Wonogiri Project Geology
The island of Java is located in the central part of Indonesia's tectonically active, east-west trending Sunda-Banda magmatic arc that marks the convergence of three major tectonic plates. Strong conjugate systems of northwest (NW) and northeast (NE) faults are the dominant structural feature at both local and regional scales.
The Wonogiri Regency, Central Java, is one of several gold prospecting areas in the southern mountain ranges of Java, Indonesia. Three types of dioritic–andesitic intrusive rocks occur in the Wonogiri area, namely, hornblende andesite porphyry, hornblende diorite porphyry and hornblende diorite, exposed in semi-circular depressions where volcanic breccias and tuffs are widely distributed. The occurrence of stockwork quartz veinlets, associated with magnetite and malachite coating along the cracks in the diorite porphyry, suggests porphyry type mineralization. This is also supported by the occurrence of polyphase hypersaline fluid inclusions in the stockwork veinlet quartz. Small-scale, artisan miners are mining quartz veins for gold associated with base metal sulfides. These veins are probably epithermal-type mineralization that overprinted porphyry-type mineralization. The Neogene intermediate to silicic hydrous magmatism in Java could have formed the porphyry-type mineralization in Wonogiri, as in the rest of the Sunda–Banda arc.
The area forms part of the Old Andesite Formation, which is host to a number of porphyry Cu-Au occurrences along the southern coast of Java, including Gunung Tumpangpitu (Banyuwangi), Ciemas (Jampang), Bayah, etc. The structural setting and erosional level of diorite intrusives are similar to that of Newmont’s Batu Hijau deposit.
Preliminary field observations indicate the area is underlain by a sequence of massive to columnar jointed andesite and dacite lava, volcanic breccia, ash to lapilli tuff, siltstone, and sandstone units, later intruded by phyric andesite, dacite and diorite stockworks/dykes. Quartz base metal sulfide veins (0.3 to 2.0 m width) mined by local artisan miners are noted as fissure filling along NE and NW trending structures. Veins are typically open-spaced with crustiform, colloform to massive textures; with 1% to 3% disseminated sulfide (pyrite-chalcopyrite-sphalerite-galena-chalcocite-covellite) hosted mainly by lava and volcanic breccia lithologies.
Typically in artisan workings “ore” material is typified by intense silica-chlorite-sericite alteration with centimetre-wide thick quartz veining/stockworks associated with galena/sphalerite/chalcopyrite fracture-filling and pyrite disseminations.
