veins and veinlets are recognized in the field. Vein & veinlet orientation is strongly controlled by N-S-trending dilational faults. Andesitic wallrocks are suffered by hydrothermal activities, thus producing three alteration zones including phyllic, argillic and propyllitic. Quartz veins contain hypogene native gold/electrum, argentite, galena, pyrite, sphalerite, arsenopyrite and chalcopyrite as well as supergene malachite and hematite. Ore chemistry indicates very variable gold content. Two samples analysed shows the highest grade of 0.61 g/t Au and 46 g/t Ag. It formed at temperature of about 290ºC and salinity of 5 wt.%NaCl eq. In Gupit prospect area, high sulfidation (HS) epithermal gold mineralization is well confirmed. It occurred in form of NE-SW trending quartz vein/veinlets and silicified host rocks. This is consistent with the mineralization orientation which may strongly be controlled by relatively NE-SW strike slip post mineralizing fault in the area. Hydrothermal alteration recognized consists of silicification (silicified host rocks), advanced argillic, argillic and propyllitic types. The typical alteration type, advanced argillic, is characterized by the presence of alunite, jerosite, native sulfur, pyrophyllite, dickite and halloysite. Vuggy silica texture is the most abundance and typical ore texture identified. Ore mineralization is typically represented by enargite, chalcopyrite, native gold, pyrite, magnetite and hematite. Ore chemistry data of 28 rock, quartz vein, silicified rock and soil samples show a very erratic grade of gold and silver ranging from 0.006 to 42,4 g/t Au and <1 to 112 g/t Ag as well as up to 304 ppb Au and 980 ppb Ag in three stream sediment samples. Those findings are new discovery LS- and HS- epithermal deposit styles in Menoreh mountain. This could open up new challenges in gold exploration in the mineralizing district.