Self-accelerating dolomite-for-calcite replacement: Self-organized dynamics of burial dolomitization and associated mineralization

Abstract

A new dynamic model of dolomitization predicts a multitude of textural, paragenetic, geochemical and other properties of burial dolomites. The model is based on two postulates, (1) that the dolomitizing brine is Mg-rich but <i>under</i>saturated with both calcite and dolomite, and (2) that the dolomite-for-calcite replacement happens not by dissolution-precipitation as usually assumed, but by dolomite-growth-driven pressure solution of the calcite host. Crucially, the dolomite-for-calcite replacement turns out to be self-accelerating <i>via</i> Ca²⁺: the Ca²⁺ released by each replacement increment accelerates the rate of the next, and so on. As a result, both pore-fluid Ca²⁺ and replacement rate grow exponentially. As brine enters and infiltrates a limestone, water/rock disequilibrium plus the self-accelerating feedback inevitably yield a process that is self-organized, both in time (as repeated dolomite growth pulses per slice of limestone) and in space (as successive slices). Self-organization in pulses and slices accounts for several properties of burial dolomites: (1) generation of dissolution porosity and its spatially periodic distribution; (2) dolomitization affects only limestones; (3) sharp field contacts between dolomitized and undolomitized limestone; (4) formation of both saddle dolomite and “late-stage” calcite near the end of each growth pulse, accompanied by Mississippi-Valley-type ores if the brine also contains Zn, Pb, Ba, sulfate, and other relevant elements; (5) “sweeping” of ores downflow with accumulation in the last position of the dolomitization front. In addition, the combination of the self-accelerating feedback <i>via</i> Ca²⁺ with the known strain-rate-softening rheology of crystalline carbonates leads to another suite of predictions that are strikingly confirmed by observation. If the dolomite-for-calcite replacement becomes fast enough to lower the local rock viscosity sufficiently, then the dolomite growth will pass spontaneously from replacive to displacive. This is when thin, self-organized, displacive zebra veins form (Merino and others, 2006), indeed displaying <i>seamless</i> contacts with their replacive walls and consisting of <i>curved</i>, or saddle, dolomite crystals. Serendipitously, both the deformation of the dolomite crystals (produced by Ca-for-Mg substitution driven by the huge pore-fluid Ca²⁺) and the seamless rheological transition result from the self-accelerating feedback <i>via</i> Ca²⁺ itself; that is why they are always associated. This detail alone strongly suggests that the new model captures the chemistry, drives, mechanisms, and feedbacks that lend burial dolomitization and its often associated MVT ore deposits their geological uniqueness.