PLATINUM-GROUP ELEMENT CONCENTRATIONS IN TROPICAL LAKE SEDIMENTS IN THE EXPLORATION FOR NICKEL LATERITE DEPOSITS

Developing geochemical indicators to guide the search for ore deposits is a long-standing goal. This particularly applies to tropical regions that often are heavily vegetated and weathered, which hampers boots-on-the ground exploration efforts. Here we present the preliminary results of a study that investigates metal contents of sediments from Lake Izabal, Guatemala. The lake’s catchment area contains several Ni-laterite deposits along its northern shore, whereas no Ni-laterite discoveries have been made in the south. Although Ni theoretically is the preferred element to search for Ni-laterite deposits, Ni is enriched in many rocks unrelated to Ni mineralization and easily mobilized. Therefore Ni signatures may represent a wide range of geological and anthropogenic processes and may not necessarily reflect the presence of Ni laterite deposits. Here we explore the usefulness of platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) chemistry in lake sediments as a corroborative proxy for the presence or absence of Ni-laterite deposits in the catchment area of tropical lakes. This approach is being tested because recent studies have shown that PGE can be significantly enriched in Ni-laterite deposits. The final goal is to test the hypothesis that local metal enrichment in lake sediments can be used to identify areas in the catchment that are most likely to host undiscovered ore systems.

We collected and analyzed 20 sediment cores that were taken in Lake Izabal along the northern and southern shorelines. The samples were collected from different depths to allow for a comparison between sediment/metal input controlled by anthropogenic processes and natural erosion. The data show that the samples along the northern shore contain substantial enrichment of PGE when compared to samples from the southern shore. Most notably, samples from the northern shore contain 4 to 14 ppb Pt and 3 to 12 ppb Pd, whereas the southern shore samples contain < 3 ppb Pt and < 3 ppb Pd. There is no correlation in the data that implies that this is related to anthropogenic processes, but rather is interpreted to reflect erosion of PGE-enriched rocks. Because only the northern shore contains Ni-laterites, our data imply that PGE patterns in tropical lake sediments can be used to pinpoint areas that are most likely to contain undiscovered Ni laterite ore.