engleski

Geochemical prospecting for gold in the tributaries of the Mentebah and Tenungun Rivers in central Kalimantan, Indonesia

In the western part of the Puttusibau quadrangle, central Kalimantan, there are surface placer workings for Au along the Mentebah river and a world war II Japanese Hg-Au mine is located west of its tributary Tenungun. In the investigated area the headwaters of the Mentebah and Tenungun Rivers drain a fault-bounded corridor of Selangkai Formation rocks. The Upper Cretaceous shallow marine sedimentary rocks of the Selangkai formation consist of shale, mudstone, sandstone, conglomerate, rare limestone and coal. The rocks are carbonaceous and calcareous with numerous Miocene intrusions of granodiorite, diorite, porphyritic dacite and andesite. In view of recent discoveries of large epithermal deposits (Kelian) in the neighboring areas a stream sediment geochemical survey was undertaken in order to locate primary epithermal Au deposits. The gold particles found in the panned heavy-mineral fraction display markedly porous, spongy, and corroded surfaces (SEM micrographs) characteristic of secondary gold from lateritic weathering profiles. Stream sediments were analyzed by flame AAS after aqua regia digestion. Gold was extracted with methyl isobutil ketone and was analysed with a graphite furnace AAS, while Hg was liberated from solid samples in a heating furnace conected to a AAS instrument. The gold contents of the - 63mm fraction of stream sediments ranged from 0.5 ppb to 70 ppb and Hg from 9 ppb to 969 ppb. R-mode factor analysis in combination with Q-mode cluster analysis and discriminant analysis of multi-element stream sediment geochemical data (Au, Hg, Cu, Pb, Fe, Mn, Sb, Zn, Cr, Co, Ni) in 106 samples from tributaries of the Mentebah and Tenungun rivers. Target samples were defined on the basis of their spatial association with areas with known mineralization and two mineralized groups of samples were defined a Au-Hg group and a base metal group (sulfide group). Using Posterior probabilities for each sample the relative probability (in %) of mineralization in the drainage area could be evaluated. The Au mineralized areas are associated Au and Hg with very high relative probabilities and some new target areas were discovered having high posterior probability indicating additional precious metal potential in the study area. Although all elements but Hg show little or no correlation with Au, the spatial distributions of Cu, Zn, and Pb show a considerable degree of overlapping. During the study epithermal, presumably low-sulfidation breccia veins containing realgar, gold, pyrite and minor marcasite were discovered in the vicinity of the village Nanga Duwa.

gold; mercury; geochemisty; deposits

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