Andreas Alfario, BSc

The Gunung Akmil high sulphidation epithermal (HSE) prospect, located within the Menoreh mountain range of southern Magelang, Central Java, presents a compelling geological setting for the potential occurrence of a concealed porphyry system. Numerous previous research studies have consistently pointed towards this possibility. This study is aimed to identify geological features and some diagnostic characteristics of HSE mineralization, to be utilized for vectoring to the possible presence of concealed porphyry system. The lithologies consist of andesite porphyry and diorite porphyry as host rock of the mineralization. The ore mineralization system is mainly controlled by ENE and NNW trending structures. To further achieve the objectives, we conducted systematic mapping and sampling from central to peripheral of the prospect. Petrography and X-ray Diffraction (XRD) to determine the type of mineral alteration and geochemical analysis using Fire Assay-Atomic Absorption Spectrometry (FA-AAS) to determine the Au content were employed. Four alteration zones were identified extending from periphery to center of the prospect including chlorite-epidote±pyrite (propyllitic), kaolinite±dickite (argillic), quartz-pyrophyllite-white mica-dickite (advanced argillic) and quartz-goethite (silicified). There is a systematic increase in silicification degree and stockwork density toward the central of the prospect. The result from five selected samples for petrography and XRD reveals the presence of several diagnostic minerals indicating acid fluid alteration such as pyrophyllite, dickite and white mica. The occurrence of alunite is relatively rare, which implies that the deposit has been deeply eroded. Previous geomagnetic survey conducted in the study area indicates a high magnetic susceptibility, which may imply the presence of secondary magnetite-enriched altered rock underneath, possibly associated with a concealed porphyry system. The result from FA-AAS analysis exhibits a systematic change of gold grade from distal (peripheral) (0.03 g/t Au), proximal (0.19 g/t Au), toward the center of the prospect (0.44 g/t Au). In summary, the systematic increase of the silicification degree and vein/veinlet stockwork density, as well as a systematic change in the alteration intensity from propylitic, argillic, advanced argillic to the silicification zones from the periphery toward the center of the prospect obviously suggests a vectoring to the ore i.e. central HSE and possible concealed porphyry system. This is supported by ground magnetic anomaly and systematic increase of gold grade toward the prospect center. The presence of pyrophyllite, dickite and white mica in the central part of the prospect indicates a transitional zone of the HSE prospect to the porphyry system.