REWORKED CLAY AS A GEOCHEMICAL EXPLORATION TOOL FOR MAGMATIC-HYDROTHERMAL ORE DEPOSITS IN TROPICAL REGIONS

The identification of reliable geochemical indicators that reflect surface-near hydrothermal alteration associated with sub-surface metallic ore deposits has been a long standing goal of ore deposit research. The search for hydrothermal alteration haloes is already an important aspect in the exploration for a wide range of magmatic-hydrothermal ore systems (e.g., porphyry Cu and epithermal Au deposits). However, the development of reliable exploration tools for tropical regions has been hampered due to the intense weathering of surface-near soils/rocks and associated element remobilization and/or redistribution.

Here we present preliminary data from a kaolin-smectite-illite deposit at Bayaguana, Monte Plata Province, Dominican Republic. The clay beds overly a back reef lagoonal facies of a Pleistocene coral reef in proximity to pre-existing dacitic and quartz andesitic porphyritic rocks. Drill core data show that the upper part of the clay deposit (a ~4 to 6 m thick continuous bed that extents for at least 5 km), is mostly composed of very plastic kaolin with heterogeneous distributions of gossans, red saprolitic material, and local quartz sand lenses. XRF analyses show that kaolin zone contains 8-23 w.t% Al₂O₃ and 4-11.wt.% Fe₂O₃ with elevated amounts of Cu (6-39 ppm), Zn (6-113 ppm) and Pb (2-15 ppm).

Drill core observations suggest that the clay has been reworked from a primary, advanced argillic alteration zone associated with hydrothermal alteration of the adjacent dacites and quartz andesites. These rocks have previously been shown to be sulfide mineralized (chalcopyrite, sphalerite, galena). The elevated Cu, Zn and Pb concentrations of the clays are interpreted to reflect disintegration of dacite/andesite-hosted sulfides, remobilization of Cu, Zn and Pb, and their subsequent deposition in the argillic zone. Argillic zones are easily eroded, transported and deposited in a tropical weathering environment where they can be reworked into clay deposits. As a consequence, we argue that the elevated metal contents in the reworked clays are indicative of an undiscovered metal deposit in proximity to the Bayaguana deposit. Geochemical analyses of similar clay deposits elsewhere may represent a new exploration tool for metallic deposits in tropical environments.