OROGENIC GOLD MINERALIZATION ON LONING RIVER AREA, LUK ULO COMPLEX KARANGSAMBUNG, CENTRAL JAVA, INDONESIA

Luk Ulo Complex, Karangsambung, Central Java was one of an accreted terrain in Indonesia formed during Cretaceous orogeny tectonic event leads to expose a various type of metamorphic rock across Luk Ulo Complex region. Together with event was the enrichment of gold related to mountainbuilding process profoundly refers to orogenic gold clan. Previous study was carried out by several researcher of placer gold occurrence in Luk Ulo region found trace that the origin was associated with orogenic gold type. Meanwhile, the origin of placer gold in Loning River, Luk Ulo Complex remains obscured. In order to answer this deficiency, a research was done by utilized several analytical method such as petrography, ore microscopy, and Atomic Absorption Spectrometry (AAS). From the result, the distribution of metamorphic rock in Loning River area was composed of garnet-muscovite-schist (green schist facies), amphibolite, and also zeolite-metabreccia. Together with the exposed of garnetmuscovite-schist bedrock, was network of localized massive quartz-carbonate vein intrusion. There are two type of hydrothermal vein recognized in the Loning River area which namely type 1 and type 2 veins. Both characterized by the cross-cutting relationship and alteration pattern. Type 1 which believed the early generations of quartz veins were strongly controlled by the foliation and deformation structure and normally to be found as segmented, sigmoidal discontinuous and parallel to the foliation of the host rock. The alteration of type 1 alteration was mainly consisting of quartz + carbonate + albite ± epidote. Whilst, type 2 quartz vein having cross cutting relationship with the metamorphic host rock. Type 2 alteration of was consisting of quartz + carbonate + albite + chlorite + pyrite ± graphite. Ore mineral found contain abundant of pyrite with minor amount of chalcopyrite, native gold, sphalerite and galena. The systematic geochemical sampling conducted using rock and heavy mineral-panned concentrate of geochemical samples. From the atomic absorption spectrometry (AAS) result, geological anomalies pointing toward the occurrence of gold mineralization in Loning River area was overwhelming. Based on the analysis, it is known that the concentration of element in each samples ranging between 2-10 ppm of Au, 2-20 ppm of Ag, 8-105 ppm Cu, 12-1000 ppm Zn, and 0.9-130 ppm Pb . Keyword(s): Loning River, orogenic gold, green-schist facies, quartz-carbonate vein