Gold mineralization in the Ajibarang and Majenang blocks, Central Java, Indonesia: Evidence of mineral fertility of the Banyumas basin

Banyumas basin is not only of interest as a hydrocarbon exploration target but also because of its potential to host epithermal gold and porphyry-type deposits. To date, very few studies have been done on the significance of basin tectonics related to mineralization, but such studies are critically needed. This paper is aimed at reviewing and describing characteristics of gold and base-metal mineralization that were recently discovered in the Banyumas basin, focusing on the Ajibarang and Majenang blocks, followed by a discussion of the mineral fertility of the basin. Various primary data collected by the authors during fieldwork with several mineral exploration companies together with data from current research results are reviewed, interpreted, and discussed. Two different styles of gold mineralization are recognized, that is, carbonate-base metal-gold/intermediate-sulphidation epithermal mineralization in the Ajibarang Block and porphyry-style mineralization in the Majenang Block, which occur in close proximity to each other (40-50 km distance) in different geological settings. The Ajibarang mineralization is hosted by a turbidite sedimentary sequence forming part of the Halang Formation, which is phreatically brecciated (diatreme), whereas the Majenang mineralization was formed in the Kumbang Formation in a volcanic and intrusive (diorite) environment. Diatreme breccias associated with gold-base metal mineralization in the Ajibarang Block may be related to remnant hydrothermal activity, recognized in Landsat TM imagery as circular structures. However, it remains unclear whether any volcanic/intrusive activity was directly involved in the formation of the mineralization. In contrast, the Majenang porphyry mineralization is clearly related to magmatism as it is hosted by volcanics of the Kumbang Formation and a diorite as the causative intrusion. The recent discovery of mineralization in both locations suggests the mineralization fertility of the Banyumas basin may be significant, making the region a valuable target for future exploration.