Regional subsurface dolomitization: Models and constraints

The "topographic recharge" and "tectonic compaction" models for subsurface fluid flow may have been overapplied to the question of the origin of regio nal hydrothermal dolomite bodies. These have been attractive models to expl ain hydrothermal dolomitization because of their obvious applicabilty to th e present day hydrology of continental interiors. However, recent modelling studies indicate that topographic recharge does not predict the uniform re gional trends of dolomite precipitational temperatures observed in hydrothe rmal dolomites, and that tectonic compaction requires an unreasonable degre e of fluid focusing to achieve precipitational temperatures equal to observ ed dolomite fluid inclusion homogenization temperatures. Topographic rechar ge also has the limitation of flushing solutes out of the system, rendering it incapable of further dolomitization. In addition, both topographic rech arge and tectonic compaction are unlikely flow mechanisms to explain the or igin of extensive open space dolomite cement because of their limited suppl y of solute. Thermal convection, on the other hand, can support long-lived flow systems that are capable of recycling subsurface solutions many times through the r ock mass. This enhances the opportunity for open space dolomite cementation . Because thermal convection can occur in confined aquifers beneath the sea bed, seawater-derived solutions may be continually added to the convection system. Added seawater would enhance the dolomitization potential of the c onvection system. The documentation of crustal scale convection systems wit hin subaerially exposed orogenic belts and the outcrop evidence of both upw ard and downward extending bodies of hydrothermal dolomite adds credence to the hypothesis that thermally driven convective flow occurred within ancie nt platform carbonates, and may have induced regional hydrothermal dolomiti zation.