Miwah Prospect High Sulphidation Au-Cu Mineralisation, Northern Sumatra, Indonesia

The Miwah prospect lies on the dilational 070¦ trending Miwah Lineament near its intersection with northerly oriented fractures within Pliocene volcanic rocks exposed between a NW trending segment of the dextral transcurrent Sumatra Fault, and the more northerly trending Samalanga-Sipopok Fault to the east. These faults reflect the continued interaction between stresses from the India-Australia plate located to the southwest, and spreading within theAndaman Sea to the north of Sumatra. A system of deep seated faults is believed to have facilitated emplacement of post-Miocene magmas and Quaternary volcanic centres at shallow crustal levels throughout northern Sumatra. Miwah mineralisation is associated with variably dipping structurally and lithologically controlled argillic and advanced argillic alteration. An oxidised supergene leached zone and a recent, thin, unaltered tuff blanket partly overlie the mineralisation. Mineralised zones are commonly brecciated and veined, and display quartz, quartz-alunite, quartz-alunite¦pyrophyllite¦kaolinite replacement assemblages, surrounded by more weakly mineralised, often vugghy quartzalunite¦pyrophyllite¦kaolinite advanced argillic, and argillic zones of quartz-kaolinite, quartz¦pyrophyllite¦kaolinite and smectite-illite-quartz. Propylitic assemblages are marginal to more intensely altered zones. Hypogene copper mineralisation is typified by enargite, luzonite, and covellite. The host mineral to gold is uncertain at present. Rare barite, native sulfur, and ?telluride also occur. Bornite and chalcopyrite have been observed in transported altered rocks in faults. Generally the mineralisation is anomalous in precious metals, locally enriched in Cu, erratic in Pb, and depleted in Zn, with a good correlation between Cu-As-Ag.