MINERALIZATION STYLE OF THE RANDU KUNING PORPHYRY Cu-Au AND INTERMEDIATE SULPHIDATION EPITHERMAL Au-BASE METALS DEPOSITS AT SELOGIRI AREA, CENTRAL JAVA INDONESIA
Sutarto, Arifudin Idrus, Agung Harjoko, Lucas Donny Setijadji, Franz Michael Meyer and Sven Sindern
1)Universitas Pembangunan Nasional “Veteran” Yogyakarta
2)Universitas Gadjah Mada Yogyakarta
3)RWTH Aachen University Germany
Abstract
The Randu Kuning Porphyry Cu-Au prospect area is situated at Selogiri district, Wonogiri regency, Central Java, Indonesia, approximately 70 km east of Yogyakarta city. The area is a part of the East Java Southern Mountain Zone, mostly occupied by both plutonic and volcanic igneous rocks, volcaniclastic, silisiclastic and carbonate rocks. Magmatism-volcanism products were indicated by the abundant of igneous and volcaniclastic rocks of Mandalika and Semilir Formation and many dioritic intrusive rocks of the Late Eocene-Early Miocene magmatism. Porphyry Cu-Au and intermediate sulphidation epithermal Au-base metals mineralisation at Randu Kuning have strong genetic correlation with the magmatism-volcanism processes. The mineralized dioritic intrusive rocks at the area, are distributed at the centre of the depression of an ancient volcanic crater. There are many intermediate sulphidation epithermal prospect area sourrounding the Randu Kuning porphyry Cu-Au. Most mineralizations both porphyry and epithermal environments are associated with the present of quartz- sulphides veins, but not all porphyry vein types contribute in copper and gold mineralization. The early quartz-magnetite veins (particularly A and M vein types) generally not contain Cu-Au or barren, while the later sulphide bearing veins such as quartz-sulphides (AB type) veins, chalcopyrite-pyrite (C type) veins, quartz-sulphides-carbonate (D type) veins are mineralized. Mineralization contain copper and gold deposits in range at about 0.66-5.7 g/t Au and 0.04-1.24 % Cu. Whereas the epithermal level, mineralization mostly related with the present of pyrite+sphalerite+chalcopyrite+quartz+carbonate veins and hydrothermal breccias. The epithermal veins and breccia lead to the occurences of silver, zinc and lead mineralization, it commonly contain around 0.4-1.53 g/t Au, 0.8-8.5 g/t Ag, 0.17-0.39% Cu, 0.003-0.37% Zn, 00089-0.14% Pb. In porphyry environment, copper and gold are likely transported together as chloride complexes (CuCl0 and AuCl2-) in magnetite stability field. The chlorite complexes then react with existing magnetite to produce free gold and chalcopyrite. On the other hand, the epithermal environment, gold and copper usually are transported in a different ion complexes. Au is transported as the thio complexe [Au(HS)2-; Au2(HS)2-S2- and HAu(HS)2-] in phyrite stability field, while Cu is preferably transported as chloride complex in the hematite stability field.
Keywords: Mineralization style, Porphyry, Epithermal, vein, hydrothermal breccia
Topic: Geology