Bulletin of the Geological Society of Malaysia, Volume 4, June, 1971, pp. 71 – 89
Conzinc Riotinto Malaysia
Abstract: The Bukit Ibam orebody was the largest single body of iron-ore exploited by the Rompin Mining Co. Ltd. between the years 1962 and 1970. The orebody was intercalated in a sequence of volcanic rocks close to a small granodiorite body. On a regional scale the igneous rocks formed part of an assemblage of volcanic and meta-sedimentary rocks of unknown but probably pre-Jurassic age, cut by numerous small intermediate to basic plutonic bodies. A number of other iron ore bodies of smaller size were mined in the surrounding area which formed a minor iron ore province.
The primary iron orebody could be divided into two distinct zones, an upper oxidized and iron-enriched zone which furnished high grade haematite-magnetite ore after simple processing, and an underlying protore containing mainly magnetite in a complex magnesium silicate gangue which required upgrading by magnetic separation to yield a saleable product. Massive limonite ore of slightly lower grade formed a distinct zone along the hangingwall and and contributed largely, together with the haematite, to a mantle of secondary boulder ore on the flanks of the main hill.
Copper, zinc and bismuth were present in minor amounts and constituted the main commercially significant impurities in the ore. Copper (average concentration 0.08%) was markedly concentrated into the limonitic ore along the hangingwall and in a zone of secondary sulphide enrichment near the ore-protore boundary. Zinc (average concentration 0.07%) was markedly concentrated in the hard limonite ores. It was also present in chloritic rocks along the hangingwall and in manganiferous concentrations. Bismuth was concentrated in the more friable zones of the high grade haematite-magnetite ore and averaged about 0.06% overall. The principal non-metallic deleterious impurity was sulphur, which occurred as pyrite in the lower unoxidized protore zone.
The ore zone lies within a sheath of sheared and chloritized rocks between a hangingwall of deeply weathered unmetamorphosed acid volcanics and a footwall of hard siliceous pyritic hornfels. A short distance into the footwall, but never less than 200 feet from the ore, is the granodiorite. Substantial shearing occurs along the footwall of the ore and along parts of the hangingwalI, and at least one strong fault zone cuts obliquely through the ore. Mineralization of a distinct type followed this fault in the hangingwall rocks.
The orebody may have originated contemporaneously with the volcanic series as an exhalative-sedimentary deposit over an intrusive at shallow depth. Subsequent folding and fracturing with further rise of the granodiorite to close below the ore caused considerable thermal metamorphism of the footwall rocks and shearing and recrystallization of the ore zone, with some redistribution of the copper and sulphur. The high grade ore was formed subsequently by supergene processes which upgraded the iron content by removal of magnesium, silicon and sulphur and oxidation of magnetite to haematite and pyrite and chlorite to limonite.