Carbonate rocks, 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. 

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.)...

A few studies reported considerable intracrystalline isotopic variations within millimetre-sized saddle dolomite crystals. For example, a 1.4%o variation in 6 0 and a 0.7%o variation in 5 C were found in isotopically zoned saddle dolomite crystals from Cretaceous carbonates of south Texas (Woronick Land, 1985). In another study, a 5.2%o decrease in 6 0 (-6 to -11.2%o) and a systematic increase in fluid inclusion homogenization temperature (127-146°C) were reported for a single saddle dolomite crystal in Devonian rocks of Canada (Kaufman et al., 1990). Further, Spangen-berg et al. (1995) reported preliminary stable isotope results for intracrystalline heterogeneities in white sparry dolomite (largely equivalent to saddle dolomite) in samples from a Peruvian MVT... 

Fig. 6. Frequency histogram of 6 C values of saddle dolomite. Note that a significant portion of the samples, particularly saddle dolomite in carbonate host rocks, overlaps the carbon isotopic composition of Phanerozoic seawater (see Lohmann Walker, 1989).

Fig. 9. Compilation of Sr isotope ratios of saddle dolomite. Horizontal arrows indicate studies that reported only a range of values for saddle dolomite occurrences in carbonate rocks. The total range of Phanerozoic seawater Sr/ Sr ratios is shown for comparison (Smalley et at.,...

C.S. acknowledges the support of an Austrian Academy of Sciences APART research fellowship and the Austrian Science Ponds (project PI 0190-GEO) during completion of this study. We thank Diethard Sanders for providing samples from saddle dolomite in alpine carbonate rocks, Reinhard Gaupp for providing some references and David A. Budd and Jay M. Gregg for helpful comments on an earlier version of this paper. 

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