Bulletin of the Geological Society of Malaysia, Volume 20, August 1986, pp. 701 - 762
1SEA TRAD Centre
2Geological Survey of Malaysia
Abstract: Results of geochemical drainage surveys for Sn are notoriously erratic and difficult to interpret. The object of this report is to examine some of the causes of these problems and suggest practical remedies.
The study area is a granitic dome, rising to 1242 m, on the eastern side of the Kinta Valley 33 km south-southeast of Ipoh, Malaysia. Drainage sediments and heavy mineral dulang concentrates were collected regionally at an average density of 1/km2 and in much greater detail from a river, the Sungei Petai, draining known Sn mineralization. At each sampling site standard procedures for collecting active sediments were employed except that, wherever possible, separate samples were collected from high and low energy environments characterized by coarse- and medium- grained sands, respectively. After disaggregation and sieving, all samples were analysed for Sn, W, As, Cu, Pb, Zn and Fe. Magnetite content was also determined.
Tin, W and magnetite content of the sediments is strongly influenced by their hydraulic environment whereby significantly greater concentrations (up to twentyfold for Sn) are associated with high energy environments. As a result, enhanced Sn values are widely and erratically distributed on Bujang Melaka and provide an unreliable guide to exploration targets. In contrast, As, Cu, Pb and Zn, which are associated with the primary Sn mineralization, are not present in the sediments as heavy minerals and their concentrations, which are not perturbed by hydraulic conditions, are much less variable. These elements can therefore provide a more reliable guide to the source of Sn than Sn itself.
Detailed studies of the behaviour of Sn indicate that the Sungei Petai acts as a natural palung: light minerals are winnowed away leaving bedload sediments enriched in cassiterite. This process is most efficient for finer grain sizes and is significantly more effective in high, compared to low, energy environments except for the finest material. This difference is responsible for erratic within-site variations. Magnetite behaves in a hydraulically similar way to cassiterite, and the Sn/magnetite ratio can therefore be used to evaluate the significance of Sn concentrations. Thus, a high Sn content but low Sn/magnetite ratio probably reflects a hydraulic (placer) accumulation. Conversely, a high Sn content accompanied by a high Sn/magnetite ratio would indicate that Sn concentrations were more directly related to an anomalous source of Sn.
The concept of hydraulic equivalence of cassiterite and magnetite was extended to cassiterite and light minerals by determining (with an F-test) the size range of light minerals whose hydraulic behaviour is most similar to that of cassiterite grains having a different size. Minus 65+100 mesh was found to give the optimum results for all finer sizes of cassiterite so that plotting values of
Snx x Wx/W-65+100
where Snx is the concentration of Sn in a size fraction, x, weight Wx and W-65+100 is the weight of the minus 65+100-mesh fraction, minimise within-site variability and maximise anomaly contrast. Resulting geochemical patterns are then very similar to those for the pathfinder elements.
It is concluded that erratic distribution of Sn in drainage sediments reflects its variable enhancement in response to hydraulic conditions. Data interpretation can be improved using pathfinder elements, Sn/magnetite ratios or hydraulic equivalence concentrations of Sn. Use of minus 80-mesh material appears satisfactory for routine multi-element surveys, but where elements dispersed as heavy minerals are of especial interest, use of very fine (minus 270 mesh) sediment might be advantageous.