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

Abstract

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 undersaturated 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 dolomitefor-calcite replacement turns out to be self-accelerating via Ca 2؉ : the Ca 2؉ released by each replacement increment accelerates the rate of the next, and so on. As a result, both pore-fluid Ca 2؉ 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 MississippiValley-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 via Ca 2؉ 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 seamless contacts with their replacive walls and consisting of curved, or saddle, dolomite crystals. Serendipitously, both the deformation of the dolomite crystals (produced by Ca-for-Mg substitution driven by the huge pore-fluid Ca 2؉ ) and the seamless rheological transition result from the self-accelerating feedback via Ca 2؉ 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.

Zebra pattern in rocks as a function of grain growth affected by second-phase particles

“…This temperature regime is also found in the literature for the area of the San Vicente mine where the involved fluids are characterized as brines with a temperature of ≈75-160 • C and a pH of ≈5 [11,43,44]. The pH in the work of Spangenberg et al [44] was determined through geochemical analyses on sulfates giving the hydrogeochemical parameters of the mineralizing brine.…”

“…The rhythmic pattern may evolve during dolomitization due to cyclic dissolution/precipitation processes focused in the white layers [7]. Another self-organizing concept which may explain the monotonic thickness and equidistant occurrence of the zebra patterns was introduced by Merino et al [10] and Merino et al [11]. In this model the crystals in the light layers displace the host rock during growth (displacive vein growth).…”

“…The growth of this second dolomite phase not only pushes away the primary dolomite in the dark bands, but also induces a stress field around the light layers. This stress field, which arises from the crystallization stress, is thought to prevent the growth of new material in the direct vicinity [11]. A more general model for the development of banded structures in rocks is the crystal zoning model [3].…”

“…It is widely accepted that the formation of the structure consists of several phases of which the dark fine grained dolomite is the first one. This first phase represents the replacement of the initial limestone by dolomite [5][6][7][8][9][10][11][12][13][14][15]. What exactly triggers the development of the subsequent phases, which build up the coarse grained regions and FIGURE 1 | Examples of zebra textures at different scales.…”

“…The light coarse grained dolomite bands are interpreted as being at least partly void filling cements which precipitated into extensional fractures [12]. Based on the high symmetry of the banding it has been stated that the process which leads to the formation of the zebra texture has to involve self-organization [7,10,11]. The pattern of the zebra dolomite is found to be much too cyclic to be related solely to pre-existing sedimentary features [7].…”