SUPERGEN GOLD-SILVER ENRICHMENT MODEL AT VEIN TYPE EPITHERMAL PONGKOR DEPOSITS, WEST JAYA

• Main Author: GIRMAY NEGA, DAGNEW
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/66691
• Institution: Institut Teknologi Bandung

Pongkor Au-Ag deposit is located in the West part of the Sunda-Banda magmatic arc, Indonesia, and is a calc-alkaline volcanic hosted vein type low sulfidation epithermal deposit. It contains 4 to 24 m thick vein ore bodies include Ciguha, Kubang Ciacau, Ciurug, Pasir Jawa, Gunung Goong, Gudang Handak, and Pamoyanan. The veins fill the rupture on the host rock created by strike-slip faulting due to the subduction of the Indian-Australian plate along the southern margin of Jawa Island, and is controlled by N330/E striking faults. The veins are hosted by pyroclastic rocks mostly lapili tuff, velcanic breccia and by hydrothermally brecciated subvolcanic andesites. Propylitic alteration is dominant and pervasive near each main vein with pyritization on the host rocks in Pongkor and argillic alteration is found as remnant pockets of erosion on the surface. Phyllic alteration has been rarely observed overprinted on propylitic alteration and strong silicification have controlled the ridge morphology following the strike of cach veins. The veins show a typical epithermal crustification texture with banded and colloform composition of guartz, carbonate (calcite, rhodocrosite, kutnahorite), and adulasia. The high grade hipogen gangue minerals show guartz ginguro banding and are rich in sulfides. Gold and silver, observed in hand specimen and polished-sections, are found as electrum-inclusions within pyrite and in guartz. Gold have been leached and reprecipitate filling micro-crackles in guartz forming worm like coloform tekstures: silver, pearcite, and acanthite also found as inclusions within guartz crystals and filling micro-crackles. Silver have been replacing the silver sulfides during the supergen enrichment and form boxworks, accicular filling crackles, dendiritik replacing achantite, and also reprecipitate as pure silver. The Pongkor deposits have undergone a surface weathering since their formation on Late Miocene to Pliocene and formed a paleo supergene Au-Ag enrichment with average grade of 49.43 g/t Au and 487.05 g/t Ag, in some places reach 748 gt Au and 210,000 g/t Ag. The Au-Ag enrichment is controlled by the sulfide (pyrite) rich guartz layers, brecciation due to micro-collapse of the ore, morphology of the host, and the sub-tropical climate of the region, and is restricted to the ore body due to the strong silicification of the wall rock. This process have formed selective supergene gold and silver enrichment paleo. Paleo supergene enrichment are currently exposed near the surface above the current water table, due to erosion and local uplifting, and are enriched by ongoing supergene process which resulted a precipitation of residual gold and silver within goethite and MnO» wad layers. In this high grade zone covelite and chalcosite have been observed replacing chalcopyrite and pyrite, kaolin and montmorillonite was resulte" from weathering of adularia and the host rock, achantite forms boxwork texture filling leached cavity, gold is also observed precipitating continuously to form worm like grain, filling the micro-crackles. The gold and silver were most probably transported by a complex of thiosulphate, Au(S203)” and Ags, and infiltrate down dip, which reprecipitate through reduction of the complexes solutions due to reaction with other hypogene sulphides, freguently pyrite and chalcopyrite, below the paleo water table. The ore bodies are still undergoing supergene surface processe. however, the complexes solutions do not reach near the current water table because of the high concentration of Manganese oxides on the current oxidizing zone. Mobile Metal Ion (MMI) analysis shows high anomaly on the top of the buried Ciurug ore body beneath the surface. Based on the interpretation of response ratio anomali, Ciurug vein may extend to the northern part of vein along strike with similar high grade ore as the current mining area. MMI may be considered as an alternative exploration tool far prediction of ore deposits like Pongkor which are usually buried under tick soil cover due to weathering.