A mass-transfer model explaining the origin of Ca and Si in the 'Pre-Salt' South Atlantic lacustrine systems

The complexity of the ‘Pre-Salt’ South Atlantic rift lakes turn these systems into a challenge because very few suitable lacustrine spherulite-forming analogs are known. The origin and nature of the carbonate-silica... [ view full abstract ]

The complexity of the ‘Pre-Salt’ South Atlantic rift lakes turn these systems into a challenge because very few suitable lacustrine spherulite-forming analogs are known. The origin and nature of the carbonate-silica precipitating fluids in these volcanic-dominated systems need to be unraveled if a refined understanding of the genesis of spherulitic carbonate and silica mineral phases are to be gained. The analysis of a particularly good analog system (the East Kirkton Limestone, Carboniferous, Scotland) allowed us to define a new hydrogeochemical model that explain the occurrence and timing of carbonate and silica phases generated in an alkaline lake overlaying basaltic bedrocks.
Using empirical and theoretical approaches, we analyse the link between metal mobilization from sub-surface volcaniclastic rocks, and the potential precipitation of carbonates, Mg-Si minerals and chalcedony in a lacustrine spherulitic carbonate setting. This suite of minerals formed at the surface can be explained by CO2 ingassing to a springwater derived from reaction of alkali igneous rocks in the subsurface with meteoric groundwater. We hypothesize that analogous basin-scale mass-transfer processes may have occurred in the ‘Pre-Salt’ rift lakes constituting a very significant geodynamic mechanism implicated in the development of the laterally extensive spherulitic carbonate deposits and silica mineralisations.