Like gold, diamonds have been known to exist in Kalimantan since the 18th Century. Diamonds have been obtained by panning in the Lardak and Kapuas rivers (Sanggau Regency) since 1836. The largest diamond ever found weighs some 6 carats. All the Kalimantan diamonds are derived from alluvial deposits, and the ultimate source rocks have never been established.
The diamond-bearing alluvial sediments are clastic rocks of 2 to 12 meter thickness, whose clasts are of quartz (yellow to pink color), hornblende, corundum, schist, slate and igneous rock fragments, in addition to magnetite, mica and gold. The slate and schist clasts are known to be pre-Permian. The other rock clasts are Tertiary in age. It was formerly considered that the Pamali Breccia of Southeast Kalimantan represents the primary source of the diamonds. However, Bergman et al. (1987) have shown that this diamondiferous formation is a sedimentary conglomerate of local Bobaris ophiolite provenance.
In 1984 Anaconda Indonesia Inc., together with PT Aneka Tambang, carried out explorations over 4,882,500 hectares of West Kalimantan, under K.P. Number DU 574 by PT Aneka Tambang. Samples were collected from the Landak and Sekayam rivers and examined in Denver, but none showed a positive indication of primary diamond. Only one sample location contained diamonds.
Diamonds are found only in streams that drain the Plateau Sandstone. There is also a correlation between diamonds in the neighboring stream and corundum-bearing rocks occurring as rounded pebbles in the basal conglomerate of the Plateau Sandstone. Paleo-current analyses were made of the Plateau Sandstone, which generally indicated a provenance from the east, but such studies were unsuccessful in locating the primary source of the diamonds. 8 Ma old minette dykes occur at Linhaisai in the northern Barito Province, but they are not considered to be the source of the alluvial diamonds, and the search for the lamproite or kimberlite sources must continue in Central Kalimantan or adjacent Southeast Asia.
The diamond-bearing alluvial sediments are clastic rocks of 2 to 12 meter thickness, whose clasts are of quartz (yellow to pink color), hornblende, corundum, schist, slate and igneous rock fragments, in addition to magnetite, mica and gold. The slate and schist clasts are known to be pre-Permian. The other rock clasts are Tertiary in age. It was formerly considered that the Pamali Breccia of Southeast Kalimantan represents the primary source of the diamonds. However, Bergman et al. (1987) have shown that this diamondiferous formation is a sedimentary conglomerate of local Bobaris ophiolite provenance.
In 1984 Anaconda Indonesia Inc., together with PT Aneka Tambang, carried out explorations over 4,882,500 hectares of West Kalimantan, under K.P. Number DU 574 by PT Aneka Tambang. Samples were collected from the Landak and Sekayam rivers and examined in Denver, but none showed a positive indication of primary diamond. Only one sample location contained diamonds.
Diamonds are found only in streams that drain the Plateau Sandstone. There is also a correlation between diamonds in the neighboring stream and corundum-bearing rocks occurring as rounded pebbles in the basal conglomerate of the Plateau Sandstone. Paleo-current analyses were made of the Plateau Sandstone, which generally indicated a provenance from the east, but such studies were unsuccessful in locating the primary source of the diamonds. 8 Ma old minette dykes occur at Linhaisai in the northern Barito Province, but they are not considered to be the source of the alluvial diamonds, and the search for the lamproite or kimberlite sources must continue in Central Kalimantan or adjacent Southeast Asia.
It is also possible that at least some of the diamonds were derived from the olivine basalts of Central Kalimantan, and from ophiolites occurring in the Suruk river area in West Kalimantan. The diamonds found in the Tertiary sediments and present day rivers of Kalimantan, Thailand, Burma and Sumatra are characteristically similar. The present geographical locations of the diamond deposits are likely to have resulted from multiple cycles of erosion and sedimentation.
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