Summary:
The HMS bearing strata of the Mandiri deposit is ancient Kahayan alluvium, which was deposited during the Holocene age. In general, alluvium has varying thickness of between 2 m and 10 m. The lithology consists of loose quartz, medium grained intercalated grey mudstone containing carbonaceous, shale and bed load stream product; coarse grain sand layer.
Alluvium, Holocene age, pale black to dark brown peat (paludal deposit); loose sands, yellowish color, fine to coarse grained, unbedded (ancient Kahayan alluvium deposit); clay grey to brownish color, very soft, locally containing plant remains (tidal area); kaolinite clay. The thickness of this unit ranges from 50 to 100 m.
Werukin Formation (Tmw), middle Miocene to Pliestocene, this formation comprises brownish black conglomerate, compact, clast consists of quartzite and basalt fragments, diametere 1 – 3 cm, open fabric with matrix of sand. Alternating with yellowist sandstone, medium to coarse grained, locally exhibit crossbedding. intercallated grey mudstone, rather soft, carbonaceous, contain sub-bituminus coal seam partly, appear as interbedded within sandstone bed with the thickness of 20 – 60 cm. The Werukin Formation has 300 m in thickness. Werukin Formation is deposited in a paralic environment. Werukin Formation is the one of main coal bearing Formation in Barito Basin.
Geologically the HMS deposit at Mandiri is a placer deposit formed in a flood plain environment by concentration of heavy minerals, mostly zircon (ZrSiO4), rutile (TiO2), leucoxene (FeTiO3,TiO2) and ilmenite (FeTiO3)). Zircon is the most valuable component followed by rutile, leucoxene and ilmenite in terms of value given to the ore. Gold, platinum and cassiterite have also been identified in the concentrate with gold recovered from the processing plant. The deposit is overlain by the Werukin Formation. The heavy minerals within the source sediments attain an economic concentration by accumulation within low-energy environments within streams and most usually on beaches. In alluvial placer deposits the medium to high energy zones on the stream are the meandering bars and channel zone. In these zones, the HM grains accumulate because they are denser than the quartz grains they occur with and become stranded. It is for this reason that alluvial placer deposits are often referred to as "strand-line deposits". The deposits are found in unconsolidated sand strata.
The mineralisation occurs as a tabular body within alluvium as a layer of between 2 m to around a maximum of 11.5 m.