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Research ArticleArticles

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

Enriquel Merino and Àngels Canals
American Journal of Science September 2011, 311 (7) 573-607; DOI: https://doi.org/10.2475/07.2011.01
Enriquel Merino
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Àngels Canals
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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 dolomite-for-calcite replacement turns out to be self-accelerating via 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 via 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 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 Ca2+) and the seamless rheological transition result from the self-accelerating feedback via 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.

  • Dolomitization dynamics
  • chemical-rheological feedbacks
  • self-organization
  • replacement physics
  • petrography of replacement
  • geochemistry's blind spot
  • dolostone porosity
  • dolomitizing brines
  • replacive-to-displacive dolomite transition
  • syntaxial zebra veins
  • MVT lead-zinc ores
  • saddle dolomite
  • late-stage calcite
  • dedolomitization
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American Journal of Science
Vol. 311, Issue 7
1 Sep 2011
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Self-accelerating dolomite-for-calcite replacement: Self-organized dynamics of burial dolomitization and associated mineralization
Enriquel Merino, Àngels Canals
American Journal of Science Sep 2011, 311 (7) 573-607; DOI: 10.2475/07.2011.01

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Self-accelerating dolomite-for-calcite replacement: Self-organized dynamics of burial dolomitization and associated mineralization
Enriquel Merino, Àngels Canals
American Journal of Science Sep 2011, 311 (7) 573-607; DOI: 10.2475/07.2011.01
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  • Article
    • Abstract
    • I. INTRODUCTION
    • II. WEAKNESSES OF THE PREVAILING THEORY OF BURIAL DOLOMITIZATION
    • III. REPLACEMENT PHYSICS
    • IV. NEW DOLOMITIZATION MODEL
    • V. DISCUSSION AND FUTURE WORK
    • VI. SUMMARY OF THE MODEL
    • ACKNOWLEDGMENTS
    • APPENDIX 1
    • APPENDIX 2
    • Footnotes
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Cited By...

  • The granulite- to eclogite- and amphibolite-facies transition: a volume and mass transfer study in the Lindas Nappe, Bergen Arcs, west Norway
  • Comment on the "Self-accelerating dolomite-for-calcite replacement model: Self-organized dynamics of burial dolomitization and associated mineralization", v. 311, n. 7, p. 573-607, by Enrique Merino and Angels Canals: Mechanisms for hydrothermal dolomite emplacement
  • Reply: (Reply to the Comment by David W. Morrow on "Self-accelerating dolomite-for-calcite replacement: Self-organized dynamics of burial dolomitization and associated mineralization", v. 311, n. 7, p. 573-607.)
  • Sulfide Replacement Processes Revealed by Textural and LA-ICP-MS Trace Element Analyses: Example from the Early Mineralization Stages at Cerro de Pasco, Peru
  • Formation of Lateral Patterns In Rock Properties By Dolomitization: Evidence From A Miocene Reaction Front (Bonaire, Netherlands Antilles)
  • Dolomites of the Boat Harbour Formation in the Northern Peninsula, western Newfoundland, Canada: Implications for dolomitization history and porosity control
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