『Abstract
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 the other
major tropical rivers such as the Congo and Amazon, and vary i
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 (Msusp/Mdiss)
appears related to the ratio of suspended to dissolved organic
carbon contents (SS/DOC), according to the relationship:
Msusp/Mdiss = Ki*SS/DOC
The coefficient of proportionality Ki 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 Ki
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.
Keywords: Cuyuni basin; Suspended, dissolved and total load; Major
ad trace elements』
1. Introduction
2. The Cuyuni river basin study area
3. Sampling and analytical methodology
4. Result
4.1. Suspended particulate load
4.2. Dissolved load
4.2.1. Cationic and anionic charges and TDS contents
4.2.2. Source control of the dissolved load
4.3. Total load: geochemical characteristics of the Cuyuni rivers
in the two geologically distinct zones
4.4. Suspended load/dissolved load relationship
4.4.1. Observed relationship between the suspended and dissolved
load contents and implications
4.4.2. Nature of the coefficient Ki
4.4.3. Correlation of the “Ki coefficients”
with hydrolysis constants
4.5. Saturation indexes of mineral phases and element speciation
in solution
4.6. REE distribution
5. Conclusions
Acknowledgements
References