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-alunitepyrophyllitekaolinite
replacement assemblages, surrounded by more weakly mineralised, often vugghy
quartzalunitepyrophyllitekaolinite advanced argillic, and argillic zones
of quartz-kaolinite, quartzpyrophyllitekaolinite 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.