Major and trace elements in river-borne materials from the Cuyuni basin (southern Venezuela): evidence for organo-colloidal control on the dissolved load and element redistribution between the suspended and dissolved load

摘要

The Cuyuni basin is a metallogenic-rich area located in southern Venezuela in the Precambrian Guyana shield. It comprises two geologically distinct zones, one with granite domes and intercalated greenstone belts in the northern and western part of the basin and the other with stratified arenaceous deposits in the south. The suspended, dissolved and total load of the rivers draining these contrasting zones show similar characteristics to those observed in other major tropical rivers such as the Congo and Amazon, and vary in composition between the different source areas.Although the river-borne material in this basin varies in composition from one river to another and contains highly variable proportions of dissolved constituents, the ratio between the suspended and the dissolved load (M-susp/M-diss) appears related to the ratio of suspended to dissolved organic carbon contents (SS/DOC), according to the relationship:M-susp/M-diss = K-i*SS/DOC.The coefficient of proportionality K-i in this equation varies from one element to another. This relationship leads us to recognize (a) the important role of organo-colloidal phases in controlling the elements in solution (b) the occurrence of a redistribution of elements between the suspended and dissolved load in the river environment.According to this analysis, coefficients K-i appear to represent partition coefficients between two phases, the suspended load and the organic-rich colloidal fraction of the dissolved load. These coefficients, estimated from analysis of the total/dissolved load versus SS/DOC diagrams, yield values which classify in an order similar to that classically proposed for the relative capacity of the elements to form organic ligands or their adsorption capacity. The coefficients actually vary as a function of the hydrolysis constants of the respective elements. Theoretical considerations show that equilibrium processes between the surfaces of suspended material and those of colloidal-type species in solution are able to explain the observed relationship. We show that such surface equilibria can be extended to the whole products in conditions which may prevail in tropical river waters, that is suspended matter primarily consisting of phases for which surface complexation is important, as is the case of clays or iron-manganese oxides produced by the present-day weathering of tropical soils.Finally, we analyse the specific behaviour of the REE in rivers of the Cuyuni basin. (C) 2004 Elsevier B.V. All rights reserved.