Saddle dolomite

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

Machel H.G. (1987) Saddle dolomite as a by-product of chemical compaction and thermochemical sulfate reduction. Geology 15, 936-940. 

Geochemical and fluid inclusion data, in conjunction with mineral paragenetic information, demonstrate that saddle dolomite is a reliable indicator of rock-brine interactions at temperatures that coincide largely with the liquid hydrocarbon window and extend well into the dry gas zone. 

Table 1 gives an overview of the published work that was analysed for the present review. Only studies that provide detailed information on the petrographic and/or compositional characteristics of saddle dolomite were included in the database. We also scanned many more research articles in the recent literature that mention saddle dolomite... 

Studies involving saddle dolomite are grouped into three categories based on their host lithology carbonate, sandstone and carbonate-sandstone mixtures. Most saddle dolomite has been reported from carbonate rocks (Table 1), particularly units associated with MVT ore deposits, whereas only a few studies provide detailed information on saddle dolomite in siliciclastic rocks. 

The amount and quality of information in these studies varies greatly and only a few of the more recent papers applied a combination of several different methods to decipher the origin of saddle dolomite. We scanned published work for the following information on saddle dolomite occurrences optical mineralogy, cathodoluminescence (CL) characteristics, elemental composition, carbon, oxygen and strontium isotopic compositions, and fluid inclusion analysis. In the following sections we shall address these types of data in detail. 

Saddle dolomite has distinct petrographic characteristics that can be distinguished from other types of dolospar, and in many samples this dolomite type can be identified macroscopically. The petrographic features make saddle dolomite a potentially useful index mineral , an aspect that will be addressed below. 

Table 1. Sources of petrographic and geochemical data for saddle dolomite occurrences (post-1980 publications)... 

Although the authors only talk about white coarsely crystalline dolomite it can be inferred from sources cited in this paper and from Qing Mountjoy (1989) that most of Aulstead et at. s (1988) samples are saddle dolomites. 

Shelton et al. (1992) reported data from 476 fluid inclusions from various types of epigenetic dolomites. Because it was unclear which data points refer to saddle dolomites only the range of values was regarded here. 

Many authors reported paragenetic relationships that allow the relative timing of saddle dolomite precipitation within a given rock unit to be determined. There is general agreement among these... 

Fig. 1. (Continued) (E) Coarse-crystalline saddle dolomite replacing host dolomicrospar. Note optical zoning of dolospar due to variable inclusion densities. Triassic limestone with epigenetic dolomite and sphalerite-galena-fluorite mineralization. Eastern Alps (Wanneck area, Tyrol). Scale bar = 500 pm. (F) Host carbonate rock completely replaced by non-planar dolospar. Note that about half of the crystals in the field of view show regularly curved crystal boundaries and/or incipient sweeping extinction, i.e. saddle dolomite. Triassic limestone with epigenetic dolomite and sphalerite-galena-fluorite mineralization. Eastern Alps (Wanneck area, Tyrol). Scale bar = 500 pm. (See also colour Plate 1, facing p. 446.)...

The ultraviolet fluorescence of hydrothermal saddle dolomite was briefly reported for samples from the Ozark region of the USA (Hayes et al., 1989). In this investigation, variations in fluorescence were found to correlate with the mineral paragenesis and CL variations. Changes in fluorescence were interpreted to be due to an interplay of activators (Pb, Mn) and quenchers (Fe). 

Figures 2-4 summarize the currently available information on the stoichiometry and minor or trace element concentration of saddle dolomite in carbonates, sandstones and carbonate-sandstone mixtures. Saddle dolomite shows a variable elemental composition, from near-stoichiometric to calcian dolomite and/or ankerite. The bulk of published major, minor and trace element data on saddle dolomite are based on electron microprobe analyses, although some authors used powder X-ray diffraction analysis to determine the dolomite stoichiometry. Others used spectrometric techniques (e.g. atomic absorption) to analyse for trace elements. We found it difficult to evaluate this published information because some authors report bulk analyses of several crystals, whereas others report intracrystalline elemental variations. Moreover, analyses are often incomplete (e.g. wt% Fe and Mn no data on Ca and Mg) and cannot be directly recalculated in terms of mol%. Further, some authors report a single measurement whereas others list hundreds of data points.

The stoichiometry of saddle dolomite varies widely, from 48 to 64 mol% CaC03. Most contains... 

The values of Fe and Mn reported for saddle dolomite are presented together in Fig. 3 because the geochemical properties of these elements are similar. Fe -i- Mn concentrations in saddle dolomite show wide variability, from <10 ppm to 30 mol% (Fig. 3), reflecting in part the compositional zoning in the saddle dolomite structure. In sandstones. 

Fig. 2. Chart summarizing the stoichiometry of saddle dolomite. The dashed vertical line indicates stoichiometric dolomite. Data from Wojcik et al. (1992) on saddle dolomite and ankerite are plotted in the limestone and sandstone categories, rather than the mixed category (see Table 1). Data published by Searl Fallick (1990) could not be included because of ambiguous data identification.

An interesting feature of some saddle dolomite is the presence of anhedral inclusions of calcite ranging in diameter from < 1 pm to a few tens of micrometres. Using electron microprobe techniques, Radke Mathis (1980) documented the... 

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