ABSTRACT

Dolomitization is so widespread that it can be accounted for only by conditions commonly encountered in nature. Most dolomites are considered secondary because they exhibit some trace of a pre-existing texture, with the crystal size determined by the original grain size and type of matrix. Microcrystalline dolomites without preserved skeletal debris are generally taken as primary, although there is no criterion to distinguish true primary dolomites from dolomitized aphanitic lime muds.

For any metasomatic alteration by aqueous solutions to take place, compounds capable of producing the reaction have to be generated in adequate amounts and be transported to the site of alteration. Bonds exist between magnesium and various inorganic and organic acid radicals and bacteria. The weaker these bonds, the less the conditions of dolomitization have to deviate from normal pressures and temperatures, and the more likely the compounds may become involved in the reaction.

Magnesium ions can be supplied by sea water directly, but limestones exposed to sea water alone do not dolomitize--even under continuous high pressure at abyssal depths. The failure to produce dolomites experimentally from evaporating sea water under conditions normally encountered in nature and the occurrence of fossil evaporites in juxtaposition with limestones support this notion. Moreover, magnesium chloride acts as an inhibitor in most dolomitization processes. Adsorption from sea water during sedimentation can take place only in the calcite, but not in the aragonite lattice; in any case it ceases before a sufficient amount of magnesium is taken up to form a self-supporting frame-work.

Weathering of magnesium-rich igneous rocks is another major direct supplier of magnesium compounds. Because volcanic soils contain only a small fraction of the magnesium available in the bedrock, a substantial removal of magnesium must occur. This can be effected by fresh ground water that infiltrates even deeply buried subsurface brines and that can lose its magnesium compounds within a short distance, e.g. in contact with bentonites. Since ground water can move humates of uranium, zinc, lead and other ores, secondary dolomitization appears to be only a special case of phreatic mineralization.

Recent primary dolomites in coastal regions can be linked to seasons of vigorous plant growth. Dolomite forming in the putrid muds of some freshwater lakes or found in fossil coals may also be associated with flourishing vegetation. Evaporation of sea or ground water commonly produces only hydrous magnesium carbonates, whereas primary dolomites appear dependent on the metabolism of bacteria or of higher plants.

Secondary dolomitization requires either a coastal plant growth in an area of calcite exposure, or active chemical erosion aided by a flourishing vegetation and a high enough relief to give ground waters a hydrodynamic flow potential for moving products of weathering and putrefaction. This is substantiated (1) by the frequent occurrence of dolomites in juxtaposition to ultrabasics or other loci of magnesium-rich ground water, (2) by a parallelism observed between magnesium content and tectonism, and (3) by the often bituminous character of ancient dolomites. In this way the prevalent dolomitization in Alberta and elsewhere of subcrops and of aquifers sandwiched between tight beds can be explained.

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