Dolomite ankerite

The fault-fill mineralization includes quartz, dolomite, ankerite, siderite, calcite, molybdenite, pyrrhotite, arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, selenian galena, marcasite, ilmenite, and rutile (Maanijou 2007). 

The major components CaO and MgO in dolomite have a minimal range indicative of very small variation in Ca/(Ca+Mg+Fe) ratios, whereas the minor elements such as SrO and MnO have a wide variation. Therefore trace components in carbonate minerals can be used as discriminant parameters to determine the origin of the carbonate rocks (cf. Yang LeBas 2004). Compositional variation of carbonate minerals from Chehelkureh on the basis of MnO or SrO (wt%) versus [Ca/(Ca+Mg+Fe)j (atoms per formula unit, a.f.u.) diagrams (Fig. 4) show that the MnO and SrO contents in dolomite-ankerite are higher than those in siderite-magnesite and accessory calcite. 

Carbonate cement content in the studied rocks varies from 0 to 45 vol. %, mostly forming the pore filling. The following carbonates were observed (Koztowska 2004) siderite, Fe-dolomite, ankerite and Fe-calcite. The term siderite corresponds to minerals from the isomorphic group FeCOs-MgCOs with 60-100 mol percent FeCOs. Most siderites fall into the interval siderite - sideroplesite (Fig. 3). 

Carbonates Calcite, siderite, dolomite-ankerite Calcite, dolomite-ankerite... 

As a result of the anoxic, low sulphate concentrations in the Me zone, carbonates expected to form include siderite and ferroan dolomite/ankerite (Gautier Claypool, 1984). The precipitation of these carbonates occurs in sediments rich in reactive detrital iron (Coleman, 1985), as follows ... 

Fig. 11. MgC03 mol% versus FeCOs mol% plot of dolomite, ferroan dolomite, ankerite (>10% FeC03) and siderites, showing a highly negative correlation.

Fig. 20. Range of temperature and isotopic composition of the pore fluids constrained for the precipitation of the analysed dolomites/ankerite (5 0 = -11.9%o to -1.5%o). Field marked for 5 0 a[er between -5 and -3%o smow (fractionation equation after Rosenbaum Sheppard, 1986).

Fig. 21. Plot of 5 OpDB versus (a) FeC03% and (b) MnC03% of dolomite/ankerite, siderite and calcite cements. See comments in the text.

Fig. 22. Plot of intergranular volume (%) versus 5 0 of dolomite/ankerite, siderite and calcite. See text for discussion.

Semiquantitative estimates of the relative abundances of siderite, calcite and ferroan dolomite/ ankerite were made by comparing corrected XRD (X-ray diffraction) peak intensities using the method of Lynch (1997). 

Oxygen and carbon isotopic trends for calcite and ferroan dolomite/ankerite... 

Fig. 7. Thin-section scale localization of authigenic ferroan dolomite. Back-scattered electron images. Scale bars 100 tm. (A) Ferroan dolomite (f) localized on non-ferroan detrital dolomite (d) that has been fractured, partially dissolved, and partially replaced by the authigenic overgrowth. Note that the outer zones of the ferroan dolomite (f ) are brighter, reflecting their greater Fe content. (B) Ferroan dolomite (ankerite) (f) localized in the vicinity of a partially dissolved and replaced K-feldspar (k). The arrow indicates euhedral termination on a quartz overgrowth.

Sr concentrations and Sr/ Sr ratios were determined for a limited set of carbonate-rich samples (Table 7). Sr/ Sr ratios in early and late calcites and in dolomite/ankerite are well above the range for marine Sr during the Phanerozoic (based on a comparison with the data of Burke et al., 1982), suggesting a predominantly silicate derived (i.e. radiogenic) source of Sr, in both early and late diagenesis. Sr concentration in ferroan dolomite/ ankerite is uniformly below the detection limit. In calcite, Sr concentration, though well above the detection limit in several samples, does not vary systematically between early and late calcites, with Sr/ Sr ratios, or with other trace elements in the calcites, either within or between samples. 

Fig. 9. Fe/Mg molar ratio versus Mn content for ferroan dolomite/ankerite.

Fig. 10. 8 0 versus 8 C for calcites and ferroan doloraite/ankerites. For the purpose of comparison with calcite, 8 0 values for dolomite/ankerite are adjusted by -3%o, to compensate for approximate differences in dolomite-water fractionation relative to calcite (Land, 1980). This correction allows for a better separation of the calcite and dolomite/ankerite data points, better revealing the contrasts in the conditions of precipitation of these phases. 

Fig. 11. Range of possible temperature and 8 0 j,jr conditions for calcite and dolomite (ankerite) precipitation. Calculated from the calcite-water fractionation equation in Friedman O Neil (1977). As in Fig. 10, 8 0 values plotted for dolomite/ankerite are adjusted -3%o from measured values to compensate for differences in fractionation relative to calcite (Land, 1980), thus allowing the same fractionation equation to be used for both minerals. (This correction effectively turns values for dolomite/ankerite into calcite values for the purpose of examining the range of temperature and 6 0 a,er conditions in effect during precipitation, and is approximate only.)...

Fig. 8. Composition of eodiagenetic (Cl and Dl) and mesogenetic (C2, D2 and D3) calcite and dolomite/ankerite cements.

Three types of dolomite/ankerite were distinguished in the Serraria sandstones based on textural... 

Fig. 10. Plot of the S CpDB and 5 OpDg values (%o) for dolomite/ankerite and calcite.

Dolomite/ankerite D2 occurs as blocky to poikilotopic, irregularly zoned rhombs (90-200 pm) which marginally replace and cover, and thus postdate, D1 (Fig. 7F). Samples cemented mainly by D2 show moderate packing, indicating a dominantly... 

Both D1 and D2 dolomite/ankerite types show evidence of partial dissolution, which preferentially affects the Fe-rich zones (Fig. 11B). Some intracrystalline dissolution pores contain authigenic kaolin-ite booklets. In some samples of the middle domain partially dissolved poikilotopic crystals of D1 and D2 are covered, and thus postdated, by thin chlorite rims (Fig. 11C). 

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