Ferroan dolomite

Silurian Mt. Wills Granite. Gold mineralisation is structurally-hosted and accompanied by the presence of pyrite, arsenopyrite, sphalerite, aurostibite, and a variety of sulfosalts (Crohn 1958). Gangue phases include quartz and ferroan dolomite. 

The phyllic alteration zone coincides with a subtle but consistent shift in the dominant AlOH peak in the short-wave infrared spectrum ( 2210 nm) to slightly lower wavelengths, consistent with an inner white mica-ferroan carbonate mineral assemblage. A preliminary analysis of hyperspectral data over the visible to near infrared range suggests that ferroan carbonates may be detected but not reliably quantified. However, TIR data allow calcite and ferroan carbonate to be distinguished, and may also detect increasing Fe content in ferroan dolomite as mineralized structures are approached. 

The decomposition of the ferroan dolomite in this experiment occurred at a temperature below that for chemically pure dolomite, which is near 807 °C. Smykatz-Kloss (15) has shown that ferroan dolomite decomposes at lower temperatures in fact, he has shown how the temperature of decomposition from differential thermal analyses can be used to determine the weight percentage of FeO in the mineral. 

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 ... 

Iron is replacing magnesium, which is revealed by their strong negative correlation (Fig. 11) (r = -0.97). Ca is also negatively correlated with Fe in ankerite and ferroan dolomite, with >10mol% FeCOs (Fig. 12a) (r =-0.76). Dolomites with < 10 mol% FeCOj show no correlation between Ca and Fe (Fig. 12a) (r =-0.16). These features indicate that in the crystal structure of ankerite and ferroan dolomite there is an increase in alternating... 

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

Fig. 12. Compositional plots of CaC03 mol% against (a) FeC03 and (b) MgC03 mol% in dolomite, ferroan dolomite and ankerite (>10% FeCOj). See discussion in the text.

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). 

Quantitative XRD estimates of proportions of the calcite, ferroan dolomite and siderite in the carbonate assemblage shows that each of these minerals locally dominates the carbonate assemblage, and also occurs in various combinations with the other carbonates (Table 2). Minor siderite is nearly ubiquitous, making up at least a few per cent... 

Ferroan dolomite (including some ankerite, > 20 mol% FeC03) (Fig. 7) is widely distributed in the Breathitt Formation. One sample of relatively early... 

Ferroan dolomite precipitation is also prominently localized around partially dissolved detrital K-feldspars (Fig. 7B). Localization is not strictly within the volume formerly occupied by the K-feldspar, but rather is crudely centred on the feldspar, extending also into the surrounding pore space. Ferroan dolomite clearly postdates quartz cementation. In a few samples there is petrographic evidence suggesting that ferroan dolomite pre-dates the formation of the late calcite. 

Most of the dolomites (including the detrital cores) have calcium-enriched compositions (Table 6 Fig. 8). Zoning within the ferroan dolomite is clearly visible in back-scattered electron images, and the higher Fe contents tend to be characteristic of the later zones (Fig. 7A). The degree of Ca enrichment, however, is not strongly controlled by Fe content and more Fe-rich portions of the crystals display a range in Ca content that is nearly as broad as that of less Fe-rich portions. Mn content is correlated positively with Fe enrichment (Fig. 9). 

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

A generally positive covariation between 6 0 and S C is apparent across the data set as a whole (Fig. 10). However, covariations between 6 0 and 6 C and other parameters, e.g. carbonate content, trace element content or trace element ratios, are absent or very weak. Oxygen isotopic values for both early calcite and early dolomite, combined with temperature constraints inferred from IGVs, are consistent with precipitation from mostly 0-depleted fluids (Fig. 11). Ferroan dolomite and late... 

Table 6. Average compositions of authigenic ferroan dolomite (including 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.

Furthermore, the fact that the late calcite postdates the ferroan dolomite, whereas the most 0-depleted dolomite necessarily precipitated at higher temperatures than the most 0-depleted late calcite (Fig. 11), provides rather weak evidence that the late calcite precipitated after the thermal peak in the basin. 

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. 

The relative temporal variation in carbonate mineralogy, with siderite followed sequentially by early Mg- and Fe-enriched calcite, ferroan dolomite, Mn-enriched ankerite, and finally late Mn-enriched calcite, documents apparent fluctuations in the availability of components for carbonate precipitation. The 0-depleted nature of carbonates across this entire temporal sequence suggests... 

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

Table 7. Sr concentration and Sr isotopic composition of selected calcites and ferroan dolomites...

Iron oxides (< 1%) occur in sandstones of the distal Caioba area associated with dissolved, ferroan dolomite/ankerite and oxidized, coarsely crystalline pyrite. These sandstones display evidence of extensive dissolution and kaolinization of feldspars. Iron oxides (2-22%) also fill pores as alternating bands with quartz/chalcedony (4-13%) in some coarsegrained sandstones and conglomerates that outcrop in the middle-domain Japoata-Penedo area. In... 

Dolomite, ferroan dolomite and ankerite occur mostly as thin (< 20 pm) overgrowths on detrital dolomite grains (Figs 5A,B,F and 6A) or, rarely, as... 

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